Substitute 7.3:1 scratched pistons by 8.1/1 or 8.3/1 Pistons

[Moca]

Dear All,
I have recently cooked and scratched both pistons and cylinder liners,on a 998, i was running STD omegas and have been advised to change for a bigger ratio either 8.1 or 8.3, they told me these could cope better with new petrol and motorways, what's your experience, do you agree?

Thanks,
Francisco

 

[Albervin]

I have had 7.3:1 pistons in the Rapide for 15 years and have no problems maintaining Australian motorway speeds of 70 mph. I have a 48T rear sprocket and have the timing set at 34º. I always use petrol that is ethanol free and usually 98 octane. 95 octane is still fine though. Go up to 8:1 or higher and you will lose some torque and probably have to re-time the ignition. Not really worth the effort for a road bike.

 

[Moca]

Thank you for sharing your experience Albervin, and your opinion would you say 98 Octane is better for our engines than the 95 Octane petrol, i tdon´t think we have ethanol free petrol in europe, any longer.

 

[Jim Richardson]

I run all of my petrol vehicles on super where available and try to avoid supermarket petrol.

 

[Howard]

I'm running 9:1 on the road. I use the highest rated petrol at whichever filling station I stop, and occasionally include a shot of octaine raiser juice from Halfords. I wouldn't worry about using 8:1s from a petrol point of view.
9:1 s were fitted when I was younger, and I might (just might) go down to 8:1 if I need a rebore. It pulls well at anything over 1500 rpm (it will run smoothly at under 30 mph in top) and it gets a boost at about 4000 rpm. That being said, I'd probably recommend 8:1 as there are rev ranges where the engine isn't as smooth as it should be.

H

 

[vibrac]

I always use petrol that is ethanol free and usually 98 octane. 95 octane is still fine though. .

Does that imply in Australia they actually let you know if the petrol has Ethanol in it? Thats not the sort of information they trust UK citizens with, how many peanuts in a bar of soap?, or how many E numbers in a pair of knickers? they will ram that down our throats ad nauseum , but whats in our petrol? - some hope

 

[ClassicBiker]

Here in the "Peoples Republic of Michigan" fuel retailers are no longer required to display the percentage of ethanol contained in the fuel they sell, unless it is currently higher than 10%. This results in a couple things 1) Pure Gas.org http://pure-gas.org/index.jsp?stateprov=MI 2)The belief that all gas is at least 10% ethanol and therefore suspect. 3) The knowledge that E85 is 85% ethanol and your grocery is going up by the minute, while the corn lobby and the oil lobby make moeny hand over fist.
Steven

 

[greg brillus]

Yes, we have 4 different fuel grades available to us, and they do let you know which has ethanol in it. 91 e10, 91, 95 and 98 are the most common ones.....I prefer methanol and avgas...but my van likes running on LPG. Has anyone got one of those Diesel powered Vincent's....?

 

[b'knighted]

I've got one that sounds like it.

 

[Monkeypants]

I switched from 7.3s to club 8s. Not a breathtaking performance change but measureable. With the 7.3s in the bike would pull up dyno hill in top at 70 mph, when coming into the hill at 70 mph then laying into the throttle. Same test with 8s and the chrono speedo needle ticks up to 72 mph half wayup the hil and hold it there.
I don't see higher compression losing torque (which is same as losing horsepower) anywhere in the range. It just adds a little bit verywhere.
But on it's own with no other change CR has a fairly small effect, according to dyno charts I have seen. Often when CR is raised substantially people also change carburation, porting , exhaust and possibly ignition, then bigger increases in output are possible.
The downside to raising CR is more vibration and more shock on motor parts.
But 8 to one has worked well for me, almost 50,000 on the engine since the switch. I burn midgrade fuel in it, tho in a pinch regular works OK.

Glen