bernmc
17-03-2007, 04:30 PM
Since getting my dirty great 3inch fujitsiwotsit system I've been thinking about turbo exhaust design.
A long time ago I made some enquiries about a full twin pipe system - separate pipes from each turbo back - and also a 2.5 vs 3inch system. The exhaust bods I spoke to (who seemed knowledgeable) told me that the twin system would definitely lose power, and that 3 inch would be pointless until I was in the 350bhp region. Something about this has never really rung true, but it's only now that I've been reading up on exhaust design that I've worked out why.
The problem was that the bunch I spoke to (in common with most exhaust shops in then uk I suspect) are probably very experienced with exhausts for Normally Aspirated engines, but have very little knowledge when it comes to turbo systems. It turns out that the design considerations are in fact very different.
Exhaust flow is enormously complex, but I've tried to simplify things to get my tiny little mind around it. Each 'pulse' of exhaust gas from a cylinder travels down the pipe as a wave. In a NA engine, you want all those pulses to line up nicely, one behind the other at perfectly spaced intervals. The situation becomes even more complex when those pulses exit the pipe, as they send a reciprocal wave traveling back up the pipe. Ideally, you want those reciprocal pulses to be perfectly spaced between the pulses coming down the pipe. Next, for a given pressure, the velocity of flow in the exhaust is greater through a narrower pipe versus a larger pipe. NA exhausts use this fact to improve 'scavenging' of the exhaust gasses: Get a pulse flowing down the pipe at high velocity, and it 'sucks' the pulse behind it along. The downside is that the narrower the pipe, the greater the backpressure there is in the pipe.
So 'tuning' a NA exhaust involves getting all the pipes from each cylinder to join up after equal lengths - to keep those pulses in a nice neat row - and finding the optimum diameter pipe from then on to get the right balance between flow velocity and backpressure. Too big a pipe loses velocity, and greatly reduces the exhaust efficiency.
So all those pumped up novas with huge exhausts are almost certailnly down on power compared to the std setup /lol
For turbo systems, however, you have to throw all that theory out of the window.
In a turbo car, the important thing is boost. Boost is generated by the turbocharger. The 'shaft power' (the energy generated by the turbine) of the turbo is determined by the pressure difference between the turbo turbine inlet and outlet - you want as high a pressure as possible going into the turbine, and as little pressure as possible on the outlet. A bit like running down a hill - the higher the top of the hill, the steeper the slope, the faster you run. Any backpressure after the turbo reduces the pressure ratio (makes the slope gentler). and reduces shaft power. Enough backpressure can even stall the turbo completely.
So for the exhaust header - the bit before the turbo - you want to get the exhaust velocity as high as possible as quickly as possible, so you actually want short, narrow tubes leading into the turbo. You will gain far more by doing this to increase turbine inlet pressure than by messing about with tuned lengths etc. The faster the gas flow, the quicker to turbo will spool up. A well designed trubo header will have pipes considerably narrower than a NA car with considerably less power.
Obviously, you don't want ot go so narrow that you 'choke' the engine, particularly at higher revs.
After the turbo, you want as little backpressure as possible - everything else is virtually irrelevant. The bigger (cross section), shorter, straighter, the better. I've read one quote from a garret engineer who says 'stick a hoover on the end of the exhaust if you can' :)
In terms of diameter of turbo systems, it seems that 2.5" is good for up to 250 BHP - beyond that you need 3". If you're making more than 400BHP, even 3" is on the small side... :o
Other things to avoid are:
sharp bends (tight radius)
compression bends - use mandrel bends
sudden changes in diameter - avoid all together, or make them gradual
The optimal shape for the pipe from the turbo outlet to the desired exhaust diameter is a graduall cone shape with wall angles of 7-12 degrees. (Actually, a hyperbolic increase (like a trumpet) is supposed to be better, but gains for the added difficulty in construction would be minimal).
Naturally, you have to play all these 'desirables' against what is practically possible and cost effective.
So downpipes that look like this:
uploaded/1552/1174145136.jpg
May look like the dogs danglies, but are probably an expensive waste of time - they're based on NA reuqirements. You're better off having a smooth-walled pipe with the minimum bends. That tight radius bend in the rear pipe will generate an enormous amount of back pressure, as will the disrupted airflow over all those welds.
The cat is the one thing I haven't mentioned yet. Again, it's essential to have as liitle flow restriction as possible through the cat (and any mufflers/backboxes etc), so a free flowing 'sports' cat is essential - otherwise you're wasting your money just doing the exhaust.
So there you go. Bigger is better.
I'm not an expert, so I'm happy to be corrected...
A long time ago I made some enquiries about a full twin pipe system - separate pipes from each turbo back - and also a 2.5 vs 3inch system. The exhaust bods I spoke to (who seemed knowledgeable) told me that the twin system would definitely lose power, and that 3 inch would be pointless until I was in the 350bhp region. Something about this has never really rung true, but it's only now that I've been reading up on exhaust design that I've worked out why.
The problem was that the bunch I spoke to (in common with most exhaust shops in then uk I suspect) are probably very experienced with exhausts for Normally Aspirated engines, but have very little knowledge when it comes to turbo systems. It turns out that the design considerations are in fact very different.
Exhaust flow is enormously complex, but I've tried to simplify things to get my tiny little mind around it. Each 'pulse' of exhaust gas from a cylinder travels down the pipe as a wave. In a NA engine, you want all those pulses to line up nicely, one behind the other at perfectly spaced intervals. The situation becomes even more complex when those pulses exit the pipe, as they send a reciprocal wave traveling back up the pipe. Ideally, you want those reciprocal pulses to be perfectly spaced between the pulses coming down the pipe. Next, for a given pressure, the velocity of flow in the exhaust is greater through a narrower pipe versus a larger pipe. NA exhausts use this fact to improve 'scavenging' of the exhaust gasses: Get a pulse flowing down the pipe at high velocity, and it 'sucks' the pulse behind it along. The downside is that the narrower the pipe, the greater the backpressure there is in the pipe.
So 'tuning' a NA exhaust involves getting all the pipes from each cylinder to join up after equal lengths - to keep those pulses in a nice neat row - and finding the optimum diameter pipe from then on to get the right balance between flow velocity and backpressure. Too big a pipe loses velocity, and greatly reduces the exhaust efficiency.
So all those pumped up novas with huge exhausts are almost certailnly down on power compared to the std setup /lol
For turbo systems, however, you have to throw all that theory out of the window.
In a turbo car, the important thing is boost. Boost is generated by the turbocharger. The 'shaft power' (the energy generated by the turbine) of the turbo is determined by the pressure difference between the turbo turbine inlet and outlet - you want as high a pressure as possible going into the turbine, and as little pressure as possible on the outlet. A bit like running down a hill - the higher the top of the hill, the steeper the slope, the faster you run. Any backpressure after the turbo reduces the pressure ratio (makes the slope gentler). and reduces shaft power. Enough backpressure can even stall the turbo completely.
So for the exhaust header - the bit before the turbo - you want to get the exhaust velocity as high as possible as quickly as possible, so you actually want short, narrow tubes leading into the turbo. You will gain far more by doing this to increase turbine inlet pressure than by messing about with tuned lengths etc. The faster the gas flow, the quicker to turbo will spool up. A well designed trubo header will have pipes considerably narrower than a NA car with considerably less power.
Obviously, you don't want ot go so narrow that you 'choke' the engine, particularly at higher revs.
After the turbo, you want as little backpressure as possible - everything else is virtually irrelevant. The bigger (cross section), shorter, straighter, the better. I've read one quote from a garret engineer who says 'stick a hoover on the end of the exhaust if you can' :)
In terms of diameter of turbo systems, it seems that 2.5" is good for up to 250 BHP - beyond that you need 3". If you're making more than 400BHP, even 3" is on the small side... :o
Other things to avoid are:
sharp bends (tight radius)
compression bends - use mandrel bends
sudden changes in diameter - avoid all together, or make them gradual
The optimal shape for the pipe from the turbo outlet to the desired exhaust diameter is a graduall cone shape with wall angles of 7-12 degrees. (Actually, a hyperbolic increase (like a trumpet) is supposed to be better, but gains for the added difficulty in construction would be minimal).
Naturally, you have to play all these 'desirables' against what is practically possible and cost effective.
So downpipes that look like this:
uploaded/1552/1174145136.jpg
May look like the dogs danglies, but are probably an expensive waste of time - they're based on NA reuqirements. You're better off having a smooth-walled pipe with the minimum bends. That tight radius bend in the rear pipe will generate an enormous amount of back pressure, as will the disrupted airflow over all those welds.
The cat is the one thing I haven't mentioned yet. Again, it's essential to have as liitle flow restriction as possible through the cat (and any mufflers/backboxes etc), so a free flowing 'sports' cat is essential - otherwise you're wasting your money just doing the exhaust.
So there you go. Bigger is better.
I'm not an expert, so I'm happy to be corrected...