924Board.org

[bass gt]

Guy's,

Has anyone gone to the trouble of having a 931 head flow benched, to determine the flow characteristics?? If so, what figures did you get?
If not, it looks like i may be the first

Steve

[endwrench]

I have never seen a flow test on the 931 head. I hope you do it and post the results. I was planning to do the same after finishing my flow bench. All the hard parts are done. I just need to get my lazy a$$ out into the cold garage and put it together
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'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

[bass gt]

Endwrench,

Chop chop, get to it man!!

[gegge]

Anyone?
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[ideola]

I am also VERY curious about this, as I would love to know what the precise VE is on our heads. There is a shop within a mile of my house that does flow testing, but it costs like $75 per hour to have it done. My unmodified 931 head is still on the bench, so if we want to take up a collection and split the costs, I'd be willing to run it over there and have it done...but he said it would take like 4 hours to set everything up and run the test. Not sure it's worth $300!!!

I did some research on building my own flow bench so I could test the head, some intercooler designs, and other stuff. But it would cost a few hundred dollars to do that too (since I'd have to also pick up the shop vac to power it all), and I'd rather put the money into the car rather than a homemade flow bench that very few people would trust the results of anyway
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erstwhile owner of just about every 924 variant ever made

[gegge]

I had my 931 ported by http://www.racing-green.com/eng/hmg.html but they never measured the head. The reason for this was that it doesn´t make much sence. It is easy to get big CFM numbers just by removing material - what you really want is to improve the flow maintaining narrow runners and low volume. But, he also said that the 931 head was not Porsches best design or work and there is definetly room for improvements. An easy 12-15% incresae in power was discovered according to them. My engine is not running , so I have to take their word for it.

But for comparison purpuse, it would be interesting to know the figures...
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[bass gt]

Guy's,

I never got around to getting my head tested, but as Gegge says, there is a lot of scope for improvement. The key is to not hog out the head too much, but to make a nice flowing profile. This is why i have raised my runners up so they sit flat with the head, rather than my original system, which required the air/fuel to turn through 135 degrees!! Not optimum. I found that at low air speeds, the engine was very sluggish, but was fine once the boost came on and aided the flow. This is why i have copied the GTR intake concept, but added injectors both into the head and further upstream. This should give the best of both worlds. The head injectors will be used first, with the upstream injectors adding fuel as the requirement increases. To this end, i have tapered my intake system with the ITB's being 40-45mm internal taper, with the head adapter being internally tapered from 40mm down to 34/35mm, as per the head ports. This should keep the air speed nice and fast.

As you know, i am running a fairly aggressive cam, so the Franco gear should help this as well by reducing the low speed overlap. I have also decided to incorporate a mild anti lag system, which should keep the intake pressure at aout 2-4psi, rather than the -25 psi experienced during deceleration. As this will be a mild ALS i don't expect a severe degredation of the engine components. Cosworth guys here in the UK run it on their street cars with no harmful effects. This is unlike the full ALS which gives +1.2-1.5 bar at idle!! This is hugely aggressive and will eat exhaust valves and turbo's in double quick time.
The other interesting issue with the mild ALS is that it allows you to run a bigger turbo than you would normally choose, as the low speed lag is effectively eliminated because the tubo is kept spinning. In fact thereis an advantage to the larger turbo due to a flywheel effect, which means it is easier to keep spinning.
Of course, the ALS can be switched off for normal track days or such, and only used on race day. The effect is that exiting a tight corner, the car is on immediate boost, much like a rally car. So less time is wasted waiting for the turbo to spool back up. This can really make a huge difference as it efectively replicates an engine of much larger capacity due to the torque being available much earlier.

Steve
_________________
Front Wheel Drive is the Devil's work.

[gegge]

For more flowbench info:

http://www.tractorsport.com/
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[gegge]

I may enlighten you with some details in this matter. I have NOT measured but got some theoretical numbers by adding to articls together
Read "Bolt on Bonanza - The sequal" from European Car:


From the article Indian Red:
Using the stock 40mm intake and 36mm exhaust valves, the Turbo head flowed an additional 25 percent more air than before the modifications, and a whopping 55 percent more air than the stock head. Clearly, by using the stock size valves and flowing more air through them, port velocities had increased radically. More port velocity and airflow combine to produce several effects; improved low end torque with better drive ability, more efficient cylinder filling and , most importantly, more power. And the stock, forged ATE valves are of a much better quality and give a longer life than most after-market valves. Larger valves were completely unnecessary to upgrade performance, even with a 2.3 liter motor, and could in fact hurt the bottom end and efficiency with lowered velocity.

Add those gave me these numbers:

_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[ideola]

So if I understand what you're saying, Gegge, you took the 25% and 55% figures to extrapolate the 931 flow figures in your spreadsheet?

I am going to be sending my head out later this month for complete reconditioning and fitment of the Integral Cam. I've read all Standard Abrasive's DIY guides, which seem to make sense, but I want to make sure there are no 931-specific issues I should be aware of. I am a porting novice, so some questions:

1. Aside from the general info on head porting, are there any 931-specific nuances I should know to achieve improved flow? What about on the intake manifold runners?
   
2. Sequence question: I'm assuming I should port the head first before having it rebuilt?
   
3. I've read elsewhere that it may be a good idea to have the intake ports exposed during media blasting to give them a good profile for creating some turbulence for better fuel atomization. Thoughts, comments?
   
4. I've also read that near-mirror finish on the exhaust ports is a good idea. Thoughts, comments?
   
5. What is the best/proper type of bit to use for porting the aluminum? Are there any gotchas I need to be aware of in terms of rotational speed, proper tools (i.e. electric drill vs. air-powered die grinder)?

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erstwhile owner of just about every 924 variant ever made

[gegge]

Yes, I used the very precise method of extrapolating the 924 stock numbers by adding 55% This was the quick-and-dirty way I am afraid. Interesting to read that the 931 head is 25% better than 924, and an average 26% gain by porting the stock 924 head would indicate that porting a 931 head is approximatly 55% better than stock 924.
The low-lift numbers of the ported 931 are more dependand of valve area than portdesign and are way too high and should be closer to stock.

The numbers does match similar two-valve designs from BMW, FORD, VW and Volvo. My conclution is that the 931 head is a ok design and even good then ported.

Have you read "Bolt on Bonanza" page 4? Look at the numbers of the intake and exhaust manifold before and after extrude honing. Might be a good idea looking into those as well. Is the 931 intake manifold larger/better than 924?

Regarding the exact sience of porting - I have to leave that to the ones with the knowledge and secrets. Smooth is good, but there is little in polishing as I understand.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[ideola]

Yep, I've read and re-read most of those articles multiple times! Every read seems to yield new and interesting insight, so I'll probably go revisit them yet again.

Re: extrude honing, I looked into it about a year ago during part of my research phase on the UWB project. Virtually everyone I talked to advised me against it. The main line of thinking wasn't so much that you couldn't achieve gains, but it was more from a "bang-for-the-buck" perspective. From what I understand, extrude honing is a patented process that is owned by one company in the US, and can only be done by them or one of their licensed shops. Virtually all extrude honing is actually done out of the facility in California. This not only adds to the cost (due to shipping), but it results in significant delays. I've heard horror stories from multiple independent sources about projects being delayed by months while sitting in a queue at the extrude hone facility. I realize it's only hearsay, but I heard similar anecdotes from multiple reputable shops from areas all of the country (Holbrook, Majestic Turbo, and European Motorworks). Granted, one could argue these shops might have a hidden agenda, but based on my dealings with them, I trust all three sources because I've generally found them to be frank and generally pretty objective.

In any event, I concluded that extrude honing was just not worth the expense or risk in terms of delay. The cost to do my entire intake tract, including turbo compressor housing, intake manifold, head, and exhaust manifold was in the $2K range, nearly doubling the cost of my top end development. Even if it yielded 10-15% gain, it just didn't seem worth the expense when similar results could be had with conventional porting. Interestingly, Autospeed ran an article a while ago on a similar process available in Australia ("Power Porting", August 2000), but as far as I've been able to determine, it is only available down under. The interesting part was the documented HP gains, but again, at what cost? And is there significant enough improvement over hand porting to justify the extra expense? IMO, probably not.

Re: the 931 intake manifold, the plenum chamber sits about two inches further to the LHS of the engine bay, closer to the brake booster, than on an NA car. So the answer is yes, the 931 intake has longer runners than the NA.
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erstwhile owner of just about every 924 variant ever made

[tuurbo]

Bass GT said:

[!tom]

It's called the Venturi effect.
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78 924 NA
5-lug

[tuurbo]

An Italian, it figures.

Very interesting, thanks!
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1980 924 turbo, MSD, Meth. Inj, otherwise stock.