Exhaust Manifolds & Systems

Rob Gaskin said:
Getting the last few 'horses' out of the engine can lead you down a very expensive and tiresome road.
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I have a lot of experience with the above having spent a lot more money than most of you would of thought possible building my Pulsar to be able to handle over 500 bhp, It's quite amazing how much difference their is between say 400 bhp and 500 bhp in the money needed keep the thing in one piece.

I also think that unless you plan on joining some kind of historic race series or drag challenge their is very little point in trying to squeeze the last few BHP out of your cars, Most of you wouldn't be able to tell the difference and extra 10-20 bhp makes on the road anyway.

I think it's much more important to make sure what you have already got is set up as well as it could possible be, That will make a big difference to most.

I think some people are getting a bit to anal about the design of headers and exhausts :eek:;).



Rob
 
Excuse Rob - I'm not having a pop here - just using your words 'cos I'm too lazy to type.

What you say about expensive power and also track days and what is dangerous is true.

What is also true is that is you spend wisely, one can have those last reliable horses. EG: a nice 2.8 set up can give you 180bhp which can be a lot of fun.....or another nice set up can give 50 more which gives a completely different sensation AND sorry to be anal but the exhaust manifold is crucial to those available and often lost horses !

Rob Gaskin said:
However a reasonably modded 240 can still produce some useful power and provide an enjoyable driving experience.
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A short-stroke 2.6 is gonna want to rev. and breath so I still think that you can comfortably use your NISMO 1 3/4. Try it on a rolling road and if not, swap down a size !
And beware the noise on a road car - your MOTs are getting stricter I think.
 
Gwyn, the size of your exhaust also depends if you want low end touque or not, personally i wouldnt go any bigger than 21/4 or 2 x 13/4.
Here is a photo of my home build twin 13/4 exhaust and the sound, it's oh so good.

Chris
 

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Mike, it is actually a attempt to copy the Clifford system, see the attached photo, i still need to put on some tips, any one knowing of a good source in UK, i canøt seem to find any with a 1 3/4 inch inlet here in DK.

Sean, the roadster belongs to my buddy, mine is aat my summer house, waiting for a overhaul.

Gwyn, if i was to use the Nismo, i would reduce it down to a smaller diameter, what is the spacs on your motor ?.

Chris
 

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I wouldn't jack my car up on its' chassis rails.....be careful man !

And how do you avoid the middle boxes vibrating against and heating up the body ?
 
Thats a similar setup to the one im making, except im not using a middle silencer, and the rear silencer is going to be a large single with the 2 pipe running through
 
Whats the advantage of running through two lots of pipes then ?, I was going to say that my old car ran with one 3" pipe and still managed over 500 bhp, Then I realised I know very little about N/A tuning so I defer to those that know better lol.

I'm going to fit a single 2.5" system on mine for the time being.


Rob
 
2fast2z said:
Gwyn, if i was to use the Nismo, i would reduce it down to a smaller diameter, what is the spacs on your motor ?.

Chris
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Hi Chris Reduce what down ? There is no chance of me getting hold of smaller dia nismo manifold & I have sold the gds, so I am going to have to use this 1 3/4 nismo for now. So what dia would you recommend for a single pipe system, also what dia for a twin pipe system ? & is there any advantage of a twin pipe system. My engine is a 260 block bored out to 2.8, 240 crank, reworked E31 head, not sure what cam is fitted, running triple 40 Dellorto's.

Gwyn
 
samuri-240 said:
This is a question I have been asking & nobody has come forward with an answer. :(
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I wouold imagine the exhaust gasses will flow faster through 2 small pipes than one large one.

As for wanting back pressure on NA engines...:) only really needed if you can't adjust the mixture to compensate for the better flow...the engine then runs leaner causing a drop in power. Quick in ...quick out that's what you want!
 
twoforty said:
Quick in ...quick out that's what you want!
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I understand what you are saying but there is more to it than that (as you know). I had a Hillman Imp years ago that I rallied, landed heavily once shearing the exhaust at the manifold. Couldn't get much 'quicker out' than that but the car lost about half it's power.
 
Rob Gaskin said:
I understand what you are saying but there is more to it than that (as you know). I had a Hillman Imp years ago that I rallied, landed heavily once shearing the exhaust at the manifold. Couldn't get much 'quicker out' than that but the car lost about half it's power.
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True but if it was a good manifold and it snapped at the point where all the pipes met it would make more power if ignition and mixture had been corrected :)

Like you say though its far from simple and all depends on what you want....low down torque or top end power.

Match the pipe to the engines flow and not too large, get rid of the 90deg angles at the back before the rear silencer. 6 into 2 into 1 of the right size would be fine for any car thats spends most of its time on the road. There are plenty of good books available on exhaust design. I bet Steve K has a pretty trick exhaust...tak a look at his :)
 
First of Gwyn, i'm not one of those "i know all about z engines and exhaust" type of guy, i have just been doing my home work before building mine exhaust.
In all books is is stated not to go any bigger than a 2.25 inch system, a dual 1.75 inch system is equal in diameter, some say that a dual system is breaking the sequenses the excaust gasset is coming out in, personally i don't care, my build is for street and fun and nothing more.
If you run a wild cam and drives at high RPM's you can go higher than 2.25 inch, if not you want to have backpreassure to obtain your low end touqu.

i assume your Nismo headder is 3 in to 2 and the collectors is 2 inch, that is also what mine is, i reduced the exhaust from 2 inch to 1.75 inch right after the collector, some say that you can reduce it even more further back.
You also have to consider the clerance under the car, on one of the photos i posted, i flipped the middle dampeners up side down and that is why it looks so narrow to the tunnel, also the parts guy got me the wrong dampeners they are 4 inch diameter and should have been 3 inch as the one in the back, the good thing is that i only paid £10 a piece, all pipes and dampeners i have used is from Swedish Simons.

There is no advantage in running a twice pipe system, except of the sound, this is just a old school way of doing it, because of the lack of bigger pipes in the old days.

Chris
 
There is no advantage in running a twice pipe system, except of the sound, this is just a old school way of doing it, because of the lack of bigger pipes in the old days.

Chris[/quote]
So why did Nissan use this system on their works rally cars? It certainly would'nt have been because of a lack of bigger pipes.
 
I would guess that the concept of running two pipes right through to the back of the carcame from experiments with secondary length which we know affects the power profile

As for wanting back pressure on NA engines...:) only really needed if you can't adjust the mixture to compensate for the better flow...the engine then runs leaner causing a drop in power. Quick in ...quick out that's what you want!
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"back pressure" is often incorrectly misquoted/misunderstood/not understood at all, actually it all centers around pulse tuning and the "5th" cycle (yes I know that there are only 4 cycles). The often termed 5th cycle refers to the point where both valves are open at around 10 degrees before and 10 degrees after TDC on the non compression stroke, as the piston can only go to the top of its stroke it leaves an amount of unburnt gases in the chamber, so we use a scavenging process to extract this last remnant of the previous burn process, scavenging is brought about by the the use of cam lift on overlap plus tuned exhaust diameteres and lengths to promote gas speed and timed "waves", the resultant -ve pressure wave (created by prior exhaust pulses travelling down the system) travel back up the systen to the exhaust port and help pull the burnt gases out and very importantly help pull the new clean fuel/air mixture through....It is the differance between a 220HP L6 and a 250HP L6

Talk to any competant NA tuner whether its pintos, A series or twin cam whatevers, the overlap period and scavenging is absolutely crucial to the engine developing anywhere near optimum performance alternatively, it doesnt happen by "magic" this process must be designed into the engine from inlet tract to exhaust tip..... hey dont take my word for it,

Take the time to read anything by David Vizard (engine God:bow:)

"With a race cam and a tuned-length exhast system, negative pressure waves traveling back from the collector will scavenge the combustion chamber during the exhaust/intake valve overlap period"
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taken from "Exhaust Science Demystified"

and dont "just" think it referes to race engines

For example read
HEADERDESIGN.COM - Header Design Concepts


Just google cam overlap, scavenging, pulse tuning etc its been written about since the first engine tuners thought "hey I know, I wonder if....."

No amount of "playing with the mixture" will create scavenging where there is non (or too little)
 
So why did Nissan use this system on their works rally cars? It certainly would'nt have been because of a lack of bigger pipes.
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And what year did they do that Nigel? look at the post below yours, that actually explains why it "should" be a benefit in using a single pipe system.
If you read all what i wrote, you would also notice that i wrote
i'm not one of those "i know all about z engines and exhaust" type of guy, i have just been doing my home work before building mine exhaust.
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This means that according to those that knows, was there no such thing as tools and pipes for a 2.25 inch system back then.

Chris
 
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