'91-95 head is even better as there is little additional cooling between exhaust ports.

One thing to keep in mind is that there are no factory studs for '89-95 heads while 32V S studs can be used with early S4 heads. Some vendors sell single studd kit for all years. This means studs must have 20mm extra thread on upper end to work with all three head styles.

 

Additional cooling between exhaust ports? That I am extremely interested in. Is there an additional oil passage or water passage between the exhaust ports, or why are the '91- heads better in terms of cooling?

 

There are pics somewhere in archives and my rennlist member page but its down now.

 

water jackets between the ports. not sure how that would affect porting limitations though.

 

928ss, vilhuer --
I've recently heard that one can make the "gts cooling mod" to the earlier heads. Supposedly, one drills one additional cooling passage per cylinder with an 8mm drill bit. If either of you two or anyone else knows how this is done, please post the instructions.
Best, Tuomo

 

Ok, I've got four extra heads + the two in the car. Scored two .4R heads on Friday!

I am thinking about doing some rnd or, more likely, having some rnd done. While thinking about this, I came across with the following idea on the web. Make one of the intake valves bigger and one of the exhaust valves smaller than stock. It's called polyquad. The idea is to induce swirl motion and reduce the blowby or cross-flow from intake to exhaust port, which will allow running more radical cams.

Anybody tried this? Anybody heard of this?

http://www.gofastnews.com/board/tech...r-concept.html

 

Be careful. That design is technically patented by vizard.

 

Saab 900's had staggered valves and port angles stock. The turbulance may be great for stock type motors, but I wouldn't suggest if high hp is the goal. On a forced induction motor cam events make a huge difference in output in my experience. Everyone says you can't have duration on forced induction motors and it's b.s. Long intake duration is perfect. Can't use too much overlap ofcourse. On an N.A. motor I would want the port floor as high as possible to keep velocity up, then you can spread out the lobe seperation angle big time to near 116 with a late intake valve close. Again, intake plays a big role. Don't forget when comparing to other vehicles rod to stroke ratio plays a big role. I know lots of people here will likely call me nuts, but I like short rod to stroke ratios on N.A. and boosted engines as long as the cylinder and piston can tolerate it. A short rod/stroke ratio means the piston will accelerate quicker down the bore from TDC and start making vacuum more quickly. In my (limited) experience, a shorter rod/stroke engine can take more cam and run well all things being equal. With the sharper movements detonation is a tiny bit less prone to happen too. On a serious forced induction motor I think durability is more of a realistic concern. I've got some pics I can post if desired of an honest over 1000hp 2JZ engine from my friends toyota supra and I think you'd be surprised by how small the ports are. Not trying to throw the thread off topic, I just thought it might help a little.

 

This is perfectly on topic.

Turbo engines can be very diverse group as far as cam selection goes. The main difference is the turbine sizing.

If the turbine is small, the ratio of exhaust back pressure (pre turbo) to boost is high. With high ratio, the typical NA high-tune level of overlap hurts as there will be a lot reversion.

If the turbine is large, the ratio of exhaust back pressure to boost is low. With narrow powerband engines, it may even be below one -- the engine operating in the cross-over mode. When the engine is running in the cross-over mode, the overlap will not cause reversion. In fact, the intake charge will help push the exhaust gas out of the cylinder and the resonance effects can be exactly tuned to push the cross-flow back to the cylinder since the operating rpm range is narrow.

So the turbo cam selection depends very much on the characteristics of the turbo engine.

 

Well, I don't want to make a turbo turbo turbo thread. I have a forced induction shop and could talk about turbocharging till I'm blue in the face. Yes, I agree that turbo and pipe sizing has a phenomenal impact on output. I've got some funny stories about crap we've thrown turbo's on.

 

Here's something that I came across today. Someone was advocating sinking the exhaust valves in deep in the head/seat and raising the intake valves. The stated logic was to reduce reversion into the intake port while making the exhaust flow better out of the cylinder.

This from a Chevy book ("How to Build and Modify Chevrolet Small-Block V8 Cylinder heads"):

"...I prefer to set the intake valve in the chamber higher than the exhaust valve. With the intake an attempt should be made to produce a valve and seat combination that sits as high in the chamber as possible, whereas I will deliberately sink the exhaust into the head a little way. Just how much the exahust valves can be sunk depends on the individual casting."

"It's usually not practical to sink valves by more than 0.020-0.030 in., and you may very well ask what the logic is for doing so. In essence, this procedure relates to the mechanics of the gas flow during the overlap period. If the intake valve is higher than the exhaust valve, it enhances the tendency for the fresh charge to pass out of the intake valve and over the top of the exhaust valve face rather than under it and on out the exhaust port."

Anybody here made any concious effort to raise the intake valve and sink the exhaust valve?

 

Hey I think I've got that book somewhere. Is it written by David Vizard? If so people better listen.

I put a couple funnies in the turbo thread.

 

Here are some flow results from Don Redmond of RMI.

These are for heads with stock valve sizes for a turbo engine. (My at this point somewhat educated guess is that a forced induction 5.0 engine wouldn't need bigger 39mm valves if spun at most at 7500 rpm.)

All testing is at 28 inches depression.

Stock RMI
Lift Intake Exhaust Intake Exhaust
050 41.9 52.0 48.5 63.9
100 104.7 105.6 110.4 115.2
150 149.2 152.8 153.2 152.7
200 198.3 196.5 201.5 203.8
250 239.2 229.2 244.5 241.7
300 269.6 247.6 280.1 262.1
400 296.9 259.2 312.6 274.4
500 301.0 264.7 320.5 279.7
600 306.1 263.0 322.1 286.2
------------------------------------------------
Avg 233.0 212.8 244.5 225.5


These are before the valve job and reflect the Porsche stock valve and valve seat angles. I expect the numbers will get a little bit better especially on low and mid lift. As you can see from the "average" numbers, there are some very nice gains. The intake at 0.350" hits the stock 0.600" lift flow number of slightly over 300 CFM, which is a nice pickup.

Although I won't be lifting the valves to .600", it's comforting to me that the flow doesn't decline with more lift at the higher lifts. My layman reasoning is that if the flow would start declining at higher lifts then the port would not be robust to small changes, instead would be somehow right on the edge. Perhaps this is all superstition and old wives tales.

I was a bit surprised to see the exhaust flow more than intake at low to moderate lifts. Focusing on the baseline stock measurements, and computing the intake/exhaust flow ratio:

Stock
Lift Intake/exhaust
50 81%
100 99%
150 98%
200 101%
250 104%
300 109%
400 115%
500 114%
600 116%

I am looking at some random people's measurements in the 928 community, and they see their baseline exhaust flow at about 15-20% lower than above. Also, their intake to exhaust flow ratios hover around 1.1 at .150" lift and at around 1.2 at .300" lift. This is most likely related to the measurement methods and the flow-bench configuration. I believe the before and after comparisons are valid for both intake and exhaust separately, but the measurement methods and the actual flow bench used can cause variations between sucking in thru intake vs. pushing out thru exhaust.

Overall average port runner velocity was also improved substantially. Some background on the measurement techniques etc. Velocity is the speed of the air moving through the port which is measured in "feet per second". Improving the speed and/or uniformity of the velocity in the port runner is important. This is an indicator of how fast or lazy the air charge will move through the port. A lazy port would need more cam timing to properly fill a cylinder as much as a fast port runner could or would. Conversely a fast port runner will need less timing. A fast port is also an indicator of a less "turbulent" port runner, and having a uniform velocity through out the port runner is a good thing. To produce the below velocity numbers, seven separate readings were taken at about an inch in front of the port divider. First reading was at the top of the left valve at approximately 11:00 o'clock, next reading over the right valve at 1:00 then the right port side at 3:00 then right port floors at 5:00 and then left port floor at 7:00, left port wall at 9:00 and last the center of the port. These seven reading at every other lift point, 0.100", 0.200", 0.300", etc. The reported data below is the average over both dimensions.

Average intake velocity: stock 128.9 FPS (not uniform), RMI 131.4 FPS (uniform)
Average exhaust Velocity: stock 350 FPS (not uniform), RMI 352.0 FPS (uniform)

Important improvement here is the consistency of these numbers around the port. Before, the intake port had a high peak number in one or two places per reading per lift. Now the numbers are more consistently higher all around the port at all test spots. This is an important improvement and shows that the port has less turbulence and better "filling" capacity than it had as a stock unit. Apparently few people normally test for this, it is important as it shows the overall ability of the port runner to flow "quality as well as quantity" of air.

The exhaust side had the same problem, stock port had high peaks spots but no uniformity. Now it has high uniformity all around the port runner and through out the lift range. This is especially important on the exhaust side for a turbo engine: Low lift good velocity numbers will spool up the turbine faster and earlier, expanding the turbos power band.

Here are Don Redmon's contact details, in case you are interested in having similar work done on your heads:

Don Redmon
don@replikamaschinen.com
Replika Maschinen, Inc
3333 Main Street
Chula Vista, Ca. 91911

So far, I have been very happy with the services provided by RMI. Services are expensive, but good work tends to be. I will continue to report on both service and the project until completion.

[EDIT: a link to photos below.]

http://picasaweb.google.com/ptuomov/...eat=directlink

 

Down here by Chula Juana? Cool.

 

One of the firms offering head porting services for 928 is 928 Motorsports. See their head porting page: http://www.928motorsports.com/servic...alve_heads.php
For what it's worth, here are their numbers compared to RMI numbers:

Intake CFM

Valve Lift 928MS stock 928MS ported 928MS improvement RMI stock RMI ported RMI improvement
0.150 142.5 158.9 12% 149.2 153.2 3%
0.200 187.1 204.9 10% 198.3 201.5 2%
0.250 225.7 244.1 8% 239.2 244.5 2%
0.300 248.3 273.8 10% 269.6 280.1 4%
0.400 267.3 311.5 17% 296.9 312.6 5%
0.500 297.0 329.3 11% 301.0 320.5 6%

Exhaust CFM

Valve Lift 928MS stock 928MS ported 928MS improvement RMI stock RMI ported RMI improvement
0.150 124.8 153.3 23% 152.8 152.7 0%
0.200 169.0 194.5 15% 196.5 203.8 4%
0.250 190.8 218.5 15% 229.2 241.7 5%
0.300 210.5 234.8 12% 247.6 262.1 6%
0.400 223.3 252.0 13% 259.2 274.4 6%
0.500 229.6 260.0 13% 264.7 279.7 6%

If the measurements are comparable, then my cleaned stock .4R heads were a better starting point than 928MS's stock heads. In theory, they should be reasonably comparable, because 928MS is using SF-600 at 28" and RMI's flow bench is calibrated to match SF-600. In practice, they may not be comparable for many reasons. To get truly comparable numbers, one should measure the heads on the same flow bench on the same day.

Assuming comparability, here's the horse race between RMI and 928MS ported heads:

RMI CFM - 928MS CFM
Valve Lift Intake Intake % Exhaust Exhaust %
0.150 -5.7 -4% -0.6 0%
0.200 -3.4 -2% 9.3 5%
0.250 0.4 0% 23.2 10%
0.300 6.3 2% 27.3 11%
0.400 1.1 0% 22.4 9%
0.500 -8.8 -3% 19.7 7%

According to this, the 928 MS heads flow slightly better on the intake side at low and very high lifts. The RMI heads flow slightly better on the intake side at max lifts of typical 928 camshafts. The RMI heads have a big advantage on the exhaust side, which isn't a surprise as the RMI heads are custom designed to work well with a turbo engine.

These CFM numbers are not the same as hp numbers. CFM is mainly a marketing tool nowadays, since the customers appear to be dumbing the issues down quite a bit. 928MS doesn't report velocities, port CSA's, or wet flow stats etc. on their page. These would give a more complete picture of the performance of these heads. Carl, how about adding those data to the porting page?

The photos in the above link are much more interesting than the advertised numbers.

Some observed differences between RMI heads and 928MS heads:

- 928MS chooses to fit 39.5mm intake valves, whereas RMI is using the stock 37mm valves. I think 37mm is enough for my 5.0 turbo engine, both based on the flow and the fact that the "big valve" 37mm intake Cosworth heads for the Subaru EJ25 turbo engines making 800 hp from 2.5L happen to share the almost exact key parameters as one of the 928 banks. With a N/A 6.5L stroker engine, I can see the merits of going to 39.5mm.

- Because of the larger valves, 928MS is unshrouding and pushing back the squish area in the combustion chamber more aggressively.

- The exhaust port CSA looks larger in the 928MS head than in the RMI head. Some claims by me: For a given level of flow, smaller exhaust port CSA is better for a turbo car. The exhaust goes sonic because of the huge pressure differential, and during this sonic period gas flow behaves very differently from, say, Mach 0.5 flow of the intake port. If one can keep the exhaust flow sonic for a long time while still flowing a large volume, the port is going to put a huge amount of energy in the exhaust flow. Therefore, a smaller CSA means faster and hotter gasses which mean more energy for the turbine (but possibly higher pumping losses.)

- The 928MS intake port is also bigger than the RMI intake port, starting from the very beginning. My eyes are seeing the beginning of the intake short side being extended downwards. This probably makes the 928MS port mid point slightly lower than RMI's. I can see the logic of raising the port, but not so much lowering it. Perhaps I am looking at the 928MS ports from a picture which is taken from a different angle.

- The 928MS intake port divider has been ground much narrower than in RMI.

- The 928MS fuel injector spray cavity is blended to the roof, whereas the RMI heads retain more of the stock shape.

- The 928MS intake valve guides are ground flush with the port roof, RMI's are not.