Re: Merge collectors
 

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.

Re: Merge collectors
 

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

Re: Merge collectors
 

Don't neglect lengthening the secondary tubes. I purposely make the secondary tubes shorter than ever needed. More often than not you will pick up even more power. There is no way to know exactly what a given motor might want for both length and collector diameter. If you do not have 4 extension tubes to move the final collector back than give me a call. I'll fix you up. The more you keep me updated with how your motor is responding to the headers or not responding the more i can help you. And don't be afraid to keep richening up the carb until you slow down.

Re: Merge collectors
 

When the PS cars, (not to mention others) started using a O2 sensor in each tube, (8 of them) is when we started running into some interesting things. It seemed that as the choke diameter on the collector was tested smaller and smaller there came a point that the O2 readings would get rich for no other reason. If the motor was leaned down, the O2 would still read rich and the car would slow down. Open up the collector diameter and things would be happy again.
Now some might think differently but my and others conclusion is that as the header was pulling harder on the engine with the smaller choke, more unburnt fuel and air with it were being pulled out of the engine instead of in the cylinders. Many have learned that this points to power left on the table as a cam timing change and other changes can utilize the increased depression on the exhaust side of the motor to make even more power with the smaller collector diameter. Not having O2 sensors in each tube, I do not know how else to determine what I call overscavaging when in the car.
Another point is if your header is too large or too long, it is also possible to hurt power straight out with too small a choke.
And yes, you do have to have extensions and adjustable choke collectors or several on hand and bring them into your testing loops. And no this is not easy, but it is real.
You can just have a bigger header and not worry about it all...:)

And some other thoughts;

A given engine wants a certain size collector diameter on a given day.

Higher altitudes can use a smaller diameter right off the bat.

A given diameter collector needs to be a certain distance from the cylinders on a given day. (that may seem to contradict the first sentence, but it does not, a larger diameter may want to be further away.)

If you keep adjusting these variables, you will find a pattern your engine likes and needs for the different conditions you run into.

If you change other things on your engine, don't neglect the above adjustments.

And I think I stated earlier, if your motor is not sensitive to these things then a larger adjustment by me may be in order. I often do that gratis too for my customers.

Re: Merge collectors
 

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):D

Re: Merge collectors
 

You are correct. The exhaust pulse may be even some what larger because of heat. Than again, how large would
would the intake charge be at the same r.p,m.? Thanks for asking me to expand on my quote. The bottom line is that we are select group of stocker guys trying to help each other. Happy Motoring.

Re: Merge collectors
 

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%.:o

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.:(

Re: Merge collectors
 

Very good info Calvin. Thanks for taking the time to chime in with your observations. I have been working on a set of 421 headers for my car and this has been some good info. I am trying for some more low end with the headers. I have straight 4 to 1 on the car now. The 421 are stepped. only time will tell.

Re: Merge collectors
 

Very good, very I nteresting information here. Thank you Mr Elston!