The faster an engine revs, the more the contents of the cylinder swish around, so combustion is completed in less time.
So as engine RPM increases, combustion speed also increases; the two effects seem to balance each other, and often spark advance is constant above a certain RPM.
The limiting factor for rpm was always said to be piston velocity. Although more modern alloys are stronger and lighter, the stresses of acceleration from a dead stop to 150 feet per second then back to a dead stop twice per revolution is not an easy thing to tolerate for pistons to handle. Smaller bore also means smaller valves., So without forced air induction, that limits flow. Lots to consider for engine designers. I do find it interesting that the 1.3 liter 3 cylinder that is in my Buick Encore GX is said to have the highest power density of any passenger vehicle except for supercars. The 1.3 ltr has a stroke significantly bigger than it's bore.
Originally posted by Daryl M: I do find it interesting that the 1.3 liter 3 cylinder that is in my Buick Encore GX is said to have the highest power density of any passenger vehicle except for supercars. The 1.3 ltr has a stroke significantly bigger than it's bore.
You are correct about the time at a given rpm, but a long stroke smaller bore engine of the same displacement as a larger bore short stroke engine does not rev as high therefore there is more time for combustion.
The limiting factor for rpm was always said to be piston velocity. Although more modern alloys are stronger and lighter, the stresses of acceleration from a dead stop to 150 feet per second then back to a dead stop twice per revolution is not an easy thing to tolerate for pistons to handle. Smaller bore also means smaller valves., So without forced air induction, that limits flow. Lots to consider for engine designers. I do find it interesting that the 1.3 liter 3 cylinder that is in my Buick Encore GX is said to have the highest power density of any passenger vehicle except for supercars. The 1.3 ltr has a stroke significantly bigger than it's bore.
The entire auto industry is investing huge amounts of money in combustion simulation using computational fluid dynamics. This has resulted in the ability of new engine designs to operate at extremely high BMEPs (high compression plus high boost) that would have been impossible a few years ago. One of the outcomes of this is the finding that a bore of around 84mm is "about right" to control combustion for extremely high BMEP operation, but the combustion process favors smaller bores. Once the OEM can get as much torque as they want out of an engine by cranking up BMEP, then there's no need to spin the engine very fast to make power.
[This message has been edited by Will (edited 01-12-2022).]
Originally posted by Will: The entire auto industry is investing huge amounts of money in combustion simulation using computational fluid dynamics. This has resulted in the ability of new engine designs to operate at extremely high BMEPs (high compression plus high boost) that would have been impossible a few years ago. One of the outcomes of this is the finding that a bore of around 84mm is "about right" to control combustion for extremely high BMEP operation, but the combustion process favors smaller bores. Once the OEM can get as much torque as they want out of an engine by cranking up BMEP, then there's no need to spin the engine very fast to make power.
With a lower average rpm, does it mean that the engine will last longer? Longer stroke engines have traditionally had broader power bands which made them more "drivable" . Or am I mistaken?
FYWIW, I had a 1996 GEO Metro (made by Suzuki) that had a 3 cyl engine, it got 50 mpg highway, 45 around town cost me about $7500 new
it had a whopping 55 HP, funny thing is, it was so light it was not the slowest beast on the road in fact it was so light that one time I discovered I had a flat rear tire driving home from work on the interstate, and since at the time there were metered on and off ramps, and I didn't want to have to get off the interstate, combined with the fact that tires could be bought for $15, I drove it home and the tire wasn't even ruined
With a lower average rpm, does it mean that the engine will last longer? Longer stroke engines have traditionally had broader power bands which made them more "drivable" . Or am I mistaken?
Ummm...
It's sort of a self-fulfilling prophesy... If your engine has a small bore and small valves for its displacement, do you concentrate on top end power or low RPM torque? (In addition to port development, like BMW's S54)
If your engine has a big bore and big valves for its displacement, why concentrate on low RPM torque when you can go for top end power?
A 5 liter engine makes 5 liters of torque regardless of its bore and stroke.
Lower cruise RPM for any engine gives better mileage and slower wear. It doesn't even have to be a "low RPM" engine. LS3's and LT1's turn <2000 RPM at 80 mph. While they're biased toward the top end, they make "enough" low RPM torque to handle tall gearing. Most of the ability to cruise at low RPM comes from transmissions like CVTs and 8 speed automatics. There hasn't historically been the capability to have a first gear >4:1 AND a top gear ~0.6:1 in the same transmission.
Compare the Cadillac 4.9--square bore/stroke but done by 4500 RPM--to the S54--slightly undersquare, but makes power to 8000 RPM--to see that there's a lot more to it than just bore and stroke.
[This message has been edited by Will (edited 01-13-2022).]
Wiki has been caught many times removing creditable and factual information to support a narrative therefore I don't go to their site for anything anymore, that's why I prefer to ask the person who actually drives the car.
[This message has been edited by Skybax (edited 01-13-2022).]
Good thing the F40 came along. One of its applications does exactly this and another is close.
The F40 still has perceived problems (well, in my view) where the top gears (such as 5, 6) are too close to each other. A factor of 1.2-ish between 5&6 isn't really worth it, unless you're driving 250 km/h and need every bit of power before/after the shift to keep from losing speed. I've never driven that fast...
On the other hand, the factor between 1&2 is huge, around 1.9, creating a no-man's land where an intermediate gear would be nice.
It seems like too much gear ratio progression, creating gaps in lower gears, and redundancy in the higher gears. Keeping the same span, the ratios could be more evenly distributed.
It's sort of a self-fulfilling prophesy... If your engine has a small bore and small valves for its displacement, do you concentrate on top end power or low RPM torque? (In addition to port development, like BMW's S54)
If your engine has a big bore and big valves for its displacement, why concentrate on low RPM torque when you can go for top end power?
A 5 liter engine makes 5 liters of torque regardless of its bore and stroke.
Lower cruise RPM for any engine gives better mileage and slower wear. It doesn't even have to be a "low RPM" engine. LS3's and LT1's turn <2000 RPM at 80 mph. While they're biased toward the top end, they make "enough" low RPM torque to handle tall gearing. Most of the ability to cruise at low RPM comes from transmissions like CVTs and 8 speed automatics. There hasn't historically been the capability to have a first gear >4:1 AND a top gear ~0.6:1 in the same transmission.
Compare the Cadillac 4.9--square bore/stroke but done by 4500 RPM--to the S54--slightly undersquare, but makes power to 8000 RPM--to see that there's a lot more to it than just bore and stroke.
Not sure I can agree with your logic about the 4.9. If the reason it was done at 4500 was bore and stroke, the LS4 would suffer the same fate, but it doesn't. Stroke shouldn't limit rpm until piston speeds start getting to 150 fps. Or so I've been told. That means that your 3.5 inch stroke engine doesn't have a big problem with spontaneous disassembly till almost 8000 rpm. I would guess that the 4.9 only revs to 4500 because it has heads, exhaust and intake designed for lugging around a giant heavy Cadillac at low rpm.
Not sure I can agree with your logic about the 4.9. If the reason it was done at 4500 was bore and stroke, the LS4 would suffer the same fate, but it doesn't. Stroke shouldn't limit rpm until piston speeds start getting to 150 fps. Or so I've been told. That means that your 3.5 inch stroke engine doesn't have a big problem with spontaneous disassembly till almost 8000 rpm. I would guess that the 4.9 only revs to 4500 because it has heads, exhaust and intake designed for lugging around a giant heavy Cadillac at low rpm.
Right, the 4.9 is RPM limited because of its heads... But the S54 is an example of a highly developed undersquare engine that can spin to 8000.
Honda K24's and Mitsu 4G64's with ~4" strokes regularly exceed 8000 RPM. On the domestic side, mountain motors run >5" of stroke to 8000 RPM.
However, most of the time discussion is around average, rather than peak piston speed. Even the Honda F20 (84mm stroke) only hits ~83 fps average at 8900 RPM.
As do GM's 6 speed automatics... but that takes a minimum of 6 gears and those have only been around the last 15 years. The newer crop of 8 and 10 speed transmission do even better. The 2.8 Babymax diesel with 8 speed transmission can get close to 30 MPG moving 2500 and 3500 full size vans... and the transmission is a huge component of that.
[This message has been edited by Will (edited 01-14-2022).]
Right, the 4.9 is RPM limited because of its heads... But the S54 is an example of a highly developed undersquare engine that can spin to 8000.
Honda K24's and Mitsu 4G64's with ~4" strokes regularly exceed 8000 RPM. On the domestic side, mountain motors run >5" of stroke to 8000 RPM.
However, most of the time discussion is around average, rather than peak piston speed. Even the Honda F20 (84mm stroke) only hits ~83 fps average at 8900 RPM.
As for aftermarket modifications on engines, there is a reason why manufacturers don't build engines like that. Longevity. A well built performance engine with parts built from special alloys can rev to higher piston speeds, but not reliably. This is where the long rod designed engine comes into play. A block with a tall deck height allows for less connecting rod angle change. That helps lessen stress on parts like rods and Pistons. Performance engines also don't give much consideration to things like heating and cooling cycles that over the years contribute to metal fatigue. Production engines, first and foremost, must last . Hotroders don't make that a high priority.
I do feel that when we talk about bottom end torque, we should not be speaking about torque at a specific RPM, but we should be speaking about horsepower at some percentage of redline.
Car engine tachometers could be calibrated in %redline, like this Boeing 737-300 tach:
Is an 8000 RPM redline engine more fun than a 5000 RPM redline engine? If gearing compensates for different RPMs, the driver might not be able to tell the difference between both engines, if the instrument cluster doesn't have an RPM readout.
In my mind, it makes a lot of sense to normalize the horizontal axis of dyno graphs to %redline, for better comparison between engines.
That should be "Percent of RPM Divided By 10", rather than "multiplied by 10", at least based on this particular gauge.
Originally posted by Skybax: I realize its off topic, but throwing numbers around I just want to elaborate, the stock Getrag 5-speed V6 Fiero's are in the 16-second range, while the stock 85-86 Muncie 4-speed V6 Fieros are in the mid 15-second range.
The difference is just due to shift points. I believe the Getrag requires a shift just before 60. I would bet a signed dollar if we put down 0-X MPH in 5 or 10 MPH increments for the Getrag vs Muncie, the Getrag would win about as many as the Muncie.
[This message has been edited by reinhart (edited 01-15-2022).]
Would this post be a good place to discuss the pros and cons of long stroke smaller bore engines vs short stroke larger bore engines? Bigger bore allows for larger valves and better breathing. Longer stroke allows for a longer time to more completely burn the air/fuel mixture making for better efficiency.
Longer stroke engines allows for a shorter time to completely burn the air fuel mixture. And it's due to the higher piston velocity and acceleration. Take for example these two 5.0L engines.
Note that Engine 1 has more piston velocity from TDC or 0* to 74* crank angle compared to Engine 2. Also note that Engine 2 needs and extra 1040rpm and an extra 3* of crank angle travel to match the piston speed of Engine 1. So this shows that since Engine 1 piston speed is higher than Engine 2's, the time to burn the air fuel mixture is much shorter than Engine 2. So a timing adjustment is needed to start ignition earlier and progressively increase it as RPM goes up. If timing is fixed specially in the long stroke engine, the piston speed will out run the flame front at high RPMs.
Longer stroke engines allows for a shorter time to completely burn the air fuel mixture. And it's due to the higher piston velocity and acceleration. Take for example these two 5.0L engines.
Note that Engine 1 has more piston velocity from TDC or 0* to 74* crank angle compared to Engine 2. Also note that Engine 2 needs and extra 1040rpm and an extra 3* of crank angle travel to match the piston speed of Engine 1. So this shows that since Engine 1 piston speed is higher than Engine 2's, the time to burn the air fuel mixture is much shorter than Engine 2. So a timing adjustment is needed to start ignition earlier and progressively increase it as RPM goes up. If timing is fixed specially in the long stroke engine, the piston speed will out run the flame front at high RPMs.
All true, but typically longer stroke engines are not designed to develop maximum power at as high an rpm as a shorter stroke engine. Your data indicates this. Your two engines assume the same deck height, right? A well designed longer stroke engine should have a taller deck height. Will that not effect crank angle? Those are the benefits a manufacturer has. They can design different size blocks for longer stroke engines than for short stroke engines. The rest of us have to use what is available. I find these mental exercises fascinating.
All true, but typically longer stroke engines are not designed to develop maximum power at as high an rpm as a shorter stroke engine. Your data indicates this. Your two engines assume the same deck height, right? A well designed longer stroke engine should have a taller deck height. Will that not effect crank angle? Those are the benefits a manufacturer has. They can design different size blocks for longer stroke engines than for short stroke engines. The rest of us have to use what is available. I find these mental exercises fascinating.
The longer stroke not designed for high rpm mentality is obsolete now. The 427ci smallblock has 4.125 bore and 4.000 stroke revving past 8200rpms and this is one of the tiny Mountain motors Will mentioned earlier. Then you have the Ricers with their K24s Frankensteins. All those are longer stroke squared engines. The advantages of a longer stroke over a short stroke are many. Using the two engines in my previous example in which the only difference is the geometry of the rotating assembly between the two the advantages of the longer stroke (Engine 1) are: -More compression. -More swept volume -More torque -Less sensitive to bigger camshaft compared to the short stroke engine. -Less sensitive to bigger intake ports compared to the short stroke engine. -Less prone to knock -It favors turbo or supercharging. Due to more swept volume, more air can be packed resulting in more power. -Very wide powerband.
The only advantage the Short stroke has is less friction. The dweling of the piston at TDC is wasted time because the crankshaft is rotating for a few degrees without any force acting on it which translates in slower acceleration. But like you, I also find these mental exercises super fascinating!
[This message has been edited by La fiera (edited 01-18-2022).]
Originally posted by La fiera: Engine 2 Bore= 4.0 Stroke= 3.0 Rod Ratio= 1.9 Displacement= 5.0L (4942cc) Piston Velocity @ 77* crank angle= 4873fpm @ 6000rpm (5717fpm @ 7040rpm)
AKA a Chevy 302
quote
Originally posted by La fiera:
Longer stroke engines allows for a shorter time to completely burn the air fuel mixture. And it's due to the higher piston velocity and acceleration. Take for example these two 5.0L engines.
Throughout all of this, I'm neglecting the effects of rod angle to simplify the math. Not so much... The 302 is within 0.030 of TDC within 11.5 degrees of TDC while the 305 is within 0.030 of TDC within 10.6 degrees of TDC. Not a big difference.
quote
Originally posted by La fiera: Note that Engine 1 has more piston velocity from TDC or 0* to 74* crank angle compared to Engine 2. Also note that Engine 2 needs and extra 1040rpm and an extra 3* of crank angle travel to match the piston speed of Engine 1. So this shows that since Engine 1 piston speed is higher than Engine 2's, the time to burn the air fuel mixture is much shorter than Engine 2. So a timing adjustment is needed to start ignition earlier and progressively increase it as RPM goes up. If timing is fixed specially in the long stroke engine, the piston speed will out run the flame front at high RPMs.
Well... The 305 is going to have 16% higher piston speed at EVERY crank angle because it has 16% more stroke. I don't think anyone is doubting that. Piston speed at constant RPM is a weird flex.
The things you're talking about are relevant to tuning, but not power production. Also, there's not a big difference between the timing required between a 305 and a 302. They're both going to make best power with max advance in the 30 degree range. Some 4 valve heads like BMWs are in the 20 degree range for NA use and teens for boost, without being knock limited. That's a big difference.
quote
Originally posted by La fiera:
The longer stroke not designed for high rpm mentality is obsolete now. The 427ci smallblock has 4.125 bore and 4.000 stroke revving past 8200rpms and this is one of the tiny Mountain motors Will mentioned earlier. Then you have the Ricers with their K24s Frankensteins. All those are longer stroke squared engines.
[QUOTE]Originally posted by La fiera: The advantages of a longer stroke over a short stroke are many. Using the two engines in my previous example in which the only difference is the geometry of the rotating assembly between the two the advantages of the longer stroke (Engine 1) are: -More compression. -More swept volume -More torque -Less prone to knock -It favors turbo or supercharging. Due to more swept volume, more air can be packed resulting in more power.
None of these follow from the basic dimensions of the engine. There is no first principles argument for this and discussion devolves to who has the better intake port. Well... a 305 will have more swept volume than a 302 because each cylinder in a 305 has 38.125 cid while each cylinder of the 302 has 37.75 cid. That's related to total displacement, not just stroke. If you calculate torque output related to cylinder pressure, torque is related ONLY to displacement, not stroke. The longer stroke engine has a longer lever in the crank, but the bigger bore engine has more bore area for the cylinder pressure to push on.
quote
Originally posted by La fiera: -Less sensitive to bigger camshaft compared to the short stroke engine. -Less sensitive to bigger intake ports compared to the short stroke engine. -Very wide powerband.
Maybe? Depends on the cylinder head & cam choice, but not on basic dimensions.
quote
Originally posted by La fiera: The only advantage the Short stroke has is less friction. The dweling of the piston at TDC is wasted time because the crankshaft is rotating for a few degrees without any force acting on it which translates in slower acceleration. But like you, I also find these mental exercises super fascinating!
Again... mutatis mutandis doesn't apply to anything in an engine. Comparing short stroke to long stroke at constant bore compares different displacements. Comparing short stroke to long stroke at constant displacement compares different bores. At constant deck height, different rod ratios... The list goes on.
The bigger bore engine can fit a bigger intake valve, which allows more air into the cylinder. That sounds like an advantage to me. The longer stroke engine needs to achieve a higher port speed just to have air get to the bottom of the cylinder when the piston is approaching BDC. Since flow through a pipe is related to the square root of pressure difference, higher piston speed does not achieve enough additional port speed to account for the greater distance the air has to travel to fill the cylinder.
[This message has been edited by Will (edited 01-19-2022).]
Again... mutatis mutandis doesn't apply to anything in an engine. Comparing short stroke to long stroke at constant bore compares different displacements. Comparing short stroke to long stroke at constant displacement compares different bores. At constant deck height, different rod ratios... The list goes on.
The bigger bore engine can fit a bigger intake valve, which allows more air into the cylinder. That sounds like an advantage to me. The longer stroke engine needs to achieve a higher port speed just to have air get to the bottom of the cylinder when the piston is approaching BDC. Since flow through a pipe is related to the square root of pressure difference, higher piston speed does not achieve enough additional port speed to account for the greater distance the air has to travel to fill the cylinder.
All interesting stuff, but your comparison doesn't address over square engines. (Stroke significantly longer than bore diameter) The 1.3l and the 2.5 LCV Ecotec both have a stroke about 15% longer than their bore size. With production engines today, excluding exotic supercars, I'm told the 1.3 has the highest power density of any production engine. (Hp per cubic inch) . If you calculate power density as hp per pound of weight, then I don't know where it is, but it is still pretty respectable. If you go by the rumbling on the net, there are already those who are planning mods to get even more power from this small engine. I am just fascinated by the amount of useable power over a broad rpm range, these new engines are achieving.
All interesting stuff, but your comparison doesn't address over square engines. (Stroke significantly longer than bore diameter) The 1.3l and the 2.5 LCV Ecotec both have a stroke about 15% longer than their bore size. With production engines today, excluding exotic supercars, I'm told the 1.3 has the highest power density of any production engine. (Hp per cubic inch) . If you calculate power density as hp per pound of weight, then I don't know where it is, but it is still pretty respectable. If you go by the rumbling on the net, there are already those who are planning mods to get even more power from this small engine. I am just fascinated by the amount of useable power over a broad rpm range, these new engines are achieving.
Very interesting stuff, I don't mean to hijack cause its related, but I once had a 1973 Vega (first body style like Ferrari) with a 1975 twin-cam 2.0 Cosworth engine sleeved and stroked to 2.3 and dialed in with the help of Duke Williams, and still running the GM fuel injection system with rebuild-able and adjustable brass sensors, that was fun! So I wonder how that would compare to your highest power density aka Hp per cubic inch with a stroke longer than bore. (my modding days are over and enjoy stock/original boring stuff now, so my hot rod memory is very limited nowadays) I was digging thru my Chevy folder looking for specs but didn't find much...
[This message has been edited by Skybax (edited 01-19-2022).]
Power density goes down as displacement per cylinder increases... it's just like that. Motorcycles have the highest.
At the lower end of power density you have the 14-cylinder Wärtsilä-Sulzer RT-flex96C turbodiesel. 108920 hp from 1562358 cubic inches. 0.07 hp / cubic inch
Power density doesn't tell you if the engine is fit for its purpose; other characteristics such as fuel consumption, power, size, weight, pollution, maintenance cost, production cost, are the actual external characteristics that matter. How an engine achieves its external characteristics does not matter, in my mind.
I don't really like how different jurisdictions tie car registration fees to engine displacement; instead of giving folks the leeway to optimize designs as they see fit to meet the true requirements (external characteristics), folks must work around meeting arbitrary registration rules.
All interesting stuff, but your comparison doesn't address over square engines. (Stroke significantly longer than bore diameter) The 1.3l and the 2.5 LCV Ecotec both have a stroke about 15% longer than their bore size. With production engines today, excluding exotic supercars, I'm told the 1.3 has the highest power density of any production engine. (Hp per cubic inch) . If you calculate power density as hp per pound of weight, then I don't know where it is, but it is still pretty respectable. If you go by the rumbling on the net, there are already those who are planning mods to get even more power from this small engine. I am just fascinated by the amount of useable power over a broad rpm range, these new engines are achieving.
Undersquare
So apparently this wasn't so clear...
quote
Originally posted by Will:
The entire auto industry is investing huge amounts of money in combustion simulation using computational fluid dynamics. This has resulted in the ability of new engine designs to operate at extremely high BMEPs (high compression plus high boost) that would have been impossible a few years ago. One of the outcomes of this is the finding that a bore of around 84mm is "about right" to control combustion for extremely high BMEP operation, but the combustion process favors smaller bores. Once the OEM can get as much torque as they want out of an engine by cranking up BMEP, then there's no need to spin the engine very fast to make power.
Modern engines with turbo, DI and variable cam phasing do amazing things with broadening torque curves, running high boost on low octane and making high specific torque.
Brand new designs of small bore/undersquare engines with highly optimized combustion spaces run absolutely extraordinary combinations of boost, compression and timing that were the sole domain of 100+ octane fuel just a few years ago. The combination of these technologies results in never-before-achievable levels of specific torque out of "normal" engines... which then allows downsizing to meet ridiculously overbearing fuel economy regs.
The result of optimizing the combustion space and charge air/mixture motion better than ever before results in gasoline engines that are almost immune to knock and running diesel-like boost levels and thermal efficiency.
[This message has been edited by Will (edited 01-20-2022).]
Modern engines with turbo, DI and variable cam phasing do amazing things with broadening torque curves, running high boost on low octane and making high specific torque.
Brand new designs of small bore/undersquare engines with highly optimized combustion spaces run absolutely extraordinary combinations of boost, compression and timing that were the sole domain of 100+ octane fuel just a few years ago. The combination of these technologies results in never-before-achievable levels of specific torque out of "normal" engines... which then allows downsizing to meet ridiculously overbearing fuel economy regs.
The result of optimizing the combustion space and charge air/mixture motion better than ever before results in gasoline engines that are almost immune to knock and running diesel-like boost levels and thermal efficiency.
Isn't all of that a good thing? Better efficiency on cheaper gas? More power over a broader rpm range at lower rpm? All of those things sound like positives to me.
I do find it interesting that the 1.3 liter 3 cylinder that is in my Buick Encore GX is said to have the highest power density of any passenger vehicle except for supercars. The 1.3 ltr has a stroke significantly bigger than it's bore.
That is a nice small engine. The output from the LT3 is 155 bhp from 1300 cc. That is very good - a power/displacement ratio of 119 bhp/liter.
Not the highest though. The Honda S2000 had 123 without a turbo being needed and the GM LNF engine had 130 ins stock form or 145 with the factory GMPP tune. (I have 188 in my Solstice but that uses a non-stock turbo unit, so may be seen as 'cheating'.
Again... mutatis mutandis doesn't apply to anything in an engine. Comparing short stroke to long stroke at constant bore compares different displacements. Comparing short stroke to long stroke at constant displacement compares different bores. At constant deck height, different rod ratios... The list goes on.
The bigger bore engine can fit a bigger intake valve, which allows more air into the cylinder. That sounds like an advantage to me. The longer stroke engine needs to achieve a higher port speed just to have air get to the bottom of the cylinder when the piston is approaching BDC. Since flow through a pipe is related to the square root of pressure difference, higher piston speed does not achieve enough additional port speed to account for the greater distance the air has to travel to fill the cylinder.
A longer stroke engine achieves higher port speed due to higher piston velocity compared to the shorter stroke engine with the same port size/volume and valves. To OVERFILL the cylinder including those couple of degrees where the piston moves slower near BDC I use cam timing or to be more precise, cam overlap. If you install bigger valves in the shorter stroke engine it will increase port flow indeed, but it will need to be revved a couple of thousands RPMs higher to take advantage of those bigger valves. To solve this problem just install 2 smaller intake valves to increase velocity in the port, like your Northstar Will. At a lower piston speeds (lower rpms) the shorter stroke engine with the one big intake valve and port will have such air stalling airflow that a 3 cylinder Geo Metro with 2 spark plugs disconnected will have more torque from 850 to 2000rpm.
That is a nice small engine. The output from the LT3 is 155 bhp from 1300 cc. That is very good - a power/displacement ratio of 119 bhp/liter.
Not the highest though. The Honda S2000 had 123 without a turbo being needed and the GM LNF engine had 130 ins stock form or 145 with the factory GMPP tune. (I have 188 in my Solstice but that uses a non-stock turbo unit, so may be seen as 'cheating'.
https://youtu.be/Td9Gz_h7Qpg This is not the same cam phasing like the L3T. 120bhp/liter in the Abarth 500 and 124 Abarth.
A longer stroke engine achieves higher port speed due to higher piston velocity compared to the shorter stroke engine with the same port size/volume and valves. To OVERFILL the cylinder including those couple of degrees where the piston moves slower near BDC I use cam timing or to be more precise, cam overlap. If you install bigger valves in the shorter stroke engine it will increase port flow indeed, but it will need to be revved a couple of thousands RPMs higher to take advantage of those bigger valves. To solve this problem just install 2 smaller intake valves to increase velocity in the port, like your Northstar Will. At a lower piston speeds (lower rpms) the shorter stroke engine with the one big intake valve and port will have such air stalling airflow that a 3 cylinder Geo Metro with 2 spark plugs disconnected will have more torque from 850 to 2000rpm.
For each intake stroke, each example engine tries to pull 38 cubic inches through the intake port. At 6000 RPM, that translates to an AVERAGE volumetric flow of 38 in3 / 5ms = 264 CFM. That volume and that flow rate are not affected by the relationship of bore to stroke. The volume rate at which the piston tries to draw through the port is by far the primary driver of the pressure difference that drives flow through the port.
Greater piston speed by itself doesn't mean anything because in the smaller bore the volumetric flow rate remains the same.
Overlap happens at TDC, man The blowdown energy that a tuned exhaust system captures is transferred back to the intake system via the overlap period... but that's the result of intake and exhaust manifold design a lot more than bore and stroke.
GM's LS7 squeezed a 2.205 intake valve into a 4.125" bore... they never had any problems with low end torque on that engine.
Going with two intake valves is done to increase curtain area, not port speed. If port speed were the overriding concern, just keep one small intake valve
More problems with low end torque are related to low speed through carburetor venturis resulting in poorly metered fuel or terrible atomization. That's not nearly as much of a problem with port EFI.
Greater piston speed by itself doesn't mean anything because in the smaller bore the volumetric flow rate remains the same.
Overlap happens at TDC, man The blowdown energy that a tuned exhaust system captures is transferred back to the intake system via the overlap period... but that's the result of intake and exhaust manifold design a lot more than bore and stroke. .
Greater piston speed means everything for maximum acceleration when comparing a long vs short stroke with the same rods or a Long vs short rod with the same stroke. True, overlap happens at TDC but the energy created by it continues even ABDC giving it a supercharging effect and that's how I accomplished my goals. And the Stroker McGirt Syndrome applies here to, "if some is good then even more is better". Not the case. In the 60* engine family I've found the perfect overlap for maximum power. In the 2.8L I got 173WHP with the perfect overlap and the wrong LSA. This gave me the most power but with low torque and lazy response. In the 3.4L I used the same overlap but with the optimum LSA for it. The result is 300WTQ from 2500 to 4800rpms and 304WHP at 6000rpms and climbing.
Pictures can say more than words, so I'm using this pictures to explain the advantage of a short rod ratio vs long ratio weather is by comparing long vs short stroke or long vs short connecting rods with the same strokes which yields the same result, different ratios.
Long stroke or short rod creates more space to be filled. Allows more air/fuel charge to be digested and compressed. Better air/fuel mixture due to the high velocity. More air/fuel means a more potent power stroke and faster spinning crank.
Coming back to this to close the tab from my crowded browser...
In the example of the 305 and the 302... halfway through the stroke, a 305 piston has increased cylinder volume (displaced) 19.06 ci. Halfway through the stroke, the 302 has displaced 18.88 ci. The 305 has the longer stroke, but the nobody doubts the 302 has higher power potential because it can fit bigger valves.
That's not to say a 305 can't be a nice engine... it sure can, but hardly anyone builds them. I've been thinking that a 331 using a 305 block with a 3.750 stroke would be a great towing or marine engine because it has a little better surface area to volume ratio in the cylinder, reducing waste heat into the cooling system.
What you're saying about long stroke applies to strokers, but at equal displacement it does not apply.
Coming back to this to close the tab from my crowded browser...
In the example of the 305 and the 302... halfway through the stroke, a 305 piston has increased cylinder volume (displaced) 19.06 ci. Halfway through the stroke, the 302 has displaced 18.88 ci. The 305 has the longer stroke, but the nobody doubts the 302 has higher power potential because it can fit bigger valves.
That's not to say a 305 can't be a nice engine... it sure can, but hardly anyone builds them. I've been thinking that a 331 using a 305 block with a 3.750 stroke would be a great towing or marine engine because it has a little better surface area to volume ratio in the cylinder, reducing waste heat into the cooling system.
What you're saying about long stroke applies to strokers, but at equal displacement it does not apply.
Yes, it applies. The 302 can be fitted with larger valves. The 305 due to the smaller cylinder bore can't. So, how can the difference be made? Matching the overlap required by the 302 is the key. For the 302 with 2.02 intake valves the ideal camshaft LSA would be 111. For the 305 with 1.85 intake valves the LSA would be 109 degrees. And with this 109 LSA the 305 will kill the 302 in a short track off the corners because the 305 would not need to be wound up at a higher RPM to get it going compared to the 302. So, the 302 at 70* overlap has 111* of LSA and 292* of intake advertised duration. For the 305 to match that it would need at the same 70* of overlap, 109* of LSA and 288* intake advertised duration. Get the idea? The 305 due to its shorter duration and tighter LSA at the same overlap than the 302 will accelerate faster, will have much better throttle response and the power will come on faster and be more explosive also due to the longer stroke (302= 3.0 stroke/305=3.48 stroke). The only advantage the 302 will have over the 305 would be on long straights, but if the 305 can accelerate sooner by the time the 302 starts to catch the 305 guess what, its time to hit the brakes. Power under the curve is king, not peak power.
