Tuning with tri-y headers

[older racer]

Originally Posted by Alan Roehrich

Tri-Y or 4-2-1 headers are almost certainly going to require a major cam change in order to make them work. If not, you were almost certainly running the wrong cam for 4 into 1 headers. It's the same as going to one or two step 4 into 1 headers and a merge collector from regular 4 into 1 headers and a regular connector. The header change creates an entirely different scavenge cycle, that's why you use them. When you make a radical change in your scavenge cycle, you need to change the cam, unless you had the wrong cam to begin with.

If you improve your scavenge cycle by any real amount, you should be creating an over scavenge with your 4 into 1 header designed cam. So you basically just pull a big part of your intake charge out the exhaust, wasting it. To take advantage of the improved scavenge cycle, you reduce the exhaust lobe and/or spread the LSA out to prevent an over scavenge condition. In some cases you can even increase your intake lobe to take advantage of reducing your exhaust lobe and/or spreading the LSA.

question? how much shorter should the ex lobe be? 10 deg @.050