I'm sure some degree (no pun intended) of ignition adjustment would allow for a better running engine throughout a variety of conditions, but all things considered equal... Would an engine perform better with a higher compression ratio and less ignition timing or lower compression and more spark advance? This is more of a "hypothetical" question than anything else. For the sake of arguement, let's ask about a naturally aspirated engine with the same heads, cam, etc, and it's all been "tuned" properly on 92-93 octane.

That's a Catch-22, octane lol. Higher compression is always better if you have the gas for it. Then you get into the aluminum head/iron head thing, for you can usually run a full point more compression with aluminum heads with the same initial ignition lead. Then you get into chamber shape--the new heads have better-designed chambers than old heads and they make more swirl. Then there's piston dome, which raises compression but kills to a degree the flame front if the dome is too high, so that's a compromise, too. Ain't this FUN? chuckle. For my part, I'd run as high a compression ratio and as much initial lead as I could all things considered. A 8.5:1 engine on 93 octane vs a 9.5:1 engine on 93 would be less responsive and wouldn't make as much power or torque. But then, an 8:5:1 engine wouldn't NEED 93 octane chuckle. But then, you could probably get away with 10.5:1 in the same engine with aluminum heads and the same timing. But then there comes a point where you can dial in TOO MUCH timing and the car will slow down. I say crank as much compression into the engine as you can get away with--the engine will be smoother, more responsive, make more power, and will have a crisper exhaust tone than a low-compression slug. This is assuming the car isn't computer controlled..... arggghhhhhhhhhhhhhhhhhhhh

Just to throw another 'curly into the above..
A 10.5:1 engine doesnt run at 10:5:1 thats static compression and basically means squat....it is the dynmaic ratio (actual cylinder pressure at a given rpm) that will determine the fuel u use.

You will have to retard the timing a few degrees on a high comp motor to run on pump gas, which will make for a lazy motor, meaning it may be makin maybe the same power and the pump gas engine, but will not rev as good, therefore it will not accelerate as good, makin it slower at the track. The high comp motor (mild SB) may make 20-30 hp more, but the low timing will kill its ability to accelerate.

From what I've read..

If you use as small a chamber as you can, a close quench, true flat-top pistons and as long a rod as you can "afford/fit", an engine's combustion process become's more eff. so that it can be higher compressioned and still run on pump gas!

I'm talking like up to 11 to 1 CR. on 87 octane stuff and higher.

Then throw in intake valve closing timing and dizzy timing to fine tune it for max. power production.

pdq67

Then you get into the aluminum head/iron head thing, for you can usually run a full point more compression with aluminum heads with the same initial ignition lead.



I dont believe in that theory too much, the engine temp is the engine temp if the motoris at 180 so are the heads Iron or aluminum. The aluminum dissapates heat faster it probrably cools down faster after you shut it off.

To speciffically address the question asked, an engine that must have it's ignition timing compromised from optimum due to too much cylinder pressure will not run as "good" as one that does not require this compromise.

There are LOTS of little tricks to improve an engine's detonation resistance or lower cylinder pressure, or even the simple act of retarding the timing on pump gas and running full advance with race gas when max power is desired.

Ya but (LOL) with the 8.5 engine you could put on a turbo and have lots more power then the 11.5. Just to be a pain in the butt. Cheers - Barry

Depends on your definition of "as good". The 12.5:1 6.00 rod 350 I just destroyed was a victim of detonation and my failure to park the car until I could make the changes necessary to drive it on the street. My brother could hear the detonation at wide open throttle over the 3" exhaust when travleing behind me.

What the others have said here holds true. You want to run as much dynamic compression as possible without detonation, for the fuel you plan to use. Cam design, piston dome shape, cylinder head shape, spark plug design, cylinder head material, ignition system design and ignition curve set up all play important roles in the design that will allow you to do this. If all of these aren't properly configured then you may end up with an engine that has it's ignition timing retarded to prevent detonation and it will be sluggish. It may not seem sluggish to you compared to the low comp motor, but it will be operating way below it's volumetric efficiency and you really haven't gained anything by running the high compression if you don't select the components carefully. The temp you see on your temp gage is only the temp at the sensor, not throughout the motor. The sensor can be located in several locations, head, intake, water neck. The temps seen on the inside surface of the cylinder head combustion chamber are much, much higher than what is measured at the temp sensor. Iron also is known for having hot spots more so than aluminum, so yes the material does matter. The only reason I'm not running aluminum heads now is I already have a set of 215 Dart iron heads. Aluminum heads are certainly something I'm considering on the next motor.

My point is too much compression or timing the motor will be sluggish, but at WOT it may still run good, just not for very long and not as well as it could if it weren't detonating. There still may be pieces of my #1 piston in the muffler or laying on the highway if it made it out that far.

Choose your components carefully, but you don't have to settle for 8.5 or 9:1 either to avoid as Smits67 puts it "the ol' death rattle".

That's a tough question to answer in general terms.

Given the choice between an engine that was a half point below the detonation point and one that was a half point above it, I'd take the lower compression and run it at it's full total advance.

In theory the further the engine is advance the more it works against itself (cylinder pressure building before piston hits top dead center) which costs horse power and parts. In reality, as has been pointed out it is a matter of dynamically juggling ignition lead with flame propagation, with detonation resistance. There are as mentioned a number of factors effect each other.

Ya pays yer money and ya takes yer chances. So who wins the Cuppie Doll?

The temps seen on the inside surface of the cylinder head combustion chamber are much, much higher than what is measured at the temp sensor. Iron also is known for having hot spots more so than aluminum, so yes the material does matter. The only reason I'm not running aluminum heads now is I already have a set of 215 Dart iron heads. Aluminum heads are certainly something I'm considering on the next motor.


I'm in complete agreement that the material does matter.

I ran my old 413ci small block with steel Vortec L-31 heads at just over 11:1 compression, and it always seem to run at or around 200° with a big 4-core HD rad and dual electric fans. Then I switched to Bow Tie Phase 6 aluminum heads which had a much smaller combustion chamber yielding 12.39:1 compression. In that same kind of street driving coolant temps now sat steady at 180° consistently. So even though the CR increased to well over 12:1 the aluminum heads made a huge difference by lowering my operating temps by 20°.

well I run 11.5:1 on Iron heads with stock (new) radiator, moroso elec water pump drive and no mech fan with Iron heads and my motor stays 160 +/- my fan doesnt even cut on until 180 and it almost never cuts on unless Im stuck in traffic.

I wasn't talking about running temp, M--I meant the ragged edge of detonation when you push the loud pedal! Without getting into specific thermodynamics, aluminum heads dissipate heat better than iron heads and in a 1/4-mile run it's a difference as the combustion temperature rises with load and rpm. It can be argued that iron heads make more power (nominal) because they retain heat better, but at the same time the initial ignition lead must be retarded more given a specific octane gasoline. I prefer more initial lead and more compression because it makes the engine statically more responsive and driveable. But hell, if 104 octane pump gas was still 35 cents a gallon this wouldn't even be an issue. chuckle Everybody has what works for them, and that's my take on it. Just keep them boys rolling!