Merge collectors

[Alan Roehrich]

Ed,
Step headers and merge collectors are similar to 4-2-1 headers, you need to do a serious amount of tuning and probably cam testing. When you use either, or both, you significantly alter your exhaust tuning, with an exhaust lobe that is large enough for a set of single diameter headers and a merge collector, it is very likely that you can create a serious "over scavenge" condition. If that is the case, you can see what happened with the right dyno data. The solution most often is a smaller exhaust lobe, as much as 4-6 degrees. You may also need to spread the LSA a degree or two. But what you may gain from all of that is the ability to run a large(r) intake lobe.

The short version of that is: if you have a correctly matched and tuned set of basic headers and collectors and cam, it is likely you will find no power gain, or a power loss with a set of stepped headers and merge collectors.

So, when/if you buy a set of stepped headers and merge collectors, and you spend $1500+, expect to buy at least one cam, and maybe spend money on a couple of dyno sessions and a track rental, so another $2000 is easily possible. It's just another example of "HP cost money, how fast do you want to go?"

[Billy Nees]

Originally Posted by Alan Roehrich

Ed,
Step headers and merge collectors are similar to 4-2-1 headers, you need to do a serious amount of tuning and probably cam testing. When you use either, or both, you significantly alter your exhaust tuning, with an exhaust lobe that is large enough for a set of single diameter headers and a merge collector, it is very likely that you can create a serious "over scavenge" condition. If that is the case, you can see what happened with the right dyno data. The solution most often is a smaller exhaust lobe, as much as 4-6 degrees. You may also need to spread the LSA a degree or two. But what you may gain from all of that is the ability to run a large(r) intake lobe.

The short version of that is: if you have a correctly matched and tuned set of basic headers and collectors and cam, it is likely you will find no power gain, or a power loss with a set of stepped headers and merge collectors.

So, when/if you buy a set of stepped headers and merge collectors, and you spend $1500+, expect to buy at least one cam, and maybe spend money on a couple of dyno sessions and a track rental, so another $2000 is easily possible. It's just another example of "HP cost money, how fast do you want to go?"

Alan, I've been telling the same story for years but nobody seems to want to listen. There are advantages to being able to take exhaust lobe away from the engine though. If a conventional header is "properly" sized to a combination, the average Racer should be satisfied. IMHO, most Racers buying off-the-shelf headers are buying too large a primary tube for their application and then being sold the merge collectors as the fix.

[Alan Roehrich]

Billy, that's what I'm seeing as well. If your headers are too big, you can "crutch" them by using a merge collector with a small minor diameter, and even better, the longer of the two common correct tuned lengths. For example, if you had 2" primaries, and you should be running 1-7/8" or 1-3/4" primaries, you could use a merge collector that had a 1/4" smaller than normal minor diameter ( say 2-5/8" instead of 2-7/8"), and if the first tuned length was 7", you could go with 14".

Unless you buy your headers from someone who really knows what they're doing (there are not too many guys like Jere Stahl selling headers these days) or you have good header software (PipeMax is what I use) most people end up with 1/8" or more larger primaries than they need. In order to compensate for that, they need to add 3-4 degrees at 0.050" to the exhaust lobe. If you can't or don't spread the LSA out, that really limits you on the intake lobe and/or the ICL.

Here's a good example. On a Stock Eliminator engine, we did the preliminary dyno testing with a set of 2-1/4" dyno headers. Then we swapped to the 2" - 2-1/8" Stahl headers with Jere's semi merge collectors. We immediately gained well over 25 ft/lb of torque, but we also lost well over 10HP at the top over a 600 RPM spread. Then I looked at the dyno data closer, and realized that at the same place I lost over 10HP, the engine flowed almost 20 cfm more! Guess where the 10+HP went. Right out the exhaust as an "over scavenge" condition. The correct fix for that is around 4 degrees off the exhaust lobe, and 1-2 degrees more LSA. The reason for the whole thing was the engine previously had a set of Hooker headers that should have been on a Super Stock engine, the primaries were too big and too short. A set of Mark Lelchook's merge collectors were used to help "crutch" that. But even that was not enough. What we know about that, is that this particular car, with that cam and Stahl headers is a killer in the 1/8 mile, but a little off in ET in the 1/4, and down around 2MPH at the top. It's getting a different combination, but Jimmy Bridges and I had decided that, if we kept running that one, it would get 4 degrees less exhaust lobe, and 2 degrees more LSA. I'm pretty sure Jimmy is ten times smarter than I am.

[Darrin Christen]

Interesting read. What would be the tell tale signs that a engine may be over scavenging? Anything noticeable in the tune up that would be an indicator of this condition?

[Billy Nees]

Originally Posted by Darrin Christen

Interesting read. What would be the tell tale signs that a engine may be over scavenging? Anything noticeable in the tune up that would be an indicator of this condition?

Pretty much an over-simplification but the car won't be responsive to timing or jet changes.
If the combo can handle it, open up the exhaust lash and make a run.

[junior barns]

Been out of racing for about 15 yrs but I can remember when we changed from our Stahl headers (2 inch) to a Davis step header (2,1 7/8,1 3/4). Didn't see much gain until we added an 18 inch extension on the collector. Really brought it to life.

What was it trying to tell me???

[Lee Valentine]

Billy is dead on,you can't seem to get it rich enough. You'll be putting jet in that makes no sense, loosening the exhaust lash and picking up shows the exhaust is pulling unburn
,ed fuel out on overlap,

[Gary Smith]

Originally Posted by Darrin Christen

Interesting read. What would be the tell tale signs that a engine may be over scavenging? Anything noticeable in the tune up that would be an indicator of this condition?

The topic of lean rear cylinders is what drove my question. We haven't even run the motor yet, not looking for gains, just don't want to go backwards. Headers are nothing special...straight 2" primaries about 40" length. Last year we made some fixes (new fuel pump) and added 13" to the collectors and picked up 2 tenths. I suspect much was from the exhaust. But everything is bigger now, with lots of other changes with same basic induction. We'll probably put straight 4" collectors on to start. FYI this is not for my stocker. It's a bracket motor built with a lot of S/S technology.

[Ed Wright]

Mine is very responsive to timing changes, likes to run richer than some others. I talked to Jere Stahl quite a while, twice when I ordered mine. Always used his stuff on my old "Jr Stockers". Tried to order a set from him when I make it a race car. At that time they had no jig for the LT1 Camaro/Firebird. Whoever I talked to then told me they could build a set for me if I wanted to bring my car there. York, PA is a long drive from Oklahoma. Talking to Bobby Warren a couple years later, I learned he had taken his there to have a set built, so they now had a jig.
Jere had me fax him my dyno data (both steped & straight 1 7/8" headers) and engine specs. I told him I had tried my friend's merge collectors at the track, and what I found.
Jere told me he saw no way it would like the 1 3/4" to 1 7/8" steps, he said the non-step 1 7/8" should be right, and it MAY like 1 7/8" to 2" steps. He didn't see it liking merge collectors, either.
I tried backing the ex lash off with both styles collectors, didn't like it. I have not tried less than 4 degree int/ex duration split @.050". Less than 3 at .200" lift. Tried wider lobe centers, even with more gear. Dyno liked it, short times suffered. Back half about the same. May need a 7" converter with that cam? Don't know. The normal cam did not like one at all.

[HawkBrosMav]

Quick question, and I know this may be more complicated than this, but as a generic rule of thumb would you need a better scavenging header setup for a cam with more or less overlap?

assume generally the same cam specs for a cam with 80 degrees overlap compared to one with say 70 degrees. only change being lobe separation. you you need more scavenging going to the smaller overlap to be as efficient?