35 replies to this topic

[skoughi]

I've just about finished getting the block/gearbox/clutch assembled and so will be turning to the head shortly. I've measured the chambers along with piston to deck etc for working out c.r. So at this stage with a BK450 gasket I'm looking at a c.r. of 10.2-1, also I'll be running it on 95 oct. I'm intending to send off my 45d dizzy to Aldon to get the weights and springs adjusted to the engine as well. I've been advised to get the c.r. down to 9.75-1 and I'm fully intending to obtain a couple of sacrificial valves and open up the chambers. Before I do though I'm wondering what folks say regarding a maximum c.r. and what problems will arise with pushing this figure too high

[ACDodd]

What cam and rockers?

AC

[skoughi]

Kent 266 and standard rockers. Also it was yourself who advised me to go for 9.75-1! I also managed to get the cam timed in at 104  deg as you suggested.

[ACDodd]

9.75:1 is the maximum you an run with that fuel. Under load at higher figures the engine runs on the verge of detonation. I find the broadest power band at 9.75:1 on this cam and rocker combination.

Ac

[skoughi]

Thanks AC, next step then is to obtain a couple of valves and a bit of studying of vizards book before some gentle grinding.

[ACDodd]

If you look on my facebook page I have just done such a head,called project decompress.

Ac

[skoughi]

Will do, thank you.

[carbon]

Skoughi - your other option is to stay with 10.2:1 and use Shell 99 octane fuel.

 

I have been running 1330 with MD266 and standard rockers using about 10.5:1 and this is not giving any issues with diet of Shell 99 Octane.

 

But I am also using 82C thermostat, and it tends to run pretty cool.

[ACDodd]

I would still wager it would run better at 9.75 than 10:1 even with 99 octane.

Ac

[carbon]

Thanks AC,

 

If you use higher lift rockers with the MD266 cam would this allow use of higher compression ratio, all else being equal?

[racingbob]

im running about 11.2 on mine with shell v power and millers octane boost

 

so about 101 all up. been like this for 2 years had no problem

 

however I don't rev the car  over 5500 except when Neil at slark

 

revved bit higher on the dyno, in fact I rarely rev over 4500 5000

 

its a mg metro cam and stage 3 head. when the head comes off soon

 

however I will reduce the comp to about 10.1

 

im not suggesting you run at that comp it realy is a bit high for mg metro

 

my race mini was 13.1

[ACDodd]

The engine will be more fussy as a result. Trust me you are always better off to under compress rather than over compress. High cr's are for yesterdays fuels. That engine will loose nothing in power If you were to run it at 9.5:1, but it would last longer as the ring loads are less. Its a road engine. It must last.

Edited by ACDodd, 25 November 2015 - 06:13 PM.

[ACDodd]

Stop reading books that base there findings on fuels we no longer have.

Ac

[skoughi]

Cheers guys for all the info, had it in my mind to have a go at smoothing out the chambers and enlarging them so it's no big issue. One of my pistons is slightly higher than the rest so that chamber needs to be 0.5cc less than the other three anyway, doing this will hopefully sort this. Will I need to start off with a tungsten bit and finish with a stone one or will I be better just taking my time with a stone bit? Only looking at removing 2cc's at most.

[skoughi]

I simply don't want an engine that I need to have the equivalent of a chemistry lesson every time I'm at the pumps adding this and adding that! Just want to buy fuel and an ice cream and go!