hotstuff
A2OC Donor
For some time now I have been keen to do some work to humpy's exhaust. I have read pretty much what has been posted on this forum regarding the removal of the catalytic converter and playing with the silencer and the piping. There have infact been a few members that have reported a negative effect as a result of a de-cat and exhaust re-work on a 75HP TDi. There are also comments that Stealth Racing have determined that the 90's exhaust is more customisable than the 75's due to the latter's "requirement" for some "back pressure" to operate more effeciently.
This line of thought has never sat comfortably with me - in all my years in the motor trade, my current profession and my logic tells me that an un-restricted exhaust should have benefits if done correctly. I have long toyed with the idea of "modding" the exhaust, and last week I did just that. Humpy's exhaust has been removed in total and been replaced with a straight through system. Two "silencers" are utilised, one in the center tunnel and one next to the composite well in the rear. Where the original system used 50mm diameter piping, the new system is made from 63mm and is stainless steel in construction and is laid much "straighter".
A quick explanation of how the exhaust system works should help to explain why there should be an improvement in performance. In an normally aspirated (non turbo) engine, the exhaust gasses travel down the exhaust in a series of pulses as the pistons goes through the exhaust strokes. These pulses happen so fast that the gasses flow - but - they are still pulses. As the pulses travel through the system, they create an area of low pressure behind them. This has the effect of "drawing" the next pulse along, thus allowing the gasses to "flow" and exit the system. Engine manufacturers "tune" the exhaust system to maximise this "drawing" of gasses which is called scavenging. It can best be described as "slipstreaming or draghting". When this scavenging effect is slowed down - be it from a catalyst, blocked pipes/silencer, or other restriction, the "flow" is slowed and is referred to as back pressure.
The difference with a turbo charged engine is that the exhaust pulses are used to spin-up the turbo turbine (exhaust impeller) and exit the turbo into the exhaust system in more of a stream rather than a pulsed flow - the scavenging is therefor dramatically reduced. The faster the gasses can spin the turbine, the faster the turbo compressor (inlet impeller) can draw and compress the inlet charge, the better the performance. The more the restriction, the less efficient the process.
Given the above, I question how almost identical engines can yield positive results in one application and negative in another, ie 75 vs 90.
So, what's the proof?
I had humpy dyno'ed some months back prior to having any work done on the exhaust, The figures recorded at the time were;
105.6HP and 75.6HP at the wheels
Today, humpy was dyno'ed again following the exhaust re-work;
112.3HP and 82.0HP at the wheels
That's an incease of around 6.5HP, and torque is up by just over 10Nm delivered around 50 rpm lower.
In addition, the curves are so much smoother and the engine "holds" its power for longer at the higher revs. I can confidently say that the car drives sooo much better, it's smoother, takes up quicker and is such FUN! Oh, and the turbo sounds great.
There is a slight down side however - the exhaust note is much louder. It makes a lovely burble and is superlative at low/medium revs and whilst cruising - but - plant it and wow - its really tells you what its doing! I'm getting used to the louder exhaust and I'm sure the extensive soundproofing that I put down in the car helps quite alot. I will do a little more soundproofing though, or may end up putting in another box.
I'll post some pics over the week-end, but to all those who have suggested that a 75HP TDi cannot have the exhaust modded to give better power or have advised not removing the cat for similar reasons - look at the title of the thread.
blue skies
tony
This line of thought has never sat comfortably with me - in all my years in the motor trade, my current profession and my logic tells me that an un-restricted exhaust should have benefits if done correctly. I have long toyed with the idea of "modding" the exhaust, and last week I did just that. Humpy's exhaust has been removed in total and been replaced with a straight through system. Two "silencers" are utilised, one in the center tunnel and one next to the composite well in the rear. Where the original system used 50mm diameter piping, the new system is made from 63mm and is stainless steel in construction and is laid much "straighter".
A quick explanation of how the exhaust system works should help to explain why there should be an improvement in performance. In an normally aspirated (non turbo) engine, the exhaust gasses travel down the exhaust in a series of pulses as the pistons goes through the exhaust strokes. These pulses happen so fast that the gasses flow - but - they are still pulses. As the pulses travel through the system, they create an area of low pressure behind them. This has the effect of "drawing" the next pulse along, thus allowing the gasses to "flow" and exit the system. Engine manufacturers "tune" the exhaust system to maximise this "drawing" of gasses which is called scavenging. It can best be described as "slipstreaming or draghting". When this scavenging effect is slowed down - be it from a catalyst, blocked pipes/silencer, or other restriction, the "flow" is slowed and is referred to as back pressure.
The difference with a turbo charged engine is that the exhaust pulses are used to spin-up the turbo turbine (exhaust impeller) and exit the turbo into the exhaust system in more of a stream rather than a pulsed flow - the scavenging is therefor dramatically reduced. The faster the gasses can spin the turbine, the faster the turbo compressor (inlet impeller) can draw and compress the inlet charge, the better the performance. The more the restriction, the less efficient the process.
Given the above, I question how almost identical engines can yield positive results in one application and negative in another, ie 75 vs 90.
So, what's the proof?
I had humpy dyno'ed some months back prior to having any work done on the exhaust, The figures recorded at the time were;
105.6HP and 75.6HP at the wheels
Today, humpy was dyno'ed again following the exhaust re-work;
112.3HP and 82.0HP at the wheels
That's an incease of around 6.5HP, and torque is up by just over 10Nm delivered around 50 rpm lower.
In addition, the curves are so much smoother and the engine "holds" its power for longer at the higher revs. I can confidently say that the car drives sooo much better, it's smoother, takes up quicker and is such FUN! Oh, and the turbo sounds great.
There is a slight down side however - the exhaust note is much louder. It makes a lovely burble and is superlative at low/medium revs and whilst cruising - but - plant it and wow - its really tells you what its doing! I'm getting used to the louder exhaust and I'm sure the extensive soundproofing that I put down in the car helps quite alot. I will do a little more soundproofing though, or may end up putting in another box.
I'll post some pics over the week-end, but to all those who have suggested that a 75HP TDi cannot have the exhaust modded to give better power or have advised not removing the cat for similar reasons - look at the title of the thread.
blue skies
tony
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). So ideally you want a variable exhaust that is matched to the revs (which dictates the amount of gas flow).
