Rich or lean........... Somthing to ponder

OK guys , I have something to discuss. Changing from a set of aluminum heads TFS Streets to a set of factory closed chamber SCJ cast iron heads, same compression or very close.

If the engine was spot on in AFR with the aluminum, with just a head swap to the cast iron heads would the engine now be fat or lean ?

Also keep in mind this swap will also go from a new efficient combustion chamber to a 40 year old design chamber.

Now lets hear some thoughts on this.

Hummm..... I'd guess it would be a little fatter because the exhaust port isn't as good in the scj's and not gettin quit as good of scavange....

Good, that's one way of thinking , Maybe we can have some others. Keep um coming

i have a question whats the diff of the valve angles betwwen the 2 of them ????

Between the streets and cj's there shouldnt be any valve angle change. I would guess lean because you are not going to have as good velocity with the larger intake port and also with the larger port your volume of air increased but the fuel stayed the same. just my 2 cents

Blake it would be interesting to have a set of exhaust pyrometers on it and see where the temp is acording to your dyno pulls it should get you close... I feel it is lean...


Randy

Blake and I were discussing this last night.

The air on this particular day was pretty good, in the 30.30 to 30.35 BP range with temps between 64F and 69F. Humidity hovering between the 50's and 60's. In this kind of air, if your tune was not changed from warmer temperatures and worse pressure then I'd say it was lean just due to the weather, in fact Mine may have been a bit lean as well. We did jet up Chuck's car a smidge and it picked up a little.

Now if the weather conditions were the same, I'd have a tendency to agree with Dave and Todd on the intake and exhaust differences. Maybe they X each other out? Thus making the tune nearly the same as before? You have a larger intake runner and a more restrictive exhaust port.

Interesting......... We need EGT or A/F ratio readings from both

Let me throw something else in the mix.

With the material being Cast iron instead of aluminum, would it not hold more heat in the combustion chamber, there for making the mixture swing towards the lean side. Then the fact of the chamber not being as efficient, would it not need more fuel to make the same power ? An efficient chamber will make more power per pound of fuel, or will it?

Keep it coming...................

I'm not sure that once the engine is up to temp you see a gigantic difference in mixture due to the materials the head is manufactured from. I'm thinking any differences are due to shape and port design. Plus cam timing requirments being different between the two.

if you were to optimize the cam for the old irons what would be the man diff between the 2 grinds ???

meaning is this cam grind that you have in there for the tfs is hurting the performance of those old irons

I'd think their could be as much as 20 degrees more on the exhaust for the iron Scj heads

I am not sure the head material will show much difference but if it does aluminum would tranfer heat away from the chamber area quicker allowing for a cooler more dense charge to enter the cylinder and would cause the irons to act rich at that point but again you still have more volume of air so maybe it equalizes out. Durn this is making my brain hurt thinkin to much!!

lghting94 wrote:I am not sure the head material will show much difference but if it does aluminum would tranfer heat away from the chamber area quicker allowing for a cooler more dense charge to enter the cylinder and would cause the irons to act rich at that point but again you still have more volume of air so maybe it equalizes out. Durn this is making my brain hurt thinkin to much!!

What about the fact that the iron chamber is hotter and burning the fuel more completely with the same jetting, would it now be more towards the lean side ?

This is good stuff

IDT-572 wouldnt the hotter chamber be counter-acted by the aluminum heads by a more efficient chamber allowing it to burn cleaner?

lghting94 wrote:IDT-572 wouldnt the hotter chamber be counter-acted by the aluminum heads by a more efficient chamber allowing it to burn cleaner?

Very well could be..............

I am going to watch the weather and when the conditions are like they were at Bowling green I am going to find the answer to this question.

I didn't do a plug check las time out , because so much was going on, but I did drop the timing from 35 to 32 and it made no difference anywhere. I thin it should have shown something somewhwere.

It feels lean running it down the track. On the top end.

IDT 572 what size carb jet are you running now? I Know with my combo(495 cj iron heads victor and 1250 hp dommie) it likes alot of jet(on the dyno in good weather 94/96 jets no powervalves)

Its at 95 square now, no power valve 1250 quik fuel.

Well now that i have a headache from thinking to much I better get back to work

me too

Lean, due to heat in the chamber, no scavenge and a poorer intake port...

Now, why the switch to the iron CJs and where the hell are my P51s?

Oh, and jet it. I'll come do it if you need me too. I think I've stuck some in there before!

I sold my Streets to Big Chuck and Laws has my P-51'S

Come on down lets tune on it

IDT-572 wrote:I sold my Streets to Big Chuck and Laws has my P-51'S

Come on down lets tune on it

I'd love to, but it's not exactly a hop, skip and a jump... When do you think you'll get the P51s on there? Lem called last week and said mine should be coming around soon.

I am going to change pistons so it will be a spell. But it will be worth it.

What are the differences in the flow numbers of both intake and exhaust between the two heads? The need to have about 1 point higher compression on an aluminum head to achieve similar power to a cast iron head may rule out the higher efficiency of the chamber on the aluminum head. I did read the static compression is the same did I not? My thinking is that if the intake port flows less than the aluminum head then there will be less signal to the carburetor which would pull less fuel, possibly making things lean. Plus if the port is not as efficient at mixing, you'll definitely need more fuel in there to make the charge burn properly. The only real way to know would be to have it on a dyno where you can record the BSFC's of both combos. Obviously the lower number the more efficient the combination.
Seems this topic is creating more questions than answers

The compression is about 3/4 point less in the steel heads, and they flow about 388 @ .800 where the Aliminum heads flowed on an average 378. But the steel heads are down about 30 above and below .500 in the mid lift.

I feel it is lean............ But I doubt this one is going on the dyno unless everybody wants to pass the hat, I'm about dynoed out.