Well, I have always known it to be true that with boosted applications, you want lower compression so that there are less chances or detonation, simple concept, makes sense and is a general rule of thumb. Well, I was talking to someone about my build, and they approached me about running high compression AND boost. My question is: generally, everything being in good condition with a freshly rebuilt motor, and a very good tune how high of a CR is safe with 10-12 PSI on a garrett T3/T4? Or to work it from the other direction, how much boost is safe on a 12:1 compression?

 

Personally if I were building a motor I would never go higher than 9:1 for a street turbo motor. If you want to buy into that high compression turbo crap, don't build anything higher compression than what you get with a flat top piston. I personally wouldn't boost a 12:1 compression motor on pump gas, but maybe that's just me.

 

I'm building my motor around 9:1 compression right now and hoping to get 500whp and tons more if all goes good. I wouldnt turbo a high compression motor...agree with turboEGhatch

 

That's kinda what I was thinking. Like I said this is what I've always heard, and it makes sense. I'm going to be running a stock SOHC ZC compression, maybe a little lower due to a thicker head gasket, so 9.6:1, I think it is. I dont know what difference there is in the ZC and D15B combustion chabers are though, so there might be some alteration there.

 

Yea well good luck with ur bluid.

 

i personly wouldn't run 12.1 CR with 12psi buit it can be done and it make lotts of power. I big thing that you need to look at is the cyclinder pressures you can get a 8:1 with 19psi and say 10.5:1 with 9psi to have the same power. two completly different setups but get the hp number every one is looking for. But also some thing else to look at is where is most of your "sprited Driving" done. On track/ light to light/ autocross/ highway pulls all demand a differnt style of power dilivery and tranny setup than the other (if built spcificly for that application) How ever say autocross and light to light racing could use a similer setup and then the question is how do you want your car to act when you are crusin down the highway at 80mph. there is plenty of information out there other than here at HT, to gain info from. DOn;t forget to look at older setups and other brands of cars to get info to see what is being done with turbos. any way kind off topic sorry but hope that helps.

 

You're on crack if you honestly believe a 10.5:1 motor at 9 psi will make anything close to the power an 8:1 motor at 19 psi will, unless we're talking about some sub-T25 size turbo that runs out of breath at 10 psi. Given something like a T3/TO4E 57-trim an 8:1 motor at 15 psi might need an extra pound, pound and a half of boost to make the same power as a 10.5:1 motor. If you're cruising down the highway at 80 you're a downshift away from boost. Lower compression means more boost and power on pump gas, so yeah, looks to me like 9:1 is the only way to go.

 

I have a friend who wants me to tune his 12:1 CR motor with skunk2 stage 2 cams running about 5 psi. i dont really wanna tune this car cuz the setup is just ask n for trouble. but im just gonna clean up the a/f ratio for him and just get the car run n good. alot of timing will be pulled!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by turboEGhatch &raquo;</TD></TR><TR><TD CLASS="quote">You're on crack if you honestly believe a 10.5:1 motor at 9 psi will make anything close to the power an 8:1 motor at 19 psi will, unless we're talking about some sub-T25 size turbo that runs out of breath at 10 psi. Given something like a T3/TO4E 57-trim an 8:1 motor at 15 psi might need an extra pound, pound and a half of boost to make the same power as a 10.5:1 motor. If you're cruising down the highway at 80 you're a downshift away from boost. Lower compression means more boost and power on pump gas, so yeah, looks to me like 9:1 is the only way to go.</TD></TR></TABLE>

I agree

 

ok so my exact details are off but as for the build i will be posting the results of my high compression and low boost motor in the spring.


There are benifits to the higher comprestion boosted motors. when off boost your car still has better power then the 9.1 cr motor.

 

that is why most people have there boost around 3k rpms so its hardly ever off boost.

 

ok then in my ever end quest for infromation what sized turbo would you run on a high compression low boost motor say 11.5:1 and looking to get 7psi with the factory 8200 rpm redline on a b16. Looking to get the quick spool say around 2-3000rpm

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by itrhatchling &raquo;</TD></TR><TR><TD CLASS="quote">ok then in my ever end quest for infromation what sized turbo would you run on a high compression low boost motor say 11.5:1 and looking to get 7psi with the factory 8200 rpm redline on a b16. Looking to get the quick spool say around 2-3000rpm</TD></TR></TABLE>

why are u wanting high compression motor if ur want it to boost around 2 to 3k rpm?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by itrhatchling &raquo;</TD></TR><TR><TD CLASS="quote">ok so my exact details are off but as for the build i will be posting the results of my high compression and low boost motor in the spring.


There are benifits to the higher comprestion boosted motors. when off boost your car still has better power then the 9.1 cr motor. </TD></TR></TABLE>

http://forums.evans-tuning.com...=1612

Holy **** yes. High compression and 100 WHP less on the top end so that you can make, what's that, like 2 WHP more before boost hits?

 

there is no advantage to running high compression with boost... EVER , if you are running pump gas


happy tony


Modified by JDogg at 2:44 AM 10/4/2005

 

I wouldn't say ever, there's this stuff called Methanol

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">I wouldn't say ever, there's this stuff called Methanol </TD></TR></TABLE>
haha

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">I wouldn't say ever, there's this stuff called Methanol </TD></TR></TABLE>

i cant get methanol at the local gas station

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDogg &raquo;</TD></TR><TR><TD CLASS="quote">there is no advantage to running high compression with boost... EVER, if you're running pump gas</TD></TR></TABLE>

Oh, i missed that in your first post.

 

lol, i figured that was implyed cause dude is talking about like 5psi or some ****... wtf ur gona do w/5psi i have no idea, lol..

 

dude, 13:1 and 5psi would make like 450whp!!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">dude, 13:1 and 5psi would make like 450whp!!</TD></TR></TABLE>

yeah, if its a 5liter motor

 

I stand corrected.

 

Go with 9:1 and size the turbo according to the power band you're looking for.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tony1 &raquo;</TD></TR><TR><TD CLASS="quote">Go with 9:1 and size the turbo according to the power band you're looking for. </TD></TR></TABLE>