Exhaust Tuning Theory and System Design

We've all heard the magazines talk about 'back pressure' (and most of what they say is utter cac) so how do we ensure we have a good, efficient, exhaust system (bearing in mind that 'back pressure, any back pressure, is a bad thing)?

Well, some history:

Back in WW2 the RAF Spitfires ran, what were essentially, open headers (they called them 'ejector stacks'). After the Battle of Britain, when they were looking to use Night-fighters, in order to cut down the exhaust glare, they ran aircraft with extended exhausts and found that engine power was increased.

Nothing then really happened until the late 50's / early 60's when the Japanese started producing 2-stroke M/C engines. - These rely upon exhaust efficiency to work.

Shortly after this people like Colin Chapman and the men at Coventry Climax started producing the 'bunch of bananas' exhausts.


So what was / is going on?



Aside from restrictions in the Silencer (which cause back pressure! and which we don't want in an efficient system!), the lengths of the Primaries / Secondaries / Collector affect the 'pulse tuning' of any exhaust.

Consider a church organ - different pipe lengths and bores make different notes. The sound you hear is due to the resonance or 'standing wave' that is set up as the air passes through the pipe. This standing wave has both a negative pressure component and a positive pressure component, the wavelength being directly related to the sound you hear because wavelength is directly proportional to the inverse of the frequency.

Now it's the same with an exhaust since it is effectively a pipe flowing gasses. Firstly we want the gas pressure in the exhaust to be lower than that at the cylinder head to assist scavenging through gas inertia. Secondly we don't want the exhaust gas of one cylinder to pressurise another cylinder.

Here comes the interesting bit. By altering the length and bore of the primaries and secondaries we can ensure that the negative pressure component of each exhaust pulse reaches the cylinder head when the exhaust valve is open, thereby further assisting cylinder scavenging. This will depend upon engine rpm and the valve opening time, ie exhaust valve duration. So, for example, on a 4-cylinder engine, we can use the negative pressure pulse from no 1 cylinder to assist the exhaust scavenging of no 4 cylinder.

What we are doing is pairing cylinders that are 180 degrees apart. This is why the stock GT6 manifold is so useless, and, more importantly, why the so called ToiletTune (Terrytune) manifold (sold by Moss) is an absolute waste of money (since it links 1,2,3 cylinders together, and 4, 5 and 6 cylinders).

Where it gets really 'trick' is if we use a wide valve overlap, ie both exhaust and inlet valves are open at the same time (hence they 'overlap'), we can use (in the example above) the negative pulse from no 1 cylinder not only to assist the scavenging of no 4 cylinder, but, because of the negative pressure and the fact that no 4 cylinder's exhaust and inlet valves are both open, this negative pulse will actually assist in sucking the new inlet charge into the cylinder. Hence gains in power and torque.

The downside is that this will only work perfectly at a given rpm. If you tune for max power you will inevitably reduce the torque lower down and 'close up' the engine's 'power band'. This is why race engines idle badly with associated popping and farting and lumpy idle rpm.

Similarly engines with a wide torque spread produce less peak bhp.


As with everything there is a series of compromises being made.



Exhaust tuning theory is actually fairly simple; it�s all about getting the negative (and, hence, scavenging) pressure pulse to arrive at the exhaust valve as it is opening. To do this we have to set the pipe lengths and diameters correctly.

The formula for Primary pipe length is:

P = [(850 x ED) / RPM] - 3

Where:
RPM is the engine speed to which the exhaust is being tuned.
ED = 180� plus the number of degrees the exhaust valve opens before BDC.
P = Primary pipe length (on a 4-1 manifold), or Primary pipe length plus Secondary pipe length (on a 4-2-1 manifold), in inches.

Generally road engines will require the manifold to be tuned to the max torque rpm whereas race engines will be tuned to work either at max bhp rpm or a speed midway between the max bhp rpm and max torque rpm.

4 -1 manifolds restrict the power band, whereas 4-2-1 manifolds give better mid-range power but reduce top end power by as much as 5-7%.

Generally speaking with a 4-2-1 manifold the starting point for Primary pipe length is 15 inches, thus Secondary pipe length is P - 15 inches. Changing the length of the Primary pipe tends to rock the power curve around the point of max torque. Shorter Primaries gives more top end power but less mid-range, and vice-versa. There is, however, little change in the peak torque or the rpm where this occurs.

Ideally the Primaries should come off the cylinder head in a straight line for around 4 inches before any turns occur.

Inside diameter of the pipe can be gained from:

ID = sq root [cc / {25 x (P + 3)}] x 2.1

Where:
cc = cylinder volume in cc.
P = Primary length in inches.

In some engines it can be useful to have a 'step' between the exhaust port and the Primary (ie the Primary bore is greater than that of the exhaust port). This tends to be the case in engines with rectilinear exhaust ports.

For a 4-2-1 system then, Primary pipe diameter is calculated as above. Secondary pipe diameter is given by:

IDS = sq root (ID x ID x 2) x 0.93

Where:
ID = calculated inside diameter of the primary pipes.

The pipe diameter can be used to change the peak torque rpm � a reduction in diameter of 0.125 inches will drop the peak torque rpm by 500-600 rpm in engines over 2 litres and by 650-800 rpm in smaller engines. Increasing the pipe diameter by 0.125 rpm has approximately the opposite effect.

The total length of the Collector and Tailpipe (to the front of the silencer) should be equal to P + 3 inches (or any full multiple of P + 3 for a road car).

Tailpipe internal diameter is given by:
IDT = sq root [(cc x 2) / (P + 3) x 25] x 2

Where P is calculated as above.

Collector length is given by:

CL = [(ID2 � ID3) / 2] x CotA

Where:
ID2 = diameter of Collector inlet
ID3 = diameter of Collector outlet.
CotA = Cotangent of angle of Collector taper (which ideally should be around 7-8� (certainly less than 10�).

The design of the collector should be such that the inlet pipes terminate abruptly otherwise the tuned exhaust pressure wave will carry on into the tailpipe and the calculations done to get the negative scavenging wave back to the exhaust valve on time will all be wrong.

good little article.

You forget two little factors (very important)

HEAT  (temperature) and Material used

Debs,
Thank you - nice(=exact), short description!

Just three things to bring up:
1/ The Hurrells were not so bad, in fact for a lot of the time they were good businessmen - they had to be!   The 6-2-1 is easier to fit to most cars, that why they marketed it, and that's why Moss don't bother with the better 6-3-1 any more (AFAIK).
2/ THE standard textbook on exhaust design is PH Smith's "Scientific Design of Exhaust & Intake Systems".  Still in print after more than 40 yeras, there aren't that many books that last that long.
3/  "Graham" Chapman?  Stop!  That's getting too, too silly!

John

Oops I meant, of course, Colin Chapman (post edited)!

Jays I've got Monty Python on the brain!

WRT the Hurrells I am afraid I disagree. While Sid (Sydney Arthur Hurrell, ie SAH) knew his stuff, his son Terry was a bandit.

The TT Mani is not a 'Performance Upgrade' despite the marketing. Actually it is a POS and no better than the stock Mani.

Yet the Marketing 'suckers' people in to spend ££££ on crap!

I shouldn't speak badly of Terry, he has had a stroke and was always a nice guy, but the fact remains he sold utter shite to a bunch of people who trusted him.

Remember the 'Triumph Tune Performance Manual'?

For example: The rear ARB. Now, just why you would want an ARB on a car fitted with a rear swing axle is beyond me, let alone on a car with a swing spring! Let alone the fact the mounts bolted into the boot floor so the ARB couldn't actually work (it just distorted the boot floor)! Yet it was marketed as a 'Performance Upgrade' and unsuspecting people bought in to the fiction.

I agree with Debs, over many years of dealing with Toilet Tune aka Mess Motors, I came to realise that their stuff was generally speaking almost worthless.

Good catalogue, no real interest in Triumphs, only MGs, sorry but true.

Eventually I met Terry in a social setting, and boy was I surprised. Nice Guy, yes, no problem there, but his expected in-depth knowledge of tuning Triumphs was exceedingly lacking.

His father Syd, is still alive, although very elderly, and living on an island in the Mediterranean, (I actually have his email address, but have promised not to reveal it)...his next door neighbour is Fred Nicklin, (ex-Standard-Triumph test driver, a first class fellow and a friend of mine).

I wanted to insert a pertinent photo here, but for some reason I cannot do so?

L

Agree on what you say on ARB, but to be fair - the ARB does not bolt into the boot floor. It is attached underneatth the diff and on the two rear vertical links.

Cheers
Nick

In which case they've changed the design Nick.

20 years ago I knew a guy with a racing Spit with a TT rear ARB and it definately bolted through the boot floor.

I believe it was Kas kastner who developed the ARB for spits and heralds. If you met a racer who had a ARB bolted to the bootfloor, then either it was a homebodge - or he had completely misunderstood how it worked. Cant even imagine how yoy would bolt it in the bootfloor. Bootbloor is not above the axles - rather behind the axles.

There is a good read on ARs here

http://members.aol.com/herald948/cc/



Cheers

Nick

Quoted from Nick B.

Agree on what you say on ARB, but to be fair - the ARB does not bolt into the boot floor. It is attached underneatth the diff and on the two rear vertical links.

Cheers
Nick

Thats a camber compensator Nick. Different beasts!

That's a 'Camber Compensator' so it act's like a bottom link and not technically as an ARB. I had one on my racing Spit many moons ago and it worked quite well before I went to a rotoflex rear end. Mine was an SAH one (I think) that I bought through some place in Leicester IIRC. It had a slightly different mounting arrangement in that it picked up on the 'pin' at the base of the vertical link - basically the mounting bracket bolted around the pin and had a nylon block that had some sideways movement.

The TT one is here (although listed as for GT6. Strangely in the paper catalogue they list it as for the Spit):

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=3114

I believe it's probably intended that the ARB mounts are to bolt through the chassis rails, similar to the front ARB? But, like I said, the guy I saw with one told me it was a TT component and it definately bolted through the boot floor - I remember we all had a laugh about it at the time doing nothing but twisting the floor!

Ooooh - I stand corrected

Should have known better than to doubt you  

Cheers

Nick

Quote, from Jon Wolfe's "Guide to racing your Spitfire", "If you use an American  rear ARB kit, like the one supplied by ToiletTune, you will need to fit chassis extension brackets extending about 30cms straight back from the towing eyes then going up to the boot floor."

Readers, I bought one, and fitted it to the Old Blue Vitesse (Rotoflex).  No idea if it did any good.
I also spent ages computing, sourcing and fitting an ARB to the Silverback (Swingspring).  It was a 205 ARB - see below.
It was no bl**dy use at all, and when I tore it off one race meeting (it was fouling a tyre) there was no bl**dy difference.

John

PS won't let me post any more pics.  Have I exceeded my quota, as on the TRR MsB?

Your images are too big John..I am paying for bandwidth out of my OWN pocket...

Each picture max of 150kbs more than big enough if compressed right and sized right.

i have one of those rear ARB kits going on ebay when i can be assed if anyone is interested hehehe

http://www.onthegosoft.com/shrink_pic.htm

You can post as many as you like John, try the above, aim for 100kbs @ 700x400 resolution etc.