Exhaust note:- firing order & pipe length effects?

[SteveB]

Here are a couple of topics that have been discussed in lots of threads I have read and wonder what experience has been gained by others who have trodden the path before.
What is the result of the 351 firing order in place of the 302 ‘normal order’ and the variation in manifold/header length on the tone/beat of the exhaust note?<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com<img src=" /><o></o>
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My car has a Ford Racing 347 engine that has a 351 firing order and the exhaust system was supplied with the deal ( I heard the system on a 302, it sounded nice). The engine is running fine without misfires and on a rolling road gives 400+ hp and 380 Lb/ft. At a distance and on song everyone says it sounds great.<o></o>
However driving the car more normally, there is a distinct beat that is very wearing on longer trips and is more prevalent below 3000rpm when there is a more distinct flat tone and harsh beat to the note. The tone is a bit like a Subaru Impretza; there’s no shortage of urge it just is not nice.<o></o>
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I have put some pic’s of the pipes and on the spread sheet I have detailed the firing orders and pipe lengths (measured for comparison with a nut and string) also the sequence of the collector events etc.<o></o>
It can be seen that #4 and #8 are the shortest pipes and with the 351 firing order are sequential in the firing order. If the pipes are half the length can this cause a double pulse effect at particular rpm’s that set up a resonance, would reworking the pipe lengths to be close to equal correct this effect? A 302 has two other cylinders fire between #4 and #8<o></o>
It can also be seen that the pulses on the collectors are dissimilar the RH pulses rotate clockwise and the LH in figure eight; would this have any effect on the exhaust tone?<o></o>
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Short of starting again, my plan is to do Option2; swap pipe #8 and #5 in the LH collector and remake the pipes of #4 and #8 to more equal length to the others. What if any effects will this have on the exhaust note?<o></o>
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If you have been here before or understand the science I’d be pleased to have your input.<o></o>
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Thanks<o></o>
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Steve<o></o>
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[Jim Rosenthal]

Steve, without getting into the details of this, which you clearly know better than I do, the original pipes were set up with the two center cylinder pipes crossing over- this worked with the old SBF firing order. What about changing to a camshaft with the original firing order- wouldn't that be easier than remaking the pipes? I think the 351 firing order is the same as the updated SBF firing order, isn't it?

 

[Jac Mac]

In attached drawing of both firing orders, note that if you flip the 13726548 order from top to bottom the firing order pattern is exactly the same as the early 15426378 order, so the motor doesnt really see this as different & neither should the human ear, the only thing the motor 'feels' is the loads imparted to the crankshaft & bearing which is exactly why Ford went to the later order.... I think the sound difference you hear might be due to the rather large spread of primary lengths, but could be due to a combo of this plus other factors like muffler, cam , plus possibly a balance issue in the rotating assy. I should also point out that the modern practice of dumping the collector into the muffler without adding a tailpipe length is fundementally flawed, the correct way is to either build the appropriate tuned tailpipe length into the muffler as per Russ Nobles first muffler system or do it like 'Dimi' did on his DRB, or better still go with totally un-PC open pipes.

 

[SteveB]

Jim, Jac,
You have come up with the things that have I have been trying to resolve.
If all the primary pipes were the same length then the firing order should be OK either early 302 or the later 302/351 as the pulses would alternate into the collectors. It is a coincidence that with the 351 order that my two shortest primaries fire consecutively so that the pulse in #5 is followed more quickly by #4 as the pipe is almost half the length, #8 is also shorter and follows in an equal period then #1 taking longer in a primary that is almost double the length. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com<img src=" /><o></o>
Is the unequal sequence creating a beat/pulse that as the frequency changes with the rpm produces resonances that combine or cancel to produce secondary 'harmonics' at various rpm. Is the 351 order amplifing the problem of the short pipes and the 302 order would mask the effect.<o></o>
I did think of changing the cam but thought that without changing the pipe lengths to be near equal there would only be a partial improvement as the 302 sequence put two long pipe firings between the two shortest primarys, however getting the pipes right would not then need the cam change.<o></o>
I could not work out if the collector pulses would benefit if both rotated in the collector rather than one being clockwise the other a fig8, does this affect the tone.<o></o>
I can correct the lengths of #4 and #8 and correct the LH collector rotation by remaking just three primary pipes, but will it help.<o></o>

Steve <o></o>
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[Jac Mac]

If you remake the headers [ and you might end up having to remake up to 5 of the primaries ] do it in such manner that the collector with #2358 has the primaries in a circle order of 2583, that way it doesnt matter if you swap the cam, the rotation goes back the other way. { check that the other collector goes either 1467 or 1764 while your at it, try to get those two short primaries up around 950/1000 mm that should cover it, Oh and think hard about doing the tailpipe in the muffler idea [ if it has not already got it ] , its worth it.

 

[Andrew Robertson]

...Oh and think hard about doing the tailpipe in the muffler idea [ if it has not already got it ] , its worth it.

Sorry for the thread drift, but a question for Jac Mac: I treated my chambered mufflers as the "end" of my system due to their reasonably large volume, and put a tuned secondary length (based on the pipemax program) into the system before the mufflers, based on stuff I'd read by Vizard. What's your view on that approach? Cheers, Andrew

 

[Jac Mac]

Sorry for the thread drift, but a question for Jac Mac: I treated my chambered mufflers as the "end" of my system due to their reasonably large volume, and put a tuned secondary length (based on the pipemax program) into the system before the mufflers, based on stuff I'd read by Vizard. What's your view on that approach? Cheers, Andrew

Thats exactly what I have in mind Andrew, as long as the 'tuned secondary' length ends in that chamber, best way is if that secondary length extends into that chamber to create an anti reversion type end rather than simply welding on to the muffler end, creates a more 'definite end to the length that way, plus kills reversion..

 

[SteveB]

Jac

Thanks for the posts, I have looked back over seveal threads on related topics.
I think that a fresh start may be the best option so I'm trying to get together a system that will look Mk II 'ish, correct the primary lengths and have rotating pulses in the collectors.
On Adams system, the diagram I have attached was for an FE that has a 302 firing order but one of the collectors has the zed sequence. If sequenced as a 351 the rotations appear to reverse but swaping 6 & 7 corrects the zed to circular rotation and is also still correct for the 302.
So that's the primary pipes as the diagram that allows #4 & #8 to curl up and round the outside and looks MkII.

I had less luck understanding the 'anti reversion' issue and quite how to incorporate a longer tailpipe in the silencers, could you expand a bit on how to do this.

Thanks for the help

Steve

 

[Jac Mac]

1..3..7..2..6..5..4..8.................................or....1..5..4..2..6..3..7..8

laid out as ; 1..7..6..4..............................or.....1..4..6..7
.................3..2..5..8......................................5..2..3..8

.................1..7............................................1..7
.................4..6............................................4..6

.................3..2............................................3..2
.................8..5............................................8..5
as long as the circular order remains as above you can rotate the pipe group to ease fabrication.

Make the exhaust up with the primary pipe cyls into the collectors in the above circular order & they work for either firing order, just the rotation changes.

Cannot put up a pic of tailpipe into muffler at moment, not on my computer...

 

[Chris Game]

Great posts guys, really helpful! I think have read and understood everything but i thought i would post and ask what you all thought of this as a design for the headers on mk1 replica?

Also does anyone know anywhere i can buy 1 3/4 inch stainless steel tubing in pre bent sections?

Thanks,

Chris

 

[Tom]

This may be slight thread drift, but I'll ask the question anyway - what's the effect of putting a catalytic directly after the collector? Most of these cars are so space constrained that if you need a catalytic to pass emissions, there's no other place to put it.

Also, as I understand it, a chamber muffler acts as a termination for the exhaust pulse, whereas a muffler which uses a perforated tube surrounded by wadding acts more as a megaphone.

 

[Ian Anderson]

Great posts guys, really helpful! I think have read and understood everything but i thought i would post and ask what you all thought of this as a design for the headers on mk1 replica?

Also does anyone know anywhere i can buy 1 3/4 inch stainless steel tubing in pre bent sections?

Thanks,

Chris

Saw this a week back. No idea if prices any good or not

Comments On Barry The Benders Web Page

4 into 1 collectors try air-cooled VW spares sites for their bug exhausts

Ian

 

[Mike Pass]

This article on exhaust design might be of interest.

http://www.bigcitythunder.com/pages/understanding_exhaust.pdf

Cheers
Mike

 

[Mike Pass]

This pic of an anti reversion exhaust pipe may help. The physics of pipes is that a gas in a pipe behaves like an organ pipe and it will set up a standing wave inside which can have a fixed set of natural frequencies. The standing waves can be so that the fundamental frequency is for there to be a quarter of a wave in the tube with a node at the port end and an antinode at the open end. There are further overtones at 3/4 of a wave in the tube, 1 1/4, 2 1/4 etc. The exhaust pulses set the gas resonating.
Additionally when a positive exhaust pulse hits the open end of the exhaust a negative pulse is sent back up the tube. The idea of a tuned exhaust length is that thsi negative pulse arrives at the exhaust valve just as it opens and this pulls the gas out of the cylinder.
This is all a bit too simple for the reality of the situation as we have the additional issue of the gas actually moving down the tube at a range of speeds and the fact that the gas temperature varies constantly. The speed of sound in the gas is therefore constantly changing so calculation becomes impossible without using a powerful computer and cfd. The engine sends pulses in a continuously changing range of frequencies. The pipe can only resonate at certain points in the range where the frequency of the exhaust pulses match the natural freqency of the pipe.
To all intents and purposes we are left with trial and error.
The anti reversion idea is to try to prevent the gas in the exhaust moving back up the exhaust tube by creating a kind of one way valve. I think they do work but are better at low rpm or where large resonances are set up in the pipe.
The idea in a collector is to use the venturi effect. This when a pulse is headed off down the outlet of the collector it leaves a relatively negative pressure in its wake. Thus the train of pulses pull each other through the collector. The same effect is used in a type of lab vacuum pump which uses a jet of tap water to pull air from apparatus.
To get the most benefit from this venturi effect the collector should designed to maximise this flow. The incoming gas should be !aimed at the outlet so that the slug of gas can best drag the next slug of gas after it. Whether the idea of rotation helps I am not sure. This can best be found out by trial and error. However it can't do any harm. If anyone wants to build up two systems and test them against each other I would be most interested in the results!
On the diameter issue a local V8 builder reckons that a 302/347 gives best power using 1 5/8" primaries. This may be a gas speed or resonance effect.
Cheers
Mike

 

[SteveB]

I have thought long and hard to justify remaking the system on my car, that had me start this thread. The original worked fine but had an odd back beat that sounded quite flat with the configuration of my Ford Racing 347.

The old stainless system is now looking for a new home!

The pictures show the start of the new system that will be built to be like the MkII in 'Adam's posts with the primary tubes as equal in length as can reasonably done. The collectors will be made to rotate the firings. From each of the two 4-1 collectors the two entries into the single silencer will be into anti reversion chambers for each with cone ends to feed into the main perf pipes through the silencer. The silencer box will be repackable. If I have the theory right the anti-reversion chamber acts as the first low pressure area and the remainder of the perf tube and tailpipe act together, I will make the tailpipe as long as practical.

It will be done in time for the Isle of Man Classic I hope.

Steve



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[Mike Pass]

Looks interesting with the equal primaries and circular firing in the collectors being the main features. I am not sure if the anti reversion effect will work well with the combined pulses after the collector. The anti reversion is usually applied in the primary pipes where there are more clearly defined and separated pulses and there is more opportunity for back flow to happen. However it won't do any harm even if the full effect doesn't operate.
Looking forward to that smooth GT40 rasp from the new system!! We might not be able to see which way you went but we should be able to hear!
Cheers
Mike

 

[David]

In attached drawing of both firing orders, note that if you flip the 13726548 order from top to bottom the firing order pattern is exactly the same as the early 15426378 order

Thanks for that. I'd been looking at the 1UZ-FE firing order and at first it appeared to be totally different from the SBF order and I was a touch worried it wouldn't sound very 'GT40'. I drew it out in the same way and realised it was identical to your diagram. I realised that Toyota count cylinders as odd one bank, even the other so cylinder numbering is:

(Front of GT40)
1 2
3 4
5 6
7 8

In comparison the SBF is numbered:

(Front of GT40)
5 1
6 2
7 3
8 4

So the 1UZ-FE order of 1-8-4-3-6-5-7-2 when drawn out/renumbered the same way as the SBF becomes 5-4-2-6-3-7-8-1 or to put it another way 1-5-4-2-6-3-8 ie identical to the early 289/302 SBF so the exact same layout should work.

 

[SteveB]

The new system is taking shape. Mick at MD Engineering sent some update photos.

Custom Exhausts | MIG TIG Welding | DPF Removal | Stainless Steel Exhaust Systems

looking good

Steve

 

[SteveB]

More progress, here are a couple of shots as the system builds up getting a bit complex now.

Looking forward now to how it will sound and run with the revised layout and pipe lengths, it will be soon.

Steve

 

[EGLITOM]

Before he finishes, he should give it a try and change all spark plugs with the system installed. looks tight.

TOM