Merge pipes without eq length headers?
 

Merge point is supposed to be a critical distance from the exhaust valve, but without equal length headers that distance isn't the same for each cylinder. How much of the "merge" effect is working if something like the factory manifold is used where the exhaust length increases by 4.5" for each of the cylinders on that side?

Makes me think some kind of "limited" but equal length header design might be worth looking into, something like the "shorty" style of headers.

Or is it enough that right and left pairs match up in distance?

If you are comparing to stock '85/'86 or later manifolds, whatever you do doesn't matter much. What I mean is a good merge to a single, or X pipe to duals, will be just as good on the stock manifolds as on any header type system. It's the merge/X pipe that matters more than what's in front of it.

unless you're building a full-on race motor, looking for every last drop of horsepower advantage, worrying about header tube length, merge junction locations, and such, is a waste of time and money.

for years MSDS was the only thing available for 928's, but Phil Threshie had introduced a tri Y header that id flow more HP.

your inquiries are better directed to those who have "been there, done that"......

I believe I listed those resources months ago for your consumption.

http://www.pelicanparts.com/support/smileys/pc2.gif

Thanks Louis,

Using the 85/86 factory manifold is what I have in mind. I posed about the same question to the guys on Speedtalk forum, and the first reply was that unless the cam has significant overlap exhaust tuning doesn't have much gain over just low restriction. Sad to say, but I have no idea how much overlap the S2 Euro cam has, just lift and duration numbers.

One of the Rennlisters with a 81 non Euro motor posted recently that his car dynoed I think it was 198 rwhp with 86 manifolds, and a modified Y pipe, "free flowing" cat and Borla exhaust. His guess is that this was good for 10 hp over stock, which seems low to me (lot of other restrictions on that car though).

I have no experience with the Euro cams so what I know that applies to S3/S4/GT may not help. I have heard that those engines are sensitive to exhaust. Go too big and you lose. You'll have to experiment and get your own data. I've known of an S4 and a GT that had documentation of power stock, with X pipe, and with good equal length headers. Header collectors feeding into an X pipe. In both cases the headers cost a few hp over the stock manifolds and X pipe.

In all my years.....seriously; I've never had a car that headers didn't kill some low-end torque and take away a bit of street drivability. The question is whether you are willing to lose a bit of every-day go for a bit of once in a while top-end go. I don't recall anyone ever having a negative about using an X-pipe, regardless of M.Y., though I don't cruise all the 928 forums near as much as I used to.

My '79 broke 200 RWHP with a gutted cat, bypassed mid muffler, ANSA rear muffler, and 5 degrees advance at the dist. Other than that it's fully stock and in a good state of tune. The 78-79 are supposed to have slightly hotter cams than the mid-early cars, so that would be right in line with the '81 that you mentioned.

To the prior post, what octane rating are you using for the 5 degrees of advance? Also, have you left the distributor stock?

My question is if changing the manifold to something equal length will have a worthwhile benefit making the Y merge pipe work better?

People tend to trade HP for torque with header tubes that are really too large for the intended application. Keep the ID down and the whole powerband can be increased. Thats the direction I see in street motors creating the most area under the curve, not just peak HP, and many have been very successful is doing it.

That said I am intentionally looking for a bit of a retro, barely street friendly nasty idling really feel it when the peak torque hits kind of motor. That sounds kind of fun, worst case I get more practice pulling the motor.

Running 87 octane; the C.R. on the early cars is really low, so pinging isn't a worry.