Highly technical question

Post by **Peetee** » Tue Jul 14, 2009 10:03 pm

I had my 1275 engined car rolling roaded recently and the compression was a bit low at 155. If I got this back to acceptable 175+ what sort of power increace would this give me?

Post by **bmcecosse** » Tue Jul 14, 2009 11:03 pm

155 is indeed 'lowish' although not terrible. 175 would be a middling figure - 200 would be the figure for a really good condition 1275 with some modifications - including raised compression ratio. I suppose 155 > 175 would be worth ~ 5 bhp. But is it just the compression ratio that's lacking -or is the gas leaking away down worn rings/pistons/bores ? Raised compression also adds up to improved efficiency - ie better mpg!

Post by **Peetee** » Wed Jul 15, 2009 10:07 am

It's unlikely to be leaking away as all 4 are near identical and I don't have any oil loss issues. I knew the compression would be low as I had reprofiled the combustion chamber (along with other relieving work) when i replaced the valve guides.
What sort of a skim would get me up to a healthy compression then?

Post by **bmcecosse** » Wed Jul 15, 2009 9:58 pm

Which head is this ?? If 202 head - then 60 thou.

Post by **Peetee** » Thu Jul 16, 2009 7:38 am

1275 Roy, so 12G940.

Post by **bmcecosse** » Thu Jul 16, 2009 3:25 pm

Ahh - sorry - still 60 thou! But - ideally unless it already is a 'big valve' head - look for one of them and skim IT the 60 thou!

Post by **Alec** » Thu Jul 16, 2009 6:01 pm

Hello Peetee,

compression pressures at cranking speed are not necessarily an indication of compression ratio.
The cam is a factor also and performance cams tend to have lower compression pressure at cranking speed.

Alec

Post by **Longdog** » Thu Jul 16, 2009 7:47 pm

Hi mines doing 190 on every pot and puts out 88 with powermax pistons stage 3 cooper s head and swiftune sw5 cam.

Post by **bmcecosse** » Thu Jul 16, 2009 10:27 pm

Yep - that's more like it!

Although this idea of hairy cams giving lower compression figures is often voiced on the Mini forums - I have to say, in the past - my mates 1293 with 649 (ie full race) cam used to give ~ 220 compression. Any less than 210 and he would have us taking the head off to grind in the valves !

Post by **Peetee** » Thu Jul 16, 2009 11:35 pm

spec is 1275 A+
Hif44
alloy inlet
large bore 3-2-1 two box system
standard cam
12G940 'standard valve' head - throats matched to manifolds and valve guides removed, de-shrounded combustion chambers.
Metro electronic dizzy.
Reputable RR set up to 60bhp. Restriction suggested as low compression.
Compression even across all 4 and measured during RR set up.

large valve head - look for one of them and skim IT the 60 thou

I have one - but it needs a lot more work.

Post by **bmcecosse** » Thu Jul 16, 2009 11:53 pm

Removing the valve guides (I assume just the bits above the bosses!!) is not really a good idea - the valve has insufficient support - can wobble about - and not seat well. Better to remove most of the inlet bosses - and taper the guides, but leave full length. The big valve head will be well worth fixing up - and skimming that 60 thou - and an MG mutro cam will make a world of difference too if you can find one!

Post by **Peetee** » Fri Jul 17, 2009 12:56 am

Sorry typo. should have read removed valve boss - the guides are new!

Post by **bmcecosse** » Fri Jul 17, 2009 12:38 pm

Ahhh - that's ok then !!

Post by **Alec** » Fri Jul 17, 2009 10:04 pm

Hello BMCE,

the other factor, which I hadn't mentioned, (and probably doesn't apply to 1275 BMC engines) is the bore stroke ratio, undersquare engines that I'm used to tend to haver lower pressures also. I can't remember what a 1098 is on that point?

Alec

Post by **bmcecosse** » Fri Jul 17, 2009 10:54 pm

1098 is the longest stroke A series engine. Can't see why that should affect the compressions though. It's just air flow and cranking speed I suppose - against compression ratio and leakage!

Post by **morrisman1** » Sat Jul 18, 2009 7:24 am

i recently compression tested my hot a15 nissan engine, three came back at 175, the last came in at 180. I can assure you that it was no slug, you could achieve sustained loss of traction. 200 psi is pretty high, do standard bmc a-series engines really get that much?

Post by **Alec** » Sat Jul 18, 2009 7:42 am

Hello BMCE,

as I understand it is the longer the stroke the more leakage there will be all other things being equal.
Basically I was pointing out that it is not a simple relationship between cranking pressure and compression ratio, although in a sound engine raising the compression should be reflected in the compression pressure with no other alterations.

For what it's worth my 2.5P.I. Triumph, (10:1 compression and a warm cam) has pressure of 155 psi, long stroke engine of course.

Alec