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NA Cylinder Head Performance

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Joined: 08 Jun 2010
Posts: 689
Location: Maysville, Colorado

PostPosted: Fri Dec 22, 2017 2:03 am    Post subject: NA Cylinder Head Performance Reply with quote

I put my previously ported head on my street car which promptly go buried for the winter before it got running due to ignition problems ands I'm having shoulder surgery soon so will be out of commission for months.

I had planned on really modifying a NA head for the track car and got started but have had to call it off as I have determined that it would take many months, but I'll report what I found.

First, I have refined my testing method and getting better more consistent readings. I started with a stock 38mm valve head as I assumed I would scrap it.

Stage 1: We put a good, modern high performance valve seat on one cylinder with 3 angles including a slight inlet taper and did a 30 degree back cut to the valve itself. The flow went up 5-10% across the board from .100 lift to .500. The most improvement was up to .300, then it started tapering back. According to the literature this is what would be expected.

Stage 2: Seeing that the valves and valve seats are recessed into head about .015" I cut some shallow eyebrows into the head on the sides of the intake valve and leaving the close bore wall stock due to potential shrouding. It looked like an obvious improvement, but measured no real improvement. We were quite surprised by this. It might be there with much further work, but I'm also cutting a combustion chamber into the head so losing compression at the same time so even a small gain would probably have a negative net result. The 40mm intake which appears to use the same seat is an even worse case as the 45 degree taper is cut straight in, but I'm not going to sacrifice another head.

Stage 3: having communicated with David Mitchell in UK I took some modeling clay and filled the terrible bowl behind the valve guide. The testing showed flow about the same up to about .350 lift at which point the flow jumped 20%. Unfortunately I continued on and the flow dropped back and became very unstable with the pressures bouncing around + or - 2" of water. I tried to improve the profile and surface of my filling and tested again with the same result.

Conclusion: The potential is there, but a very long development time would be required.

With 10 days left and needing a cylinder head fairly soon after my recovery. The plan is to do some port smoothing with minimal enlargement and match the inlet port to the manifold. I don't use the gasket as it is over size making the gas slow down then speed up again, then do a good valve job. I also have a lot of plumbing and electrical work to deal with also and it really is more important to get the car running than drag the last bit of power out of it.
Poco a Poco, '80 future track car
'77 924
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Joined: 27 Jul 2007
Posts: 1014
Location: Sweden

PostPosted: Fri Dec 22, 2017 3:41 am    Post subject: Reply with quote

Nice to read about your findings

Been there, done that...

I have the 3 angles, port smoothing and plastic in the bowl and on the floor making a D-shaped duct. The transition from valve seat to combustionchamber is also smooth and not like stock. Like a 4 angle job. Stock valves, HD springs, solid lifters and a cam with 0.55" lift. 251 deg @ 0.05". Pulls strongly from 2000revs with stock CIS all the way to 7000!

If you can find Applebit results from porting the 924 they would tell the same story. Stock and ported flow ~135cfm at 0.03" and the stock stalls at 0.04" and doesnīt go higher than 150cfm opposite to ported that reach 180cfm at 0.06".

There is therefore no point of porting unless a highlift cam is used. You just move the revrange without increasing volumeefficiency. And of course the other way around, no point of using a hot cam without adressing the stall at 0.04"

An extended valveseatjob, smooth the short side radius and donīt enlarge to inlet gasketsize! Quick'n'dirty but good enough!!!
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT clone LSD Coilovers
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black with low ET "Phonies"
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Joined: 05 Oct 2008
Posts: 79
Location: Guatemala

PostPosted: Sat Jan 13, 2018 8:19 am    Post subject: Reply with quote

WOW that is interesting!!
Porsche 924 1977 XK Euro
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