low vs high compression ratio for FI

Orphan · 01-13-2010, 05:47 PM

I think its important to realise there have been books written about this it is that in depth and there are so many variables to consider. This reason why a lot of aftermarket builds tend to run lower CR with boost as they don't have the experience/funds/technology the big manufacturers have to develop new systems with the emmisions and reliability that OEM demands. These guys have huge resources and engineering minds at work as well as unlimited engines and dyno time to get it right - its a lot harder when you have your ONE engine that you don't want to loose haha not to mention not that many tuners are adept with mapping timing to intake air temp for added safety, for aftermarket use you don't really have to have a super accurate plot, the main thing is to have an accurate cut down point so when the intake temps reach it the timing backs off.

There are just so many little things involved that its quite hard to sum up even a majority of the main points so it will be interesting to see how in depth this thread goes.

FYI I found a M3 build running the S65 4L V8 with 12.0:1 CR and running 16psi from a centrifugal supercharger, I guess one benifit is that because they are running a centrifigul they can run an intercooler as well as you don't have the radient heat coming from the exhaust side of the turbo so intake temps can be kept quite manageable, that said I have seen some turbo builds that the intake charge was only a few degrees above ambient by the time it got to manifold the intake cooling was that efficient.

Like I said in my first reply you have to figure out what you want from the build to really decide which way to go. As Kastley has said above higher CR and lower boost will give you a more linear power and torque curve making it smoother overall and easier to drive hard but a lower CR + more boost gives you more of that kick assosiated with turbo vehicles, no real difference in roots/twin screw supercharged vehicles as they generate boost directly proportianate to the RPM so power is quite linear.

01-13-2010, 05:47 PM	#13 (permalink)
Orphan Base Member Join Date: Aug 2009 Location: Brisbane Posts: 13 Drives: SC400 Rep Power: 16	I think its important to realise there have been books written about this it is that in depth and there are so many variables to consider. This reason why a lot of aftermarket builds tend to run lower CR with boost as they don't have the experience/funds/technology the big manufacturers have to develop new systems with the emmisions and reliability that OEM demands. These guys have huge resources and engineering minds at work as well as unlimited engines and dyno time to get it right - its a lot harder when you have your ONE engine that you don't want to loose haha not to mention not that many tuners are adept with mapping timing to intake air temp for added safety, for aftermarket use you don't really have to have a super accurate plot, the main thing is to have an accurate cut down point so when the intake temps reach it the timing backs off. There are just so many little things involved that its quite hard to sum up even a majority of the main points so it will be interesting to see how in depth this thread goes. FYI I found a M3 build running the S65 4L V8 with 12.0:1 CR and running 16psi from a centrifugal supercharger, I guess one benifit is that because they are running a centrifigul they can run an intercooler as well as you don't have the radient heat coming from the exhaust side of the turbo so intake temps can be kept quite manageable, that said I have seen some turbo builds that the intake charge was only a few degrees above ambient by the time it got to manifold the intake cooling was that efficient. Like I said in my first reply you have to figure out what you want from the build to really decide which way to go. As Kastley has said above higher CR and lower boost will give you a more linear power and torque curve making it smoother overall and easier to drive hard but a lower CR + more boost gives you more of that kick assosiated with turbo vehicles, no real difference in roots/twin screw supercharged vehicles as they generate boost directly proportianate to the RPM so power is quite linear. Last edited by Orphan; 01-13-2010 at 05:53 PM.