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Cylinder Head gasket (slimmer)
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dreamgts  



Joined: 29 Dec 2009
Posts: 504
Location: malta
PostPosted: Tue Apr 10, 2012 7:25 pm    Post subject: Reply with quote
In my first post I asked or wondered if a slimmer head gasket could be fitted to aquire a bit more CR from the engine, this method is easy to do on other heads without having to go to extravagant expenses and work, especially at this stage anyway.

So it seems that it cannot be done or else there is doubt about interference, I know there are better ways to increase the CR and I do not blame you ideola for not throwing out half assessed assertions.

Yet at this time I cannot go the full way since this will throw back the finish date by some time again, Iv already used up about 2 years which includes the time i was in hospital and resting at home due to illness i had, just to get the body work and paint prep work ready and some minor mechanical work ready too. So if I go all the way it will just have to wait for now. I do have a spare engine anyway. I will though enquire with the engineers about certain ideas I have and see what they say. Thanks anyway.


dreamgts
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dreamgts  



Joined: 29 Dec 2009
Posts: 504
Location: malta
PostPosted: Tue Apr 10, 2012 9:10 pm    Post subject: Reply with quote
Iv just been comunicating with David Newman of Newmans Cams UK. He states that there will be no type of interference when using the .027'' cylinder gasket, with standard or bigger valves and his POW 280/480 CAM which is fast road cam having a power band width bewteen 1500and 6000 revs, duration on inl. and exh =280, valve lift and cam lift of 0.480, ATDC =110, BTDC = 110.
http://www.newman-cams.com/pdf/porsche.pdf
I still will check with my engineer though . It seems theres always a solution with research to save a lot of work , time and money. Savings on all 3 is definetly a plus. He also added that with my setup an increase of 30 to 35bhp will be achieved if using 40 DCOE webbers, using 45 DCOE with the slight increase in CR might bring the figure around to 40BHP gain that plus the weight savings on the car are what im looking for presently. For some reason he suggested I lighten/balance the flywheel but to be modest on this modification.

I once remember having this problem of interference on a 2lit Ford OHC, I think if I remember correctly that we machined either the valve seats or the valve spring seats to correct this. Not sure which though at the moment.

Thanks
dreamgts
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ideola  



Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
PostPosted: Tue Apr 10, 2012 9:33 pm    Post subject: Reply with quote
I'm sorry, but there is NO WAY to maintain non-interference when going with bigger valves unless you modify the stock piston. ESPECIALLY if you reduce the distance of the valve to piston crown by reducing the gasket by .024". I will respectfully disagree with Mr. Newman on this point and refer you to the pix posted above for further scrutiny.
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ideola  



Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
PostPosted: Tue Apr 10, 2012 10:32 pm    Post subject: Reply with quote
From this article (using the code tag to preserve the table spacing...):

Popular Hot-Rodding wrote:
CR Power Gains
The theoretical thermal efficiency (E) at any given CR can be predicted by the following formula:


Where E = the Otto cycle (thermal) efficiency, R the compression ratio and k the coefficient of adiabatic expansion for air, which is nominally 1.4. Rather than flogging through a lot of calculations, the quick reference chart (Figure 2) has been made up. To use this chart locate your original CR horizontally. Next, locate the new CR down the first column. Where the two intersect is the gains that can be expected. For instance if the CR is raised from 9:1 to 12:1 we find the two values intersect at the box with 7.7 in it. This is the percentage increase that can theoretically be had by raising the compression from a lower to a higher level.

Code:
     ORIGINAL CR
NEW    8:1   9:1   10:1   11:1   12:1   13:1   14:1   15:1
9:1    3.5                     
10:1   6.5    2.9                  
11:1   9.2    5.5    2.5               
12:1   11.5   7.7    4.7   2.1            
13:1   13.6   9.7    6.6   4.0   1.9         
14:1   15.4   11.5   8.3   5.7   3.5   1.6      
15:1   17.0   13.0   9.8   7.1   4.9   3.0   1.4   
16:1   18.6   14.5   11.3  8.6   6.4   4.4   2.8   1.4

Now anytime we use the word "theoretically," it usually implies that there is not much chance of achieving as much in practice. Here the good news is that because intake valves do not close at BDC, the gains computed in Figure 2 tend to be a little on the low side.


From this table, you can see that a full point of CR increase will yield about a 3% gain on our motors. 125 BHP (Euro-spec) x 3% = 3.75BHP. Pretty measly, eh?! You'll have to decide if half a point of compression is worthwhile to take a chance on losing the non-interference aspect of your engine.

For me, because the timing belt is so easy to do on our cars, I feel that the issue of interference is way over-rated. Most modern cars are interference design. To achieve the biggest possible HP gain, I'm going more radical, and going to 11:1 on my NA project (120 BHP [US-spec] x 5.5% = 6.6BHP). Of course, it also involves additional stroke, Stage 1 cam, Cometic gasket, big valve head, free-flowing header, etc. so I expect the gains to be multiplied. I'm also encouraged by the fact that the stock motors went from 120 BHP at 9:1 in US-spec to 125 BHP at 9.3:1 in ROW-spec. That's a 5 BHP gain, or ~4% increase from .3 increase in CR...
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dreamgts  



Joined: 29 Dec 2009
Posts: 504
Location: malta
PostPosted: Tue Apr 10, 2012 10:37 pm    Post subject: Reply with quote
If there is interference then there are 2 solutions, Either I keep the standard gasket and go for the bigger valves or else go for the thinner head gasket and keep the standard valves which according to Newmans wont hit the piston., It seems that the latter is more a solution for me at this stage.In any case the engineer has to do some measurments.

Thanks
dreamgts.
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dreamgts  



Joined: 29 Dec 2009
Posts: 504
Location: malta
PostPosted: Tue Apr 10, 2012 10:47 pm    Post subject: Reply with quote
Ideola Iv seen that table just yesterday and as usual you have been very helpful. On this particular engine I need to go the easy way since I at least want to see the car in some way on the road, I have my reasons for this urgency yet it is my wish to do so at the moment. At least if something happens and the car is ready and driveable it wont be such a burden on my family in either way. I though have the option of doing up a spare engine more properly as you describe yet that will have to wait a bit for now. Adding 3 % of power is as you say measly at the moment. So I will consider everyones advise and make the best decision I can out of it. As usual you all have been a great help and I appreciate it.

Thanks a lot

dreamgts
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2 PORSCHE 924 (CURRENT PROJECTS)
2 CLASSIC CORTINAS (FINISHED)
1 CLASSIC FIAT(FINISHED)
1 CLASSIC FIAT 132 2000CC(JUST PURCHASED)
4 SUV'S
2 SEDANS
1 OPEL PANEL VAN
WIFES SUZUKI ALTO
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