Reversion and turbocharging
 

I know it's common practice to keep the step between the exhaust port and the header, but how important is it on a turbocharged vehicle? Should there be any steps to prevent reversion, or would the pressure in the header before the turbo make the step useless except to slow flow by an increase in volume?

On one hand, I kind of feel like a small step around the outside of a port will do very little to prevent exhaust gas from entering the cylinder again, especially at the pressures in a turbo header. Also, it seems like a sudden increase in volume would decrease exhaust velocity.

On the other hand, maybe the step will help even more than on a naturally aspirated engine because pressures are higher. So far, I'm leaning more towards the idea of eliminating the step and having nothing but smooth transitions from the valve all the way to the turbo, but I don't have the experience many of you do. Has anyone tried matching the port and header primary on a forced induction car? Did you see any significant results? Thanks!

Speaking from the theoretical point of view I would say that keeping the exhaust path as smooth as possible is the best bet with a turbo charged engine. Typically the back pressure is twice the boost. Thats far more than enough pressure to make the step insignifigant in preventing exhaust gasses from getting back into the cylinder.

I think your also correct about the change in velocity due to the increase in diameter. Ideally you want your exhaust pulses to travel all the way to the turbine with as little loss in energy as possible. Although the increase in diameter will increase pressure via bernolli's principle, a change in the gasses velocity means some energy is required to do it. If you were to change the diameter you would want to do it gradually to loose as little energy as possible. Also an increase in volume of the exhaust manifold also increases turbo lag.

Turbocharged engines are a different beast when it comes to the exhaust system. There is a lot more to consider.

i read a really good conversation on this on a turbo buick board... now if i could only remember what board it was, id link ya... lol.

Reversion is an issue with any type of induction. An improvment over just the step we have found that machining in a small grove on the larger lip helps even more. Also, we found that the small CNC marks left by porting (or just the porting marks) are better left as is & NOT polished smooth....in watching a wet flow demonstration we see the molecues along the surface form small "ball-bearing" looking particles on the rougher surface allowing the flow to carry a higher & smoother mixture velocity, where the "polished" I have used for years actually caused turbulance along the surfaces affecting velocity. That was an eye opener!

Ahh there are already so many different views on what to do as far as porting heads on turbo cars... and now this! I always thought the smooth exhaust runner was agreed to be better than rough...