Exhaust research - FYI

[boofhead]

As most of you know I am always looking to improve my knowledge so I have been looking at exhaust systems lately. I was aware the X pipe is generally better located further down the system than a H pipe would be so I was looking into X pipe location and it affect. Since this is a Tech forum (most of the time ) then I came across this article - thought others might be interested.

http://www.r3vlimited.com/board/showpos ... tcount=220

I found it very interesting.

[hybrid]

Unigroup engineering eh... I had them dyno my Nissan.
Good bunch of guys.

[Ausjacko]

The mate who does the e-types makes his own exhausts and was telling me about this a fortnight ago. His insight comes from a guy in the States who raced nascar and they are very into the harmonics and tube lengths etc. He was saying also that there is another wave that travels around the pipe that is associated with the harmonics. Nascar used some form of internal lip on the pipe to disrupt the wave travelling further back up to the port and head. Mind you, they are revving those things to 9k!

[ozbilt]

I had found on the 5.0 EFI blown engines that having the X just under the front drive shaft uni was the optimun position for that engine. That was running a 2 1/2" pipe.

Good news was it also served as a drive shaft loop and allowed positioning of the pipes so the gearbox could be removed without touching the exhaust .......

[ozbilt]

The mate who does the e-types makes his own exhausts and was telling me about this a fortnight ago. His insight comes from a guy in the States who raced nascar and they are very into the harmonics and tube lengths etc. He was saying also that there is another wave that travels around the pipe that is associated with the harmonics. Nascar used some form of internal lip on the pipe to disrupt the wave travelling further back up to the port and head. Mind you, they are revving those things to 9k!

NASCAR teams run a different exhaust for each track (actually different cars and engines too).

[boofhead]

There are quite a few harmonic lengths both for the intake and the exhaust. In the
exhausts case you have the primaries, secondaries, main tubes (to the H or X pipe) then
the secondary main (to mufflers) and then the final. All have different harmonic lengths with possible different tuning options. Harmonics vary depending on the tube diameter.
Then there are tricks used to help stop reversion - such as small restrictions (a little
like a rocket engine) or short inner tube. For us there is little we would really be
concerned with though position and type of balance tube is fairly low hanging fruit.

Oz - all very good points to further the advantages of an X over a H pipe.

[ozbilt]

We also used to run stepped headers on the race car (sports sedan) and had an exhaust system for both engines. One was a short circuit setup and the other for long circuits.

[boofhead]

Indeed - often higher performance engines use a stepped primary which also help reflect reversion pulses so they are minimal by the time they get to the exhaust valve. You do not want to pollute the next intake charge with old exhaust gasses. A lot of the old-school rough idle we all love is from reversion.

[ozbilt]

Another thing we did on the sports sedans (and I have carried it onto some of the road cars I have built) was to have differing lengths in the system for each side. That way the release point at the rear did not bounce off each other amplifying the exhaust note.

[boofhead]

Ok I see your logic - cool.

Here is one people do not think about often. When you have a set of headers that are a 4 into 1 design - you want to make the primary pipes go into the collector in the firing order for that bank. This makes the pulses go around a circular pattern in the collector so the previous pulse cause high vacuum close to the next pipe helping draw the gasses out of the primary.

The issue here is the sequence can have differently timed pulses due to 2 ignitions on the same bank in succession, e.g., HO firing order is 13726548 which means 1-3 and 6-5 are on the same bank which changes the affect of the vacuum pull. This is why an optimization is to have an exhaust the crosses over from one bank to the other so even pluses in the collector. A good example is a GT40 header system.



See how it crosses over to connect to the opposite collector.

[ozbilt]

Ahh, what would they know. Like what did that car ever win ? ......

[ozbilt]

The original Ford GT (GT40) would have been the 289 therefore ..... 1-5-4-2-6-3-7-8, however looking at the pipes, it appears to be a HO at least. Is that correct?

[boofhead]

Yes 289 and indeed as far as I am aware would have been the original firing order.
So what you want to do is say for 1-5-4-2-6-3-7-8 you want every second ignition event to be directed to one bank and the other set to the other. You end up with one side being 1-4-6-7 and the other 5-2-3-8 so there are two cross overs for each side.
In the case of a HO cam it is 1-7-6-4 and 3-2-5-8
Looking at the image I think your correct as to me it looks like it is set up for the HO firing order.

[ozbilt]

Yes 289 and indeed as far as I am aware would have been the original firing order.
So what you want to do is say for 1-5-4-2-6-3-7-8 you want every second ignition event to be directed to one bank and the other set to the other. You end up with one side being 1-4-6-7 and the other 5-2-3-8 so there are two cross overs for each side.
In the case of a HO cam it is 1-7-6-4 and 3-2-5-8
Looking at the image I think your correct as to me it looks like it is set up for the HO firing order.

I have just been checking original images in the Ford & Shelby racing books I have & they all are the same config you pictured. Does this mean way back then they were playing with the HO firing order?

Interesting to me from the history perspective ......

[ozbilt]

2 late models & HO from what I see. The second is Roush so (I assume) has to be a HO.