Dong Fang DF 250 RTB Bobber Information

[axa]

9.0:1?

we have a winner.
your uncanny barnone.

So i must say, i didn't expect it but, indeed the CR we read for the loncin 167FMM is indeed as advertised.

So here is the rundown.
Measuring with a liquid displacement method. really there is no other feasible way:
https://drive.google.com/file/d/0B-M...it?usp=sharing

I didn't ignore the givens and actually measured the stroke at exactly 65mm and bore 67 indeed... The rest was:

Bowl 22cc = 22000ml³ (second take 22.2cc)
TDC to deck 2mm = 7051ml³
Gasket 1mm = 3526ml³
Piston dome 4cc (maybe 5cc)= -4000ml³
Stroke 65mm = 229167.404
229167+22000+7051+3526-4000:22000+7051+3526-4000
(229167.404+28577)/28577
= 9.02 CR

when i double checked a second measure of the bowl, i got the a hair over the graduated measurement line so if you call it 22.2 you get
= 8.96 CR

so it seems that the CR is indeed 9.0

BUT, It was very difficult to measure the piston with the molding clay i had on hand, it was much too hard and didnt conform to the dome and valve relief profile well, and didnt provide a deep enough void. So i estimate i may be a bit wrong on that measurement.
If the numbers are rerun for say a 5cc dome, it comes out to be quite different at 9.31, much like the advertised DF 9.2, so that is plausible as well.

Thus it will be worth while to get a softer clay and remold the piston for measurement, 0.3 is much too large a margin of error to work with. But right now i believe the number is 9.0.

So starting with 9.0, as increasing it by machining, here are the options:
Reducing the gap by 35mil yields a 25470ml³ volume for a 10.0 CR
20mil -> 9.5
50mil -> 10.5

Given confirmation on the temp, and piston dome, I will resist the urge to go straight to 10.5, 10.0 might be a more reasonable starting point...

I should note, reducing the cylinder head depth is not the only option, the cylinder itself can be reduced as there is a 2mm piston to deck height gap. Though popularhotrodding showed no power decrease for deck heights up to .125mil deep. so either or...

But despite what you may hear around these parts, going from 9.0 to 10.5 will only yield a theoretical 4.5% power increase:
http://www.popularhotrodding.com/tec...on_ratio_tech/

Unfortunately~60mph + 4.5% does not = ~80mph, not in our 3rd dimension anyway