Muff to Crankcase shims and gaskets

[Howard]

Hi Phil

Stop worrying about calculations to the nearest thou - we're talking 1950s not formula one. Start with a paper gasket under the muff, if the piston corner (top of the parallel side) is level with the top of the liner (as Bill says) or slightly below, stick the head on, and run it. You don't know what's going to suit you best, so start with the compression ratio the piston was designed for, you can always reduce it by adding compression plates if you're not happy. Lots of us run at 9:1 quite happily.

Good Luck.

H

 

[BigEd]

Well said. Get it running and give it a try.

Hi Phil

Stop worrying about calculations to the nearest thou - we're talking 1950s not formula one. Start with a paper gasket under the muff, if the piston corner (top of the parallel side) is level with the top of the liner (as Bill says) or slightly below, stick the head on, and run it. You don't know what's going to suit you best, so start with the compression ratio the piston was designed for, you can always reduce it by adding compression plates if you're not happy. Lots of us run at 9:1 quite happily.

Good Luck.

H

 

[Bill Thomas]

Hello Phil, Just a thought,If you think it is a 9-1 and you might have a mk2 cam in it, I remove the edge of the valve pocket on the piston, The valves used to rub on inside of the pocket, And check the oil feed hole is inline with the hole in the barrel, Good Luck Bill.

 

[vince998]

The volume of a standard Vincent cylinder head is 103.5 cc. The swept volume is 499 cc. Total is therefore 602.5 cc. If one had a flat topped piston the compression ratio would be 602.5/103.5 = 5.8:1. That is why pistons have domed heads. Each millimetre of swept volume is 499/90 = 5.54 cc so if the total depth of the compression plates and gaskets is 2 mm there is an increase in unswept volume of about 11 cc bringing the effective cylinder head volume up to 114.5 cc. The aforesaid flat topped piston would then give a compression ratio of 5.36:1.
In order to have a compression ratio of 9:1 one has to have an effective cylinder head volume of 62.5 cc. Increase this by the 11 cc above and one gets a compression ratio of 7.79:1, that is, near enough to 8:1. The combustion chamber shape is not ideal but lots of us have done it and been happy with the results. Also lots of us have also used 9:1 and been equally happy. Enjoy!

Anybody know how much volume a spark plug has between the electrode insulation and body (i know a fair bit of degreaser can be sprayed in their when cleaning the plugs!!) I was just wondering what effect the second plug on twin plug setups has on the CR?

 

[davidd]

Vince998,

The net combustion chamber volume is measured with the head off, a plexi glass flat pressed to the sealing surface with some light grease and a hole drilled in the edge of the plexi and positioned at a high spot. The plugs to be used are installed and fluid from a burrette is poured in the hole. This gives you a good idea of the net combustion chamber volume. Additionally, the piston dome volume has to be calculated. Some tuners have a selection of indexing washers for the plugs so they can be positioned for height and electrode position in the chamber.

Most of the discussion has been about the mechanical compression ratio, which can be much higher than the effective compression ratio, ie. when the engine is running.

David

 

[timetraveller]

When I did the original measurement years ago there was an extended nose plug in the plug hole. If the volume inside a plug was one cc, and that probably varies from plug type to plug type, then the compression ratio would drop from 9:1 to 8.86:1, not enough to worry about.

 

[Howard]

When I did the original measurement years ago there was an extended nose plug in the plug hole. If the volume inside a plug was one cc, and that probably varies from plug type to plug type, then the compression ratio would drop from 9:1 to 8.86:1, not enough to worry about.

I'll go with that, but just how much of a change should we worry about? How much of a Shadow's extra 10 horses came from raising compression by 0.4:1? I reduced my twin motor to 8:1 and put it back to 9:1, 2 years ago, with no noticeable change in power or vibration - not at up to 4500 rpm anyway.

Is this a different thread?

H

 

[BigEd]

Re: Is this a different thread.
It is the thread it has been posted in.;-) In this case, I think it is relevant to Phil' Baker's original question/post.

I'll go with that, but just how much of a change should we worry about? How much of a Shadow's extra 10 horses came from raising compression by 0.4:1? I reduced my twin motor to 8:1 and put it back to 9:1, 2 years ago, with no noticeable change in power or vibration - not at up to 4500 rpm anyway.

Is this a different thread?

H

 

[timetraveller]

Not just the compression ratio but also larger carbs and assembly with specially chosen parts, which were nearer to the design specs than some of the parts manufactured. I started riding Vins in 1957, only a couple of years after Vincents ceased production. Within a few years I was tuning my own bike and those of a few friends. This gave me the opportunity to inspect more parts than some other people and cams, as one example, were a mess of malformed rubbish which would mean searching through a box of them at dealers before I could find examples without flats on them. I suspect that some of the improvement was due to this selection of parts.

 

[Howard]

20% power increase from slightly increased compression and carb size? I suspect that a bit more than "some of the improvement was due to this selection of parts", assuming all Raps were 45 bhp and all Shadows 55 bhp. Pity they weren't supplied with a Goldie style certificate showing measured power, that would be interesting now.

H