Inliners International Forums General Discussion Hi Performance dividing siamese ports on chevy 250 head

I'm in process of dividing the Siamese port and will take photos to show progress. Any one try this or know of any info on it. I know about lump ports but that does not eliminate the velocity drop of the big double port bowl

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You don't get that much of a drop in velocity as you may think. Don't forget that each intake port is shared by 2 cylinders, so you aren't creating that much of a pressure drop compared to that one large port only feeding just 1 cylinder.

Also Tom's dyno tests showed that there was very little HP lose when dynoing the same head with the bosses removed and no lump in place with theoretically poor velocity vs. the exact same head with the lump bolted in place with theoretically more velocity.

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I discussed doing this with Tom Langdon. He said GM tried it and it hurt the performance. FYI

Yes, the Brazilian GM 12 Port head is a good example of this, but you have to think outside the box to make it a benefit instead of a detriment, and GM didn't do that.

You mean like putting the intake on one side of the engine and exhaust on the other so you have room to separate the intake runners?

If so then yes.

I don't see a way to get enough room to make this work well. Will it work OK? Yes Maybe. Will it be an improvement? Not likely.

No, not a crossflow head. But a divided port siamese head has already been done 20 years before Tom did it and it had great results even above what is capable with a fully ported brazed lump race prepped head.

What is this unobtainium and where do I find it?

In 1963 we took a stock head and divided it with 3 cast counter weights cut down and brazed in to make dividers, with an algon fuel injection and sent a new AHRA record by more than a second.

That was then this is now. A divided head would run smooth but not make the power most think they want today.

Also back in the mid 60's went to Ohio same town as Ohio George to look at a 12 port that had the intakes on the plug side it was already cast but the ports were narrow like the new chevy heads did not think it would work, the ports were low to the valve not tall like it needs to be.

I did make a bolt in divider that worked I always tried to use a divided port just makes sense, in the 60's guys would try and remove the divider on supercharged V8's and they would not run at all. Guess you really need a divided port, Just get a Kirby 12 port and be done with it, you will be money ahead.

If you can not afford that, a bump port works very good Kay Sissell was way ahead of his time and Mike Kirby is very sharp on what works and what doesn't.

My 2 cents on dividing ports.

Well I divided mine. Just finished, came out real nice will take some photos. I used high temp JB weld putty to do it. Then ported it so it would swirl going into combustion chamber. Stay tuned for the photos in the next couple days. I put in 1.88 valves on the intakes and enlarged the ports by grinding down the head bolt boss and some on opposite wall. Looks almost as big as exhaust ports but not quite.

You should get the head on a flowbench and see what you accomplished. But unless you remove the bolt bosses entirely, you wont have any gain at all. Tapering or thinning the bosses enough to have any gain will result in collapsing the roof of the intake ports when you torque those head bolts down. You'll have better results by totally removing the bosses and dividing the port in that manner.