Siamese ports

[6bangerbill]

Does anyone understand what occures inside a siamese type intake port?
The late chevy 6 groups 1,2 ; 3,4 and 5,6 intake ports together, so that two cylinders feed from each of three ports. I have read that Jim Headrick in 1978 flowed 319 cfm @ 28" of h2o thru one of these ports producing 650 hp. THe guy was a wizzard with a chevy and suposedly ground his cams with different lobe centers on different cylinders to compensate for the siamese effect. I assume that these ports act as a common plenum to feed only two cylinders, but dont have a clue about how valve timing would compensate for anything. It would seem that port air velocity would not be sufficent for intake ram tuning because each cylinder sees such a large port section area.Maybe one port robs wet fuel from the adjacent cylinder inside the port, since adjacent cylinders dont fire 180 degrees appart. I know that the siamese ports work better than single ports with an IR set up with Weber carbs because each cylinder draws from two venturies instead of one. Commonly IR systems choke off airflow due to extreem air velocity in venturies.Notice that prostockers dont use IR systems. I understand that sonic velocity is the max speed that can flow in a port without some sort of boost.

 

[xctasy]

The Minis used Megadyne and American VP-8 cams with scatter patterns to compensate for the robbing of adjacent pulses from neigbouring cylinders. They conclusively work, just like varing compression ratios on cylinders prone to detonation. David Vizard found considerable extra power from doing very complicated testing with a raft of Crane camshaft masters. He found that after a few weeks of figures gathering, after doing five runs per test, that he came up with a few patterns. He then varied the lobe centres or inake opening and shutting, or the 50 thou figures to compensate for certain intake pulses.

In the A-series Mini engine, there was no way to pulse tuning the siamese ports, but there were ways to appease the sharp intake pulses. I don't know what LCA, what variance in intensity or what of the huge cost of the machining operation from new billet cams. Ouch!

The division of intake ports works on triple DCOE 45 Weber equiped Holdens worked. They just sleaved the siamesed section, and added an pined through alloy section. It reduced the port size, but helped a little. Thus the 9 port Chevy head, which is what the Holden L6 was based on, became a 12 port head. Some think the smaller port is bad, some think the scrambeled cams can make up for a bad 9 port.

All I know, is he who sows the dyno work in sweat, wins the green wreath.

 

[6bangerbill]

From what you wrote it seems that the siamese port has two wave forms bouncing in each of the three ports. And intake closing must be timed to swallow pressure waves that super impose or subtract pressure when combined.With a 153624 firing order,1 and 2 fire 240 deg apart,3 and 4 fire 180 deg and 5,6 fire 120 deg apart.What a mess! I think I will stay with a ford 300.However if some one could figure out the valve timing required to swallow super imposed 2nd or third waves, a 25 to 20% boost in hp would be possible.

Do you think that L head or flathead engines actually can benifit from intake wave effects? Or do the waves just bounce off the inside of the head and die? Wave length as i understand is measured from the valve to the port mouth.Maybe it does work.

My old flathead seemed unresposive to the number of carburators i used. I tried 3 twos,4 twos and a single 4 barrel carb with no measurable difference. It ran consistent 12.50s in my t roadster back in the 60s on recapped slicks. THose were the days with a flagman starting a drag race at "yellow belly" drags in dallas texas. If you started before the flag, the cars returned for another try. Flatheads were spotted almost half track against the "new" overheads.