Exhaust tail pipe size

[R59]

I think you'll find it's all to do with cost vs emissions vs noise.

Get all three right, and you have a strangulation filter fitted up front, with a wheelbarrow handle at the back.

Many of the aftermarket exhausts with big tail chimneys often lose power compared to std. Some of the rolling road tests on Sky1's "Vroom Vroom" programme have shown it.

One lad had fitted an induction kit (making the front end noisy) with a hoooooge chimney, and lost over 10bhp from std. Not only was it very noisy, it make him look a complete wayne kerr.

Piper used to make a system for the Vauxhall Nova/Opel Corsa GTE/GSi, which gave a genuine 8bhp increase. One problemo..... 108db!!

Rob.

[Al Weyman]

Well take it a stage further and just run a straight through system it would probably have made 15bhp plus an endorsment on yer licence everytime you took it out. Incidently why do light aircraft in this country have to be so bloody noisy?

[greenamex2]

Originally Posted by Al Weyman

Incidently why do light aircraft in this country have to be so bloody noisy?

Because they can?

[Al Weyman]

But why can they, we can't and they can't in Germany as I understand it, I am right on the spot where the learners from Elstree bank round when doing circuit and bumps, now I am not complaining and live and let live but I bet on a hot summers afternoon these are more annoying than what the Combe nimbys have to put up with and yet people listen to them, why?

[greenamex2]

Because they can?

The people round Combe have put in complaints about "those noisy race cars" when there hasn't even been a single car going round the circuit!

You'll probably find they are also the same inconsiderate t**sers that set off industrial strength fireworks at 00:00-01:00 at every opportunity.

There is no logical solution because people are illogical, irrational and self centered.

[woodyracing]

hi, i know what you mean about having an exhaust too large, I had an XR2i 16v before and I fitted a 3inch tail pipe which i thought was quite modest at the time. All the power went up the rev range and at low revs it was very unimpressive. The cone type air filter seemed to make a difference but the problem was that it was sucking up the hot under-bonnet air aswell as from the cold air pipe.
If the optimum exhaust size for my MR2 is say a single 2.5inch, I dont get why manufacturers make "performance" pipes which are twin 3.5inch! same manufacturer does a twin 4inch for the turbo!

[Smokey 6 litre]

what are the diameters of your primary pipes comming off the manifold?

[woodyracing]

I'm pretty sure they are 2.5 inch.

Andy.

[greenamex2]

Originally Posted by woodyracing

I'm pretty sure they are 2.5 inch.

Andy.

My 218BHP 1600 only has about 1.7 inches primaries. And this is about what QED said I needed for the 2 litre vauxhall in my kit car for upto 270BHP.

Any bigger than the above and we measured a significant loss in low down power for no extra. or reduced, power at the top end.

[TEAM78]

Originally Posted by Denis Bassom

Because they can?

The people round Combe have put in complaints about "those noisy race cars" when there hasn't even been a single car going round the circuit!

You'll probably find they are also the same inconsiderate t**sers that set off industrial strength fireworks at 00:00-01:00 at every opportunity.

There is no logical solution because people are illogical, irrational and self centered.

firstly Chucky, that picture is priceless
as for the MR2, I would go 3" based on experience
finally as for Castle Combe,as far as im aware the Combe residences are fine with the circuit except for about a few t**sers who moved into the area and decided they would ruin Castle Combe.
The circuit was there before them so what the hell was going through their minds moving to a place where they didnt like the noise is anyones guess
They effectively won by stopping the F3 and british GT's ever racing there with the noise limits.
at the least these people have to be inconsiderate and self-centred to of done what they did here as they cost local people some serious dosh in the long run.

I wonder what they could do if they moved to Heathrow?

could they stop all 747's from taking off?

[greenamex2]

Originally Posted by TEAM78

I wonder what they could do if they moved to Heathrow?

could they stop all 747's from taking off?

Probably. And then complain when they had to drive to Gatwick to go on holiday!

[phoenix]

As a general rule, on a four cylinder engine with all four cylinders exhausting from one tailpipe, the AREA of the tailpipe should be no less than the area of one primary (obviously), but no greater than the area of the four primaries.

For optimized performance with good scavenging (again on a four cylinder engine sharing one tailpipe) the area of the tailpipe should be 1.778 x the area of one primary. This equates to a diameter 1.333 times the diameter of one primary. This is not so obvious a number, but remember that on a four cylinder engine there is really only one cylinder exhausting at any one time, plus a little bit of scavenging from a nearly empty cylinder, so the tailpipe doesn't need to flow four times what one primary flows. Good scavenging needs good exhaust speed, and the correct diameter tailpipe helps maintain good exhaust speed which gives a good "draw" on the cylinder being scavenged. Too large a tailpipe kills scavenging dead. Larger tailpipes have a larger internal surface area than small ones. This causes more drag on the exhaust flow than smaller pipes which slows the exhaust down and can actually increase back pressure.

So:

1.25" primaries should have a tailpipe of 2" (50mm)
1.5" primaries should have a tailpipe of 2.33" (60mm)
1.75" primaries should have a tailipe diameter of 2.67" (67.7mm)

and so on. A little bit of adjustment either side to suit available material is ok, but don't stray too far from these dimensions.

Although these sizes may make you think that such an exhaust would be too restrictive, it will not be, so try it first on the dyno/rolling road before you make too hasty a decison.

Good luck

Martin

p.s. I have assumed that you have the primary diameter optimized to suit the exhaust valve diameter and lift to start with.

[Al Weyman]

I very much doubt the primaries are anything like that size are you sure you are not measuring the external diameter of a cast manifold! I have 17/8" primaries on one of my 5.7 engines and quite frankly it is too big and I purchased these when I ran with a 6.6, the ideal size on a full house small block chevy 5.7 is 1.75" any bigger and you move the rev range too high and be absolutely hopeless at revs less than 4000, better to be too small than too big IMO.

[Smokey 6 litre]

we done some work on a Lola Ford Group C car, it had 4 2 inch primaries on each side of the V8 so we build 2 4 inch tail pipes, then when i went to see it at the silverstone classic i lifted the cover to look at the back only to realise it was covered because during practice someone had wiped out the back end. all my hard work gone to waste.

[phoenix]

Originally Posted by Smokey 6 litre

we done some work on a Lola Ford Group C car, it had 4 2 inch primaries on each side of the V8 so we build 2 4 inch tail pipes, then when i went to see it at the silverstone classic i lifted the cover to look at the back only to realise it was covered because during practice someone had wiped out the back end. all my hard work gone to waste.

2" primaries would be required on the Ford 305 5 litre engine making peak power at 9000 rpm, which is what the best Lola Ford T70's were fitted with. A similar engine making max power at a more modest 7000 rpm would require 1.75" primaries. Bigger primaries and too large an exhaust tail pipe would kill scavenging (and therefore torque and power) in the lower revving engine.

Tail pipes would not need to be larger than 68mm for the 9000 rpm engine and 60mm for the 7000 rpm engine. A four inch tail pipe would not have enhanced the engines performance, I am sure of that.

[Smokey 6 litre]

Originally Posted by phoenix

Tail pipes would not need to be larger than 68mm for the 9000 rpm engine and 60mm for the 7000 rpm engine. A four inch tail pipe would not have enhanced the engines performance, I am sure of that.

I dont design it, i just build what im told to build. i'm sure they had their reasons. They looked good tho.

[norman-normal]

Tune it for the middle of the RPM range you will be using. My guess: not more than 2". Most likely 5000 to 5500 RPM is where you will be most of the time.

[zefarelly]

interesting reading . . .Phoenix makes sense, I have 1.5" primaries, 2 1/2" tail and system on an old engine like mine knocking on 95 BHP per litre isn't too shabby so it can't be that far off the mark, sounds nice too.

Obviously it will be better when I fit a pair of 6" tail cans with sparking devices

[norman-normal]

Originally Posted by zefarelly

interesting reading . . .Phoenix makes sense, I have 1.5" primaries, 2 1/2" tail and system on an old engine like mine knocking on 95 BHP per litre isn't too shabby so it can't be that far off the mark, sounds nice too.

Obviously it will be better when I fit a pair of 6" tail cans with sparking devices

An interesting tidbit that I can't really account for: when the exhaust is nearing optimum performance they sound better, especially the lenght of the pipe, they will start resonating as the power starts to peak. I have observed this phenomena on a dino.

[woodyracing]

what happens if your length is too short ?

[norman-normal]

You are referring to exhaust pipes aren't you????
A shorter pipe will move the power and torque up the RPM range. For most production engines that are front mtd, termination at about the rear axle is pretty near optimum: power boost=4000RPM. For a "hot V8" exiting ether side just aft of the driver works well. The problem is with rear engine cars. it's difficult to get enough length with out folding them back and forth; this is why a lot of people with rear engined cars loose power when they remove the stock system.
Two necessary elements are involved: the necessity for some back pressure and the bonus of the scavenging effect of the impulse wave traveling traveling back and forth along the pipe.
It all works (diameter & length) the same on the intake manifold pipes but different numbers of coarse

[woodyracing]

yes exhaust pipes

my car is mid engined, the downpipe finishes on the left hand side then doubles back on itself with the rear silencer, there is a tail pipe on the right handside and it doubles back on its self again internally and has a tail pipe on the left hand side. I was thinking of removing everything after the downpipe, just having a 90 degree bend and a small rear silencer exiting on the left hand side. This would save a lot of weight but would also be very short compared to the original. The other consideration is to make the new silencer exit from the right hand side thus having a longer pipe.

[phoenix]

Originally Posted by norman-normal

An interesting tidbit that I can't really account for: when the exhaust is nearing optimum performance they sound better, especially the lenght of the pipe, they will start resonating as the power starts to peak. I have observed this phenomena on a dino.

And so it should! The basic theory behind manifold and exhaust pipe length tuning is based on organ pipe theory. A tuned length exhaust is literally that - a length at which the pipe has a natural frequency of resonance which matches the resonance of the engine at a particular engine speed. The complication is that the speed of sound in hot exhaust gas is higher than the speed of sound in the cool air inside a church or cathedral, so the resonant lengths have to be based on average exhaust gas temperature rather than ambient temperatures.

[woodyracing]

Originally Posted by woodyracing

yes exhaust pipes

my car is mid engined, the downpipe finishes on the left hand side then doubles back on itself with the rear silencer, there is a tail pipe on the right handside and it doubles back on its self again internally and has a tail pipe on the left hand side. I was thinking of removing everything after the downpipe, just having a 90 degree bend and a small rear silencer exiting on the left hand side. This would save a lot of weight but would also be very short compared to the original. The other consideration is to make the new silencer exit from the right hand side thus having a longer pipe.

any recommendations ? shorter and lighter or longer and a bit heavier ?
thanks.

[norman-normal]

I do have a recommendation Woody: the Toyota engineers are very smart fellows and they have designed a package. A well designed package is the only thing that will work well. Don't buy any ex-pipe package that is very different than factory unless you modify the entire engine$$$$. If you do you will loose horse power (but it will be loud) The same goes for any modification.
If you can figure a way to get cool air to the air filter (K&N?) next a chip installed in computer (usually easy thing to do even for a beginner).
If you want more than that, turbo is the only way to go and the best way to do that is to sale yours and buy a turbo model or buy a half cut from Japan. Much better than a turbo kit and cheaper in the long run.