Someone help me out here. My turbo project is nearing completion. And I am thinking of the various little things I can do to beef it up. I was thinking about the cam and lifters, but realistically, the cam and lifters CAN'T help in a turbo application. If you're pressurizing a container to X psi, then every point within that container has the same pressure. This is how our vacuum lines work. I can't remember who's law it was, but I heard it in dynamics class. So apart from the issue of allowing enough air fast enough, there is nothing a cam can do really to help, cause no matter how aggresive the cam or how high the lifters, you're still only going to get X psi in the cylinder. Right? The only thing you can do to help a turbo is make the valves larger, and port the heads, and that's only if you're running higher boost.
interesting theory, however if that were true then how would a cam on a naturally aspirated engine make any difference? It certainly wouldn't change the air pressure in the intake manifold. I suspect even under boost, there is not enough stock duration to allow the pressures to total equalize. And of couse don't forget about exhaust, and overlap.
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11:27 PM
Jul 11th, 2005
watts Member
Posts: 3256 From: Coaldale, AB, Canada Registered: Aug 2001
Trust me when i say.... it makes a HUGE difference!
The turbine is being spun by the exhaust gas as you know. The intensity, heat etc leaving the engine makes a WORLD of difference.
My mild aftermarket cam: I was making about 5-6psi. Locked off the wastegate (pinched the hose off!) and got about 11. My custom turbo cam: I was making about 5-6psi. Locked off the wastegate... Ummm... I'll never do that again! Nailed the gauge off the end. 20+
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01:33 AM
88gtNewb Member
Posts: 922 From: Surrey, BC, Canada Registered: Aug 2004
The only thing you can do to help a turbo is make the valves larger, and port the heads, and that's only if you're running higher boost.
Anyone agree?
-Shawn
Not true. It is all about airflow, not PSI. 15 psi can be very different. For example, 15psi on a 3.4l DOHC requires a much larger compressor than 15psi on a 3.4l OHV, as the DOHC flows a lot more air. Therefore, in order for the compressor to make the 15psi, when the DOHC is sucking in so much air, is to flow a lot more. More boost does not mean more power, in fact, when you modify a supercahrged motor (Same concept except the turbo controls boost using a wastegate), you often drop PSI, but gain hosepower. This is becuz you gained flow, and more air is actually getting in your cylinder bores.
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02:32 AM
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
Originally posted by AaronZ34: Not true. It is all about airflow, not PSI.
RIGHT a good trubo cam will give you much more power. with a turbo, many things that improve your performance DROP the boost, because more air is being pushed into the cylinders, instead of sitting in the intakes under pressure. like porting the intakes & putting in a high-flo intake - you'll see a drop in boost, but a great improvement in power. anyways, a good turbo cam has lots of duration, and very little overlap. the CompCams 260 is a good "off the shelf" cam that'll be good for turbo.
Originally posted by SCCA FIERO: How do you figure? They are both 3.4 liters in capacity.
3.4 is the physical displacement if there were NO HEADS on the engine. its the "ideal" number if there was unlimted air flow. but the reality is, the air has to go thru the intakes and the heads/valves to get into the cylinder. at 3000 RPM, the air has 0.02 seconds to fill the cylinder. even under pressure, it not likely the cylinder is actually gonna get filled. so, adding more airflow is what ya need. as much as you can get. 5 PSI on a garden hose is better than 15 PSI thru a sipping straw.
For example, 15psi on a 3.4l DOHC requires a much larger compressor than 15psi on a 3.4l OHV, as the DOHC flows a lot more air.
In theory...Yes!!...in application No!! Yes the DOHC flows more, however when picking youcan use the same comp. wheels as long as they are well matched...for example a 58-61mm comp wheels will be good for both apps...HOWEVER their respective exhaust A/R should be different...the DOHC motor will need a bigger scroll...
FOR turbo cams; Just from sitting with Chris West @ WCF,churning his brains... I have learnt a lot from "turbo cams" it is really more of a black art when selecting a perfect turbo cam for your particular application. Does a turbo cam create power..well it is really dependent and subjective...I think most people on this string are correct to a point, but a lot has to be considered... Yes with a well designed turbo cam, less psi can be ran because it is really about flow not psi...psi ONLY refers to restriction within the intake. Everything you have learnt about N/A cams are not the same when going UNatural. The biggest variable here is that the xhaust side of the turbo. The turbo reaches a point called crossover, where the pressure differential between that of the intake and exhaust reaches a ratio of 1:2 or even worse 1:3. This is why people try as much as possible to unrestrict bottle necks within the aforementioned system. The effect 'tries to' brings this ratio closer to 1:1. The net effect is less backpressure...how are all this connected? Well every street application (turbo ) most have x backpressure it is inherent...the turbine is a restriction!...the best way to make a turbo cam is to actually measure the point where crossover starts and what the ratio is...the only other thing is to unrestrict the system and make a better guess which will be within the 1:2 ratio. When it is time to make the 'turbo cam' the tuner/turbo grinder designs the lobes based on several specs eg heads, turbo, xhuast A/R etc and then pick the valve events...the by products of the valve events are what we are accustomed to seeing...the wider LSA, shorter LSA, long & short duration, high lifts and no overlap to minimal... Again dependent on application you can either be frugal and pick a 'turbo cam' based on what you think...a good tuner will ask supporting questions; the trend is to get a turbo cam that is bias on the intake, this school of thought assumes the intake needs help and the turbo supports this...this cams usually will have negative overlap, which are good for low boost to about 14psi...the other school bias the exhaust and actually add lower LSA to get overlap...this has the effect of helping the motor breathe more and still help the turbo spool faster...usually this are those with bigger turbos especially bigger A/Rs...overlap is employed because you can 'bleed boost' and still create power from the net effect of overlap... To me cams with a single pattern are better, because now you aren't depending on the recepient engine's inadequacies...the turbo makes the power, obviously the bigger the cam, the more power potential...however there is a caveat with turbo cams, a wrong turbo cam will be disastarous, ruin drivability and streetability... IMO...durations need be less, lift more, LSA between 112-114... regardless your specific application dictates the valve events which are the more important things to consider...hth
------------------ the ORIGINAL 3800 II v6 intercooled turbo...
[This message has been edited by nocutt (edited 07-11-2005).]
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02:08 PM
AaronZ34 Member
Posts: 2322 From: Colorado Springs, CO Registered: Oct 2004
In theory...Yes!!...in application No!! Yes the DOHC flows more, however when picking youcan use the same comp. wheels as long as they are well matched...for example a 58-61mm comp wheels will be good for both apps...HOWEVER their respective exhaust A/R should be different...the DOHC motor will need a bigger scroll...
I'll be the first to point out that I am pretty new with trubocharging, but I have done a lot of research on supercharging. I compared the 3800-II to the 3.4l DOHC, thinking they'd use a very similiarly sized compressor. Once I got deep into figuring out the exact Adiabatic Efficiency of each unit, at a certain boost, on each motor, I found that the 3.4l DOHC actually needed a much bigger compressor than the 3800-II in order to keep the compressor at maximum efficiencies. So I figured the same was true with the turbochargers. From what we know so far, the best compressor for the 3.4l DOHC (Lightly modded to near stock) is a GT35r, as the T04-60e was a tad bit on the small side.
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09:41 PM
PFF
System Bot
Fierobsessed Member
Posts: 4782 From: Las Vegas, NV Registered: Dec 2001
Yes with a well designed turbo cam, less psi can be ran because it is really about flow not psi...psi ONLY refers to restriction within the intake.
Everything else sounds good, but I'm deliberatly going to pick this one statment apart just alittle. While it is totally technically accurate to say that PSI is a measure of restriction of flow, It doesn't mean you won't have pressure buildup if your intake is perfectly efficient. Its just a question of HOW efficient.
Ideally when we have a naturally aspirated engine, we are attempting to get as close to or in some cases even beyond atmospheric pressure into the cylender before the valve closes. Atmospheric pressure in the cylender at that point is when you have achieved 100% Volumetric Efficiency. In other words your engines displacment is equal to the amount of air it takes in per 2 revolutions of the crank. When you introduce boost, you are deliberatly forcing air into the cylender to attempt to bring the pressure inside the cylender beyond atmospheric pressure (100%+ VE). Your engines inneficiency at this point would really be a measurement of your boost minus your actual cylender pressure at the time the intake valve closes, it's a theoretical number and cannot be measured. The better your engine's intake flows the closer the two numbers are, the more efficient your engine is, and you will make more power given the same level of boost. Even if the difference is 0, and your engine is perfectly volumetrically efficient. You still are making boost, and have PSI. And that is not because your engine is innefficient.
Turbo cams are VERY important to get your engine to the point where it has maximum efficiency on both the intake and exhaust. Just like the intake, the exhaust back pressure is a measure of how innefficient the turbo is, and the pressures on the exhaust can be quite excessive when the turbo is spooling. Usually there are only four remedies for this, a larger turbine scroll and wheel or a smaller compressor, a less restrictive exhaust header or manifold, or less restriction after the turbo. Some may say that you want to eliminate overlap with the turbo cams, but I say otherwise. A little overlap is a good thing, it helps get the hot exhaust gasses out as well as increases the oxygen count by exhanging some of the remaining exhaust gasses trapped in the cylender for fresh cooler and more combustable air. This is less of a problem with a supercharger because exhaust pressures are FAR lower, trapping less exhaust gasses in the cylender. So there are some gives and takes in that category, and that may make cam choice vary slightly. Either way, a naturally aspirated performance cam and most stock cam's are not the greatest idea for when you are planning to boost, unless you can vary there timing to suite your needs.
Thanks for listening to my ramblings...
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10:23 PM
85-GT Member
Posts: 365 From: Dover, NH, 03820 Registered: Mar 2005
To defend myself however... I did know that a cam would make a different. I was just discussing the logistics of it, which is, to say the least, interesting. I'm running a Compucam 2030. My turbo right now is only running 4psi, cause it's still being tested. But when the wastegate hose split, it flew up over 20psi. Scary moment to be sure. I'm hoping the cam won't be a limiting factor. Again, I'm not trying to maximize power per se... just get it to run reliably for now.
... So I figured the same was true with the turbochargers. From what we know so far, the best compressor for the 3.4l DOHC (Lightly modded to near stock) is a GT35r, as the T04-60e was a tad bit on the small side.
Aaron...I have to honestly disagree, however you do have a point. The thing is power is very subjective...I have seen two 3.4DOHC motors using a smaller unit than the t04E 60 pull over 400hp at the wheels using mild boost...while the GT35R utilizes a 61mm compressor wheel (meaning it can potentially outflow the t04e 60), it is possible to attain respectable power with a smaller unit...remember we are talking broad curve here, not peak... IMO, I think it depends on your overall output...meaning what you really want to do..."the best compressor" will then become subjective to those who have done otherwise...don't you agree? Just a different point of percpective...that is why turbos are so much versatile
Fierobsessed... I am not sure I am following you? Look at the example Aaron provided above, about a GT35R versus a t04e 60...both turbos will make 7psi gauge, but they will not flow the same volume of air at that relative pressure...do you agree? so therefore unrestricting the intake still will not make any difference to our gage pressure...even though efficiency has increased on both the hairdrier and the engine...
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11:41 PM
Jul 12th, 2005
Fierobsessed Member
Posts: 4782 From: Las Vegas, NV Registered: Dec 2001
7 psi with any turbocharger on the same engine is going to provide you with the same improvments in the power made, however if the smaller one is running at 70% adiabatic efficiency because it is spinning at or near its max impeller speed, and the other is spinning at a slower speed, running at 85% AE, then you will have a slight advantage with the larger one. Because it is drawing less power to provide the same boost, and also doesn't heat the air as much. But then again, the smaller one will probably perform better because its spooling earlier, giving it better torque, response and drivability. Trade off's again.
The pressure Vs. Flow is NOT determined by the turbocharger at all. It is a sole function of displacment and intake restriction. If you have a HUGE blower or turbocharger pushing 7 PSI into the intake, then the flow rate would be exactly the same as if a T-25 was pushing 7 PSI. Because both are pushing the same flow through the same restriction, therefore building up the same pressure. But the bigger one might be closer to its efficiency range helping reduce losses from innefficiency.
Start on that page and select camshafts there is a very good write up in my opinion regarding the concerns on the subject. My preference is the stock cam or a little bit above due to the tried and proven idle characteristics. I believe the best approach to a good outcome in the turbo engine is to make sure the exhaust restriction is minimal first to help move the crossover point where the exhaust pressure catches, equals and then exceeds the intake pressure, is moved further up the boost level curve. Or in short high exhaust efficiency needs to be high on the list.
with a pushrod motor my prefferance is to use a mild cam with high lift rockers - this gives you the valve lift of an aggressive cam without all the overlap that is detrimental to turbo performance.
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09:16 AM
85-GT Member
Posts: 365 From: Dover, NH, 03820 Registered: Mar 2005
You need at least 112 degrees lobe separation and I believe that puts you into the safe zone, another reason you need to stay out of overlap is if you are running a MAP sensor which depends on the appropriate manifold pressure to maintain a desent idle, overlap reduces idle vacuum and will cause an opening throttle response signal to be sent by the MAP sensor when it is actually closed = poor to no idle and rich mixture. If the cam works well with a MAP sensor you should be in the safe zone. Those guys are way high on the Crane series Cams for the V6, both run $170 complete with lifters from Summit Racing and probably the same from Jegs. Go to the Crane site below to view the cam spec according opening and closing points for entering in a program to determine overlap, should be minus 3 degrees so no overlap
Would that be a good cam for a stock 3.1 turbo from a grand prix? Only mods are mild ported and pocket job on heads, port matched top end, and obviously gonna hafta do something new for the exaust. Ive spent a lot of time reading suggestions and looking over cams, but i just cant seem to find a sutiable one. Id really appreciate it if someone with experience would just say "Buy this...."
If you do not have access to a performance chip to take advantage of the cam, given that you are going to gain quite a bit of performance from the TGP engine swap any way, my advice is to leave the stock cam which often perfroms very well with aftermarket turbo systems anyway alone even more so since this is the cam that came with the turbo engine. Find a way to optimize the current design. You could really make some headway with a turbo or turbine housing that will breath better than the stock turbine housing as well as look into having some aftermarket headers retrofitted in place of the GM econo special. The higher breathing cam is going to be choked long before you reach its full potential since you are going to loose a little bottom end in exchange for more up top. Your transmission of choice needs to be considered in the scenario to.
Leave the cam alone and pick on the exhaust side of the engine now that you have worked on the intake. Once you have done that you can purchase an aftermarket chip that will allow you to run a bit more boost. http://www.turbograndprix.com/
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07:56 PM
PFF
System Bot
Jul 13th, 2005
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
Originally posted by 86GT3.4DOHC: Would that be a good cam for a stock 3.1 turbo from a grand prix? Only mods are mild ported and pocket job on heads, port matched top end, and obviously gonna hafta do something new for the exaust. Ive spent a lot of time reading suggestions and looking over cams, but i just cant seem to find a sutiable one. Id really appreciate it if someone with experience would just say "Buy this...."
quote
Originally posted by Pyrthian: RIGHT a good trubo cam will give you much more power. with a turbo, many things that improve your performance DROP the boost, because more air is being pushed into the cylinders, instead of sitting in the intakes under pressure. like porting the intakes & putting in a high-flo intake - you'll see a drop in boost, but a great improvement in power. anyways, a good turbo cam has lots of duration, and very little overlap. the CompCams 260 is a good "off the shelf" cam that'll be good for turbo.
CompCams 260 grind is a good off the shelf cam for a turbo. it is not a "turbo cam", but the overlap is low, the profile is square, the lift is good, the RPM range is great. this is not the Crane 260
[This message has been edited by Pyrthian (edited 07-13-2005).]
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08:37 AM
Dec 10th, 2005
3.8 SC Member
Posts: 577 From: On the Great Lakes-Ohio Registered: Dec 2003
I recall reading that the cam in the TGP is a stock cam. In my opinion the stock cam or the 2030 Comp Cam is as far as you should go on a conservative turbo car. I have heard and read a good bit between the two theories and believe the best benefit could be had with a stock cam already tested and proven, and the perfect turbo choice and exhaust system. You should keep in mind how the engine will run when it is not under boost. I like the formulas and calculations but prefer to keep things in simple terms; the cam that will give me a tolerable idle, keep the computer happy and allow the most air into the cylinder and most exhaust out the tail end is the best one. If you are a 0-60 person the stock cam will likely serve you better.
