Comp. Ratio, Someone Double Check My Calcs.?

[Rippthrough 98]

Just wondering if anybody would be so kind as to double check my CR calcs. before I take the head to be skimmed?

Had to take an estimate of compressed head gasket thickness as I can't find any figures for it, unless someone knows?

Anyway, my calcs point to needing a touch over 30 thou off to take it up to around 10.4 from 9.3?

 

Edit:- Forgot to mention I measured it as a 1mm skim would remove 6.5cc of volume.

Edited June 20, 2006 by Rippthrough

[petert 550]

Do you have a CR calculator? There's plenty available on the net that take all the variables into account. This one is my favourite. It's in inches, but that's ok for me.

 

http://cochise.uia.net/pkelley2/DynamicCR.html

 

The downloadable exe is down the bottom of the page.

Edited June 20, 2006 by petert

[Rippthrough 98]

Do you have a CR calculator? There's plenty available on the net that take all the variables into account. This one is my favourite. It's in inches, but that's ok for me.

 

http://cochise.uia.net/pkelley2/DynamicCR.html

 

The downloadable exe is down the bottom of the page.

 

 

Cheers, was hoping someone knew the rough thickness of the gasket though, I've just used 1mm in the calcs.

[petert 550]

The XU gaskets are 1.32mm if that helps any. I doubt a TU gasket would be any thinner.

[Rippthrough 98]

The XU gaskets are 1.32mm if that helps any. I doubt a TU gasket would be any thinner.

 

 

Ok, I'll take that as a figure and re-do the calcs.

Anyone got an old head gasket they could measure?

 

Edit: - comes out at just under 30 thou....

 

Think I'll just take .30 thou off and have done with it.

Edited June 20, 2006 by Rippthrough

[trogboy 17]

Are you sure that is enough? My back of a fag packet calculations require ~1.8mm skim to make that increase!

[Rippthrough 98]

Are you sure that is enough? My back of a fag packet calculations require ~1.8mm skim to make that increase!

 

That'd be about 10cc less chamber volume?!

 

edit: gah, wrong figure for displacement per mm in the head, should have been half that, so your probably nearer....

Edited June 20, 2006 by Rippthrough

[jonah 1]

Forgot to mention I measured it as a 1mm skim would remove 6.5cc of volume.

 

If that's correct then I also get 30 thou... (but I didn't need to know h/g thickness for that calculation )

 

But 6.5cc per mm skim can't be right unless this engine has a very large bore and very short stroke! How did you measure that?

 

 

edit: Doh, beaten to it!! 60 thou in that case then.

Edited June 20, 2006 by jonah

[trogboy 17]

Do you need the head gasket thickness?

 

I used the compression ratio and the displacement of the bore to calculate the compressed volume for each of the different ratios. Then used the difference in the compressed volumes and the surface area of the bores to calculate the depth of skim required. This makes the potentially crappy assumption that the combustion chamber is a clylinder the same diameter as the bores though so may be an underestimation.

[Rippthrough 98]

Do you need the head gasket thickness?

 

I used the compression ratio and the displacement of the bore to calculate the compressed volume for each of the different ratios. Then used the difference in the compressed volumes and the surface area of the bores to calculate the depth of skim required. This makes the potentially crappy assumption that the combustion chamber is a clylinder the same diameter as the bores though so may be an underestimation.

 

 

It's not near a cylinder

3.75 cc for the first 1mm removed, less after that.

[Rippthrough 98]

Thanks for the help guys, otherwise I would have accidently buggered that up

 

Going to deck the block and take the remainder off the head as there's quite a bit to be removed.

Edited June 20, 2006 by Rippthrough

[Rippthrough 98]

Right, who's the best to get a vernier pulley off? As the cam timing's going to be retarded a few degree's I expect from removing so much material, how much I have no idea, can it be worked out somehow?

 

1mm of material has been removed. Decided to go more conservative so we can still run 95 fuel if we have to.

Edited June 23, 2006 by Rippthrough

[christopher 5 1 Cars]

Right, who's the best to get a vernier pulley off? As the cam timing's going to be retarded a few degree's I expect from removing so much material, how much I have no idea, can it be worked out somehow?

 

1mm of material has been removed. Decided to go more conservative so we can still run 95 fuel if we have to.

 

I think they are much of a muchness (Piper, catcams kent)

as far as I can tell..All bloody expensive...Mine is from Kent which is just fine and uses allen keys.

 

The TU3 alloy block uses a 1.5 mm gasket (at least that i what I measure).

[Rippthrough 98]

I think they are much of a muchness (Piper, catcams kent)

as far as I can tell..All bloody expensive...Mine is from Kent which is just fine and uses allen keys.

 

The TU3 alloy block uses a 1.5 mm gasket (at least that i what I measure).

 

 

Probably compresses down to about 1.3 though? It's near enough, this is by no means a high budget engine build.

 

In fact, the total cost so far is £67.

[trogboy 17]

I think I read that the catcam verniers are the easiest to adjust accurately - something to do with 2 discs and hole alignment meaning that can be set 1 degree either way dead easily. More expensive than a kent or piper one so not really inline with the bidget engine build.

 

I got mine by posting a wanted add on the forum and seeing what I got offered. The 8 valve vernier pulley is the same for the TU and XU engines so there are usually loads about.

[Rippthrough 98]

I think I read that the catcam verniers are the easiest to adjust accurately - something to do with 2 discs and hole alignment meaning that can be set 1 degree either way dead easily. More expensive than a kent or piper one so not really inline with the bidget engine build.

 

I got mine by posting a wanted add on the forum and seeing what I got offered. The 8 valve vernier pulley is the same for the TU and XU engines so there are usually loads about.

 

 

Fair enough, ebay and for sale section trawling it is then.

[Rippthrough 98]

Fair enough, ebay and for sale section trawling it is then.

 

 

How far does one tooth on the standard pulley advance the timing? I think it's much too far, but just want to see what people think, as the pulley will be retarded by 2mm to take up the extra slack in the belt, would advancing by one tooth still be much too far for good running (think centre to centre on teeth is 6mm-ish).

 

See, told you it was a cheap build

Edited June 27, 2006 by Rippthrough

[brianthemagical 1]

how would head skimming change the valve timing, the pullys will still be in the same place and what not. i'm confused and slightly embarrassed i can't work it out. thanks for the clarification.

[trogboy 17]

The distance between the cam and crank pulleys is reduced by skimming the head so to keep the same tension in the timing belt you need to rotate one pulley relative to the other. In this instance you end up retarding the camshaft relative the to crank.

 

If say the diameter of the cam pully is approximately 120mm (seems about right) the circumference would be 377mm (pi x diameter) so skiming 1mm off the head would retard the valve timing by ~1degree (377/360)

 

I don't think the standard set up is this accurate anyway as you are never 100% spot on with the timing marks once the belt is tensioned up and you can only alter it a tooth one way or the other.

[Rippthrough 98]

Only option is perhaps to try it and see then?

[brianthemagical 1]

i do understand as your only tensioning one side of the belt, thanks for the help.

 

Only option is perhaps to try it and see then?

 

whats the worst that could happen?

[Rippthrough 98]

i do understand as your only tensioning one side of the belt, thanks for the help.

whats the worst that could happen?

 

 

Well as long as we check there's enough valve clearance, the worst that could happen is it runs like a bag of spanners.

 

-phillip.