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| V8 vs V6 (Page 8/9) |
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Will
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JAN 20, 01:29 PM
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| quote | Originally posted by Daryl M:
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. |
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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.
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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).]
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Skybax
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JAN 20, 04:03 PM
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Excellent explanation and simply put!
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Daryl M
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JAN 21, 02:26 AM
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| quote | Originally posted by Will:
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.
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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.
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BillS
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JAN 23, 01:56 PM
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| quote | 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. |
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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'.
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La fiera
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JAN 23, 06:37 PM
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| quote | Originally posted by Will:
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.
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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.
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La fiera
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JAN 23, 07:05 PM
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| quote | Originally posted by BillS:
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'. |
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https://youtu.be/Td9Gz_h7Qpg
This is not the same cam phasing like the L3T. 120bhp/liter in the Abarth 500 and 124 Abarth.
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Will
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JAN 24, 05:54 PM
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| quote | Originally posted by La fiera:
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. |
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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.
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La fiera
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FEB 10, 10:51 PM
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Will
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JUN 02, 09:35 PM
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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.
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La fiera
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JUN 02, 11:10 PM
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| quote | Originally posted by Will:
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. |
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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.
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