Open Pipe / Megaphone design - any ideas?

[Paul Thompson 'Hooligan']

Hi all -

We may have the opportunity for some high speed testing where there are no noise restrictions.

Does anyone have any knowledge of exhaust design for open pipes? -

From the collectors, what sort of length/diameter (taper?) design?

I have seen both straight pipes and tapered pipes on originals but have no knowledge of flow or design charateristics so any ideas / thoughts would be much appreciated.

We will run with full wide-band targeted Lambda control via Motec ECU and engine is 347ci with TFS Heads + Kinsler Inj bodies.

Thanks

ps - forgot to mention - primarys are all approx 40" in length, 1 7/8" dia.

[Bill Bayard]

Paul,

Just curious, but how did you manage 1 7/8" primary

tubes? Weren't the flanges a bitch to get bolted up? I have

1 3/4" pipes, and they were no fun to bolt up!

I assume that your heads are Stage III, to take advantage

of those huge primaries. [img]/ubbthreads/images/graemlins/cool.gif[/img]


Bill

[Paul Thompson 'Hooligan']

Hi Bill -

Could well be a mistake there - Primarys may be 1 3/4 - I'll check.

I presume you refer to the flanges bolting to the heads. for which Roy had steel adapters cut from 5/16 plate which bolt to the heads with flush c-sunk bolts. The header flanges then bolt to these adapters with wider spaced bolts and adapter is profiled to match the bore changes as best as possible. Bolting to the steel adapters enables us to pull the bolts pretty tight and as they have been in and out a few times, we do not damage the threads in the heads.
(hope that makes sense?)

The heads have been heavily modified, both in US and by Roy but I don't have any flow figures.

[Kalun D]

The Mk V that I know has a curved cone going from the collectors to the tip. It starts about 2.5"dia and ends about 3". (with 1-5/8 primaries, adjusts size accordingly) About 24" long. It's pretty hard to build because not only is it a evenly tapered cone but it's curved with a very slight radius that's even along it's entire length. I think it's curved so it clears the back of the trans but then drops down to fit in the exit hole. In truth it just looks like an exercise in difficulty for the fabricator. I believe that first a straight cone is constructed and then it's sand filled and heat bent.

[Adam C.]

Paul,

I did a big writeup on this very topic a while back, did you see it?

http://www.gt40s.com/ubbthreads/show...=&fpart=1&vc=1

Adam

[Paul Thompson 'Hooligan']

Hi Adam -

Nope [img]/ubbthreads/images/graemlins/blush.gif[/img] - but I sure have now!
You detail secondary pipes, 2" to 4" taper 36" long and also compare with straight 4" from collector to exit.

From what I see, 4" straight wins at top end and as its a lot easier to fabricate, that's what we'll go for.

Unless anyone else knows different?

Many thanks Adam - A great Technical post.... [img]/ubbthreads/images/graemlins/grin.gif[/img]

[Malcolm]

Paul

I have plenty of 4 inch pipe left over fom my cess pit change over. That should make you go like stink! [img]/ubbthreads/images/graemlins/grin.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]

More seriously, I have a contact in Mychett who may be able to help. I will endeavour to pop in and see him in the next few days as I have another query for him anyway.

Malcolm

[Bill Musarra]

Adam,
I have read all the post so far. I may have just missed it, but I have one other question. We hear a lot lately on stepped exhaust headers, where the diameter changes along the primary lenght. Can you comment on these and tell if there is much to it. I was considering changing my exhaust diameter from 1 5/8 to 1 3/4. I talked to the people at Heddman and one of there outsourcing people. Heddman was not interested in making a set of crossover exhaust as it would take to much time and resources vs. what they would be able to recoup in sales. The outsourcer was very interested, except he could not make the multiple bends required for single pipe per cylinder setup. He would have to weld up the straights and bends. He said he could make the stepped headers without any proplem. From what I have read (I will read almost anything technical on engines) the stepped headers are the hot setup these days (at least in the drag racing circles). Is there any advantage to using the stepped design, and if there is, what are the better sizes for such a design. I don't know if your software can handle this sort, but your thoughts would be appreciated. Thanks
Bill

[Adam C.]

Bill,

I did take a quick look into stepped headers some time ago. I didnt' see anything to write home about, and so didn't persure it any further. I think that if there is something there it is probably small.

For instance, in the other post I compare 1 5/8" and 1 7/8" primary diameters. If the difference here is small, then why would one that starts out 1 5/8" and then changes to 1 7/8" half way down really change anything?

One possible small benefit is that by allowing the exhaust gas to expand some as it travels along the primary, the flow losses are reduced some. This is not very important on the exhaust side though.

Alot of people are doing stepped headers these days, so there is probably something to it, but I don't think it is a huge breakthrough.


Paul,

The 4" stuff may look a little silly. Maybe just run whatever the collector diameter is (usually 3").

Adam

(By the way, I just noticed that some of my graphs have wrong titles in the other post, sorry about that.)

[Paul Thompson 'Hooligan']

Once again - many thanks Adam

3" pipes would fit a little easier too - can't wait to hear what it sounds like... [img]/ubbthreads/images/graemlins/grin.gif[/img]

[Adam C.]

Bill,

I was doing a literature survey for a paper we're going to publish next year and came across this one:

Dermot O. Mackery, John G. Crandall, Glen F Chatfield, and Malcolm C. Ashe "Optimization of exhaust-pipe tuning on a 4-stroke engine using simulation" SAE paper 2002-01-0002

Although the primary focus is optimization techniques, the effects of straight and stepped headers were investigated. They found that if both designs were optimized (i.e. the correct lengths and diameters for each configuration were used), there was no performance difference between the two.

If you want to check out the paper, just go to www.SAE.org and enter the above paper number. It'll cost you $12 though.

Adam