Compression Ratio help, high PSI?

[AndyB72]

I've recently bolted on another 295 head to my 1100 engine (+80 SW5). I cc'd the head and skimmed it to get it down to 24cc. I've just done a compression test and the gauge is showing between 220 and 230 PSI (15bar) across the 4 cylinders. This seem a bit high compared to previous engines I've built. Have I made a c0ckup with my calculations?

[Smiffy]

Been there do that, it's about 13.4:1 with flat tops on a 998 so even higher on your 1100.

[COOPERCO]

Hi Andy,

I get those kind of figures from my short stroke 1300, it never gives any bother & runs on standard V power with no additional octane boosters. It's done about 8,000 miles never had a head gasket change etc, infact it only came out early on to change a bulk ring! You may have to limit the distributor advance to stop det'. The 8 port injection car is 12:1 with an S dizzy on points, the advance is limited to 10 degrees (20 at the crank) that also runs on V power with no det/over heating etc. I'd give it a try & see what happens.

Mike.

[littler]

my s is about 10.5:1 and gives 190 normaly.

our mustang racer puts out 220PSI and we run that on super and its ok on the track.

should make a proper torquy motor

[Smiffy]

You can't compare a 8 port with a 1100 motor, or for that fact a 1300!!! It's way to high for a 1098 motor, especially with the MILD SW5 in there.

[graham in aus]

If the Pistons are flat top, I'd say you have high compression!

I've just done one (1100), but it had bucket dishes in the pistons, from memory, 060" over, the head was 23cc, but compression was around the 10.2:1 mark.

Not done dynamic compression tests yet, engine still on the stand waitiong for a lightened / balanced flywheel!

[mk1]

Looks a bit hight to me.

Comes out as 12.6 : 1 using the patented MK1 C/R Calculator "Its ugly but it works"

viewtopic.php?f=10&t=13

On the setup you are using a 5.5cc dish piston would give around 10.5 : 1.

[AndyB72]

Thanks guys, looks like I need to go back to school. I've got another 295 head casting somewhere with more meat, just need to have it reconditioned/unleaded.

[mk1]

Andy,

My calculation was a worst case scenario, it is not likely to actually be as high as that, but it is pretty likely to be around the 12 or so to one mark.

[Vegard]

Is that with a SW5? If so I think you've got a too high CR as already mentioned. Should be easy to calculate though.

[littler]

doubt the started will like it ...