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CR - can it be true?
https://www.ausmini.com/forums/viewtopic.php?f=5&t=72325		 Page 1 of 2
Author:  Asphalt [ Mon Aug 08, 2011 11:34 pm ]
Post subject:  CR - can it be true?
Hi!

I'm building a 998 at the moment. Calculated CR is 10.7:1.

Flat top pistons (A+), 0.5mm clearance to deck, 21.4cc chambers (12G940), 0.75mm gasket gap, 73mm gasket bore. Std. stroke, std. bore.

I'm using a 1275cc A+ camshaft (230/252°) - the exhaust valves don't hit the block with that cam, even without a head gasket. So if I calculated my CR correctly, I could do without valve pockets. I do realise they aid flow around the exhaust valves - but I'm more the lazy sort of person :D

With a stage 1 thingy and stuff - what HP could an engine like this produce? :)

Thanks!

Jan
Author:  lmbm [ Tue Aug 09, 2011 12:04 am ]
Post subject: 
Hi Jan,

Me and my father in law built a similar spec for his clubman some years back,

only differences: MG Metro cam and dished pistons, lightened flywheel ( 1kg or so ). HIF38 on a MG Metro manifold. No block cut outs either.

not sure about the power, but with 3.65 diff is good enough to keep up with a stock 1275GT.

if you have the chance pick an MG cam and get more decent RPMS!

cheers
Luis
Author:  blue_deluxe [ Wed Aug 10, 2011 12:03 am ]
Post subject: 
Just thought its funny that a question from someone from Germany is answered by someone from Portugal on an Australian forum.

And you might get close to 50 hp from that. Going off 36 for a 998 plus a bit more for the head.
Author:  Asphalt [ Wed Aug 10, 2011 12:34 am ]
Post subject: 
:lol:

Thanks! So Cooper territory :) That's just fine. But I'm going for a even shorter 3,76 diff with a A+ gearset. I think I might have build a 3 speed gearbox there :roll:

Cheers,
Jan
Author:  Kennomini [ Wed Aug 10, 2011 6:52 pm ]
Post subject: 
Well Matt (the mini man) had a 8v bmw head on a stock 998 and that made 60bhp.
One of my old 998 engines that was full stock had 30.9bhp which compared to two other 998's on the dyno day mine was the best :) .
With the standard head (you should work it to help the flow) your using I'd guess around low 30's too 40bhp.
Author:  VulcanBB18 [ Thu Aug 11, 2011 4:19 pm ]
Post subject: 
Hi Jan,

I used one of those exact cams in my last 998, it was :

1040cc
~10:1 CR
295 head with 1.3" In 1.15" Ex (+ extensive mods)
Minispares inlet + HS4
Freeflow exhaust + RC40
3.44:1 final drive on 10" wheels.

It easily kept up with stock 1275 engines, and I had quite a few people surprised it was a 998 on a 3.44!

With the 940 head you may loose some low-end torque due to the larger ports.

Wish I had put it on a dyno now :(

cheers,
Author:  Asphalt [ Tue Aug 30, 2011 6:36 pm ]
Post subject: 
First impression: wowsers! :D

A 998 at heart, but almost as strong as a std. 1275 :) Only lacks a bit of torque, which gets obvious going up-hill. But apart from that: I'm surprised how well it goes. And bearing in mind it cost me less than 50€ to build - the outcome is fantastic :)

Image

Cheers,
Jan
Author:  lmbm [ Tue Aug 30, 2011 7:06 pm ]
Post subject: 
998's with some nice mods are very pleasant to drive!

You even saved on engine paint :lol:

BTW, nice rocker cover in there! :wink:

Cheers
Luis
Author:  Asphalt [ Tue Aug 30, 2011 8:07 pm ]
Post subject: 
lmbm wrote:
998's with some nice mods are very pleasant to drive!

You even saved on engine paint :lol:

BTW, nice rocker cover in there! :wink:

Cheers
Luis


Hi Luis!

Yes! And so smooth. Somehow I missed the smoothnes of a smal bore engine :)

I HAD to save on paint, to counter the extra-weight of the double valve springs :lol:

And that rocker cover gets surprised 'Oh!'s every time :) And wehn I tell people I imported it from Oz, they have that judging "lunatic!" view in theyr eyes :lol:

Anyway... I'm off to build the next 1300 engine... Moar powar!! :lol: :twisted:

Cheers,
Jan
Author:  lmbm [ Wed Aug 31, 2011 6:20 pm ]
Post subject: 
Quote:
And that rocker cover gets surprised 'Oh!'s every time And wehn I tell people I imported it from Oz, they have that judging "lunatic!" view in theyr eyes


Hi Jan.

yes i knew the cover was from Oz.. love them! Nice period item...

what's the origin of these covers ? i suppose they were an aftermarket brand ?


cheers
Luis
Author:  drmini in aust [ Wed Aug 31, 2011 7:16 pm ]
Post subject: 
Yes Luis they were aftermarket. Unlike the Bling flimsy ones coming from China now, these were sand cast.

ps I've got one too...
Image
Author:  MORRIS COOPER S 501 [ Wed Aug 31, 2011 7:34 pm ]
Post subject: 
C/R how can you calculate or find what the combustible ratio of your engine is?

The specs of the engine in my van

is 998cc
Bored 60"thou to 1046
12G295 head intake valves 1 5/16" exhaust valves 1 3/32" 1100S valves
Wade 240 Cam

I've been told by previous owner that it's C/R is 9.55:1 does this sound right?
Author:  mini_mad_matt [ Wed Aug 31, 2011 7:54 pm ]
Post subject: 
C/R is calculated by the (swept volume + clearance volume) divided by (clearance volume)

So the swept volume is 1/4 of the engine size (being that there are 4 cylinders making up 1046cc) = 261.5cc This is the volume the piston will displace.

The clearance volume, is the volume of the combustion chamber, the piston dish and piston to deck height and the gasket volume.


So to calculate your static compression ratio, you would need to know the piston dish, and the distance from the top of the piston to the deck height so you can work out that volume of undisplaced air, the thickness of the gasket when crushed to work out that volume, and the volume of the combustion chamber.

The cylinder head may have had the surface machined, chambers modified, valves recessed etc which would all affect it's volume.
Author:  MORRIS COOPER S 501 [ Thu Sep 01, 2011 12:48 pm ]
Post subject: 
the only info I have on the pistons is the weight- 308g(which doesn't help anyone) but I do have the chamber volume 21.4
Author:  justminis [ Thu Sep 01, 2011 1:15 pm ]
Post subject: 
MORRIS COOPER S 501 wrote:
the only info I have on the pistons is the weight- 308g(which doesn't help anyone) but I do have the chamber volume 21.4


The chamber volume you quote (21.4cc) is for an unmolested 12G940 (1275) head, rarely accurate now! You have said your cylinder head is 12G295 (Cooper), which unmolested was about 28cc. Not uncommon for these heads to be surfaced tho, but to get down to 21cc would require significant material off, probably more than would be good for the head.

Only way to work out your CR accurately is to measure those bits you have, eg, combustion chamber volume in the head, piston dish etc. Then you can use the formula provided above.
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