[Hostile_1LE]

I am thinking of bumping my compression up in December when I have my motor out. Currently I am running too little compression (11.5:1) for my cam. I still want to be able to run pump 93 octane without having to us octane booster, but I want to give my cam some more comp to play with.

What is the max compression I can run and still be able to put 93 in the car and have it be happy.

Currently I am HCI with:

SS3
GPI Ported Heads
MSD
103mm
Roto-Fab
1 7/8" catless LT's
E85
Full 3" straight pipe
GPI Tuned
Car weighs nearly 3900 lbs with me in it.

My guess is 530whp based on the runs with others cars that I've gotten.

[Hostile_1LE]

[m6-lt1]

No expert but the Mustang GT runs 12:1 and works on 93 fine. Not sure if this has anything to do with it but Mustangs run DI and Port injection. It’s my understanding that Chevy switching to DI on the LT1 is what let them bump compression from 11:1 on the LSX engines to 11.5:1. Based on that I wouldn’t think the Mustang having port injection as well is what lets Ford run 12:1. Again not an expert but I think Ford runs larger gaps on the coyote piston rings. No idea if that’s why they get away with .5 more compression or not.

It is my guess that anything above 12:1 is too high for 93. I have read on these forums that people see knock on their logs with 93 however I can’t remember if that’s the LT4 or the LT1 or both.

I think you’re going about it the smart way by trying to stick to 93. Some people go to high for 93 and run e85 only but then you gotta test the quality e85 you’re getting and you can no longer road trip the car as a good chunk of the country doesn’t have e85.

To confuse you even further, the current Miata runs 13:1. No idea what so ever why they can run that high.

[s346k]

i would speak directly with whoever is tuning your car. because ultimately how well the car runs on a handicap like that will point directly at them.

[NYblack1le]

Originally Posted by m6-lt1

No expert but the Mustang GT runs 12:1 and works on 93 fine. Not sure if this has anything to do with it but Mustangs run DI and Port injection. It’s my understanding that Chevy switching to DI on the LT1 is what let them bump compression from 11:1 on the LSX engines to 11.5:1. Based on that I wouldn’t think the Mustang having port injection as well is what lets Ford run 12:1. Again not an expert but I think Ford runs larger gaps on the coyote piston rings. No idea if that’s why they get away with .5 more compression or not.

It is my guess that anything above 12:1 is too high for 93. I have read on these forums that people see knock on their logs with 93 however I can’t remember if that’s the LT4 or the LT1 or both.

I think you’re going about it the smart way by trying to stick to 93. Some people go to high for 93 and run e85 only but then you gotta test the quality e85 you’re getting and you can no longer road trip the car as a good chunk of the country doesn’t have e85.

To confuse you even further, the current Miata runs 13:1. No idea what so ever why they can run that high.

Coyote is a DOHC. Completely different ball game. The static compression means nothing. 4 camshafts and vvt make things like this possible along side DI.

[Joshinator99]

Originally Posted by NYblack1le

Coyote is a DOHC. Completely different ball game. The static compression means nothing. 4 camshafts and vvt make things like this possible along side DI.

Bingo. Dynamic compression > static compression

[m6-lt1]

Originally Posted by NYblack1le

Coyote is a DOHC. Completely different ball game. The static compression means nothing. 4 camshafts and vvt make things like this possible along side DI.

Gotcha. Didn’t think that mattered but now that I think about it makes sense. Same thing with miata I guess since they run 13:1 which I still find amazing.

[Joshinator99]

Originally Posted by m6-lt1

Gotcha. Didn’t think that mattered but now that I think about it makes sense. Same thing with miata I guess since they run 13:1 which I still find amazing.

Yes they can use the independent cam timing to bleed off cylinder pressure…

[Megahurtz]

As its already said, its all about dynamic compression. The Engine Builder, if they spec'd the cam, should have the info you seek.

That said, 12:1 static typically isn't an issue with 93 pump but the camshaft design will be a factor in how much timing you can put in it.

[laynlo15]

Leave it at 11.5.

[FlukeSS]

Originally Posted by Hostile_1LE

YOUTUBE VID

That was a good run between the Mustang and camaro.

[Hostile_1LE]

Originally Posted by laynlo15

Leave it at 11.5.

Why? I’ve got a cam in it that needs a higher dcr, and the only way to get that is a higher SCR. Plenty of guys out there running 13:1.

[FlukeSS]

Originally Posted by Hostile_1LE

Why? I’ve got a cam in it that needs a higher dcr, and the only way to get that is a higher SCR. Plenty of guys out there running 13:1.

The other issue is if you plan to Turbo/SC it later.

If you run the higher compression I believe that eliminates possibility of using a Turbo/SC?

The lower compression allows you that lee-way if I am not mistaken?

[Hostile_1LE]

yes, high compression is less boost friendly but I don't plan on boosting it. besides, it would need a shortblock rated for at least 1000 for me to consider boost, which I could easily get in a lower compression.