A bit of history (part 23) – Exhausted

Now that I had committed to the new turbo choice, and the old GT3076 had gone to a new home, I could start to move on with my planned supporting mods to help the turbo realise its potential. The first of which was to reduce exhaust gas back pressure on the outlet of the turbine wheel.

This is an area of tuning that really seems to cause confusion, and one that I’ve always tried to think really simply about: the turbine wheel has hot gases entering it from the engine, these gases spin the wheel as they flow past it and out of the turbine housing. The aim in this system is maximise the amount of energy the exhaust gas imparts to the turbine wheel, and hence the amount of energy going into spinning the compressor wheel to make boost.

The best way (in my mind) to do this is the maximise the pressure gradient across the turbine wheel- as in this case pressure represents the energy in the exhaust gases. So high pressure (& temperature!) on the engine side of the turbine wheel, and low pressure on the exhaust side…the drop in pressure (and temperature) representing the energy that has gone into the turbine wheel.

In order to aid this therefore, we want to aim to do everything we can to reduce the pressure on the exhaust side of the turbine wheel…and the best way to do this (as you see in top spec drag race cars) is to remove the exhaust, which gives no restriction to the gases exiting the turbo and so forces preventing the pressure dissipating. As I read in one of my old-school forced induction books- the best exhaust for a turbo is no exhaust.

However- as I said: there is a lot of confusion around this area, and I think this is in part due to how naturally aspirated exhausts are designed…which is to take advantage of exhaust pulse scavenging. In a nutshell this aims to take advantage of the pulses in the exhaust gas caused by the exhaust valves opening and closing, and seeking to combine the gases from each cylinder in the exhaust in a way that reduces interference from these pulses between cylinders (and the resulting pressure waves that restrict the gases exiting the cylinders).

In addition, this tuning aims to keep exhaust gas velocities as high as possible to keep the gas flowing through the exhaust, away from the cylinders, and out of the back pipe…and in order to do this, a larger exhaust is not always better. In fact- a smaller diameter (within certain bounds!) can aid exhaust flow by helping maintain exhaust gas velocities more than running a massive cannon in which the gases simply stall.

Now back to turbo charging: in this situation I’ve heard it argued that you don’t necessarily want to go too big on the exhaust for similar reasons- i.e. to aid scavenging. In my mind (and I am happy to be told I’m wrong), this doesn’t really apply to turbo exhaust systems for the simple reason that the exhaust gases all flow through the turbine wheel. In doing so the exhaust gas pulses from individual cylinders are negated as the gases mix and are pressurised in order to force their way past the turbine wheel.

That’s not to say that you cannot use scavenging to aid the flow of gases into the turbo (and this is exactly what twin scroll exhaust manifold and turbine housings seek to do), but I still believe the best exhaust post turbo is no exhaust, or as reasonably large as we can go without bursting our eardrums!

So…theory aside, I wanted to remove the perceived restriction in my exhaust system which tapered to 2.5 inches in order to mate with the HKS back section. I had kept it as it was on the original car I imported, but the time had come to let go of the drive to maintain how the car originally looked in order to try and improve performance.

After a lot of research I ended up talking to Pete (who runs a small outfit called PTR exhausts), and he talked me out of a 4 inch system…in terms of cost to benefit, and my desired power outputs, a 3 inch system was perfectly adequate. He had just completed a full exhaust system for a 650hp Lotus Exige, and after looking at my build he felt a similar design would work.

I wanted to keep a single exit system (mainly to keep weight down), and wanted a removable second silencer, giving me the option of a track legal system (in terms of noise output), whilst still being able to remove the second silencer and run at slightly higher noise levels, but slightly higher potential power outputs.

I also asked for a removable section that would either vent the waste gate gases back into the main exhaust (as per my previous exhaust), or the option of a vent to atmosphere screamer pipe. Childish for sure, but lots of fun when noise limits are not an issue!

Needless to say, I was extremely happy with the skills Pete put into fabricating the whole system, complete with a very functional looking 3 inch tail pipe…no more 5 inch cannon for me…no pretences, just nicely functional…