Compression Ratio: Aluminum vs. Iron head

[johnj92131]

Does anyone have real world experience to share? I have an aluminum Hesco Jeep head, but it may not be right for what I want.

How much higher a compression ratio can the aluminum be run and still avoid detonation on 87 octane gas? From my youth, I recall British sports cars used aluminum heads and ran about half a point higher compression compared to iron heads. But that was in the '60s with cheap premium gas. Lately, I have read a few posts saying 1 to 1.5 points higher is possible with aluminum vs. iron. But that seems a bit much right now.

So does anyone really know the answer from their own experience, be it Chevrolet or anything else?

Thanks

[FlyinRyan]

My frame of reference is ~300 vehicles, most being trucks with Small Block Mopars, about 50 or so I6 trucks.

It is not as simple as stating "XYZ compression ratio will only run on ABC octane if you have an aluminum head" . Its about the big picture. Cyl pressure resultant from the hard parts used, in conjunction with valve timing events (read: camshaft) and head material.

Iron will retain heat, the equivalent being approx 1-1.5 points CR.
However, this can be offset by using a camshaft with more overlap.

Running a stroker on 87 gas is a losing battle, regardless of parts used.

[johnj92131]

My frame of reference is ~300 vehicles, most being trucks with Small Block Mopars, about 50 or so I6 trucks.

It is not as simple as stating "XYZ compression ratio will only run on ABC octane if you have an aluminum head" . Its about the big picture. Cyl pressure resultant from the hard parts used, in conjunction with valve timing events (read: camshaft) and head material.

Iron will retain heat, the equivalent being approx 1-1.5 points CR.
However, this can be offset by using a camshaft with more overlap.

Running a stroker on 87 gas is a losing battle, regardless of parts used.

Understand it is complicated with the cam choice and the actual quench area height being major items. Also know the "magic" a good ecu tuner can perform. Huge respect for that skill. Also understand that by sticking with 87 octane fuel, I am pushing some of the tuning "magic" off the table.

Can I really run a full point or more compression just because of the aluminum head?

Here is my situation. I have a Hesco head and I have a cast iron H.O. head. I am looking to build a stroker for my 91 Comanche. The truck is an automatic with 2 wheel drive. It is my "Home Depot" truck for around the house or at the rental house. Location is San Diego, which means the truck has to run at sea level and sometimes goes to 6000 ft. Once in a while it goes to the desert. But most of the time it just gets used in the city.
So, since I have the aluminum head anyway, how can I make the best use of it? Seems I should be able to bump the compression up by some amount, for any given octane fuel over the similar cast iron head. The question is how much??
I want to stick with a factory cam and the renix cam seems to give the lowest dynamic compression. It also seems to be better tuned to low end torque than the H.O. cam or even the latest OEM cam. I am also planning a tight .035 quench gap. So the dish in the piston will be machined or not to reach the final static compression (what ever that turns out to be).

[Jim K in PA]

Running a stroker on 87 gas is a losing battle, regardless of parts used.

Ryan - could you explain/qualify this statement a bit more? What do you mean by a "losing battle"?

Thanks.

[akadeutsch]

I think what he means is...Why "upgrade" your engine to run the same fuel? This stroker is capable of so much...why tune it down?...It is reliable in stock form. And if you plan to tune it down then why use an expensive aluminum head? My 2 cents. Sell the expensive aluminum head and put the money into a factory rebuild kit...Perhaps an RV style cam to boost low end torque. If your quench is going to be as tight as you think you are going to have to dish your pistons quite a bit to use 87. And why do you need such a tight quench to run 7:1 compression any ways? I guess I am assuming that you desire that C/R but you said 87 octane so I assume you will not be running a 9:1 SCR engine. If your question was simply, how much compression can an aluminum head handle verses an iron head I think there would be a more clear answer. My question is, Why are you running an expensive aluminum head on a basically stock engine? I think you would see similar gains with a "simple" DIY pocket port and port matching job to your stock head; for much less money.

[johnj92131]

I think what he means is...Why "upgrade" your engine to run the same fuel? This stroker is capable of so much...why tune it down?...It is reliable in stock form. And if you plan to tune it down then why use an expensive aluminum head? My 2 cents. Sell the expensive aluminum head and put the money into a factory rebuild kit...Perhaps an RV style cam to boost low end torque. If your quench is going to be as tight as you think you are going to have to dish your pistons quite a bit to use 87. And why do you need such a tight quench to run 7:1 compression any ways? I guess I am assuming that you desire that C/R but you said 87 octane so I assume you will not be running a 9:1 SCR engine. If your question was simply, how much compression can an aluminum head handle verses an iron head I think there would be a more clear answer. My question is, Why are you running an expensive aluminum head on a basically stock engine? I think you would see similar gains with a "simple" DIY pocket port and port matching job to your stock head; for much less money.

Why use an expensive aluminum head? Answer: I happen to have the head anyway from another 4.0 engine. Problem for me is the head was set for a 10:1 compression on the 4.0 and I want to use it on a stroker, so the compression will be even higher on the stroker. Even with a 30 cc dish in cast pistons the compression is 10.15:1 Have not checked the dish on pistons for the supercharge stroker yet.

Now, Why not sell it and run a stock head, then pocket the extra money? Yes, I have thought about that. If the wife had any clue about what the aluminum head cost, I would be in deep trouble. So selling the head is an option. Ultimately, I may end up using the aluminum head and running premium gas. As Ryan noted, that would allow different tuning with more horsepower/torque. But thought I should try to get an answer about RUG before I start the engine build. According to Hesco, the head will permit up to 11:1 compression on 91 octane gas. Don't really want to be that high for a street engine.

Does anyone here run the Hesco head on their street driven Jeep? Or is the head just a racer item?

[akadeutsch]

Yes Sir I understand being wifed. At least they let us play a little.
Is it possible to take any material out of the Combustion Chambers in the aluminum 10:1 head that you have? removing head material would allow less piston dish.
Also, Which Con rods are you using? If you plan to use the 4.0 rods you could always opt for the shorter 258 rods and go with custom pin height pistons. I guess that would be heavily scrutinized by the misses as they would be very expensive. Good luck.