I have read alot of posts were people have said things, like you are more able to get away with running higher compression on pump gas, if you run aliminum heads. Less chance of detination. Is there any truth to that? Or am I misreading? Lots of guys say the are running close to 11.0:1 on pump gas. Thats were I would like to run. Is it a pipe dream?

 

Aluminum heads are a big contributor to compression tolerance. 11:1 compression is definitly possible with the right type of chamber, quench and cam.

 

A Lt-1 or Lt4 motor with computer control and reverse cooling can "approach 11:1" compression but keep in mind the computer controls spark advance if things get out of hand.
The induction system is also a "anti-tuned system" with its short runners. A long runner induction system will have a much peaker
torque spike (cylinder pressure curve) in the power curve.
These motors like a long runner TPI tend to only tolerate a 10:1 cr.
A carbed motor with aluminum heads will tolerate about 10.5:1.
How much you can vary this depends on your actual combination and the running conditions but keep in mind the +/- varience is not that much.

Some people claim to be able to do this, that and the other
and succeed (or foul themselves) in varing degrees but you'll always be riding the edge.

Riding the edge (of detonation) is expensive, over time.

Third gen Fbodys have a cramped engine compartment and a limited radiator size which contributes to hot underhood running conditions as compared to the ideal.
This really limits what you can practically get away with in a daily driver that operates in the real world.

 

So many things contribute to detonation that it's hard to give you a definitive answer. Is it possible, sure, will just bolting on a set of aluminum heads allow it, probably not. I've seen engines able to run close to 11 to 1 with iron heads, and I've seen engines with aluminum detonate at 9.5 to 1, both having similar timing curves. Factors include, but not limited to
Timing
Air/fuel ratio
engine temperature
air temp
cooling system flow in the heads
antifreeze/water percentages
weight of vehicle
gearing of vehicle
load
exhaust backpressure
camshaft timing
camshaft overlap
combustion chamber design
piston dome design
spark plug heat range
egr functioning
type of gas used (not necesarrily octane)

Also lt1 and lt4 engines use reverse cooling to lower detonation sensitivity

 

Aluminum gets it's detonation resistance from the fact that it ABSORBS heat so much more quickly than cast iron. General wisdom is that ALL ELSE HELD EQUAL you can generally get away with about 1/2 point more compression than cast iron. My experience generally agrees with that estimate. Expect no miracles.

I'll make another additon to the list of things that affect detonation resistance (aluminum heads or not). MILEAGE!!! A brand new fresh engine you can often get away with relatively high compression for a time. Once you get some carbon built up on the internal parts (valves, combustion chambers, piston faces) the octane requirement to stay out of detonation rises quite a bit. Meaningless on a race engine or one that sees very limited street duty BUT on a street engine that sees thousands of miles pass under the tires it can make a BIG BIG difference.

All the "super special" high-compression pump gas engines I've seen or heard about generally don't live long lives on pump gas. This is usually why.

 

Great, that was the kind of info I was seeking. I am building up a 383 with AFR's and a HSR. The cam is 234* adv. with a .525 lift. The rotating assembly will be forged. It will be street driven on nice days, and flogged hard at the track. If its possible to run 11.0:1 with this set up, would you suggest it? Or should I stick with 10.5:1? Just trying to squeeze as much HP/TQ as I can. Thanks for you input.

 

Don't squeeze that way. Compression isn't the be-all, end-all of making power. General wisdom is that 1 point of compression is worth about 4%. Half a point would be worth 2%. On a 400HP motor that's only about 8 HP. Probably not enough to risk living on the "bleeding edge" of detonation.

BTW- I have found that estimate to be about right. High compression will FELL like it gives you more than that since it props up low-midrange torque by more than that, but peak-to-peak, it's about right.

You'll make more power with slightly lower compression and optimum spark advance than you will with slightly too-high compression and retarding the spark advance to stay out of detonation.

 

Thanks for pointing that out Damon, sometimes I need some one to kick me in the head when I'm thinking too much, thats why I post the idea so some one can point out what I'm not seeing.

 

Aluminum heads aren't better for detonation because they ABSORB heat, but because they DISIPATE heat much more effiecently than iron. It's not how much heat they can take, its how much they can get rid of.

 

Good point. They basically "carry" the heat from the combustion chamber into the coolant more quickly. Sorry for the less-than-accurate terminology.

 

The '72 PU Truck in my sig has high compression and iron heads. With modern combustion chamber design and camshaft technology you can run compression that seems over the top. I also feel that compression is an important key to making maximum power in any given engine combination. True, that having so much compression that it causes you to have to retard timing alot is that magical point of "diminishing returns" The truck I have has iron vortec heads and .100 domed pistons coupled with the head gasket I run comes in with the compression at 10.65 to 1. I run 92 octane gas and a bottle of octane booster with 34 degrees of total timing and it doesn't clatter at all. My Ram Air TA clatters more than my truck does. So, it all revolves around your particular combination.:rockon: