merge collectors ?

[rod]

the guy at the header shop made me a very nice set of merge collectors, megaphone exits with a goilet inside.
not having any experience with these....it appears that the carb. needs to be jetted up several sizes to compensate for the increased 'pull'. [from the few runs i have made]
your opinion and or experience would sure help and be appreciated.

thanks
rod in AZ.

[Tony Curcio]

I have a dyno session scheduled for December 30th, when I intend to look into that very same question. I'll have wide-band oxygen sensors in both the straight and the merge collectors to see how they effect the mixture.

When I bought the merge collectors at a flea market, I bumped into Super Gas racer Tom Goldman, who told me to expect to richen 3 steps, just as you have found.

If you want to find out my results, e-mail me at tony@brentwoodmanagementcorp.com, on or after the 30th.

[68camaroSS]

why not post the results? we would like to know.

[Tony Curcio]

I'll be glad to post something, now that someone has asked.

[Tony Curcio]

I used 4 of 11 pulls on this issue. The result was the opposite of what I expected.

The AFR for straight collectors was 12.47 and 12.48
The AFR for merge collectors was 12.20 and 12.18

So the mixture was richer as a result of installing the merge collector, the equivalent of .002 or .003 change to secondary metering rods.

There was a horsepower gain, about 4.5 average, with no other changes. It responded to being richened even further, so it isn't clear if the gain had more to do with needing to be richer in the first place, or the merge collector. We'll work on this some more at the track in the spring.

[Mart Jeltema]

I have been down that road on many of they engines I dyno for other people. The O2 and EGT's can lead you down the wrong road many times. Always tune for best HP. I look more at the BSF # more than anything and have found that the merge collectors straighten out the fuel curve and increase torque more than a peak HP increase. That being said your power #'s are more useable and consistant through the curve.Happy Tuning and Happy New Year.

MJ aka PM

[Adger Smith]

Tony,
It is probably the carb booster signal that the collectors changed.
As you would expect the collectors are making a change in the acoustics
of the engine, through the ex.
It may not have the same results with another carb. Some carbs are more sensitive than others.
So much for saying you always need 13.2 A/F.....
Can you say it needs the A/F ratio the engine wants?
I would bet it is not a high compression combination.

[Tony Curcio]

First, I want to say that when I wrote that the result was opposite of what I expected, it is probably more accurate to say "different than what I understood". I was told I'd need to jet up to realize the full potential of the merge collector, and this is what happened. But I thought the AFR would go leaner upon installing the collector.

After reviewing the log book for test & tunes on July 28 and Nov 7, I'm now reassured that this carb was jetted optimally for the real life conditions we encountered. I was concerned that it was too lean going into the dyno session, but the temp was only 10-12 degrees lower yesterday that it was on Nov7th. The dyno setup was still making more power even after a radical move from .050 to .040.

So, I think this means the extra exhaust velocity was making it want more fuel?

-Adger, you are correct, it's a 255 HP 350, which has 9.00 to 1 compression.

I am not sure I understood the question, "can you say it needs the A/F ratio the engine wants? ". I suspect that on track tests will show that it doesn't want to be quite so rich- is that what you mean?

-Mart- I will look over the BSF again. thanks for your comments.

[gmonde]

what was the size on the merg collector vs the straight collector i know from the o2 sensor just by adding collector extensions(14") on 3 1/2 hooker headers it changed the afr by.13.00-12.50 and it picked up in the 60 ft and the bung is close as i can get to the primary out lets as possible ,,,your merge collector does it choke down??? or is it the same size as the straight ??? gmonde

[Adger Smith]

Tony,
What I mean is that you should give the engine what it wants to make power. Don't say it needs to have (as example) 13.2 AF ratio. Do like MJ says. Tune for best power. That might change going from Dyno to car, too.
I have run into customers that will tell me:
1. It has to have ---- A/F ratio to make power
or others that have an EGT meter and tell me:
2. It has to have ------ EGT... Usually they find out real quick they are off base with their assumptions if they want the engine to make the best power.
I have found that, Usually, lower compression engines make more power on a little richer mixture.
They can also like a degree or two more timing than the same combo with hi compression. Some times that changes in the car. (gearing/converter/Load in the car being different than dyno)
My guess it has to do with the charge density in the combustion chamber.
The richer mixture may increase the C/R a little and make the mixture molecules pack tighter and burn better. Isn't it said that Combustion is just a chemical reaction where heat is extracted from the A/F mixture? Just as a side note, some of the guys I know that run Comp say they can't see much with the merge collectors on the Dyno, but the car usually goes down track faster. I'm with MJ on how the merge collectors might help on the fuel curve, esp in a car going down track.
I might suggest you go back and check your tuneup with both collectors at different acceleration rates on the Dyno. You might find that w/ the merge collectors there is a real difference in the faster rates. (back to that signal at the carb booster)