Who likes maths.

Im planing on puting together my 1426 at some stage in the future.

bores are 74.7mm (triumph 2.5), stroke is stock and the block is decked so the come up flush, assuming a stock 12g940 head, how high is my compression?

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speed costs....how fast do you want to go...

You will also need to provide at least combustion chamber volumes of the head. Are the pistons flat tops?

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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

hehe thanks 1018, yes, pistons are flat tops, stock 12g940 is 21.5cc.

Just thought it would be an interesting excercise, i have done the maths, just want to make sure im not nuts!

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speed costs....how fast do you want to go...

I don't know what you got but when I just did a really quick calculation (not taking into account the volume created by the H/G etc) I actually got 18.9:1

Obviously this will be a little higher than it will be (as I said before I did this REALLY quickly) but I didn't expect it to be that high.....I'll have another look at it

<edit> I think I f**ked it up...hang on a bit longer

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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

Ok, I have no idea then. I keep working it out and getting really high compression ratio's each time.

e.g.

Stroke: 81.28mm
Bore: 74.7mm
Chamber Volume: 21.5cc
Gasket: 2.5cc (guess)

Putting all that in gives me a C/R of 15.84:1 - which is pretty high still. Maybe someone else can see what I'm doing

Without taking into consideration my gasket guess I still get 17.57:1

Sorry jbeenz I must be doing something wrong (unless your numbers are around the same).

<edit> Working off stock 1275cc sizes (70.6mm bore, 81.23mm stroke & 21.5cc chamber volume) in order to get 8.8:1 compression (stock) there is 19.23cc taken up elsewhere (not part of the stroke / chamber volume etc).

Working off this number for your engine I get 9.73:1 compression. I don't think that this is enough - if your pistons are coming to the top of the block and there is less than 30cc's in the chamber volume + gaskets etc (again 30cc is a guess - I wouldn't think there would be that much per cylinder) then your C/R should be more up around 11.57:1

I'm so confused

Hopefully someone else can clear this up

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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

Thats about what i got, i allowed 3cc for gasket, so mine was a little less....perhaps i could use diesel

thanks!

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speed costs....how fast do you want to go...

Area of cylinder = 2 Pie r2 x Height

Edits to include MadMatt's terms for clarity:

= 356.2168 cc's per cylinder. Plus 21.5cc head chamber and 3.5cc head gasket;

Total volume of 381cc, (CV + SV)

Compressing to 25cc (CV). So total volume/CV =

15.24:1 compression ratio, static.

Nick, we must be missing something in working out static compression ratios?

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68 Deluxe.... it's shiny..... I like shiny!!

I did an edit just before ^^.

At least someone else is getting high compression ratio's too!

I hear that methanol is a cheap racing fuel Jed, just a pity because your car will use more methanol to produce the power

Decompression plate anyone?

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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

i think im going to need a bathtubed head! id like to get down to 11-12 to 1. Plus a real lumpy cam i guess!


Block pic!

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speed costs....how fast do you want to go...

Jed, I have just done the maths. Still assuming 2.5cc gasket size you will need a combustion chamber volume of about 29.9cc's for 12:1, 31.4cc's for 11.5:1 & 33.1cc's for 11:1

I wish you luck with your bath tub head

There isn't too much meat between those bores now......But it will be so much fun - 1426cc's........

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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

Don't worry, my usual calculator worked out a crazy high C/R too

Big bathtubs for you

Where's Dr Mini when you need him

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Stop Licking the Dog...I Don't Care Who Started It

out with the angle grinder

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speed costs....how fast do you want to go...

You need to put a dish in the piston for a start, you only need squish at the edges of the piston. I would say 6-8cc should be attainable. Add to this your gasket and bit above the ring land before you start grinding the combustion chamber.

Daniel

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Videos - Turbocharged A-Series on Dyno & at Wakefield
Mafs waz not my strong subgeckt at skkol

Allow 3.5cc for the gasket please

The formula is combusted volume(CV) plus the swept volume (SV), then divided by the Combusted Volume (CV)

so,,, seeing that CV divided by CV = One

Now all we have to do is divide the SV by the CV ,,, & then add one (for the CV divided by CV bit)

get it????

heaps of people muck it up & forget to add the one

NOw!!!!!!!!!!!!!!!!!! In saying that you will definately want to CC (Volumetriclly measure) the space above the piston at TOP DEAD CENTRE,,, If there`s a tiny bit there then good ,,, add that into the chamber volume with the 3.5ccs for the head gasket

Now,,, You will NEED to CC (Volumetrically measure) the cyl head chamber too, if there`s been "ANY" previous chamber griding &/or facing then you are all only guessing

Do it right,,, buy a burrett off e-bay, the 50mm plactic ones are cheap as chips ($23) i paid for my last one

get one of those, & cut a piece of flat perspex out to suit a little bit bigger than the bore, drill a hole in the guts of it for a filler hole,,, a smear of grease over the block & head faces & fill the dam things up & measure them both

then & only then will you get any accuracy

i do this crap probably more than anyone (maybe except GR) but still,,, this is how it`s done

NO GUESSING !!!!!!!!!!!!!

Class dissmissed

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.