z8cw

No real estate...everything was so tight, there was no room. I would think that a properly tappered down pipe would scavenge the exhaust ports pretty well. In any case it remains to be seen.

z8cw

Thanks, the fab work was done by a local engineering shop, Saridea, they are the skill-fulls. As for the ports, that is what I did, reduce the overlap of a full PP. The problem is you need to weld the exhaust port shut and then report the housing. The semi PP intake port can be smaller which will reduce timing. In general, I like overlap, turbo or not. Extending the secondary in the side plate for a street port will give you mostly later closing. That will yield some increase in power. For more power, you cut a small bridge which will mainly increase your opening timing, which will increase overlap. So, there you go....

Will see what this set-up yields in due time...just love the manifold

Owen

iTrader: (1)

Thanks, so I guess the only way to decrease overlap is to make the port smaller...I was hoping to be able to move the port up.

I don't know if I have the capability of welding the exhaust port shut too...

You're not worried about blowing boost out of the engine?

Owen

z8cw

Welding the housings I wouldn't recommend anyways. you need to respec everything after you done...

As for blowing boost out the engine, are you refering to the overlap or my housings not being plane?

Owen

iTrader: (1)

I was just thinking in piston-terms when both valves are open, the intake charge gets pushed straight out to the exhaust or exhaust gas gets reverted into the intake tract.

Not a huge problem for NA but more of one for forced induction. So the theory says, I have no personal experience in this.

Owen

z8cw

I have no personal experience either but hopefully will soon have. My thinking on this was that the two areas, exhaust and intake are pretty well seperated even without the apex seals seperating them. The little hump between the intake and exhaust port will hopefully do its job. In any case, I forgot my timing numbers..it has been a while, but looking at the exhaust "stroke"; exhaust port opens, burned gas is rushing out with high velocity, exhaust chamber is close to its largest volume. Rotor reduces chamber volume helping to empty gases. Just before the intake port opens chamber size is reduced to what about a quater of maximum? So most of the exhaust gas has left the chamber and pressure is reduced substantielly. Now the intake will start to open and the exhaust port is still open. So what do you think will happen. I guess it depends on the pressure in the intake runner. If the pressue in the intake is less than what is left in the combustion chamber and exhaust runners, some exhaust gas will be drawn in to the next intake cycle. That will dilute the intake charge slightly. You have a lot of exhaust gas leaving the combustion chamber with high velocity, so in order to reverse direction you would need a pretty big pressure drop. Still this is probably why PPs or extreme Bridge ports have bad idle and little power at lower rpms. With higer Rpms and/or positive boost pressure in the intake, the intake charge pressure will probably be higher than the remaining exhaust pressure and some of the fresh intake gases might find there way into the exhaust. Exhaust gases leaving might add some extra velocity to the fresh gases entering the chamber, which will help to fill the chamber. Of course pressure waves coming back through the intake and exhaust manifolds will play a role in all this. The possibly little wasted intake charge will help to ensure you have a perfectly filled combustion chamber. Actually this is where the PPs are getting their 120% VEs. In a turbo application you might have this help spool your turbo.

I am brabbling, so enough and anticipating lots of corrections.

Owen

iTrader: (1)

I completely agree, it all depends on intake pressure in the runners. And like you said, the only real pressure/gas loss is at lower rpms (unless you're boosting at idle). At higher rpms, the exhaust port is closing "faster" too, not much time for the intake charge to bleed off.

I can't wait to get started! Won't be for another 6 mos at least tho...:-(
Owen