what's the highest compression ratio to run on 87 Octane??

[Chuck78]

what's the highest compression ratio to run on 87 Octane?? I'm looking at possibly going with stock 850 jugs and pistons for my GS 750, But also considering some high performance pistons that will give me 10.25 or 10.5:1 compression. Will either of those necessitate using 89 or possibly 91 octane with stock GS750 cams? I've been told by multiple people on this forum that the 750 cams actually have a longer duration than the 850 or 1000, there for bleeding of a little bit of that access compression as compared to a shorter duration mild cam. This is the only thing that I can see definitely working in my favor.

[bellucci]

I have 10.5 comp. I have run on 91 octane on the street. Summer will bring out the badness in lower octane. Same motor at the track takes 110. I was going to say something like unequal flame progression causes detonation, but I'm not that smart. Until the real smart guys chime in, my answer is 9.5.
Curt

[trippivot]

at 10.25-1 you'll be fine -- just retard the timing 2 or 3 degrees and adjust the idle speed up to compensate. use a colder heat range spark plug anyway.

I started to have issues at 11.5 to 1 and 34 degrees total advance on a 1085 2 valve engine -running B9ES- went right to leaded 95 octane

and sunnoco 110 after going 13+to 1

112 and 116 not necessary unless turbo or nitrous.

[OLDENUFF]

I have 10:25 in my otherwise stock 1168 and run reg. gas with no problems.

[Chuck78]

Well that's encouraging! The manufacturer of the 10.5:1 72mm (920cc GS750/850!) pistons that I am buying ( Discontinued NOS) Recommends using 91 octane for them, But told me that they can mill .040" off of the tops of the domes and enable me to run regular pump gas. They also suggested that if I didn't want to do that that I could install a thicker cylinder to crankcase base gasket, but did not recommend a thicker head gasket. I would think that either would cause a much taller than wanted quench distance

[RichDesmond]

Just out of curiosity, what's the objection to running premium? Performance costs money, and you're willing to spend the bucks on a big bore kit, so....

[Mekanix]

Got any pics?

Where is the quench on these?

Is there even a quench area on the top of the dome that milling would hurt?


Some rough calc's show that just increasing the bore to 72mm your up to 11.6:1 From an 8.7:1 stock CR I think, with a stroke of 56.4mm


So to reduce to 10.5:1 they must have made the dome shorter to reduce the compression.


All the pics I find show the stock piston as a Dome and the new larger ones are flater toped with a quench area around the sides but lots of space on top.


If you go the gasket route.
Lifting the jugs with a gasket is going to cause one more problem. What do you do about cam timing ?
Your timing probably just jumped xx degrees just from that 0.040 in height change since the front part of the chain is tighter and pulling the cam's further ahead while tensioner lets out some slack.
Point is, you would have to degree as well. But then again that might be something you want to play with anyways.

And for the quench area, Say for instance you had 0.043 (just head gasket) for the clearance. Add another 0.040 to that and you would have a 0.080 quench area. IS that even useful anymore ? I don't know I'm just asking. From what I've read and been told 0.040-0.060 is a good range.

Milling might be the better way to go to make it the easiest solution with the least amount of work.

[Chuck78]

You made me wonder, do the factory overbore pistons have less of a dome than std size stock pistons? I wonder... probably so, but that'd be an awesome oversight by the engineers if the cc's of the domes were the same between the 69mm std bore 850 & the 70mm +1 overbore replacements! That'd mean a bump up in compression & displacement!

[trippivot]

copper base gasket thickness choices no base,.005,.010,.020
most copper head gaskets are .046 or something close to it.

leave the pistons at 10.25 hgt. check basic deck height and add .040 for rod bearing sling out. add for the head gasket - don't want to smack the head with the piston

if you mill head surface remember the piston valve pockets will be in wrong place and need custom clearancing.

now after all that you can put grease on a piston and use fluid to check EXACTLY what your ccv is and calculate a finished/final comp ratio

choose the base gasket thickness last after the whole top end is matched mated measured.

gallon per gallon -- higher octane gas sits in the ground tanks longer and goes stale before you buy it. cheap gas is the freshest you will find- more people buy it and it is replentished more often than hi grade.

ever hear complaints about a bike running bad with expensive 91 in it?

[Chuck78]

I had the local Suzuki dealer look up 70mm 850 replacement pistons for me, part # 12111-45110-100, but they are no longer available. maybe a google search could turn up some NOS pistons. I'll just get these 920cc GS750/850 pistons & wait for a deal on a set of 1979 850 cylinder jugs. It will make a good winter project!

The 1980 and newer compatible 850 jugs will work with some grinding and cost about $30& up. The '79's bolt directly on but are much more expensive($80+) & less plentiful.,

[OLDENUFF]

Just out of curiosity, what's the objection to running premium? Performance costs money, and you're willing to spend the bucks on a big bore kit, so....

Higher octain gas burns slower to fight preignition. So, slower burn means less performance.

[850 Combat]

I have 10:25 in my otherwise stock 1168 and run reg. gas with no problems.

I think that the four valve head can tolerate more compression at any given compression.