Highest compression w/out running premium fuel?

[Robahr24]

What is the highest compression you can run and still use regular grade fuel. Also, since I'm thinking about making my engine a stroker with doing some head mods. What is the best way to raise compression to the max for regular fuel?

[yossarian19]

I don't think you'll get any reliable, simple answers here.
The fuel required is going to be dictated by more than just static compression ratio. There is also block & head materials (iron & iron here), shape of the combustion chamber, fuel injector spray pattern & spark to consider.
I'm losing my train of thoguht, but the idea is this: with aluminum heads & a cooler plug, you could run 85 octane at 10.5:1 compression. In our engines, iron block & heads, maybe the number is 9.5 - but whatever number you pick is going to be highly in the "gray" area - because your coil pack, plugs, head work, piston dish etc will all play into it.
Maybe 5-90 or Dino could chime in, call me a fool and give you a straight answer - but I think you are looking for a guarantee that your engine will or won't require X fuel rating at Y compression.... and that is a tough one.

[Robahr24]

I would be using stock head and block if that helps in any way. And Im not just looking for a exact answer, but what is a rough estimat most people are unning in there strokers if they are running a little higher compression and regular fuel.

[5-90]

HAH!

You'll not catch me answering this question definitively, I've yet to find a reliable answer melself. And I've been looking around and asking...

There are really too many factors at work here: forced induction or no? Open or closed combustion chamber? (I believe the OEM AMC242 chamber is "open" - but don't hold me to it as yet. I've been working on axle info too much of late.) Quench compliance and quench distance? Head gasket bore and design? (yes, it can make a difference.) Mean intake and air temperature? Is there a heat shield between the intake and exhaust? (It can be important on a side-draught head like ours.) Fuel temperature? Water or Water/MeOH fog?

Speaking very generally, the more you can do to keep combustion chamber/intake air/fuel temperature down, the lower your ONR will be.

Also, camshaft selection has a good deal to do with ONR - the Octane Number Requirement (ONR) of the fuel isn't so much a function of the static compression ratio as it is of the dynamic compression ratio - not the ratio between BDC volume and TDC volume so much as the ratio between IVC volume (Intake Valve Closing - the piston is already on the way up) and the TDC volume. Build six different engines with the only difference being camshaft timing, and you have six different engines with the same SCR but different DCRs, and therefore different absolute ONR figures (although all are likely to be covered by pump gas.)

Dino - if you've got something you can use to prove me wrong, I'd love to hear it! But, you've some idea of the research I've been working on (you helped me with a good deal of it...) so if there is, it's probably something you've found fairly recently...

[wolfpackjeeper]

I run premium in my stroker.

In the winter I can run a GOOD mid grade with positive results. But with the summer heat I need more octane.

[yossarian19]

Well, there you have it: with a 4.7 at ambient air temps around 90-105 F, you'll probably need to run premium, but maybe with cowl hood, header wrap & true cold air intake you could *maybe* run mid grade, if you relocate the IAT sensor as well... blah blah blah

Build it, run it & see for yourself.

[Talyn]

On my previous stroker I was able to run midgrade in the summer with these specs:
.043" gasket:
SCR: 10.11:1
DCR: 8.63:1
Quench: .0564"

I'm going to try to push for regular with my new build, which will have lower compression and a better quench.