22 replies to this topic

[Its a min]

Hi all.

 

Currently, I've an unmodified CAM4180 head on a 1989, 998 A+ (high compression) with flat top pistons. The only mods are a Minispares Stage 1 kit. 

 

My understanding is that this should have a compression ratio of approx 9.6:1 and the the chamber volume of the head should be around the 25.5cc. In my garage, I've what appears to be an unmodified 12G295 head with the measured average chamber volume of approx 28.3cc.

 

I've seen various posts on TMF on how to calculate how much needs skimming to achieve the desired compression ratio but my thinking is, and probably completely wrong, that a quick way to get this figure, if I wanted the same compression ratio, is to fill the chambers of the 12G295 with 25.5cc and therefore have a compression ration of approx 9.6:1.

 

This leads to some questions for you kind folk.

 

1. Is my thinking correct or are there other factors which would skew this theory?

2. Should I be looking for a different compression ratio to get optimum/enjoyable performance or is the standard 9.6;1 ok?

3. What performance improvements would i see using a 12G295 with a compression ration of 9.6:1 over the standard CAM4180?

4. I read somewhere that a compression ratio of approx 10.1:1 would be a good ratio to aim for. What improvements would this give over a ratio of 9.6:1?

5. What other mods to the head should I consider if I decide to have the 12G295 head skimmed/modified?

 

Appreciate any/all views.

 

thanks.

 

[Ethel]

That would be one way of getting an indication, it would be more useful to identify and measure the chamber with the largest volume as that would be the one to work to. Probably a better alternative would be to transfer the outline of the chamber on to graph paper to you can count the squares inside the outline to determine the area then calculate the depth required. It'll be easier to get the exact chamber volumes by grinding the chambers.

 

A bigger concern is not uncovering any holes by skimming. I forget, but I think there's an oil passage way that's worth checking for depth. There's something to be said for telling an experienced head modder to get you a certain CR than just telling somebody to skim X thou of a head.

 

9.6 sounds a reasonable target for today's petrol. How well your head flows and more so how wild your cam is will have an effect.

[DeadSquare]

9.6 is perfectly OK for road use, using standard grade fuel.

 

It is only for competition use that you need to venture above 10.

 

To get "enjoyable performance" from your 998, you could get your spare head modified to suit, and fit a supercharger.

[Fast Ivan]

 

I've seen various posts on TMF on how to calculate how much needs skimming to achieve the desired compression ratio but my thinking is, and probably completely wrong, that a quick way to get this figure, if I wanted the same compression ratio, is to fill the chambers of the 12G295 with 25.5cc

 

and then measure the distance from the face to the liquid with a depth mic, that will give the amount to skim

[DomCr250]

Many moons ago, we used to take off between 60 and 80 thou to restore the ratio on a standard 998 with dished pistons, not scientific but it worked.

 

To work out the available thickness measure the depth of the head and then measure down the rocker oil feed hole ..it will give you the thickness, you need 60 thou as a margin of error after skimming - they leave the factory with something like 100 to 150 thou as standard.

 

And to answer question 3 - it together with a good intake and a LCB it transforms a 998, makes a really nice little unit that will rev for ever and keep going for years and years without a rebuild.

 

I've had 3 engines with 12G295's on them, and loved them - more smiles per £ than anything else than pumping your 145's up to 50PSI on a wet day.

Edited by DomCr250, 24 April 2019 - 12:10 PM.

[hhhh]

I wouldn't put a 12G295 on without at least removing the protruding exhaust valve guide bosses. They really restrict the ports.

[ACDodd]

Don't go over the top with compression. 9.6:1 is plenty for modern fuel.

Ac

[Its a min]

Thanks everyone

 

 

 

A bigger concern is not uncovering any holes by skimming. I forget, but I think there's an oil passage way that's worth checking for depth. There's something to be said for telling an experienced head modder to get you a certain CR than just telling somebody to skim X thou of a head.

 

 

Which leads me to my next question...does anyone know of a good machine shop in, or around, the London area? I've found some but the reviews haven't been the best.

 

Cheers again.

[Ethel]

I wouldn't put a 12G295 on without at least removing the protruding exhaust valve guide bosses. They really restrict the ports.

 

Makes sense if you're going to the trouble of removing and stripping a head for skimming. Then you'll need to recut the valve seats, so  go triple angle...

[johnv]

Thanks everyone

 

 

 

A bigger concern is not uncovering any holes by skimming. I forget, but I think there's an oil passage way that's worth checking for depth. There's something to be said for telling an experienced head modder to get you a certain CR than just telling somebody to skim X thou of a head.

 

 

Which leads me to my next question...does anyone know of a good machine shop in, or around, the London area? I've found some but the reviews haven't been the best.

 

Cheers again.

 

http://cmesuk.com/ do my stuff .. no complaints at all!

[nicklouse]

I would just get the current head sorted  as the cost will be similar to commissioning the 295.  and you could end up with a better head.

[gazza82]

Thanks everyone

 

 

 

A bigger concern is not uncovering any holes by skimming. I forget, but I think there's an oil passage way that's worth checking for depth. There's something to be said for telling an experienced head modder to get you a certain CR than just telling somebody to skim X thou of a head.

 

 

Which leads me to my next question...does anyone know of a good machine shop in, or around, the London area? I've found some but the reviews haven't been the best.

 

Cheers again.

 

Depends what you want them to do .. simple rebores, crank regrinds, fitting pistons to rods I can vouch for CKM in High Wycombe

 

CMES I think are in Bracknell and build the engines for the HDRC Academy series (A+ based Marina inline engines) .. but they aren't cheap!!

[DomCr250]

 

Thanks everyone

 

 

 

A bigger concern is not uncovering any holes by skimming. I forget, but I think there's an oil passage way that's worth checking for depth. There's something to be said for telling an experienced head modder to get you a certain CR than just telling somebody to skim X thou of a head.

 

 

Which leads me to my next question...does anyone know of a good machine shop in, or around, the London area? I've found some but the reviews haven't been the best.

 

Cheers again.

 

http://cmesuk.com/ do my stuff .. no complaints at all!

 

These guys do all my machining too - they are brilliant, quick and reasonably priced, know their way about an A series too.

[Its a min]

Cheers everyone.

[albirhiza]

I recently did this myself (1987 998 park lane, high compression flat tops) targeting a 9.5 CR to go with SW5 cam. I will reinforce what the experts on here have previously recommended on carefully measuring your own build. I did a ton of calculations using min/max published values and came up with a range from 23.5-25.5 cc required in the head to achieve 9.5. Since the flat top pistons aren't decked to block surface, that was where i found the most variability in published vs my actual values. In the end, i needed the final combustion chamber volume in the 295 head to be just under 24.5cc to hit 9.5 CR. My machine shop used the cc measurement I gave them to determine the amount to skim. It was eyeopening for me to see how easily you can pick up 1-2 cc between all the measurements and in turn the effect of that on your desired CR.