Page 1 of 1

What Squish to use

Posted: Tue Oct 08, 2019 9:34 pm
by geezagillard
Hi All......know the KR1's respond to a bit more compression so whilst I had my motor apart for a rebuild thought about taking a little bit off the head. I am aware of the safe limits for squish (.7+) but was more concerned about keeping the bikes ability to put any commercially avaiable fuel in. What are the same limits for pump fuel? (91 plus)
If I reduce the squish to 1mm or below where do I stop before detonations starts with our worst octane 91?

Re: What Squish to use

Posted: Tue Oct 08, 2019 10:45 pm
by KR-1R
difficult question to answer, when there many factors that will cause detonation; CR, number of cylinders (per displacement) gear load, engine wear, carbon build up, ignition timinig jetting and environment that play parts.

because what is safe at 12°C (winter) wont be safe when that engine is in an environment of 22°C (summer).
I have a 2.3 IL4, it was designed to run on 95. It did used to run ok on 91 (younger) but now it has over 270,000km it will pink when pushed to accelerate in 5th gear. In summer it will do it in lowest gears when presented with a steep hill but lower loading at higher ambient temperature. Using 95 it has no problems and is notobly stronger in the 100kph top gear roll ons. For THAT car it is a difficult decision at the pump each time given how NZ pricing varies so much by region and retailer discounts you might have.

I dont think anyone racing would contemplate using 91 for a cost reason when 95 and 98 are available (10-15% more$) given you will face prolonged higher revs and temperatures.

a smaller squish and higher compression negatively affect ability to over rev which something you want when you need that last eak of speed to pass someone on the track

I think the scans you had of the SP manual has compression at 8.0:1 - which wouldnt be for use with 91!

Re: What Squish to use

Posted: Thu Oct 10, 2019 5:52 pm
by JanBros
it's not the squish that matters , it's the compression ratio.
in fact : the tighter the squish, the higher the compression ratio can be before you run into deto-problems .

minimal squish height is 1% of stroke, but for road use you'd better aim for the 0.7like you said.

but 91 in a high performance 2-stroke ? for me that's a no-go (not even available over here, only 95 and 98). I only run on 98 and very good and expensive the end , it's cheaper than engine-rebuilts. learned that over the years :roll:

Re: What Squish to use

Posted: Fri Oct 11, 2019 7:46 pm
by geezagillard
Thanks for the reply’s .....I guess I was hoping someone had done the cc calculations for 91 or 95 octane and had translated that to the KR1 heads already. Might just shave it down to 1mm as a rough guess and assume 95+ will be safe enough.

Re: What Squish to use

Posted: Fri Oct 11, 2019 9:03 pm
by KR-1R

Re: What Squish to use

Posted: Fri Oct 11, 2019 9:59 pm
by Binetta Steve
Wow this is what this forum is all about. A technical question followed up by sone serious practical answers. On Facebook this would have elicited responses from folk who suggest putting a bit of coat hanger in the cylinder and measure it in inches.

What an oasis of common sense in a sea of amateurs. Long nay the forum continue