One thing ill say is that sometimes things that shouldnt work do work and you only find out by trying. Pick hp level and figure out your target boost/rpm to hit it, select turbo, select compression raito/cam , advance cam if you are running some backpressure. Advance it even more if you are running a lot of backpressure. Increasing the CR is the same on the ex stroke as advancing the cam as far as minimizing the egr effect goes. It also explains why the ex valves can be a lot smaller than the intake valves. You will only be able to get the most out of it if you pick a narrow area to make the most power which means you will need a really good converter. You cant have an engine that will make a bunch of power at x boost then drop it and make about the same power at a lower boost. It all has to go together or it will be a turd. I picked up about 25whp per psi on the last high backpressure application but only from 20-24psi. It really needs a higher stall at vacuum/low boost than it has for optimal response (or N2O). It picked up less at lower boost levels. Bigger compressors wont do chit for the power at lower revs if the engine cant use the air. Focus on the ex side. There is a lot more to be gained there. Think about the ex plumbing between the turbocharger and the ex valve and the pressure in there. Next think about the intake pressure. Finally think about the overlap period and the valve opening and closing times with particular attention to the ex valve close and the position of the piston in the cylinder at/around that time and the mechanical CR. It is impossible to stop reversion when you have higher ex pressures than intake pressures but you can do several things to minimize it which is what you need to do if you want to get a high hp/ci with a limiting factor like very high backpressure which is present in just about all of the hard running heads up classes. If they could use any ex side on the turbo they wanted and use a bunch of N2O to spool they could pick up huge power.
Exh. biased cams..
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One thing ill say is that sometimes things that shouldnt work do work and you only find out by trying. Pick hp level and figure out your target boost/rpm to hit it, select turbo, select compression raito/cam , advance cam if you are running some backpressure. Advance it even more if you are running a lot of backpressure. Increasing the CR is the same on the ex stroke as advancing the cam as far as minimizing the egr effect goes. It also explains why the ex valves can be a lot smaller than the intake valves. You will only be able to get the most out of it if you pick a narrow area to make the most power which means you will need a really good converter. You cant have an engine that will make a bunch of power at x boost then drop it and make about the same power at a lower boost. It all has to go together or it will be a turd. I picked up about 25whp per psi on the last high backpressure application but only from 20-24psi. It really needs a higher stall at vacuum/low boost than it has for optimal response (or N2O). It picked up less at lower boost levels. Bigger compressors wont do chit for the power at lower revs if the engine cant use the air. Focus on the ex side. There is a lot more to be gained there. Think about the ex plumbing between the turbocharger and the ex valve and the pressure in there. Next think about the intake pressure. Finally think about the overlap period and the valve opening and closing times with particular attention to the ex valve close and the position of the piston in the cylinder at/around that time and the mechanical CR. It is impossible to stop reversion when you have higher ex pressures than intake pressures but you can do several things to minimize it which is what you need to do if you want to get a high hp/ci with a limiting factor like very high backpressure which is present in just about all of the hard running heads up classes. If they could use any ex side on the turbo they wanted and use a bunch of N2O to spool they could pick up huge power.
So if your looking to drop back pressure (which I have read is a problem with many of the billet wheel turbos with them being more efficient?) it would be beneficial to run the bigger exhaust housing and maybe advance the cam a bit?
Jerryl
Tall Unvaccinated Chinese Guy
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Bison’s advice (as usual) is right on the $$$$.
"I" 100% concur with the advice as it confirms the FMEA, calcs and research I have done. Thanks Bison.
For those interested in more BP discussion, read the thread in the link below. IMO, one of the better no BS threads on this forum.
http://www.turbobuick.com/forums/turbos-turbo-related-parts/144094-boost-vs-back-pressure.html
In addition, try this one with the advice from JDEstill.
http://www.turbobuick.com/forums/turbos-turbo-related-parts/89955-back-pressure-check.html
From what ive seen the billet wheel turbos are running lower backpressure on the same engine vs. cast wheels. Im not sure if the comparisons are accurate since the ex wheels are not all the same and will have a big effect on backpressure. If your not running the engine hard then a cam with more duration wont dont anything for performance. The mass flow through the engine and the turbine will determine the backpressure for the most part. A larger a/r wont drop the backpressure much. There is no evidence of more efficiency with billet compressor wheels at low boost pressures. Precision doesnt post any compressor maps so its hard to say what the actual efficiency is. Id expect more efficiency when you start running the compressor really hard which you wont be able to easily do without a lot of wastegate pressure.
Turbo6Smackdown
Well-Known Member
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I sort of thought boost would throw a monkey wrench in there, in terms of a perfectly matched setup. Ahh well, at any rate, I now know the direction I must go. Study back pressure & it's results... for now. As always, thanks bison, for the solid advice.
Stay small and you wont regret it. Advance it. Less than 135mph in the quarter no need for more than 212@.050.
I know some may want to throw rocks at me for suggesting this, but if you really want to study how the many variables can have such a big effect in the making of a cam spec selection, grab a good engine analyzer program and play with it. I'm not going to suggest that it will give outputs that should be treated as gospel, but with enough experience you can start to see trends and understand some of the things you are searching to learn.
In my limited experience, I feel the cam needs to be the last item selected in the long list of targets, specs, and variables. The cam will need to be closely matched to the abilities of the heads to meet your target goals.
For instance, a cam can be spec'd to help make up for some short comings of the heads being used in relation to your target goals, but could be the wrong choice when used with a much better flowing set of heads.
One thing to keep in mind, the one specification that seems to dictate the rpm range of the camshaft the most is the intake duration number. If you've picked a rpm range for the engine then the intake duration should follow that choice with the flow ability of the intake port also taken into consideration. The other cam specs will follow the needs of the heads and the power curve.
It's pretty much guaranteed that the more duration you run on the intake, the higher the rpm range and hp output. Assuming all other variables are optimized.
edit: The exhaust duration will depend on the rpm range of the engine, the amount of exhaust volume that needs to be expelled and how well the exhaust valve, port, exhaust system, turbine housing and wheel are sized to do the job of evacuating the cylinder properly. Generally, if the setup is restrictive, the exhaust side will need more time to expel the exhaust, to a point. High exh bp throws a wrench into that one.
A sim would also let you see how reversion comes into play with different amounts of backpressure.
With enough time on a sim, you could cut down on the amount of questions you have about picking certain components. In other words, instead of having to try 8 cams on a dyno to come up with one that fits the bill, you may only have to try 3 after having done some of the narrowing down on a sim.
edit: It also seems silly to me that a head porter would be suggesting a camshaft before he has even finished porting the heads and checked to see how they flow.
In my limited experience, I feel the cam needs to be the last item selected in the long list of targets, specs, and variables. The cam will need to be closely matched to the abilities of the heads to meet your target goals.
For instance, a cam can be spec'd to help make up for some short comings of the heads being used in relation to your target goals, but could be the wrong choice when used with a much better flowing set of heads.
One thing to keep in mind, the one specification that seems to dictate the rpm range of the camshaft the most is the intake duration number. If you've picked a rpm range for the engine then the intake duration should follow that choice with the flow ability of the intake port also taken into consideration. The other cam specs will follow the needs of the heads and the power curve.
It's pretty much guaranteed that the more duration you run on the intake, the higher the rpm range and hp output. Assuming all other variables are optimized.
edit: The exhaust duration will depend on the rpm range of the engine, the amount of exhaust volume that needs to be expelled and how well the exhaust valve, port, exhaust system, turbine housing and wheel are sized to do the job of evacuating the cylinder properly. Generally, if the setup is restrictive, the exhaust side will need more time to expel the exhaust, to a point. High exh bp throws a wrench into that one.
A sim would also let you see how reversion comes into play with different amounts of backpressure.
With enough time on a sim, you could cut down on the amount of questions you have about picking certain components. In other words, instead of having to try 8 cams on a dyno to come up with one that fits the bill, you may only have to try 3 after having done some of the narrowing down on a sim.
edit: It also seems silly to me that a head porter would be suggesting a camshaft before he has even finished porting the heads and checked to see how they flow.
Turbo6Smackdown
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Don, where would I get such a simulator at?
better run some compression with that one
The problem with our little motors is!!!!! It lives where the ait is thick on both sides of the valves.
If you will study the stock cam it has the exhaust valve opening kinda early in respect to other turbo engines. This is due to the chitty flowing exhaust ports.
If you can reduce the back pressure pre turbine by say 1 psi. It's like adding another psi of boost on the intake.
Boost is nothing but a function of air backing up in the intake.
The is a thread here that was started along time ago talking about the need for bigger exhaust valves instead of intake valves
http://www.turbobuick.com/forums/ge...68993-increasing-exhaust-valve-size-head.html
If you will study the stock cam it has the exhaust valve opening kinda early in respect to other turbo engines. This is due to the chitty flowing exhaust ports.
If you can reduce the back pressure pre turbine by say 1 psi. It's like adding another psi of boost on the intake.
Boost is nothing but a function of air backing up in the intake.
The is a thread here that was started along time ago talking about the need for bigger exhaust valves instead of intake valves
http://www.turbobuick.com/forums/ge...68993-increasing-exhaust-valve-size-head.html
You will not gain what you gain adding 1psi of boost reducing the backpressure 1psi pre turbo. Increasing the compression raito and advancing the cam will do a lot to help if backpressure is getting high.
Brian
When raising the boost don't it raise the back pressure in the exhaust?
It's to my understanding that one needs @ least 1 psi of back pressure to make 1 psi of boost. when i measured my car a long time ago with a open down pipe i was in the area of 1.5 to 1 . thru the mufflers i was 1.8 to 1.
sorry for the noob questions :biggrin:
Raising boost will eventually raise ex pressure. How much depends on mass flow and the cam/turbo used, and the engine speed. No it doenst take at least 1 psi to make 1 psi of boost. Its possible to have higher manifold pressure than ex pressure but not on these engines with small turbos. Usually N2O is used to spool with when they build an engine for max power/ci with a turbocharger. You could have 45psi in the intake and 40 in the ex. Common on a lot of the dyno queens you see posted around the net.
