Merge collectors

[dmir55]

I tied a set of headers together on Line's dyno and picked up five foot pounds of torque with no other changes. A two into one set up. Back to Jack Davis, he told me he made a set of two into one collectors for a NASCAR team. When they ran their car at Michigan the car was so much faster that Nascar banned the two into one set up. Hence they went to the "X" pipe. All we are trying to do is to keep fifteen pounds of atmospheric pressure from running back up the pipes. The fewer openings you have for the exhaust pulses to exit the better headers and collectors will work. Just think of how small that exhaust pulse is at six to eight thousand or more r.p.m.

[Billy Nees]

Originally Posted by dmir55

Just think of how small that exhaust pulse is at six to eight thousand or more r.p.m.

I'm not understanding what you're trying to say. If X cfm of air and fuel is going in then X cfm of air and fuel must be coming out. The exhaust pulse may be getting faster(and stronger) but it's not getting smaller!

[Exhausted]

Originally Posted by Alan Nyhus

Calvin, a couple of questions. I've got a set of your 4-2-1 headers on my 327/275 stocker motor and want to spend some time on the dyno this winter doing some camshaft work.

- Is backing up the exhaust duration by 4 degrees is a good place to start. Your thoughts?

- If an engine really is over scavenging due to the header design, what are some of the things to look for on the dyno or the track?

Thanks for joining in. -Al

Alan, A 421 header is probably not going to overscavange anywhere as easily as a 4into1 might. The battle folks have with cams and 421's is the change in reversion at the bottom of your power curve. If the headers are helping protect torque, you can go places with cam timing that you could not before. This includes retarding the whole thing, widening the lob center and therefore having to shorten the exhaust duration, etc.
There are lots of times you can go much faster without changing anything because of the broader power curve. If you already have your cam timing advanced to the max with lobes as big as possible, then you will not see so much of a benefit bolting on a 421 header.
Making more power at the top of the curve is a matter of how well built the header is from a flow viewpoint. Too many tight bends and lousy port to flange transition is where most fall down. Most of that is right at the flange. It is very important and overlooked.

[Alan Nyhus]

Thanks, Calvin. Initial dyno work was done with a decent set of 1 3/4 dyno headers with a good merge collector. A set of good 1 5/8-1 3/4 headers with a very good merge collector was worth 8 hp. over the 1 3/4 dyno headers. The motor was freshened up (including a different cam) and we started with the 1 5/8-1 3/4 headers for a baseline. Then we put your 4-2-1's on and with no other changes picked up 11 hp and a big bunch of torque starting at 4,400. I had expected to see the torque pick up but wasn't expecting the h.p. increase. -Al

[Exhausted]

Originally Posted by Alan Nyhus

- Is backing up the exhaust duration by 4 degrees is a good place to start. Your thoughts? -Al

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)

[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.