Question : Exhaust Port D Shape - Reversion and Turbo

[Schmieder]

Well, the real question is about the exhaust port. I know the D shape is smaller on the heads versus the exhaust manifold where they meet.

I was told the size difference is for reversion control and is not to be ported to gasket match the exhaust manifold.

But, if true, when a turbo charger is present reversion is nullified by the turbine blades. Does this mean a turbo could allow the porting of the exhaust ports to gasket match the exhaust manifold?

Or should they still remain a specific size for velocity? Possible to blend for a smoother transition into the exhaust manifold?

[Driver_10]

Well, the real question is about the exhaust port. I know the D shape is smaller on the heads versus the exhaust manifold where they meet.

I was told the size difference is for reversion control and is not to be ported to gasket match the exhaust manifold.

But, if true, when a turbo charger is present reversion is nullified by the turbine blades. Does this mean a turbo could allow the porting of the exhaust ports to gasket match the exhaust manifold?

Or should they still remain a specific size for velocity? Possible to blend for a smoother transition into the exhaust manifold?

To answer your question bluntly, NO... Dont worry about blending the transition. The turbine creates a restriction to flow and hence creates exhaust reversion. Besides, you want the exhaust pulse to carry high pressure and speed. Its the expansion of the exhaust in the turbine housing that creates force to spin the turbine. (Hence the reason the exhaust is always cooler after it leaves the turbine.)

[ForcedFirebird]

Driver is correct. Think about the turbine blade being a pinwheel and blowing on it through a sipping straw, then trying to make is spin blowing through a paper towel tube.

[Joseph Upson]

There is an exception to the above, anything done to the exhaust ports that will help a naturally aspirated engine should help a turbocharged engine simply because the issue of exhaust pressure doesn't come into play until you're under boost which is a small percentage of the time.

A properly selected turbine or turbine housing on the large side will take longer to exceed the 1:1 boost pressure to exhaust pressure ratio, catalytic converters and mufflers likely produce more back pressure under normal conditions than a properly sized turbo. I've mentioned this several times before, but my 3100 turbo motor with a poor 7ish:1 compression ratio still made near 28 mpg hwy with the performance 4spd manual spinning around 3100 rpm and a turbine on the small side .63.

I agree though that there's no need to gasket/port match on the exhaust side but if there's something you can do to make the exhaust leave the port a little faster I'd do it as the more boost pressure you run the more the small restriction becomes a big restriction to flow.

I've read that one of the greatest benefits to larger exhaust valves on a boosted engine for example, is that there is more area for exhaust to escape immediately upon the valve cracking, on the other hand there is more stress on the valve train as a side effect because there is just that much more valve area for the gas pressure to work against the camshaft trying to open it.

I have one exhaust port to go on my heads, I polished the combustion chambers too.

[Schmieder]

There is an exception to the above, anything done to the exhaust ports that will help a naturally aspirated engine should help a turbocharged engine simply because the issue of exhaust pressure doesn't come into play until you're under boost which is a small percentage of the time.

A properly selected turbine or turbine housing on the large side will take longer to exceed the 1:1 boost pressure to exhaust pressure ratio, catalytic converters and mufflers likely produce more back pressure under normal conditions than a properly sized turbo. I've mentioned this several times before, but my 3100 turbo motor with a poor 7ish:1 compression ratio still made near 28 mpg hwy with the performance 4spd manual spinning around 3100 rpm and a turbine on the small side .63.

I agree though that there's no need to gasket/port match on the exhaust side but if there's something you can do to make the exhaust leave the port a little faster I'd do it as the more boost pressure you run the more the small restriction becomes a big restriction to flow.

I've read that one of the greatest benefits to larger exhaust valves on a boosted engine for example, is that there is more area for exhaust to escape immediately upon the valve cracking, on the other hand there is more stress on the valve train as a side effect because there is just that much more valve area for the gas pressure to work against the camshaft trying to open it.

I have one exhaust port to go on my heads, I polished the combustion chambers too.

Good advise, thanks. I had already polished the exhaust pocket and runner. I used a Cross Buff kit after I removed casting defects (tits). The polish is really nice.

The intake/exhaust valves were deshrouded as well. They were deshrouded exactly how WOT deshrouds their race/boost heads.





So the exhaust port remains polished but not blended/ported at all.

Thanks for all the advise fellas.