Making CR compatible with cam

[PeterBurgess]

About four years ago we sold a Fast Road MGB head to a customer and dynoed the standard engine with K&Ns and nice exhaust at 86 bhp at the wheels, standard cam and around 9.4:1 CR. Customer rebuilt engine +20 thou" and fitted a Piper HR285 cam, he did not increase the CR to match and we reckon he was about 9.5:1, we tell folk to aim for 10.5:1 to match the 285 cam. The engine was flat on the rollers and maxed out with a dismal 90 at the wheels.with less power than the standard based engine below 3500 rpm. We dynoed on a Thursday and the next day at 8 am the owner was on our doorstep so we could skim the living daylights off the head! We raised the CR to 10.3:1 as we couldn't skim any more off the head! Today we ran the higher CR engine, excellent results, engine sounds very sharp and maxed out at 96 bhp! We haven't gone down the route of dynamic CR, relying on 35 years of pushing as far as we dare within limitations of fuel and materials and having a good knowledge of what we can get away with for each spec of cam. Blue line std based engine, dark red line 285 cam with low CR, lighter red line Cr up to 10.3:1

[stevieturbo]

Is it so repeatable that a few years apart 10hp is an accurate figure ? ( although still yes a good percentage increase )

If he needs more cylinder pressure and power...add a supercharger lol.

[227bhp]

PeterBurgess said:

We haven't gone down the route of dynamic CR

You have, that's exactly how it works. It doesn't take 35 yrs of anything, it's simple physics and can be calculated.

[PeterBurgess]

Hi Stevie, after running the inertia rollers for nearly nine years we are finding the repeat testing very consistent year on year unless something has changed. Coastdown losses were withing 0.3 bhp for the B as well.Happy the differences are representative of the three differing states of tune. Like the testing on 45 vs 3 angle seats I wouldn't have chosen to test low to higher Cr as it is so time consuming. The customer did the head off and on again so not much time unpaid for me apart from third dyno session of an hour and a half or so, very worthwhile to confirm the effects. Nothing new though to be honest as I buggered up a head skim on a B about 25 years ago, engine was flat, took head off, skimmed tons off, refit, head, engine flies, apologies to customer who had a long day at our unit, lesson learned

227bhp, happy to be enlightened please? I just have numbers in my head for CRs relating to cams we use and build specs. Worst one was a TR4 rally head for use with Outer mongolian fuel during the mongol Rally. So low it could have been a WW2 TVO engine