stdyhand

What is the stock compression for the 91 GT 5.0?

What is the maximum compression that can be run for 87/89 pump fuel?

That is all that's offered in my area and when I get a 347 I need to decide on the compression. I will likely be running N/A only so the higher compression with the aftermarket H/C/I should be beneficial when it comes to torque at least.

As I've said before, my goal is 400hp to the wheels and most importantly as much torque as I can get. I doubt it will see much track time except to check ET's.

I was hoping for 10.5:1 or somewhere in that area.

67mustang302

Gonna depend on the build. You can have an 11:1 engine that runs fine, and a 9:1 engine that detonates. Cam is going to play a huge role.

Tha Boss

I would say around 11:1

Venomantidote95

The heads type will add to the result, the rule of thumb is 10:1 with iron and 11:1 with aluminum (heads), but thats with 93, BUT camshaft like stated above can affect it, if efi the tune also! My 347 is 10.2:1 and i can run 87 and have no issues.

4rdnut

87-89 octane? 10:1 compression max with at least 4.11 gears if you want the engine to live.

Venomantidote95

WTF do the gears have anything to do with this post? and WTF do 4.11 gears have to do with the life of an engine? deedadee! slap yourself in the face!

What kind of car and mods do you actually own a geo?

sn95_331_GT_ yellow

11:1 with alum heads

check out your cam specs to see what your dynamic compression ratio is thats what really affects what you can do.

Venomantidote95

I have never seen a dynamic compression anything on my cam cards. How could a manufacturer quote you anything to do with c/r?
they have no clue what your combination is , measurements etc!

67mustang302

Gears will matter actually, and so will vehicle weight. But it's more accurate to state that engine load will matter. A lighter vehicle with more gear will be less prone to detonation....same engine in a heavier vehicle with less gear might burn up in 2 seconds. Partly due to how quickly the rpm rises....the faster the engine can zip up, the more quickly you get fresh cool fuel into the chamber to keep it cool, and the less time there is(progressively) for detonation to occur. That's why detonation is more common at lower rpm than higher....just not enough time at high rpm(though when it does happen it's devastating).

Dynamic compression you can figure based on IC point for the cam and static....but it's only a ballpark. Since a TRUELY accurate compression figure isn't available(unless you know piston crown-wall clearance when hot, along with ring land height, and ring to ring land clearance when hot etc etc). And who's to say when the cylinder actually starts compressing? Maybe when the valve is closed, maybe slightly before.

There's a ton of factors that affect what compression you can run.

Venomantidote95

It is an inaccurate statement to make (use this gear or else!) thats all i was saying!
If you have or built an engine that has to have a certain gear to rev itself, then something went wrong somewhere!?

Dosent that have more to do with power output, engine balancing, and internal combustion principle, than a certain gearing to obtain the ability to rev at a certain rate?

I mean i do completely understand the concept of subing gears for the lack of power persay to get your car moving?!

I mean the original thread is c/r vs pump gas right? not what gear do i need!