Max Compression on pump gas?

Whats the maximum compression that can be run with a 455 on 93 pump gas? Will be using TA Stage 1 aluminum heads, SRP pistons and probably the 413 cam...gotta deal with power brakes.:3gears:

 

I would also say 10.5 to 1 also. I'm just under 10.2 to 1 and can run 89 octance with no pinging. But I did polish the snot out of the tops of the pistons and the combustion chamber, even removing the casting button in there to prevent any chance of a hot spot.

 

I agree with the 10.5 cr. I'm running 10.4 with Stage 2SE heads with a slightly milder cam-no ping problems with 93 octane. Of course I probably couldn't hear it anyway with the windows down and 3" pipes and Spintech mufflers.:laugh:

 

it depends on the c.c. shape and piston dome. if it is a flat top or reverse dome i would go up to 11 to 1. like larry said cooling system has to be spot on and so does the tune up. i had a sbc that had flat tops and aluminum heads with 11 to 1 and ran pump gas. iron heads 10 to 1 or so. maybe 10.25 to 1.

 

I run 11.1 with iron heads without an issue.
My buddy runs a SBC @ 12.5 aluminum heads without an issue.

We both run Sunoco pump gas

 

I am so jealous of you guys that can find 93 octane at the pump. The best I can find here is 91. :ball: :ball:

 

Max Compression on pump gas?

There are too many variables involved to make an across the board projection on what is the max one can get away with.

Its fun to discuss though. :TU:


Static or Mechanical compression ratio will depend on atmospheric pressure, where youre located (elevation), boosted or non-boosted, these affect the running cylinder pressure of the particular build.

A "too big" cam with a large amount of overlap will bleed off cylinder pressure at low engine speed and allow an increase in cylinder pressure with a given fuel. Low gears and a light car will help lower the load the engine see's. Combustion chamber design and piston surface/shape may also contribute as will keeping those surfaces smooth to fight detonation. Pump gas also varies from location to location and season to season.

All this is why most folks only try to find a safe number rather than the max, which is never safe.:TU:

 

Static or Mechanical CR depends ONLY on the mechanical configuraqtion of the engine.

Dynamic or Effective CR does depend on elevation, atmosphere,etc, etc,,

It's not overlap which explicitly determines bleeding off cylinder pressure, it's the intake valve closing event. Overlap is a secondary effect which sometimes but not always correlates with cyl press.

 

There's a gas station/car wash up here in Scottsdale that sells stuff up to like 109 or something but anything more than 91 and it gets really pricey

 

Dang, I stepped in it...recon thats what I get for not re-reading what I typed.ou:

Mechanic/Static compression is completely dependent on the physical parts used to assemble a given engine.

Everything else is dynamic or what the engine actually sees under running conditions.

Thanks Scott for catching my gross error...:TU:

 

No better than 91 octane pump gas out here in the land of Fruits and Nuts.:rant:

 

We have 94 octane here, I am running 11.5:1 with the aluminum stage 2 heads and thought I'd have to lower the compression. Mike at TA suggested a 260/260 degree .640 lift cam with the 11.5:1, I am actually using 1.8 rockers so it works out to .700" lift. Works good on the 464 and makes 670 horse on 93 octane.

 

Well hey Mike Jr, what's the answer?

93 octane and your Stage 1 heads.

 

Hey Larry I think you have a typo 260/260.Will you race Sunday if so Adam and I will go?
Jamie

 

Not sure if anyone has done it but I'd propose that anyone who runs more the 9.5:1 on pump gas with iron heads (approx 10.5 w/AL) go to the track, make 3 baseline runs, add in at least 5 gals of race gas and and run another 3 runs. I suspect you'll see an increase in performance. Any takers?

 

Been there, done that. You're leaving out an important variable though, ignition timing. When timed safely on 93 octane, my 10.8:1 engine with the 308S cam did no better even with race fuel.

Of course, when I adjusted the timing to take advantage of more detonation resistance with the higher octane, it did better. I would never had tried those settings with 93 octane for fear of hurting it.

Devon

 

Devon, thank you for making my point! You can't run that much compression and keep your car in top hp tune. When you get beat, nobody will care if your (nobody specifically) timing is retarded.

 

I have 10.6 to 1 and run 37* total timing. Shell 93, no issues.

John

 

John, Couple questions...

Cast iron or Aluminum cylinder heads?

How long has this engine been running 10.6:1 and 37 degrees of timing?

How often do you race it at the track?

When racing at the track do you use 100% 93 octane?

Do you have monitor AFR, if so, what is it at WOT (end of the 1/4)?

Thanks for your help!