[66GT289]

engines, one with aluminum heads and one with cast iron.
I'm assuming the engine with the aluminum heads would
require a slightly richer idle mixture than the engine with
the iron heads. Remember the engines are identical, only
the head material differs......
Does this make sense or am I mistaken?

 

[FALCONAROUND]

Why would it need to be richer?

 

[66GT289]

I'm assuming it would.
In the real world example, with nothing but a swap from
iron to aluminum heads, when warming up I'll get a random
back fire on the air cleaner side. (Only if you open the
throttle a little bit, but do so quickly)
It's interesting to note that once the engine is warm,
this situation goes away. Idle is normal, throttle response is
normal and if anything, the lifters on the intake side are on
the loose side, definitely not tight.... intakes not shutting is
where you'd think that this problem would be coming from.

 

[ujt389]

The only difference is the iron heads will make a little more power (if the head design is exactly the sam) than aluminium ones. the only difference it the heat transfer and expansion rates.

 

[tonys10sec306]

Like ujt said. the iron head will make more power

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<font size=-1>[ This Message was edited by: tonys10sec306 on 5/21/06 4:21am ]</font>

 

[Fomoco8]

The aluminum headed engine could probably run a little richer due to the fact that they will allow for a little more timing before detonation occurs

 

[67Coug]

Aluminum absorbs and transfers heat more rapidly than cast iron so the combustion chambers are running cooler with an aluminum head, especially during warmup. So a lower percentage of fuel is being vaporized. Liquid fuel doesn't burn so in effect, you are running on a leaner mixture. Lean mixtures ewill cause an engine to 'pop back' throught the carb. The fix ias to increase the accelerator pump shot-set to a longer stroke or bigger squirters. My theory is that with aluminum heads you NEED more compression to offset the loss of thermal efficiency-heat is power! That is, heat inside the combustion chamber, not in the intake manifold.

 

[mavman]

On 2006-05-20 17:39, 67Coug wrote:
heat is power!

Now, keep that fact in mind whenever building an engine and you're on your way to making some serious power. That's how they're squeezing 200 HP out of 1.4 Liters....at 13,000 RPM on jap bike engines. Get the air in, get it as hot as possible without burning itself, and get the bad stuff out.

Same principle with diesel engines. Hotter=better to an extent. You can only go so far though until the bad stuff happens.

 

[66GT289]

Thanks 67Coug, I've run the valves at least three times
for adjustment. The intakes are now on the loose side,
so I know that isn't the issue. About 10 minutes after
startup, throttle response improves significantly and
the random backfire on the air cleaner side totally
stops happening. It's almost like you threw a switch or
something....
Plugs are perfectly clean and show nothing unusual.
I'm going to richen it slightly on the idle side and see
how that works out.