E: Engine Squish

[Robert Watson]

What grade of gasoline did you use with the 9:1 bike?

Hi Bruce. The Rapide (Woolly) has 9.4:1 Arias forged pistons and HD ignition. I generally run it on the local chevron 94 as it has no ethanol BUT have toured extensively on it and been in places where there is nothing but 87 regular with 10% ethanol, I have to tell you in honesty it doesn't run any different and doesn't make more funny noises that usual!

 

[royrobertson]

Hi Roger. My Racer has 11.3 compression and twin plugs. It is on 40mm Amal smoothbores modified by myself and I run it on a 50/50 Esso best and Avgas. It may run happily on Esso best (Zero over here in most of the UK) but I have never tried as I have aways been able to get Avgas. In NZ I Ran on 100% Avgas as I was told their petrol was low octane.
roy the racer

 

[Monkeypants]

What grade of gasoline did you use with the 9:1 bike?

94 Octane ethanol free most of the time but also some 91 Octane ethanol free. The bike has series D points/coil ignition, so no helpful second advance curve as with Robert's HD ignition. It worked just fine though. Never tried 87 in that one.

Glen

 

[Nigel Spaxman]

My bike has a very similar ignition system to Roberts bike. I have the second advance curve controlled by the vacuum switch, but it really doesn't seem to be necessary at all. Sometimes I have run it on the slow advance curve which gives 34 degrees full advance, and sometimes I have tried running it at 39 degrees, it really doesn't seem to make any difference to the engine at all, even the gas mileage stays the same. I don't think I have ever tried 87 Octane in it. Maybe with the low octane the second curve would be useful.

 

[vibrac]

Ignition point may not be noticeable on a road bike but I can assure you that when looking at the power and touque curves on a dyno there is a difference.

 

[Nigel Spaxman]

Vibrac, I know what you are saying about dyno results etc, but there are a lot of things you can find out on a road bike. People who keep on experimenting can find out a lot. For example running with one ignition setting back and forth to work all week on one tank and then doing the same thing on a second tank full, if you measure the gas mileage and it is identical then that is something that might be pretty hard to measure on the dyno. You also have lots of time, weeks really to test different jets and settings. Also many people have a Dyno hill and they can test acceleration results on the hill. On my Goldstar I had an ignition advance lever, it was pretty easy to find the optimum setting on a hill with that. Besides that there are some bikes where when you advance the ignition to much that it will ping, and if you retard it to much the engine won't rev out. Sometimes the dyno results and tuning don't seem to give a really good result on the road either. I have found that on my Vincent that the timing between 34 and 38 degrees doesn't make much difference to anything. Probably I should try running on lower octane gas to find out more. When you retard the ignition to much the engine starts to feel soft and it makes more noise. I think on an engine that is softly tuned (not 11:1 compression) maybe it doesn't make very much difference. Also sometimes you race a friend up a big hill and you can find out for sure whose bike is faster!

 

[roger v]

Hi TIm, I ran the racing Ariel 350cc and Model 50 Norton engines at 40 thou squish. Both had new flywheels, mainsheets and 35mm pins, the weak area of the Ariel being the cases. Both revved to 8500 rpm and did not display any contact when stripped. I would imagine 40 thou on a relatively low revving Vin would be fine.

Hi My road going Venoms use a Viper [ 350 ] head squished to forty thou and from memory about 6mm wide . Thinking about doing the same with a 90 mm bore piston but as the land is so narrow , I wonder if it is worth it . Could always put a bit of weld in the std head between the valves and machine / dress to give a comp of say 8: 1 . Any thoughts out there

 

[davidd]

Roger,

I could be wrong on this, but I don't think squish is a good street mod. Squish heads prefer two spark plugs and wildly retarded ignition to make power. Most Vincent owners prefer to run advanced ignition. If you are not comfortable running 16-20 degrees advance, save your money. You need all three mods to get a noticeable increase in power. Anyone of the three will not produce very much power.

Raising the compression closer to the maximum octane that you can easily purchase would be a better mod for the street. In the US it would probably be 10:1, but 10:1 will give you some decent power alone. You can also install compression plates if you wish to adjust it to a lower compression in the future.

David

 

[LoneStar]

Raising the compression closer to the maximum octane that you can easily purchase would be a better mod for the street. In the US it would probably be 10:1

Do you mean race gas that's easily purchased, or pump gas? The highest octane at most US pumps is 93 (R+M/2 method) - are you really able to run a 10:1 CR on a Vincent with that fuel?

 

[davidd]

Do you mean race gas that's easily purchased, or pump gas? The highest octane at most US pumps is 93 (R+M/2 method) - are you really able to run a 10:1 CR on a Vincent with that fuel?

Sorry, I should have been more specific, but most Japanese bikes run at 10:1, like the Hyabusa. It is the cam that yanks the compression ratio so low. There is no way that any modern bike can be running more compression than the pump gas will allow. If your Ducati has 10.5:1 pistons, then the cam timing has been designed to lower the compression to work well on the pump gas of the country in which it is sold.

I can use my racing cam on the street because it lowers the CR from 11.23:1 to 8.1:1. I could always run it on pump gas. The amount of compression ratio loss for each cam is a bit different. The Mk2 loses a minimum of 1 point. so if you have an 8:1 piston in your engine with a Mk2 you are likely running no more than 7:1 and it is probably closer to 6.75:1, depending on the cam timing.

If you want maximum performance on the street, you need to calculate the dynamic compression ratio of your cam and then add the difference between the static and dynamic compression ratio to your piston CR. This will get you closer to the 9.2 dynamic compression ratio that you can run on US premium gas.

Ultimately, I think it is much easier to run your CR closer to the maximum that you can get away with on pump gas than to add squish.

David