Merge collectors

[Exhausted]

Originally Posted by Exhausted

It seems that 10-15 years ago, most engine builders all had more exhaust duration than needed. (I think this is a result of relying on dynos, IMO). It is less the case currently, but I am getting less feedback these days.
Going from a 4into1 to a 421 it is a safe bet. I am always willing to discuss those decisions with folks if they would like,and I do often, but the cam grinders are probably more up on direction to go there.
I will say that a 421 header will almost always add bottom end power no matter what size you make it. But horsepower upstairs is very easy to lose or miss by breaking flow rules and using wrong tube size. (read; you should get what you pay for)

Revisiting this thread I thought of another point about cam timing I would throw out. Adding duration to the intake with the same exhaust lobe has the same effect as shortening the exhaust lobe.
If your intake lobe duration and location is compromised by needing to maintain a certain bottom end power level for the hit, than shortening your exhaust isn't gonna help much. I think I posted before, if you let the headers protect the bottom end power, (and they can) you can move the intake to places you could not before.
I think it is certainly possible that the "421 needs less exhaust duration" thinking was a benefit because that also changes the effective lobe center
which is what the motor really wants. Tight lobe centers as a rule help larger lobe cams to run better but at the expense of a shorter power curve.When you only have three (or two!!) gears the broader your curve the faster your gonna go, all else being equal.
If you are not willing to bring the exhaust system into the equation your tuning options are expended sooner.

Overall, on the issue of "should I try a merged collector?", if you have the funds, and your engine builder shrugs his shoulders, pick a header builder and start with their recommendation. Outside of that it is a crap shoot. Successful guesses probably run around 20-30%.

I would also say if you have not tried simply lengthening your collector, do that first. If there is no effective change by doing that fairly cheap modification, than a merged collector probably won't either. Your header is not very close to what the engine wants and changing the collector is not gonna help.