exhaust port/manifold matching? any tips?

Obin Robinson · 06-10-2001, 08:09 AM

hello,

i'm wondering if anyone knows any good tips on how to PRECISELY match cylinder head exhaust ports to the exhaust manifold?

i heard about the gasket method, but is there any other way that is even more precise?

offhand, you should see just how much casting "flash" is in the 924 block. you can clearly see how an engine (not just the 924, any engine) is molded at the factory.
thanks!

obin

Dave951M · 06-10-2001, 08:42 AM

There is a more precise method, but in practice it's just like the gasket method. Each gasket is made with some "slop" in the bolt holes. If you make a template that removes that slop, you can scribe a very accurate line on each side. I made one for a 124 motor a while back on our laser. It took a try or two to get it right and would have been much easier with a set of drawings. Another way to eliminate the slop is to use shoulder bolts. Past that, you're looking at big cnc machining bucks.

Dave951M

Obin Robinson · 06-10-2001, 08:48 AM

hmmm,

yeah, i noticed the "gasket slop" and that's why i was wondering about the gasket method. the only other way i thought about doing this would be to make an exhaust gasket out of plastic, and then simply try to match it to the the exhaust ports.

do most other Porsche engine builders use the "gasket method?" i heard that they did.

thanks!

obin

Dave951M · 06-10-2001, 09:54 AM

The reason most engine builders go with the gasket method is that the gasket can actually interfere with the passage. If the openings are the same as the gasket, the chances of interference are reduced substantially. When installing the gasket after matching the ports, we used to "tack" them in place with a dab of grease or two to make sure they didn't move while we mated the parts back up and essentially negate what we had just done.

There's also a lot to be said for just removing the casting flashing. You could make a template from some plexi or similar material, and make your own witness marks to do the scribing. If the studs are already removed, then just a cross hair for each location might work ok. Wild hair idea, try smearing something like fingerpaint on one surface, mating them up, remove and see where there is paint on the other surface. Never tried this one, though. If you do go with plexi, get some extra. Since you're already grinding, match the combustion chambers. It's not hard to cc the head yourself. Do that one after the valves are in. You'll need some form of graduated pipette to do the job. About any hobby shop that has chemistry stuff would have something to do it with.

If you are having work done on the block, consider knife edging the crank and align boring all bearing seats. In the casting and mass machining, the bearings for the crank and anything else are not bored absolutely concentric. This could cause some binding on the crank or cam or whatever, thereby robbing horsepower and potential top rpm. For a engine freshening, this is way overboard, but if you've already got it apart....

Dave951M

[This message has been edited by Dave951M (edited 06-10-2001).]

Roland Kunz · 06-10-2001, 05:40 PM

Hello

An other trick is to use the "step" methode. The following port is slightly larger avoiding inteerferences also the steping generates turbulence ( Grenzschicht ) on the wall and and the step will stop backflowing.

A simple way is to use shootpening to flow the ports. You will see any overstanding and the shootpening is made with airflow with likes to find the lowest resistance.

Shoootpening will not abrasive to much so you have to start with the basework manually.

Just want to mention that carbs don´t like to much polished ports. FI loves super smoth surfaces.

Grüsse