9:1 or 8.5:1 comp

[Don Ganso]

This is really just a curiosity question here. i know that an exact answer would be hard to come by so best estimate.

How much more boost could you realistically run on 93 octane with 8.5:1 than with 9:1?
How much better gas mileage could you realistically expect with 9:1 over 8.5:1?

Please do not just throw random numbers out there; if there is no reliable answer then say so... Couldn't find anything related to this on the search or on GOOOOOOOOOOGLE, so thanks for your time either way.

[The Shaolin]

Depends entirely on the tune, engine, turbo, etc. There's no set number anyone can give you.

[arthursc2]

^^

Too many variables.

[Don Ganso]

And that's all she wrote.

[tristanlee85]

I'd do at least 9.5:1 (seriously), but I am also an idiot.

[TypeSL2]

I wish I had gone with at least 9:1. I hate the fact that I put 8.5s in my car. Although, the damage to the tops of them when my valves met them probably would have been quite a bit more extensive without the dish in them.

James

[Donnie]

I'd do at least 9.5:1 (seriously), but I am also an idiot.

+1.

When I was still seriously considering introducing forced induction to an LL0 in one of my cars, I was only pricing out stock compression slugs.

[punkboySL2]

i believe that my forged sleeved motor is using 9:1 pistons with a lightly decked head and flat valves and I hope to run at least 25psi through a t3/t4. That being said I'd go at least 9:1 on the pistons. You'll make more power per each psi.

[Don Ganso]

i believe that my forged sleeved motor is using 9:1 pistons with a lightly decked head and flat valves and I hope to run at least 25psi through a t3/t4. That being said I'd go at least 9:1 on the pistons. You'll make more power per each psi.

On 93 octane? I don't know much about tuning, but that sounds pretty impressive for sure...

[S.Bretz]

I was running 10:1 at one point. That bitch screamed. I wish I had the golden eagle block then because it was lauching headgaskets. It never pinged (that I could hear) up to 15psi. Over that and I got some 100oct from one of the local Sononco's.

[Don Ganso]

Dayum. Learn something new every day!

[arthursc2]

I have 8.5's for 2 reasons
A) its what was available at the time, for the right price
B) I am a tuning noob, and less risk of detonation.

[S.Bretz]

My current engine is 8.5. Before I had to replace the head gasket, it was about 8.7.

As long as you ae not shooting for 400hp, I would say to stick in the 9 to 9.5 range. That keeps the throttle responce crisp and you will be safe.


My first engine was 9.0 and it ran 10psi at about 250whp for 2 yrs with no issues. ...it took 5 more psi to start breaking stuff (the rods pretzeled up).

[Don Ganso]

My current engine is 8.5. Before I had to replace the head gasket, it was about 8.7.

As long as you ae not shooting for 400hp, I would say to stick in the 9 to 9.5 range. That keeps the throttle responce crisp and you will be safe.

My first engine was 9.0 and it ran 10psi at about 250whp for 2 yrs with no issues. ...it took 5 more psi to start breaking stuff (the rods pretzeled up).

So what if I am shooting for ridiculous power? I'm not now of course, I've got to practice "teh art of teh tune" but if I was in the future, 8.5 is a better bet?

[S.Bretz]

Yeah, lower comp would be easier to hit larger numbers with...but it will require a couple more lbs of boost.
High compression will get you there with less boost, but you will need to use some super expensive fuel with really high octane.