High compresion.

[Adam_Winstone]

Those head volumes for a small cc kit seem very large!? Are you sure? How are you measuring them?

Don't worry about corrected compression for now, I'm trying to get my head around the geometric first.

Adam

Edit: OK, guessing that 64.5 bore gives something like 185 (guessing!) and the figure of 29cc being more 'in the ballpark' (32 and 34 were getting very large for a small block!) then this would give something like 7.4:1 geo... certainly not high compression for a standardish barrel on today's fuels. I'm way beyond looking into corrected at this time of night but a high exhaust port would cause this fairly low geometric to become a very low corrected. However, don't start to think that the corrected numbers should be anything like the same range as geometric... a well respected UK tuner and successful racer once quoted me corrected target figures of 6.25:1 for road/tour and 7.5:1 for road/race. Just as your geometric looks low'ish so does the corrected figure that you've calculated.

Adam

[Adam_Winstone]

Hi Mike,

The 29 figure is closer to the mark, which you previously measured at 34 (don't forget that you need to measure just the combustion chamber volume... although it is important to allow for the volume of the squish band at a height of 1.5mm (e.g.)). Giving something like 7.4:1 geo.

If your method was showing 5cc difference between your 2 heads (34 became 29 measured on motor), let's guess that the other was 31 - 5 = 26cc, giving a geo of 8.1:1 (please note that I'm using the 185 guess at 64.5 bore as I can't be arsed to calculate it... soz). This still would not be crazy high on a standard barrel, I'd be pretty happy with that, providing the ignition timing was set to no more than 19 degrees, although I'd test it and consider running as low as 17 even with standardish porting (testing is the key, rather than trying to run at quoted settings).

Corrected ratios vary MASSIVELY as the exhaust port height makes a huge difference to trapped mix, especially if running a box pipe that doesn't stuff mix back up the pipe. As such a head that works perfectly on one 64.5mm bore barrel may well be extremely high (or low) on another.

I've got the calcs that I did for Rusty's iron/alloy 185 when it was taken to 200cc. I'll see if I can dig these out.

Adam

[Adam_Winstone]

Rusty's 185 to 66mm (200) iron alloy Stg IV had the combustion chamber volume increased to give a final head volume of 25cc (chamber + squish band), which resulted in a 9:1 geo and 6.27 corrected.

This bike ran perfectly to Ireland (Euro and back) and a number of other local'ish runs... until the thin chrome rings wore the soft liner again, which was the cause of previous rebores at 175 and the thinking behind going to 66mm bore (the % width of exhaust port gets smaller as the bore increases). Soft iron liner + hard chrome rings = bad combination!

What the figures above show is that 25mm head volume is not too high (less than it would have been if fitted as supplied for RB20 use... noting that the RB's corrected would have been lower because of the higher exhaust port) so both of your heads would give considerably lower comp ratios (geo and corr'), and that accuracy of measurement is VERY important. Your bench measurement of 34 and assembled measurement of 29 shows just how easy it is to introduce errors that throw the results out. Even if your smaller 29cc measurement is the correct one, this is not high, especially if you think that Rusty's 25cc head was not hard to kick and ran well/cool/reliably (... other than the soft liner!).

Adam

[TS1-200]

Interesting, Adam well, we'll see how it goes as is, and possibly look to increase compression ratio if Nec.
by fitting the other head or maching down the one that's on there,

[Adam_Winstone]

Depending on what the port spec is like, the low compression head (if indeed it is low) should allow you to run slightly more ignition advance, which in itself should reclaim a fair bit of torque, without overheating. Like all other aspects, it is all about testing and balancing one element against others... not that I need to tell you Mr. R

Cheers for now.

Adam

[Spanish Fly]

Stiff kickstart? "It was okay on the other case"



SF

[TS1-200]

No,kicked over fine with plug out as normal. It will be back in the scoot in a few days will keep updated.