Custom Exhaust sizing

[campbellju]

The more I look at my datalog and torque curves, the more I think my 2.5" elbow and exahust is limiting the engine. I always knew it would be less ideal than a drainpipe attached to the engine but I wanted to maintain some off-boost drivability. I have achieved this and the car is very smooth with the torque delivery like an NA V8 from 2500rpm. I believe there is a better compromise in there and as I need to take the exahust off to weld in a hole for my W/B lambda I thought I may as well just change it at the same time.

Reading around, the downpipe is a major restirction and that should really be 3". However, if that's 3", why not leave the rest of the system as 2.5"?

I've seen Scooby exhausts with a 4" downpipe that goes to a 3" system. This to me makes more sense than a 3" elbow and system. As I'm going for a custom build (done by the same bloke as did Jones_gtir), I may as well get exactly what I want and I'm not afraid to get something different.

0.Cheap Drivability, 3" downpipe with 3" to 2.5" section and the system and backbox the same.
1. Drivability, 3" downpipe, 3" to 2.75" to 2.5" system, existing 2.5" back box.
2. Top End and bring on the boost surge, 4" downpipe if pos, 3" system and back box.
3. Oh I don't know, do what everyone else does 3" downpipe and system etc... Baa

The way I see it, the biggest resistance (back pressure) is at the elbow where the gases are hottest, if you're exhaust is the same size after that then its just overkill. So if option 3 works well then option 2 or a variation on option 1 should work better overall.

I've got the RR this saturday which will be a good benchmark to show how restrictive or advantageous my current system is. I'll be counting my pennies and getting my order in after that so if nothing else it will have been useful experience but I only want to do it once.

Jim

 

[Fazz]

Hi jim,

I've been looking into this too in anticipation of a similar problem.. and I agree with your second line, or option No: 1.

The downpipe will prob be the main issue so just mod that to suit and see if that helps..

I'll be interested to see your results!

 

[campbellju]

Chatting with Kieron, he's had a lot of success with just a 3" downpipe and 2.5" system on an old car. Current favourite is option 1, I'm looking into prices this week through local blokey.

 

[stumo]

when you look at the std elbow you'll see how restrictive it is compared to even a 2.5 inch elbow!!!

 

[tro||]

Chatting with Kieron, he's had a lot of success with just a 3" downpipe and 2.5" .

3" elbow and front pipe tapering to 2.5 to match the decat ;-)

the standard elbow is the worst part of the whole system.

mark turbo also got very good results with a divided elbow with vent to atmosphere screamer

 

[boucherie]

3" elbow and front pipe tapering to 2.5 to match the decat ;-)

the standard elbow is the worst part of the whole system.

mark turbo also got very good results with a divided elbow with vent to atmosphere screamer

my mates currently designing and welding me up a custom downpipe using a divorced flange so he can install an screamer to atmosphere while the downpipe is 3" and then either taper off down the old system or 3" side exit were not sure yet which way to play it!

 

[campbellju]

How much is he charging you for that? The only thing that has previously put me off the divided pipe is the ones I've seen are around 2" bore for the main part which I'm worried will still limit top end?

When you taper off down the old system, how big is your old system and how long is your taper? I've just been debating with a process services engineer the advantages of 2.5" vs 2.75" vs 3" but am still none the wiser as to what's optimum. All we thought was the more gentle taper might look better but is less likely to have an affect on the torque curve tuning than something like a deliberate 2.5". Without nay expert knowledge of turbo systems though he correctly said that for optimum turbo performance you just want a short side exit 3" pipe

 

[boucherie]

he hasnt given me a running cost yet, but i supplied him with the pipe and he's gona let me know the labour charge!!

correct me if im wrong but by having a outlet hole the same size (or slightly larger) than the turbine wheel you will still be able to flow the inital gases out quick enough without building up a 'back pressure' on the wheel

il let him know that a side exit is the best for turbo performance, all i need to do now is order up a couple of silencers!!

if none of the above makes any sense then i apologise for my dumbass abilities!

 

[campbellju]

correct me if im wrong but by having a outlet hole the same size (or slightly larger) than the turbine wheel you will still be able to flow the inital gases out quick enough without building up a 'back pressure' on the wheel

il let him know that a side exit is the best for turbo performance, all i need to do now is order up a couple of silencers!!

You and me both, I learnt a lot this afternoon, I'll have to paraphrase as we talked for over 30mins and I'm still no expert. This is an area that seems to have a lot of rough science as the scientific calculations can get quite complicated but on a turbo it gets easier again.

Turbines work on a pressure drop across the blades providing the speed. It the pressure is the same on both sides then the air just sits there. The lowest pressure you can have is atmosphere so the closer you can bring atmosphere pressure to the exit of the turbine the better. The downside is you get 800degC gases in your engine bay. The next best solution is a huge pipe that exits just outside your engine bay. This is of course very very loud.

This is the "best" method for a turbo trying to achieve a peak HP figure. Once the turbo comes into its operating range, the pressure drop is so high across the blades it will spin up very quickly. BTW, this services engineer used to race motobikes so knows a lot about NA exhausts. His opinon is that "nothing, nothing, bang" could also be achieved on an NA by increasing the bore but in his opinon was not the optimum design for racing where the preferred option was a flatter predicatable torque curve. He also hates turbo's on bikes for the smae reasons.

For an NA, there is no turbo forcing air into the engine but it its possible to carefully tune your exhaust to suck it out. When I said the lowest pressure available was atmosphere I lied, its vacum. The way you create a vacum in the exhaust is a bit like waves on the sea. For every peak there is a subsequent trough and the greater you make the peak the greater the subsequent trough (vacum).

In an exhaust, these waves are the pulses from each cylinder firing like a rifle shot down the exhaust. As each pulse forces its way through the exhaust, it creates a vacum behind it so when the exahust valve opens on the next cylinder it sees a vacum rather than atmosphere and moves out quicker. At high rpm though, the pipes will need to be tuned far differently than at low rpm where the pulses are less regular. At high rpm you need a bigger bore so the pulses can exit quickly but a lower rpm where you are reliant on the vacum to make the engine more effective, on the same exhaust, the pulses are just pushed out rather than being sucked out.

Back to turbos. Although on a turbo engine, the vacum is less obvious between pulses its still there just more muffled. So when you're off boost, this scavenging (vacum) affect is helping to create a lower pressure area after the turbine that helps the turbo to spool up. But then as soon as the turbo spins up on my car the same pipes are limiting the volume of air that can be released creating a build up of air in the manifold to exahust that reduces the pressure drop across the turbine as both sides are stuffed with air. This is why my car takes longer to hit full boost as its having to shove all the blooming air out the way.

Unfortunately this is all from the monkey rather than the organ grinder. My mate was also talking about sonic vs gaseous pulses and the effect the changing pipe bore size has on each at any given time. At this point I'd be blagging it so I won't try.

For me though what was interesting was through out the entire conversation, he never used the word back pressure once when he was talking about moving from big pipes to smaller pipes etc and how it affected the flow of gas.

For turbo cars looking for high horsepower, back pressure has no advanatge but off boost and when coming on boost, correctly setting up the exhaust bore and length at certain points can help to suck out the air and make the turbine spin sooner.

On the having the same size pipe outlet, IMO, it dpends on the rate of expansion of the gas and the length of the pipe. What you're talking about sounds logical as you're going 2" to 3" reasonably quickly.

Right my head is about to go pop and my fingers are sore.
:yield: :yield:

 

[boucherie]

far to complicated for me but i get the general idea of where your coming from!!

My head hurts!!!!

 

[Fusion Ed]

Back to turbos. Although on a turbo engine, the vacum is less obvious between pulses its still there just more muffled. So when you're off boost, this scavenging (vacum) affect is helping to create a lower pressure area after the turbine that helps the turbo to spool up. But then as soon as the turbo spins up on my car the same pipes are limiting the volume of air that can be released creating a build up of air in the manifold to exahust that reduces the pressure drop across the turbine as both sides are stuffed with air. This is why my car takes longer to hit full boost as its having to shove all the blooming air out the way.

I think you'll find that relatively speaking there will never be any kind of vacuum after the turbo. Although there is to a certain extent pulse effect a turbo really does slow down and soften the pulse to a point where you can ignore it, and instead concentrate on aiming for the least resistance possible from the system as a whole. To get any effect of good pulse timing, you need to dump the stock manifold for a correctly designed tubular one..

 

[campbellju]

To get any effect of good pulse timing, you need to dump the stock manifold for a correctly designed tubular one..

ah ha, well spotted ;-)

 

[Fazz]

@ jim - did you get this fitted yet?

 

[campbellju]

@ jim - did you get this fitted yet?

Photos going up shortly. I nursed it home though as fuel was low and I was concerned about how the map would respond. WB lambda is fitted as well now.

Whilst showing the bloke who did the job around the engine bay, noiticed a loose hose on the inlet manifold. This has been the culprit of the off boost lean misfire problem :doh: . Nothing to do with the mapping

Aside from that good news, I'll start the fine tuning myself on the mapping.