Quote:
Originally Posted by older racer
question? how much shorter should the ex lobe be? 10 deg @.050
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There is no answer for that question, it is way too broad.
An example would be a combination that has a relatively large and lazy exhaust port, usually having 4 into 1 headers on the large side. A properly designed and built Tri-Y header, with merges at both Y's and on the final exit, could allow 6-8 degrees off of the exhaust lobe at 0.050", as well as an increase in the LSA of 2 degrees. That could allow 2-4 degrees more intake lobe.
This is something that requires very serious dyno testing, as well as track testing. There is no shortcut, none. Unless you find a person with an identical combination that has done all the work and is willing to share. It's a really nice guy that will share the results of spending $10K and 80 hours or more to find a tenth.
As discussed in a thread about merge collectors, Tri-Y headers are another one of those things that should be a "near end of development" endeavor for a given combination. You can probably find cheaper and easier tenths in your combination, unless you've been flogging it seriously for ten years.
Probably one of the smartest guys on this is Larry Meaux, the developer of PipeMax software, he is a cylinder head guy, who has a flow bench, a dyno, and an incredible love for this stuff. Larry will be the first to tell you that once you get past the first steps of correct length and diameter sizing on headers and collectors, 99.9% of all racers will quit working and spending money. They won't even spend the time or money to find what can be had with merge collectors.
You often need a significant cam change just for one or two step headers and merge collectors. One of our engines, for example, will suddenly use 20 cfm more air above 6400 RPM, yet lose 12HP, when we switch to one step headers and a semi merge collector. As such, we know we have a serious over scavenge condition. We did not have time for a new camshaft, and given that we gained a ton of torque, we had to skip the cam change. We broke it before we had time to get back to the cam. But I now know we need 4 degrees less exhaust at 0.050", and 1-2 degrees more LSA. We also know we can run 2 more degrees at 0.050" on the intake, to compensate for a weak intake port we can't fix on these heads.