Getting ready to do headwork on my 7M and I've been reading not to port match the exhaust manifold to the head. that i'll loose the reversion dam. what is the reversion damn? what do i do? just polish the exhaust side and port the intake?
What's the reversion dam?
- Thread starter MA70witBoost
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Supramania Contributor
There are a few ways you can do this.
First, the dam is a lip that's created around the inner diameter of the runner, and it disturbs the laminar flow at the head gasket/port, creating an effect that at lower speeds, helps to keep exhaust gas flow from backing up into the combustion chamber.
This effect improves low end power and tourqe. And it has a very limited effect on flow on the top end, so limits power nearly so little you can't tell. (especially on a turbo charged engine, where at WOT under boost, the exhaust gas is under pressure, and going to escape anyway it can anyway to the turbo where it can find a pressure drop, and escape out the down pipe/muffler etc.)
So here is my advice:
If you are using a stock cast iron manifold, you can improve the flow somewhat by smoothing/porting the manifold at the ports so it's about 1 or 2 mm LARGER than the gasket holes. Do not port the exhaust ports smaller than about 1 to 2 mm SMALLER than the exhaust gasket.
So, if you lay the gasket on the head, you see a lip of a few mm of port all the way around it.
And if you lay the gasket on the exhaust manifold, you see no metal, but a tapered volute for each port.
In my view, and from personal experience, this makes a neat stair step that gives you the turbulence you want at lower speeds, and does not seem to give up anything on top at all. (First step is the head, then the gasket, and it drops off into the volute on the manifold runner.)
If you are using a stainless tube manifold, then I'm not sure how critical this is. (When I was working on my last Manifold, it was fun to actually see the effect blowing compressed air down a runner, and watching the one next to it suck porting shavings out of the tube next to it.. Yes, it creates a serious low pressure area in each runner next to it, so is the lip really needed on a tubular manifold? IDK for sure, but hey, it's not going to cost me anything to leave it like I just described, and that's how it did it anyway, since the exhaust port is plenty large enough if you leave 2mm showing on the gasket, and then "venturi/volute" the exhaust manifold runner opening.)
Just my .02psi on this.
Good luck, and wear good eye protection when porting. (I reccomend welding gloves with a long gauntlet, long shirts, long pants, boots and if porting the head, get a carbide with long curved and open cutters v/s the tighter "pineapple" cut that works on cast iron better. A large can of WD40 is good too. Spray it on the runner, and then port. When you get low on oil, apply again, and it will keep your cutter from clogging up much, if at all.)
I also did not bother to final polish the ports with stones, as the "golfball" effect of the carbide was nice, and I coated the runners anway. (So they were sand blasted, coated and the entire head baked with all the porting and thermal coatings etc.)
I don't reccomend that level of insanity to anyone.
First, the dam is a lip that's created around the inner diameter of the runner, and it disturbs the laminar flow at the head gasket/port, creating an effect that at lower speeds, helps to keep exhaust gas flow from backing up into the combustion chamber.
This effect improves low end power and tourqe. And it has a very limited effect on flow on the top end, so limits power nearly so little you can't tell. (especially on a turbo charged engine, where at WOT under boost, the exhaust gas is under pressure, and going to escape anyway it can anyway to the turbo where it can find a pressure drop, and escape out the down pipe/muffler etc.)
So here is my advice:
If you are using a stock cast iron manifold, you can improve the flow somewhat by smoothing/porting the manifold at the ports so it's about 1 or 2 mm LARGER than the gasket holes. Do not port the exhaust ports smaller than about 1 to 2 mm SMALLER than the exhaust gasket.
So, if you lay the gasket on the head, you see a lip of a few mm of port all the way around it.
And if you lay the gasket on the exhaust manifold, you see no metal, but a tapered volute for each port.
In my view, and from personal experience, this makes a neat stair step that gives you the turbulence you want at lower speeds, and does not seem to give up anything on top at all. (First step is the head, then the gasket, and it drops off into the volute on the manifold runner.)
If you are using a stainless tube manifold, then I'm not sure how critical this is. (When I was working on my last Manifold, it was fun to actually see the effect blowing compressed air down a runner, and watching the one next to it suck porting shavings out of the tube next to it.. Yes, it creates a serious low pressure area in each runner next to it, so is the lip really needed on a tubular manifold? IDK for sure, but hey, it's not going to cost me anything to leave it like I just described, and that's how it did it anyway, since the exhaust port is plenty large enough if you leave 2mm showing on the gasket, and then "venturi/volute" the exhaust manifold runner opening.)
Just my .02psi on this.
Good luck, and wear good eye protection when porting. (I reccomend welding gloves with a long gauntlet, long shirts, long pants, boots and if porting the head, get a carbide with long curved and open cutters v/s the tighter "pineapple" cut that works on cast iron better. A large can of WD40 is good too. Spray it on the runner, and then port. When you get low on oil, apply again, and it will keep your cutter from clogging up much, if at all.)
I also did not bother to final polish the ports with stones, as the "golfball" effect of the carbide was nice, and I coated the runners anway. (So they were sand blasted, coated and the entire head baked with all the porting and thermal coatings etc.)
I don't reccomend that level of insanity to anyone.
Think of it as a non return valve for the exhaust port, if the Header port is larger than the exhaust port in the head you have an effective dam.