hi all,
ive got my engine in pieces at work, goin to be rebuilt
soo whilst its out im gonna change a few things:
change the cam
up the compression ratio
and do some port work
but i dont know how much to tell the machine shop to take off the head?
and not to sure on what cam to chose?
my cars an spi with a rc40 and decat already
ive tried the search but i cant seem to find the answer im lookin for
thanks for any help
richard
Compression Ratio
Started by
Rich1275
, Jul 23 2008 07:22 PM
3 replies to this topic
#2
Cooper-202S
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- Members
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- 201 posts
Mini Mad
Posted 26 July 2008 - 12:01 AM
Richard,
I am in the process of rebuilding a JC converted Spi which have dropped a valve seat and burned out a piston. In searching out a machine shop that would replace the seats for me I picked up some useful information which may be of interest.
The general opinion is that the Spi's are temperamental where cams are concerned thats why the likes of Kent and Piper list specific cams for injection engines. Depending on what your looking for a couple of phone calls to either will yield a great deal of useful advice, Simon at Morspeed was also a great help, it seems to be a balancing act between drivability and all out top end power.
Personaly I'm having a replacement from an Spi reground to give extra lift married to a set of 1.5 roller rockers.
With regards to increasing the compression, anything above 10.5-1 and your going to end up using premium unleaded all the time (expensive unless your only doing low milage). I went beyond this (11.5-1)with another engine some years ago and it now lives under a bench awaiting the luxury of a track day car that i can run on Ron 104 ( race fuel ) because it pinked no matter what we did.
I'm sure someone listed the calculations for sizes and CR's on the site somewhere but i couldn't find them before posting, they are not for the mathmaticaly faint hearted.
You dont mention a doing compression test in your origional post.
If the block has done over 50K then you are well into the area of a re hone and new rings at least if not a rebore. Increasing the compression without a good seal will burn out pistons realy quickly.
If there is no evidence of the origional honing ( Cross Hatching ) on the bores then they need to be checked for size to determine the wear. the tollerances between piston and bore are a lot smaller than you might think.
The one area that will yield real gains is the port work , comparing the Cooper head (which was produced by mini sport for the cooper garage) to a standard head there seems to be a lot that can be done. In reading some realy old articles about performance improvements to the mini there also seems to be some value to matching the exhaust port sizes and profiles to the manifold which seems to be neglected on some of the stage 2 heads available.
Another area not often considered is getting the crank wedged its not ridiculously expensive and yeilds a real improvement to general performance, likewise getting the flywheel balanced to the crank helps too.
I trust the above is of use and that i havent tried to 'teach Granny to suck eggs'
Regards Paul
I am in the process of rebuilding a JC converted Spi which have dropped a valve seat and burned out a piston. In searching out a machine shop that would replace the seats for me I picked up some useful information which may be of interest.
The general opinion is that the Spi's are temperamental where cams are concerned thats why the likes of Kent and Piper list specific cams for injection engines. Depending on what your looking for a couple of phone calls to either will yield a great deal of useful advice, Simon at Morspeed was also a great help, it seems to be a balancing act between drivability and all out top end power.
Personaly I'm having a replacement from an Spi reground to give extra lift married to a set of 1.5 roller rockers.
With regards to increasing the compression, anything above 10.5-1 and your going to end up using premium unleaded all the time (expensive unless your only doing low milage). I went beyond this (11.5-1)with another engine some years ago and it now lives under a bench awaiting the luxury of a track day car that i can run on Ron 104 ( race fuel ) because it pinked no matter what we did.
I'm sure someone listed the calculations for sizes and CR's on the site somewhere but i couldn't find them before posting, they are not for the mathmaticaly faint hearted.
You dont mention a doing compression test in your origional post.
If the block has done over 50K then you are well into the area of a re hone and new rings at least if not a rebore. Increasing the compression without a good seal will burn out pistons realy quickly.
If there is no evidence of the origional honing ( Cross Hatching ) on the bores then they need to be checked for size to determine the wear. the tollerances between piston and bore are a lot smaller than you might think.
The one area that will yield real gains is the port work , comparing the Cooper head (which was produced by mini sport for the cooper garage) to a standard head there seems to be a lot that can be done. In reading some realy old articles about performance improvements to the mini there also seems to be some value to matching the exhaust port sizes and profiles to the manifold which seems to be neglected on some of the stage 2 heads available.
Another area not often considered is getting the crank wedged its not ridiculously expensive and yeilds a real improvement to general performance, likewise getting the flywheel balanced to the crank helps too.
I trust the above is of use and that i havent tried to 'teach Granny to suck eggs'
Regards Paul
#3
Rich1275
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On The Road
Posted 28 July 2008 - 09:56 PM
thanks for the info.
i did a compression test to start with and found number 2 was down, so took the head off to discover a BIG score down the bore...so i took it to work and striped it all down... found that the gudgin pin had slid out and made that nasty scoring down the bore also in the process found the diff was nakered and most of the bearings were gone, tought it was a bit odd on a 33,000mile car. guys reckoned it could have been run outa oil in the past.
away was sent of the the machine shop and its gonna be bored and sleeved and have a crank grind.
i think the cam i would go for a kent md274 but would this give me any problems come mot time and would i need to use venier timing gear?
i did a compression test to start with and found number 2 was down, so took the head off to discover a BIG score down the bore...so i took it to work and striped it all down... found that the gudgin pin had slid out and made that nasty scoring down the bore also in the process found the diff was nakered and most of the bearings were gone, tought it was a bit odd on a 33,000mile car. guys reckoned it could have been run outa oil in the past.
away was sent of the the machine shop and its gonna be bored and sleeved and have a crank grind.
i think the cam i would go for a kent md274 but would this give me any problems come mot time and would i need to use venier timing gear?
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