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MG Midget and Sprite Technical - Combusion Chamber Volumes

Hi folks,

Sorry, I have not been about in the past few weeks. I went and got a job. Boo! :-)

The plus side is I have money for projects again. Yay!

I am thinking about prepping a cylinder head. Simple stuff like tidying up the ports, converting it to unleaded, maybe tidy up the combustion chamber and raising the compression ratio slightly.

Two questions (to begin with):

1. What would be a good, safe C.R to aim for? 9.5? 10?
2. Could anyone shed some light on the figures in the second table here http://www.teglerizer.com/triumphstuff/spitheads.htm I know these are for <1500 engines, but I can't seem to relate the chamber volumes etc. to the engine capacity and compression ratios. Also, what is the clearance volume?

Or are they just rubbish?

Cheers,
Malcolm
Malcolm Le Chevalier

Less is more on cly heads

id increase or lower CR using cometic cly heads

be back in a bit
Prop and the Blackhole Midget

Malcolm, if you haven't already seen it you should get hold of a copy of Vizard's book. Lots of information there with interesting background on the theory of it all! Well worth reading.

My 1275 is at 10.5 CR and runs well on standard fuel (UK RON 95) I think that is about as high as you can go for normal fuel
GuyW

OK,

Think I have got it. Clearance volume is the entire volume at the top of the cylinder when the piston is at TDC. i.e. combustion chamber + gasket + piston volume (piston vol. not stated in that table). Compression ratio is then the swept volume + clearance volume / clearance volume.

Just pondering whether to start with a 1300 head (as this would give 10.5 C.R as stock) and relieve the chamber slightly to reduce C.R to ~10. Or start with a 1500 head and skim.

You can get 1300 heads with bigger valves too, so that might be the way to go.

Any thoughts on max compression when running premium petrol (97 RON or higher)?

Cheers,
Malcolm
Malcolm Le Chevalier

Im back

rather then relive the cly head thur combustion chamber grinding

...id use the comtic cly head gasket... you can get them as thin as 1/32 of inch to over 3/8 of an inch thick

What i learned about modifying cly heads is less is more when it comes to grinding.... the biggest area to work id the blob that bolds the guilds ... you want ro grind that out so its a gentle slop not a 90 degree angle

next clean out all the sand casting crap thats on the port walls and smooth out any humps and ridges in the ports

polish the exhaust ports and apply or leave a textured intake ports

Air flows faster at the bottom on the ports then at the top

dont widen out the ports as that creates more volume which means a slower moving fuel charge

install stainless steel wasted stem racing valves over sized ... hardend seats high quality springs and retainers... ect ect

and you got a nice head

also go to the junk yard and get several busted heads and grind on those 1st ... about 3 heads before you do your head ... to get a feel for what your dojng... and just go hog wild on em ... once you have got that out of your system then do your own head

prop

and thats
Prop and the Blackhole Midget

I presume by Piston Volume you mean the volume above the piston and below the block deck, not actually the volume of the piston! Generally this is the dish in the piston plus the volume given by the piston clearance to the block surface X area of the piston top.

Pretty sure the 1300 head (1275?) doesn't give 10.5 CR as standard. Where did that figure come from? Or is it a Metro 1300 head?
GuyW

Start with a 1500 head, a mm off will take you near enough 9.8 ish to 1. The chambers are fine. Fit the MKIV large inlet valve, three angle in and ex seats, mod the backs of the valves with a 30 degree cut. Blend in the valve throats to the larger inlet valve, blend the ex throats and away you go.
Peter
Peter Burgess Tuning

Forgot to mention

for a street car i wouldnt go for max CR ...

1. to many changes are happening to fuel ... 15 years ago our fuel was still non ethinol and primium 98 octan was at almost every pump....now all our fuel is ethinol based at 10% and going to 15 % shortly and our primium is 91 octan and all of it other then boat and airplane fuel is ethinol based

so a common hot rod at 11 CR wont even work kn todays gas on the street lime they easily did 10 years ago


the 2nd reason for not going to a max CR is tunability. If your CR is lower fhen you have more optons for dissy timing and dissy curves ... where as your very limmited with a higher octane ... which means lower performance on the street but great if its a track only car not for the street

something to remember thats a big misconcption is fuel octan ratings

the higher the number of fuel octan means the slower and cooler the flame... where as the lower the octan number the faster and hotter the fuel burns. Most people think the opposite and think a faster hotter fuel burn is what makes the engine run faster a d better....but thats not true

the slower and cooler the fuel burns is what gives the engine more power because it burns longer and more complete giving more power to the time the pjston is in its sweet spot of travel during the ignition process...

as you know you dont want to run out of explosive charge high up the rpm range if you do the engine pings and detonation occures

vizard covers this very well but its a long ardous read and needs to be read about 5 times to let it sink in fully

suffice to say CR + fuel octan is about ignition timing and how fast or slow dawgs and springs inside the dissy reacts to the speed of the burning flame of the fuel charge will determin how well your engine preforms at the top of the rpm range.

just remember on the street you will spend ALOT more time at the bottom of the rpm range then you will at the top...very few people need 4700 rpm to idel at a stop light. Most city street driving is under 3000 rpm and interstate highways and autobonds are around 3500 and 4000 rpm


prop
Prop and the Blackhole Midget

Sorry on 3rd paragraph .... higher octan should read higher CR


.
Prop and the Blackhole Midget

Listen to what Peter Burgess says. He knows cylinder heads better than anyone. Better still, get him to work your head for you. You won't be disappointed.
Mike Howlett

But a 1500 head? On an A series?
GuyW

Its a Triumph engine. I thought everyone knew that! :-)

Yeah, by piston volume I meant the extra bit between piston and deck. I couldn't think what else to call it.

Thanks Peter B, your experience and advice is always much appreciated.

I fancy doing the work myself, as with everything. The doing is half the fun.

Malc.
Malcolm Le Chevalier

Peter,
does your book not advocate smoothing the ramp on the plug side of the chamber to unshroud the inlet a bit (or is that one of the others, hmmm)?

Malc.
welcome back :-)
I'll look out the relative pages for you next week.
I did more or less what you are planning on the mates machine but didn't bother skimming after working on the chamber as we used a +.060 over bored block to compensate... have to say it is as smooth as silk but not had in on a dyno to check what the output is.

Best of....
MGmike
M McAndrew

Forgot that. Should have checked your vehicle profile! Dohh!

Explains your 1300 head comment too doesn't it!
GuyW

And the link to a bunch a Spitfire specs ;-)

No worries Guy.

Malc.
Malcolm Le Chevalier

This thread was discussed between 27/09/2016 and 28/09/2016

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