19 replies to this topic

[087dave]

Hi all

I have a 12g295 head which has 28.5cc
I have done my cr calculation below
The pistons r flat top

Piston at TDC 1.90cc
998 gasket. 2.7cc
Ring land. .6cc
Cylinder head. 28.5cc
Total chamber vol 33.7

Swept vol 998/4 249.5

249.5+33.7=283.2 total vol

Total vol /com vol 283.2/33.7

Cr 8.4

So is this correct?

If I want to get the cr to 9.5 how do u work out
How much to skim

Thanks

[ACDodd]

If this is a STD bore engine you need 24.1 cc in the head. Simply level the head and run into the chamber 24.1cc of liquid. Measure down to the surface of the liquid. This is the amount to skim form the head to get the required CR.

 

AC

[Gremlin]

Always wondered how you measure down to the fluid, we did it by working out the cross sectional area and putting up with the fact that the 'walls' taper down slightly, worked out .061" was needed to take mine to 10:1

[087dave]

Tks AC

Yes it's standard bore/stroke

Would I have to worry about the oil feed hole when skimming?

[Gremlin]

Poke something down there and measure it, no good putting a vernier spike down as the hole has been drilled so it is 'tapered' at the bottom, as standard you'll have about .160" so your fairly safe unless your skimming a ridiculous amount off

[ACDodd]

You need to check its position. If in doubt don't skim as much. Better to run a lower CR than a deck which is too thin.

AC

[087dave]

Always wondered how you measure down to the fluid, we did it by working out the cross sectional area and putting up with the fact that the 'walls' taper down slightly, worked out .061" was needed to take mine to 10:1

.160" that's about 4mm right?

That's a lot of material

[Gremlin]

Yeah just over 4mm, I didn't skim .160", thats how much 'head' there was between bottom of the oil hole and head gasket face

[Vipernoir]

With good untouched 295s becoming scarcer, would you be interested in swapping for one that has already been skimmed ?

 

Overbored 1100s need 29cc or more...

[timmy850]

How did you measure your 1.90cc Piston at TDC? Did you have the piston at TDC and measure the volume to the top of the block? If you did it this way it would include the ring land volume I'm pretty sure...

 

If you bore it out to +060 using flat top pistons, skim the head back to 27.5cc, and deck the block a little you could easily get 9.5:1. This way you won't need to skim the head so much.

 

I've got a 12G295 with 28cc chambers, but I'll be using a 1098 crank and flat top pistons, which bumps up the CR as it has an extra 100cc of total volume...

[087dave]

With good untouched 295s becoming scarcer, would you be interested in swapping for one that has already been skimmed ?
 
Overbored 1100s need 29cc or more...

No thanks

[087dave]

 
I've got a 12G295 with 28cc chambers, but I'll be using a 1098 crank and flat top pistons, which bumps up the CR as it has an extra 100cc of total volume...[/quote]

Sounds interesting ....so if u have a 1100 crank no skimming?

[Gremlin]

[quote name="087dave" post="3091045" timestamp="1403986843"]

 
I've got a 12G295 with 28cc chambers, but I'll be using a 1098 crank and flat top pistons, which bumps up the CR as it has an extra 100cc of total volume...[/quote]

Sounds interesting ....so if u have a 1100 crank no skimming?[/quote]You'd have to calculate it

[087dave]

I need to take .060 thou

Timmy850 I'm interested in the 1100 crank idea could u let me know
More....

[Cooperman]

The 1100 engine has very different characteristics from the 998. Whilst the 1100 has more capacity, it needs a cam which works best between 3000 and 5000 rpm as to rev it beyond 6000 is asking for a snapped crankshaft. But the 1100 will give more torque at low to mid range.

You won't get more power from and 1100, but if geared higher with a mild cam it will give great mid-range torque.

I once had an 850 into which I fitted an MG1100 engine. It was great to drive around town and the torque was excellent.

But a 998 will give just as much power, maybe more, if the ability of the 998 to really rev is used.

Edited by Cooperman, 29 June 2014 - 01:05 PM.