Aluminum heads
- Thread starter Dan s
- Start date
-
I actually thought about that. But I know for a fact I will be putting a motor together sometime soon so I just want to go ahead and buy a good set of heads. My 318 is not worn out. Actually I have 2. Bought them both with a fresh rebuild. Long story. One is in my truck with a nine inch rear end and 3.73 gears and true track. The other is in my dart. Dart has the bigger cam. I have know idea what size is in either. The truck has a wienand in take and headers.
j par
Well-hung Member
When you use words like lumpy cam and 700 lift I want to go back to my first comment and just say keep asking questions...
I find that I have the opposite problem. No one will race me at a light.
Dual quad tunnel ram sticking out of the hood and the pipes crackling... I'll rev it up to four or five grand and the side pipes are deafening when they hit them... then I engaged the two-step and it sits there and crackles at three and a half Grand popping and crackling at the low rev limit...
They don't even try....
All while praying to God they don't call your bluff.....No one will race me at a light.
Dual quad tunnel ram sticking out of the hood and the pipes crackling... I'll rev it up to four or five grand and the side pipes are deafening when they hit them... then I engaged the two-step and it sits there and crackles at three and a half Grand popping and crackling at the low rev limit...
They don't even try....
j par
Well-hung Member
That's your line...
When I said .700 lift I was referring to what the head would be good for. I know I’ll never get that big of a cam. A lot of people on this site are in a league that I will never be able to get to. I work to much and more money than I have got. A set of these heads for me is a big deal. Now , should I put them in my little teener that I know very little about except that I bought it from a junkyard right after the truck it was in was totaled. The motor is a fresh rebuild except for thrashing it has got from me. Probly not. Will I ? Sure. it will give the younger crowd where I live some experience and a chance to play with something most of them will never afford.
lead69
hopeless car junkie
Yes yes,it has saved the day more than once and earned a place amongst spare parts!
yellow rose
Overnight Sensation
Compression ratio and cylinder pressure are not strictly results of mechanical factors.
Confusing compression ratio and cylinder pressure is a common mistake and problem for amateur engine builders. They are related, but not the same, as commonly thought. Compression ratio is strictly a result of mechanical influences such as bore, stroke and combustion chamber volume. Cylinder pressure is pressure inside the combustion chamber when the engine is running. Cylinder pressure is influenced by compression ratio along with several other factors – for example:
1. Altitude – The higher the altitude, the lower the cylinder pressure: resulting in reduced performance like lowering the compression ratio, but the compression ratio remains the same.
2. The material the cylinder head is made of effects pressure too. The aluminum cylinder head will dissipate (that means draw off) heat from the combustion chamber much faster than cast iron with no effect on the compression ratio. But it will lower the running cylinder pressure. Lower temperature means lower pressure, and lower pressure means less power. That is why you should increase compression ratio when switching to aluminum heads, or you can leave it alone and reduce your power, all other things being equal.
3. The position of the camshaft relative to the piston position as well as the position of the intake and exhaust lobes to each other influence the cylinder pressure too, but not compression ratio. Confusing compression ratio and cylinder pressure has caused many damaged engines on one hand and losses of power on the other.
Pressure or more correctly cylinder pressure is what pushes the piston down. More pressure – more power. If the pressure is not at its maximum you are giving away power. If the pressure is too high (for the octane you are using) you are looking at detonation.
Another problem in the confusion between compression ratio and cylinder pressure is the octane of the fuel being used. The octane of the fuel (pump gas with 85 to 93 octane for the sake of this discussion) determines how much pressure the fuel can tolerate without detonation. Detonation for your engine is what cancer is to humans – bad news! 93 octane gasoline can tolerate more cylinder pressure without detonation, than 85 octane gasoline, like duh, you knew that didn’t you? Aluminum heads can tolerate approximately 30# more cylinder pressure than iron heads with the same octane gasoline, without detonation. There are ways to cheat that system too. With rod length, quench heights, cylinder head material etc. None of which affect compression ratio, only cylinder pressure.
Not understanding the difference in compression ratio and cylinder pressure is a fools game that will cost you horsepower or a damaged engine, and is usually played by flat earth people.
Confusing compression ratio and cylinder pressure is a common mistake and problem for amateur engine builders. They are related, but not the same, as commonly thought. Compression ratio is strictly a result of mechanical influences such as bore, stroke and combustion chamber volume. Cylinder pressure is pressure inside the combustion chamber when the engine is running. Cylinder pressure is influenced by compression ratio along with several other factors – for example:
1. Altitude – The higher the altitude, the lower the cylinder pressure: resulting in reduced performance like lowering the compression ratio, but the compression ratio remains the same.
2. The material the cylinder head is made of effects pressure too. The aluminum cylinder head will dissipate (that means draw off) heat from the combustion chamber much faster than cast iron with no effect on the compression ratio. But it will lower the running cylinder pressure. Lower temperature means lower pressure, and lower pressure means less power. That is why you should increase compression ratio when switching to aluminum heads, or you can leave it alone and reduce your power, all other things being equal.
3. The position of the camshaft relative to the piston position as well as the position of the intake and exhaust lobes to each other influence the cylinder pressure too, but not compression ratio. Confusing compression ratio and cylinder pressure has caused many damaged engines on one hand and losses of power on the other.
Pressure or more correctly cylinder pressure is what pushes the piston down. More pressure – more power. If the pressure is not at its maximum you are giving away power. If the pressure is too high (for the octane you are using) you are looking at detonation.
Another problem in the confusion between compression ratio and cylinder pressure is the octane of the fuel being used. The octane of the fuel (pump gas with 85 to 93 octane for the sake of this discussion) determines how much pressure the fuel can tolerate without detonation. Detonation for your engine is what cancer is to humans – bad news! 93 octane gasoline can tolerate more cylinder pressure without detonation, than 85 octane gasoline, like duh, you knew that didn’t you? Aluminum heads can tolerate approximately 30# more cylinder pressure than iron heads with the same octane gasoline, without detonation. There are ways to cheat that system too. With rod length, quench heights, cylinder head material etc. None of which affect compression ratio, only cylinder pressure.
Not understanding the difference in compression ratio and cylinder pressure is a fools game that will cost you horsepower or a damaged engine, and is usually played by flat earth people.
I am curious as to how you are measuring this pressure change difference between aluminum and iron heads on a running engine? In other words , where is the proof?
well, we know aluminum dissipates heat faster, and a drop in temp will drop pressure. Like a balloon in a freezer, it shrinks, or a tire in winter loses pressure. I could be wrong... just the natural thought of aluminum and iron. It's why you can run more compression with aluminum heads???
Agreed, but those instances take time to make much difference. Combustion in a chamber happens pretty fast in comparison. I would like to see actual proof of much difference in pressure there really is.....
Only way to really prove it is to make an aluminum head and cast head 100% in the same mold with no difference and dyno them. I'm just using my logic, but I could be wrong. I'm open to better science as well.
To me, this statement seems like they actually tested it. I would like to see how they did it in a running engine. I am not saying they did not measure it, I would like to see the instrument they used. I hope they are not just talking about a compression gage. I would like to see how the pressure difference between Iron and aluminum reacted to varying RPM and load, and coolant temp.
I wonder if quench was considered in the Hughes finding. considering most aluminum heads are closed chamber.
yellow rose
Overnight Sensation
It has been done and there is no way that the cores can be the same with aluminum and iron.
You'd either have an aluminum head that was so thin a baby fart would blow it apart, or a CI head that was so heavy, and the water jackets so small it would weigh a ton and couldn't be cooled.
Again, I read the Hughes post and he didn't give any "science". It just regurgitated the same old fact that aluminum dissipates heat faster. So what? Refer to my post in the racers forum.
Let me see if I can sum it up without typing a book.
The rate of change in increase of temperature of combustion is so fast, that the rate of change of temperature of the coolant can't accept that temp increase.
IOW's, no matter how fast the head materiel can reject or dissipate heat, the coolant can only take so much of an increase. And coolant accepts heat much slower.
And then, we can discuss how many BTU's the coolant can reject, at what load does heat dissipation become relevant?
For example, you have an 11:1 CI headed engine and cruising down the road, temp and load stabilized, and when most detonation occurs is right here...when you transition into tip in. The load goes straight up, the engine rattles and you either mash on it to get through it or back off the throttle to stop it.
Explain to me, with science, how an aluminum head can conduct heat so quickly as to remedy that.
I can also tell you this observed fact. Detonation almost never occurs at WOT. Almost never, and to get that your tune up has to be so jacked up that nothing will fix it except correcting the underlying issues and head materiel won't do that.
Low RPM, high load detonation is the real enemy, and an aluminum head ain't going to fix that.
yellow rose
Overnight Sensation
I've been testing it, real world since 1980ish. Hughes is wrong. And it needs to be corrected.
- Thanks. The coolant stabilizing the rate of which heat can dissipate from the aluminum is something I hadn't though about.
- I think the concept of what I spoke of in Jpars thread still has some reality, but I also agree with your post in detonation and especially the coolant stabilizing the rate of which heat is able to leave the head.
- I will say the way I thought of it is "not all wrong, not all right either" if that makes sense.
That’s been done, published, video tapped, posted, discussed before.
yellow rose
Overnight Sensation
Certainly you are not all wrong. If you are loading an engine to detonation in a steady state, where everything is stabilized, then there is some help getting the heat out of the head quicker...IF the cooling system can deal with it.
We are usually discussing engine in transient response and not steady state so that makes a big difference.
Last edited:
And yet, the comment temp still rises to the same temp level......
I wasn't trying to imply that it couldn't be tested. I just wanted proof that THEY had, and real world numbers, not just a blanket statement of 30#s.
Personal experience on my own cars has shown me that iron is not the enemy of compression.
yellow rose
Overnight Sensation
So you are saying you'd tell someone with aluminum heads they can run 11:1 on pump gas but the exact same engine with iron heads you tell the guy to only be 10:1? That's wrong and crazy and the science and experience don't prove that out.
-
