Filling or getting exhaust crossovers filled??

[dyeager535]

Yeah I was reading up on combustion temps, and apparently they exceed aluminums melting point every time the cylinder fires, but the amount of time that heat is in the cylinder isn't long enough to cause damage, again under normal operating conditions. Melted pistons are obviously possible though!

With a plug between two exhaust valves, you'd be seeing double the heat output, which would heat that area up more. With combustion temps exceeding aluminums melting point on every firing (assuming the melting point of pistons is somewhere around 1200* and combustion is somewhere around 1500*) the only reason pistons don't melt is because they don't have time to absorb the heat. Double the amount of time the heat produced on every combustion event stays in the cylinder and the result would be a melted piston in short order. I mean, you can melt aluminum over a wood fire.

In any case, I'm just having fun analyzing the possiblities. If you've seen them built and run them this way, I'm sure there is no issue, and there is no reason to argue with success. Besides, even if they started to melt or break apart, I'd expect they'd go downstream which wouldn't be catastrophic.

 

[sled_dog]

Combustion temperatures are far higher than the temperatures you are quoting.

4000-5500* actually.

Heres the way it breaks down. 1/3 goes to power production(forcing piston down), 1/3 goes out the exhaust, and 1/3 is absorbed in the cooling system.

Why don't pistons melt? They are cooled. Sure you couldn't touch one but they are cooled by oil and engine coolant. Little of the exhaust heat is absorbed into the head. The heat makes it move out quickly, as well as the fact that it is being pumped out.

We just spent a few days learning this exact subject in Engine Management Systems(cooling and oil systems were the lessons).

 

[dyeager535]

Dang, numbers I found must have been for a two stroke or something (no idea what would be that low)...just the first numbers found in a search.

Little of the heat is absorbed into the head/piston, but increase the amount of time that heat dwells (or overall cylinder temperature) and surely the amount of heat produced would quickly surpass the ability of the cooling mechanism. (or else you'd never see melted pistons from lean mixes, right?) A piston/cylinder sees a cold intake charge every 4th stroke. Exhaust never sees a "cold" charge, and siamesed exhaust ports double the time a part sees exhaust heat. I'd expect the wall between exhaust ports 3 and 5 to easily exceed the temperature of the wall on say, 1 or 7. In any case, if you can make a head out of aluminum, no idea why a plug couldn't be poured of it either.

I have a feeling there is a lot of theory on cylinder temperature, effects, dispersion, etc.

 

[sled_dog]

I am certain you are right on that. I should have put a disclaimer that my knowledge comes from the teachers of Wyotech. Are they the most knowledgeable folks? I have no idea. It all sounds good.

 

[ntsqd]

I has assumed that we were discussing SBC heads. I saw the Pontiac ref, but thot that was only to point out that it can be done. When you say "siamesed ports" are you refering to the nearness of the SBC center exhaust ports or?

FWIW, at that engine shop they build a HUGE number of 460's for one customer. It's a slow speed low load stationary engine application and they have singlehandedly run the entire country out of 460 Badger pistons, several times. Those engines go out with a plate welded into the intake side of the heat riser port on each head.

 

[sled_dog]

we are talking SBC heads. I was confused by his siamesed comment too but I believe he means the nearness of the middle ports.

My Pontiac comment was that I've looked up a lot of Pontiac performance stuff over time(dad owns a 69 Firebird) and that was the only place I spotted mention filling the exhaust crossovers but figured it would apply to a Chevy too.

 

[Mad-Dog]

I paid just over $900 for the Dart Iron Eagle 180's fully assembled with ARP studs, Comp guide plates and .030" head gaskets.
The intake ports actually flow out at 174cc and they have great exhaust ports, not as good as AFR's but who cares since i did'nt want aluminum heads.
Remember that the heads dictate the cam choice and you'll do fine, 180's are as big as you want for a 350 cid motor to keep velocities high at low speeds and still make close to 450 w/o power boosters.

 

[mountain4x4]

Alum fill

I was just talking about this a little while a go with a guy that drag races his BBC, as I'm building a new BBC for my '74 k20. He heated up the heads in the oven, and filled the exhaust crossovers with aluminum, then contoured them to match the other cylinders. SBC would be the same idea. The machine shop I use does the exact same thing, for a large fee. I'd just cap the port where the inntake manifold gasket goes, filling the head is probably not worth the hassle if youre not racing the motor, but I do not know what the difference in power really is... My intake manifold has no provisions for exhaust x-over, thats good enough for me.