Compression and Power

[david.g.chapman]

It's been 1000 miles since an engine rebuild over the winter, and everything seems to be going well.

I was curious about one thing. The old compression readings on the engine were 170-180 psi (before the top rings started breaking!) .

I now have new OEM pistons with larger valve cut outs - supposedly to eliminate a clash between valves and pistons if the chain breaks (for heads that hav'nt been shaved too much). My ex-federal Stromberg head still has plently of meat left on it.

I now have compression of 160 psi across all cylinders - nice and even. Power seems about the same, but I was wondering what the theoretical difference such a drop in compression would make. Could you restore it with some sort of crafty long-reach spark plug?

Dave Chapman.

[ppnelan]

How about 'adjusting' the valve timing to increase the compression readings - see the other ongoing thread on a related issue: www.lotuselan.net/forums/viewtopic.php?t=16736

Matthew

[rgh0]

It sounds like your piston change has dropped the compression ratio by about 0.5 say from 10:1 to 9.5:1. This change is worth around about 3 to 4 hp loss according to my simulation data.

The compresson pressure is altered with valve timing by changing the point after bottom dead centre at which the inlet closes. Closing the inlet earlier increases measured pressure closing it later decreases pressure. The specified timing for a sprint cam is 66 degrees after BDC.

Making the closing point earlier will increase slow speed compression pressure as measured by the guage and increase torque at low engine speeds but reduces air actually breathed in at higher engine speeds reducing peak torque and power. The selected cam timing is a balance of a range of these factors.

cheers
Rohan

[david.g.chapman]

Thanks for the replies - an interesting point about the effects of valve timing. I have one sprocket mark up and one down at TDC - it looked to be obout half a tooth out in total when I set it up last.

I might do some experimentation next winter ...

Dave Chapman

[bill308]

Assuming 105 bhp at the the flywheel and an initial cr=10:1, thermodynamics would predict :

105*9.5/10=99.8 bhp @ 9.5:1 cr

Assuming a 15% driveline loss:

0.85*105=89.3 bhp @ 10.0:1 cr
0.85*99.8=84.8 bhp @ 9.5:1 cr

The predicted difference/loss:

89.3-84.8=4.5 bhp @ the rear wheels

This assumes all else is the same. Not too different from Rohan's estimate.

Bill

Bill