Question For Engine Builders

[Truckzilla46 17]

Ok just curious why most boosted applications run 9.1 compression and others run 10.1 + compression. I assume there must be a purpose built reason for this?

Jason

[monty1269 29]

9.1 is pretty low.... usually 9.5.

When you say "others" who dat?

[Truckzilla46 17]

9.1 is pretty low.... usually 9.5.

When you say "others" who dat?

Just a loose number. Gt500 /hellcat and most builds I see are mid to lower 9s then some 10s

[srt5939 159]

they last longer. some of those extreme guys run high compression and get new engines for every race.

[Cnhman2008 1]

Lots of variables ie Fuel, type of induction ,amounts of boost ect.

[Dave Weber 2]

Lots of variables ie Fuel, type of induction ,amounts of boost ect.

Agreed you also have centrifugal vs twin screw with a completely different boost curve.

[El Diablo 72]

So if you had two motors built identical with the exception of one being 9.5:1 comp verse another at 10.1:1 with the same blower, obviously the lower compression motor you would have to spin the blower faster to make the same boost. What is the advantages or disadvantages in the two ?

[lakersfan7272 59]

More dynamic compression and a likely need for better fuel on the higher CR motor. More power too.

[Hemi31 194]

More dynamic compression and a likely need for better fuel on the higher CR motor. More power too.

. Thing is cam choice plays a bigger role in dynamic than does the static compression of an engine. A higher compression motor is going to make more power but you will be working harder to control detonation.

[El Diablo 72]

. Thing is cam choice plays a bigger role in dynamic than does the static compression of an engine. A higher compression motor is going to make more power but you will be working harder to control detonation.

^^^That is what I thought E.

[BayouTiger 88]

In my case I went with 10:1 because first having a Techco, then a Maggie I knew that I was gluing to be limited by the blower, and would never be pushing the boost level. With a goal of 650 I would rather go with more compression and less boost on top of more displacement. If my goals were strip based, I would go with less displacement, lower comp, and more boost.

[Chally 5]

The reason you run lower compression with boost is that, with boost, the air going into the cylinder is more dense than in an n/a application, so you don't have to compress it as much to make the same power. Which in turn helps with detonation issues and pretty much what everyone else in this thread has mentioned.

[Hemi31 194]

The main goal on pump gas is to keep the final static compression ratio about 17:1 MAX on pump gas. Now 11.5:1 compression with about 7 lbs will be right there opposed to 9:1 with 14 lbs. Theoretically being they are both the same compression and all else being equal they should make the same power....but we all know there are too many other variables.

[turbofreek 27]

ive ran high compression with boost with a turbo. had not issues with detonation or timing problems. car was an absolute beast from off throttle to on boost. 11.5:1 compression and 14 psi ran 21° of timing on a ford with regular 110 fuel. ran like a scalded ass ape. 306in windsor designed ford. flat top relief pistons. made massive power and torque across the board from 2600rpms and up.

i plan on about 11:1 for my newgen and an 82mm billet turbo maxed out. dont know what the timing will be either with single plug coil setup and not dual plugs like normal. probably about the same as regular guys on here due to ignition lead will be different with only one plug to fire the a/f.

[Hemi31 194]

ive ran high compression with boost with a turbo. had not issues with detonation or timing problems. car was an absolute beast from off throttle to on boost. 11.5:1 compression and 14 psi ran 21° of timing on a ford with regular 110 fuel. ran like a scalded ass ape. 306in windsor designed ford. flat top relief pistons. made massive power and torque across the board from 2600rpms and up.

i plan on about 11:1 for my newgen and an 82mm billet turbo maxed out. dont know what the timing will be either with single plug coil setup and not dual plugs like normal. probably about the same as regular guys on here due to ignition lead will be different with only one plug to fire the a/f.

im gonna gues 13-15* timing.

[turbofreek 27]

yea i was thinking that as well. timing wont be very high these combustion chambers are far better than a ford windsor head. ill start with a low boost setting and work it up and down together. boost up and timing down to see what it likes. 8-10psi to start. my big tune issue will come with the traction control and wheel speed tuning window. i know most if not all of you dont have that to play with but it is another factor in engine tune for drag racing. fun fun fun.

[junior.666 7]

We did a 2013 jeep with simple forged 6.4 with

stock heads twin 5862's and stock cam. Made ridiculous torque and power on 9-11 degrees timing. 1075 tq and 1018 hp on 18.5 psi. Not sure what the compression was but I'm sure 9.5 highest.