Compression Question

**gashkerwin** · 10-21-2008, 03:00 PM

So, I've got this idea that I'm missing something here. Maybe timing is involved moreso than I think, maybe the bore to stroke ratio plays in here. Maybe cam lobe profile for duration and overlap is what is the real thing at play here.

So the general consensus seems to be that much above 11:1 compression in our cars is a bad idea on pump gas, which living in PA I will call 93 octane the "best" we can get.

I certainly understand why too much compression is bad. What I want to know is why can't we safely go higher than 11:1? If you look at all those street bikes out there, they're all running compression between 12.5 and 13:1 obviously on pump gas.

So what gives? These little freaking engines are cranking out some REAL power. They're low on torque, but they're shorter stroke relative to bore, than our engines are, which leads to higher revving/hp, lower torque numbers.

Are these little buggers just not built for durability like a car engine is? There are 10s of thousands of LS1s out there with near or over 100k on them by now I bet, but I don't know that I've ever seen a sport bike with more than 15-20k. Are they running cam profiles that are radically different than an LS1?

**TempleU_WS6** · 10-21-2008, 03:07 PM

Side note, where in Southeast Pa do you live.

Im no expert but I think it has to do with size, the smaller ci/cc the engine is the higher the comp ration and RPM.

**gashkerwin** · 10-21-2008, 04:22 PM

I'm in Pottstown.

I've never heard anything like total displacement (or as is more likely the case displacement per cylinder) making any difference, but I suppose that is a possibility. Something about cylinder wall and head surface area realtive to volume perhaps...

**Mr. Luos** · 10-21-2008, 04:30 PM

11.6:1 is about the max I would run with aluminum heads and a good tune.
I run 11:1 on my stroker.

The bikes can get away with it, because of the cooling, and the cam profile.

Dynamic compression and static compression can both effect what you can and can't run for timing.

**gashkerwin** · 10-22-2008, 03:52 PM

Alright Luos, I suspected cam made a difference as I said, but anyidea what the big difference is? More Overlap? A ton of intake duration like in a lot of turbo motors? I would think that you wouldn't want too much exhaust duration...

**Mr. Luos** · 10-22-2008, 03:58 PM

Hard to explain...

Static compression doesn't account for valve events.
Dynamic compression is determined by valve events, and everything that determines the static as well.
This may help....
http://www.empirenet.com/pkelley2/DynamicCR.html

**typhoon1110** · 10-24-2008, 04:36 PM

dynamic or cranking compression has more effect on the octane of fuel, altitude and humidity play a big role too. there are many things that can be done to help, you can get away with 12 to 1 if you set it up right.

**gashkerwin** · 10-24-2008, 05:27 PM

Well obviously if you set it up right you can get away with all sorts of nonsense.

I've started actively waiting for there to be a station nearby that sells E85 before I get heads. The package I had picked out was exactly installed on another car by a local guy and he did 48x hp and 45x tq at the wheels on 11:1 compression.

I think I'm going to get my heads milled down to give me 12.5:1, and running the E85 should be able to keep my timing at about what it would be to run the 11:1 on 93 octane. Hopefully both numbers would be in the 500s at the wheels on that setup. On my stock bottom end.

Will I need to dump a ton of alcohol in there? Yes.
Will I be risking my bottom end? I can't see any way around that. I'll still be running the stock rear until I see where that engine setup makes its power in the powerband before deciding what rear end gear ratio to go with in a new rear.

Am I a bit crazy? I sure am... crazy like a fox. I want something unique, and a what I'm looking at will be that alright.

**bvinton** · 10-25-2008, 05:48 AM

There are a number of things that affect the maximum compression ratio you can run and fuel, timing, head design and material are just a couple. Other factors (and this list is in no way complete) are: chamber design and shape, piston design and shape, squish, quench, intake port design and shape, intake shape, design and material, coatings, A/F ration, fuel atomization, carb/FI, valve timing and profile, exhaust system efficiency, ambient air temp, intake tract temps, head and block material (alum v. iron), combustion chamber contamination (is oil getting by the rings), vehicle weight, gearing, altitude and more.

Also, are those people sure that they're really running the comperssion ratio they think they are? I've torn more than one engine that the owner thought had higher compression than it did. They usually think it's in the 10.5-11:1 range based upon advertised chamber volume but when cc'd come out to 9.0-9.75:1.

I'm running a little over 11:1 with 93 octane and I'm pushing it. But, I live in Florida at near sea level, where it's stifling hot and humid most of the time and my car weighs close to 4k lbs. The 4.10 gears and manual trans help keep the load on the engine tolerable but even with aluminum heads and longer duration cam, I've had to back off the timing and run her a little rich.

I hope this provides some food for thought without muddying up the water too much!