Compression

[Anonymous]

What is the max. compression that can be run using UK unleaded fuel?
Mine is currently 12/1 and seens OK, but I currently have the engine apart, so I can easily raise this while the head is off.

 

[JJ]

Th UK must have a much better grade of pump gasoline than the USA does. The highest CR I'd run here is about 10.5:1, and that would require 92 octane minimum.

 

[Anonymous]

I think ours is 95, but please correct me if I'm wrong
Shaun
PS we pay 70p/litre about ?3.15/gallon = about $5.6/gallon so I would hope our fuel is good!

 

[Anonymous]

The new sportbikes have CR of 12:1 and run fine on pump gas.

 

[Anonymous]

smaller pistons can usually tolerate a higher compression ratio on the same grade of gas. why I DONT KNOW OR FORGOT.

 

[Anonymous]

I think ours is 95, but please correct me if I'm wrong
Shaun
PS we pay 70p/litre about ?3.15/gallon = about $5.6/gallon so I would hope our fuel is good!

I believe i wil stop complaining about the price of gasoline. 8O

 

[Anonymous]

PS we pay 70p/litre about ?3.15/gallon = about $5.6/gallon so I would hope our fuel is good!

I believe i wil stop complaining about the price of gasoline.

Which is why they value cars that have good fuel economy in Europe. Most of that price is taxes. We can't even get higher gasoline standards for cars in the U.S., which is technically quite easy to do.

As for the higher compression ratios - the GSXR1000 has 12:1. The pistons are not small. In reality, the aluminum in the cylinder head allows for a higher compression ratio than a cast iron head (which was used in early muscle cars).

 

[Anonymous]

Modern bike engines can run higher CR's on pump petrol for a number of reasons, cylinder head design being the most important; combustion chanber shape, the bore/stroke ratio, valve number and shape, head cooling, fuel delivery efficiency etc etc all promote efficient burning.

So Shaun it depends on what your bike is. I'd say 12:1 is pretty good. My GS1000 will run fine on 95 UL but I put super (97 octane) in anyway. I have Wiseco 10.25:1 pistons, but they are reputedly a bit lower than this.

I know that if a went to 12:1 on my bike I would get detonation on 95 fuel and possibly on 97 too, as the head design is not as good as later bikes.

 

[Anonymous]

Mine is a '82 GSX1100 with 1169 overbore. I did not have any problems with detination with the 12/1 ratio. But reading the comments above it would seem this is the highest I should go.
But could I SAFELY go higher if I use Super Unleaded 97 octane? - is there much to be gained by raising the compression any more?

 

[Anonymous]

Mine is a '82 GSX1100 with 1169 overbore. I did not have any problems with detination with the 12/1 ratio. But reading the comments above it would seem this is the highest I should go.
But could I SAFELY go higher if I use Super Unleaded 97 octane? - is there much to be gained by raising the compression any more?

I would say no and no.

You're going to get lots of probs going higher than 12:1, not just detonation, but valve/piston clearance etc.
In fact I'm surprised you're getting away with 12:1!
If you need more power go down a different route...cams? carbs? Turbo?
What power is the bike making now?

 

[SqDancerLynn1]

If you are running 12/1 you are all ready on the edge of detonation with the GS design on pump gas. In Calif we have to deal with 91 octane dishwater I think the way the octane is figured is different in Europe
reg 87 plus 89 prem 91 You can also get 100 octane unleaded at the racing shops ($5 gallon, maybe more) for street use and leaded racing gas for OFF ROAD USE ONLY