14 replies to this topic

[lapider]

Hi all.

So my 998 87 Mayfair gave a compression reading of 151 psi give or take 3psi for variation. That makes 10.3 bar according to my compression tester.

My question is. With a 12g295 head. Should I be looking to modify my cylinder head at all to achieve a greater compression.

I believe I have the flat top Pistons.

Also I take it this means I have a 10.3:1 comp ratio. Or am I wrong.

Bearing in mind I am trying to achieve a nippy road, driver friendly 998. Which embodies nice power charicteristics. Not a bhp topper.

[lapider]

Hi all.

So my 998 87 Mayfair gave a compression reading of 151 psi give or take 3psi for variation. That makes 10.3 bar according to my compression tester.

My question is. With a 12g295 head. Should I be looking to modify my cylinder head at all to achieve a greater compression.

I believe I have the flat top Pistons.

Also I take it this means I have a 10.3:1 comp ratio. Or am I wrong.

Bearing in mind I am trying to achieve a nippy road, driver friendly 998. Which embodies nice power charicteristics. Not a bhp topper.

[Cooperman]

You have to measure and calculate, then finally machine to get the desired result.

 

How to calculate compression ratios is on here if you search the data base.

[Spider]

You'd be wise to measure everything before modifiying the head or anything else, but usually the 12G295 will give a lower static CR as the chambers are bigger than it's lesser brothers, BUT these are all pretty old now and most have been through lots of hands, so CC the head and the block.

 

Compression pressures measured from a Compression Test bear little resemblance to your actual CR, though they do show to some degree the Dynamic CR of the engine, this is a different number to the Static CR and is much more meaningful, but again, to accurately and properly arrive at that number, some measurements need to be made and then a calculation - probably not the thing to be doing on New Years Day!

[Dusky]

I follow Moke Spider on this one, there are al lot of variables when doing a compression test, static compression is much more important, and much more accurate.

[germini30]

My 998 with 12G295 head has 180-185 psi across the four cylinders. It goes lovely.

[timmy850]

The compression test you've done is more a check of engine health than the compression ratio. The compression ratio is the relationship between the uncompressed cylinder volume and the compressed volume. The bigger the head and piston dish volume you have, the lower your CR will be. 

 

A stock 12G295 head on a stock 998 will give you a C/R under 8.0:1

 

To get a C/R up in the 10.0:1 region you'd need a decent skim on the head, and use flat top pistons too. 

[coopdog]

The car is coming out at last?!

I always use an app on the App Store called "mini head" very good for working out the CR

Like it's been said you need to work out what cc the head is ATM, chuck me a message if your stuck!

[limby2000]

http://www.gtsparkpl...RatioCalc.html#
Just something I come across. My 12g295 had to have 80 thou off but that was with dished 1098 pistons and 40 thou over rebore. I seem to remember being told 88 thou if everything else is standard with flat top 998 piston,s. You have to do your measurement,s as they all come up different, plus mine was left with just 2.2 mm of metal above the oil gallery (lot,s really).

[Dusky]

88 thou is a lot. Mine needed a 40 thou skim, standard 998 bore, flat top pistons.
If i recall right you need atleast 80 thou left, so 2.2 mm is not that much (i could be wrong tho)

Edited by Dusky, 01 January 2016 - 07:59 PM.

[limby2000]

What was your c/r on 40 thou Dusky?,was that with a standard deck height.

[Dusky]

What was your c/r on 40 thou Dusky?,was that with a standard deck height.

9.75:1 

THe block was skimmed to get the pistons flush with the deck . Using the copper performance gasket ( 2.8 cc compressed ) /

With the standard deck it might be as low as 9.3:1 , measuring is the key here. 

[limby2000]

Yeah, 88 thou now seems a bit excessive, perhaps that was with dished pistons.How much did the shop take off your block to bring pistons to top.

[Dusky]

Not a lot, the dished pistons are the biggest problem
It all depends between blocks tho, all down to the super accurate machining tolerances of BMC ;)

[Cooperman]

There is an oil hole which feeds the rockers and it goes from the rocker mounting surface towards the face of the head. When skimming you need to measure the depth of the hole and compare it to the depth of the head. After machining you really need a minimum of 0.040" between the very bottom of the hole and the head surface. I believe that the original amount of metal between hole base and head surface is 0.130" (+/- a bit for BMC's creative tolerances!) so about 0.090" can be machined from an 'as original' head. But there won't be many 'as original' 295 heads around these days.