12 replies to this topic

[Pauly]

Hi all,

Think I've got my head around compression ratios, but my maths skills are shocking so I need someone to double check for me?

Theres 2 ways I may be having the block machined. 1 is standard bore & pistons (which have 8cc dish, 21962 piston) and another is re-bore +20 = 1293 with ae 21251 pistons which have a 8.4cc dish.

So calculations for option one, at the moment the comp ratio is 9.4:1 (standard HLE)

All calcs are done according to vizard.

To get 10.25:1 comp ratio.

10.25 - 1 = 9.25
1275/4 = 318.75
318.75/9.25 = 34.4 (total chamber volume required)

Add ringland (0.7cc) piston dish (8cc) piston to deck height (37 thou = 3.7cc) and gasket thickness (4cc)

0.7+8+3.7+4 = 16.4 (which is what the other things above add up to)

So...

34.4-16.4 = 18cc (and the size the head combustion chamber volume needs to be for 10.25:1)??

Option 2

10.25:1 comp ratio

10.25 - = 9.25
1293/4 = 323.25
323.25 - 9.25 = 34.9 (total vol)

0.7+8.4 (dish for 21251's)+3.7+4 = 16.8

so...

34.9 - 16.8 = 18.1 (new head chamber vol)???

Now if I wanted to skim so much off the block deck (say 10 thou) would I subtract 10 from 37 (current piston to deck height?) giving me 2.7cc?

Thanks guys.

Paul.

Edited by Pauly, 24 November 2009 - 06:06 PM.

[GraemeC]

Just quickly skimmed that.
Should your piston to deck volume not increase as the bore increases in the second option?

Also - why are subtracting 1 from your desired Compression Ratio?

Edited by GraemeC, 24 November 2009 - 06:59 PM.

[GraemeC]

Ignore my second point - I've read it again and understand the subtraction of 1 now.

I have never really worked it out this way - I start with a head volume and then work out what CR that will give me. With that in an excel spreadhseet you can then tweak the head volumeto change the CR to what you desire.
Your way is perfectly valid though.

[Pauly]

Ive done a bit of button pushing on the calculator and a +20 rebore will give me a difference of around .3cc, so its too small to take into account?

Also when working out the compression ratio I get 10.26:1 with a 18cc head chamber (first option I did) but have worked out I actually need a 18.05cc chamber to get it to 10.25, will the head modifier skim the head to give me an exact 18.05 chamber or is it best to just have it skimmed to give me 18cc?

Paul.

[Sprocket]

Head volume 21.4cc
BK450 gasket volume 3.4cc
Ringland volume 0.5cc
deck volume 4cc
Piston dish volume 8.4cc

Skim the block, its easier to calculate and you also improve the squish. Idealy you bring the pistons flush with the top of the block, but to do this you need to dry build the engine with the new pistons, measure how much has to come off, and then take the block back for it to be skimmed.

[Pauly]

I would say that some will have to come off the head aswell??

sprocket, when I use your numbers for calculating my current comp ratio I get 9.5 instead of 9.4?

Paul.

Edited by Pauly, 25 November 2009 - 09:09 AM.

[GraemeC]

I get 9.45 for a 1275 and 9.56 for a 1293

I wouldn't worry too much about such a low variance. Do achieve your desired 10.25:1 you would need to reduce the piston to deck clearance to 0.012" (12 thou).
I would suggest getting 25 thou skimmed off the block and running with that (unless the head needs skimming).

[Pauly]

Right, just taken all my stuff into the machining place and I now need a +40 rebore due to wear, so what cc is that?

Im looking for 10:1 comp ratio now.

Just worked it out and if I skim 20 thou off the top of the block with a +40 overbore will give me roughly 10:1 without having the head skimmed etc.

Phew think ive cracked it.

Paul.

Edited by Pauly, 25 November 2009 - 11:33 AM.

[Sprocket]

DO NOT skim the block before a dry build, UNLESS you are 110% sure that the new piston crown hight is the same as the original piston!!!!!!!!!!..............

[Cooperman]

As 'sprocket' says, you MUST do a trial build after having the block re-bored and before having the pistons pressed onto the rods. What you do is to linish down the old gudgeon pins to make them a slide fit into the new pistons, then remove the piston rings (carefully!) and build up the engine without bolting anything up too tight. Then you measure the distance down the block to the piston crown at TDC. You an also chek the ring gaps whilst they ae off the pistons. The gap should be between 0.008" and 0.015".
The 21253 is a much better piston than the 21251, especially if you really want to rev the engine over, say, 6250 rpm.
The 21253 is a higher comp ratio piston and sits higher in the bore than the 21251, so that's why a trial build is so vital.

[Pauly]

Ok, ive told them not to skim anything until ive done a trail build.

I'll be using 21253's but why not have the gudgeon pins pressed straight on?

[Cooperman]

Because if, perchance, the block has previously been decked for different pistons you could find the 21253's stick a few thou out of the top of the block. It's easy to machine a bit off the pistons and quite acceptable, up to a point, but if the pistons are already pressed onto the rods, then it's the very devil of a job. If they stuck out too far to skim a bit off, you would still have 'new' pistons which you could return in exchange for ones with a lower gudgeon pin height.
A trial-build is always vital if you are to have success with building engines. I know a lot of builders don't bother, and that's why there are some poor quality rebuilt engines out there.

[Pauly]

Ah right gotch ya. Its all standard because I measured everything and worked out its current comp ratio which is 9.4, but I will do a trial build because of the increased crown height.

Paul.