Skunk Workz

Bore a cylindrical hole and sleeve it,insert by heating the head and cooling the sleeve....or just TIG-weld it to the right size(carefully...you don't want so much heat you warp the head).

Then you'll have to ask the engineers at Porsche...my guess is that they were in a "70's & 80's max-flow-frenzy" and supersized it...quite a bit too much.

Pauerman

Skunk,

My 2.5 T head vs 2.7 N/A head have drastically different intake ports - turbo being round and N/A being oval.

I measured my 2.5 port and got dimensions of approx 40.15mm at the opening and 47.2mm at the largest point. The 2.7 has an opening of 49.20 and it transitions to the largest point of 58.01.

That's a big difference in size and considering your comment about the 951 intake port being large enough to accomodate a 3.2L engine, why do you think Porsche would increase the port size and shape on a cylinder head for an engine with only .2L increase in displacement compared to the 951?

Jeremy,

The 968 T w/ 8 valve used a different cylinder head that had smaller exhaust ports compared to the 2.7 N/A - not sure if was it had ceramic liner or not.

Skunk Workz

From what I know,most older Porsche ports are too big...The 2.7 Turbo S I think has the smallest port...this should the best base to work from,if this is correct. (Even if some tuners say they are $hit because they don't flow tons...they don't have to). Keeping the speed/heat in the exhaust manifold/crossover is essential for making good spool. Guess the liners help prevent the cooling system from "robbing heat" that is needed to spin the fan. How much it helps,I have no clue. Stainless headers/crossover (with thermal wrapping/coating) also helps keep the heat in.

Skunk Workz

49.2 in width,I guess...what height,with it being oval? Haven't measured any of the 2.7NA's,they're not really common over here....

I have no idea,really...the 2.7 has a bigger intake valve,maybe they saw some relationship between the port size and valve size or something like that. You'd need to ask Porsche about that one...I have NO idea why they wanted to do that.

TurboTommy

Skunk,
I'd be interested in knowing how you measure your VE.

125% VE means you have 25% more air molecules than would otherwise occupy a fixed volume (cylinder). The only way that can happen is if the pressure goes up by 25%.
That would mean that on a NA engine that has 14.7 psi absolute pressure available (at sea level, for example), there would be an effective static pressure of 18.4 psi when the intake valve just closes.

On a turbo car that runs 12 psi boost ( 26.7 psi absolute), the pressure achieved in the cylinder would be 33.4 psi. That would be like achieving actually over 18 psi boost because you have a VE of 125%.

Sorry, I don't believe this for a second.

Pauerman

On either side of the injector cut out the port measures 40mm vertically. The vertical section of the port tapers gradually smaller until the base of the guide.

Skunk Workz

Yes...inside the cylinder,and nowhere else.
Exactly.

At that exact rpm where you achieve the 125%,yes,the cylinder would "see more boost than you have". But 125 is hard on a turbo engine,as the pressure goes up the speed of sound goes down...which means you'll have "sonic choke" at some part of the intake cycle,robbing some % of the VE. But you are still over 100%...for a much wider range than a big port gives you.

No problem...I can't make you belive it,but I don't really care...I have done it,and seen it work. So what other people belive,is their business.

Skunk Workz

Ok...so if you compare the cross-sectional area of the two ports at a point just before the valve guide,what do you get? Which port has the smallest cross-section? The round NA-port is about 47mm in dia...this is about 1735 square millimeters...what is it for the 2.7's oval port?

Pauerman

On the 951 port, if you consider 47mm to be the diameter at the largest point w/out considering the area lost from the hump in the casting before the guide, the best vertical measurement I can get on the oval port is 34mm. The hump in the casting for the guide begins much sooner in the oval port compared to the round port. So, measuring before the guide, I found a width of 58mm and a height of 34mm on either side of the casting hump.

Don't know if this is right, but since the port is oval I averaged out the sum of both cross sectional areas and came up with 1775 square millimeters.

TurboTommy

The question still stands:
How are you measuring? How do you know you're achieving 125% VE?

Skunk Workz

That means that for an increase in size from 2.5 to 2.7 liters you just have an increase of 2.5% port area compared to the 8% displacement increase...the port is smaller relative to cc's.

Skunk Workz

I got the number from back-calculating a Yamaha R6,600cc I tuned. Had 122 hp on the wheel from 600 cc's. My software program predicted 124.9% VE on that one. At least the same number is achieveable on the 944 NA, too. On the turbo it would be slightly lower if you were to run lots of boost,then you'd need a slightly larger cross-section in the port to avoid too much "sonic choke". But nowhere near as big as the stock port is.

Ben Z.

Do any of these flow/velocity arguments apply to the exhaust side of the equation?

Skunk Workz

Yes,it has some influence...Lindsey did a modification for the exhaust port for the 951,but I am not sure how it worked for them... More speed creates lower pressure in the exhaust resulting in better scavenging...and the "high-speed escaping molecules" exerts enough "punch " due to their speed you get lower exhaust blowback,resulting in a cleaner intake charge.

Pauerman

I previously calculated the area of the oval incorrectly - the formula I should have used is L x W x 0.8 which equals 1577 square millimeters for the 2.7 head.

I remeasured my 951 port and figured 46.5 mm as the average diameter so that equals 1720 square millimeters.

So according to my measurements it seems that the 2.7 oval intake port has approx 8% smaller cross sectional area just in front of the guide compared to the 951 intake port.

Skunk, this decrease in cross sectional area will contirbute to increasing the velocity of the intake charge right? Given these measurements, do you think the port sizing on the 2.7 N/A head is any better matched for a 3.0L turbo application compared to the 951 port and its 2.5L displacement?