27 replies to this topic

[oltonlad]

hi all, i've got a 93' 1275 carb engine, as you may know these engines run a lower compression ratio than the coopers etc, would i be able to have the head skimmed to raise the comp ratio to a decent level or is it a case that the pistons in my engine are low comp ones?

cheers

[Noah]

I think you can raise the compression ratio by skimming the head.

It depends they had some seriously low CR models. I think that mine 94 carb 1275 is 9:1? I think probably wrong.

[oltonlad]

stuff it. i'll have 40 thou taken off the head........or even slap an MG metro head on it and have that skimmed and machined for unleaded.

[Artful Dodger]

well i was SO wrong!! time for a bit of read and learn!!

A better way is lower cc dish pistons. Std pistons have a 8cc dish. You can get hepolite ones with 6cc. This is the best way to raise the CR. if you deck the block as well (skimming the deck do it did flush with the piston crown when piston is at TDC.)

Mine for example is a modified head, but sill a near standard 22cc combustion chamber, decke and 6cc dish pistons and I have a 10.5:1 cr.

Edited by Artful Dodger, 25 January 2013 - 07:22 PM.

[ACDodd]

ok, to clear this up the engines are not all the Same CR. The Sprite engines are all 9.4:1 CR Which is perfect if you plan to use 95 octane fuel. Metros range from 8.8 to 10.5:1 and included 9.4, 9.75 and 10.1 inbetween!!

The heads are also not the same. Unleaded factory head are nominal 23cc. Leaded heads are nominal 21cc. For a Fast road engine on 95 octane fuel leave the compression where it is.

AC

[Noah]

AC do you mind if I PM you with some questions?

[ACDodd]

Fill yer boots!

AC

[oltonlad]

ok, to clear this up the engines are not all the Same CR. The Sprite engines are all 9.4:1 CR Which is perfect if you plan to use 95 octane fuel. Metros range from 8.8 to 10.5:1 and included 9.4, 9.75 and 10.1 inbetween!!

The heads are also not the same. Unleaded factory head are nominal 23cc. Leaded heads are nominal 21cc. For a Fast road engine on 95 octane fuel leave the compression where it is.

AC

AC, if i leave the comp ratio as it is how can i get a bit more power out of it............would fitting an MG metro (large inlet valve) head help, i would obviously have it converted for unleaded, and i was thinking of a maniflow freeflow manifold as i hate lcb's....and a maniflow exhaust.

[ACDodd]

The biggest limitation you have with this specific engine is the cam and head.

See below for a spec to really make these engines come alive, compare the first 2 with the last engine, the last engine is a bolt on modified sprite unit, the average torque is virtually the same at the 1380 from 2 to 5krpm, which is where you drive, and also remember this started as a 50-52hp engine and with only bolt on's ended up as 82hp!! His only wish was that he kept the CR a bit lower to make the engine less fussy. The big news also is massive torque right off idle, a smooth idle and excellent MPG.




Here we go, this is a special one for me. It the first featuring one of my new cams;


The engine is an MLMotorsport customer unit carefully screwed together by Andy Davies.
This one is a Fresh (500ish miles) 1330cc built to MLM road spec. It features a ACDodd rebuilt Morspeed stage 3 head (35.7 x 29.4mm valves mega +0.060" pistons, this combination giving 9.75:1 CR. ACDodd - RS cam installed at 106 degrees. It uses Minisport 1.3 roller tip rockers, Single HIF44 SU carb, cone filter, Custom ACDodd needle, Ignition utilized an ACDodd Custom curved Lucas 65DM4 electronic dizzy, the usual freeflow intake and exhaust system. Fuel 95 Ron Unleaded.

Produced figures of 85.9hp@ 5500rpm (flywheel), and 91.6lbft @ 4000rpm which is impressive given the spec.

Here are the exact figures if anybody want to plot them, runs taken in 3rth gear, power at wheels was 76.5bhp peak.
Correction factor 0.98672 today, DIN 70020

Rpm Bhp Torque(Lbft)

1000
1500
2000 29.53 77.56
2500 40.95 86.02
3000 51.35 89.90
3500 60.41 90.65
4000 69.81 91.66
4500 77.53 90.48
5000 83.23 87.43
5500 85.92 82.04

The result is I am very happy with the new ACD - RS cam more torque for the road base engine than anything else I have tested to date,



The 2nd engine is a carefully assembled customer built unit.
This one has run about 5000 miles and its leak down figure were very good for this mileage 10 to 17 %. The engine features 1380cc capacity with Omega pistons. It features a Minispeed stage 3 head, giving 10.5:1 CR. It runs the SW5-07 cam with 1.5 roller tip rockers. Fueling is via a Single HIF44 SU carb, cone filter, Custom ACDodd needle, Ignition utilized an ACDodd Custom curved Lucas 65DM4 electronic dizzy, the usual freeflow intake and exhaust system. Fuel 98.5 Ron Unleaded.

Produced figures of 92hp@ 6000rpm (flywheel), and 89lbft @ 4000rpm which is the most powerful sw5 cammed 1380 I have tested to date.

Here are the exact figures if anybody want to plot them, runs taken in 3rth gear, power at wheels was 79.2bhp peak.
Correction factor 0.98672 today, DIN 70020


Rpm Power(Bhp) Torque(Lbft)
1000
1500
2000 28.19 74.03
2500 37.92 79.67
3000 48.66 85.20
3500 58.73 88.13
4000 67.79 89.01
4500 76.19 88.92
5000 83.23 87.43
5500 88.94 84.93
6000 91.96 80.50




Another surprise of the day was this modified 90's sprite, these were the most detuned of the factory 1275's with only 52bhp, well for this owner something had to be done;

This one has covered around 40K miles miles. The engine features a stock A+ 1275 bottom end inc stock sprite cam. It uses customer home modified head (35.7 x 29.4 valves),appox 10:1 CR. It uses 1.5 roller tip rockers. Fueling is via a Single HIF44 SU carb, cone filter, Custom ACDodd needle, Ignition utilized an ACDodd Custom curved Lucas 65DM4 electronic dizzy, the usual freeflow intake and exhaust system. Fuel unknown.

Produced figures of 82.5hp@ 5500rpm (flywheel), and 89.3lbft @ 4500rpm. This is very very good output considering the very mild cam used. Good news for those with sprites, as this lot is all bolt on with out the need to remove the engine to get this huge power!

Here are the exact figures if anybody want to plot them, runs taken in 3rth gear, power at wheels was 76.5bhp peak.
Correction factor 0.98672 today, DIN 70020

Rpm Power Torque
1000
1500 21.48 75.21
2000 30.88 81.08
2500 40.27 84.61
3000 49.34 86.37
3500 58.06 87.13
4000 66.79 87.69
4500 76.52 89.31
5000 80.55 84.61
5500 82.56 78.84

AC

Edited by ACDodd, 24 January 2013 - 12:16 AM.

[david036]

Those engine specs etc make some really interesting reading...

Just a quick thought, are you going to get a higher CR by using flat top pistons?

[ACDodd]

You would getr a higher CR by using flat top pistons as long as the deck height remained the same as the dished version.

AC

[Cooperman]

I built an engine for an Endurance rally car which must have a 'standard' engine. The car was a 1990 1275 Cooper.
So I bored it to +0.060" i.e.1330, as allowed by the regs, fitted 21253 pistons as they are listed as the standard fitment now the originals are NLA. The block was 'decked' to bring the pistons right to the top at TDC. The Metro GT head I fitted was skimmed (service repair!!) to give 10.7:1 C.R. and an MG Metro cam was fitted and very accurately timed. The head had the 'steps' around the valve seats removed and the inlet and exhaust tracts were 'cleaned up' and the guide bosses were 'dressed down' a bit. Standard carb was HIF44. The Haynes 'book of lies' says the inlet valves on that car are 35.6 mm (they are not) and I had that documentation in case the scrutineers wanted it. Inlet manifold was cleaned up a bit and matched to carb and the carb had a few minor tweaks inside it. Distributor was re-curved by Aldon. Rocker gear was early Cooper 'S'.
This was really a 'blueprinted' engine and when set up it gave 84 bhp at the flywheel at 5800 rpm. I can't recall the torque, but I fitted a 3.44 final drive and it went really well.
I must say I can't recall ever spending more time on super-careful assembly.

[racingbob]

do dynotunes vary a lot my engine only gave 73 bhp at 5400 and 78 torque at 3900

has a nicely ported head mg metro cam med substack and maniflow full system, 3.4 dif

and showing 210 compression each pot

just had it done at Slark

[oltonlad]

i just want it to have a bit more "go", i dont want it to be rapid as such, i was thinking along the lines of a kent 266 with an MG metro head, are the MG metro pistons or 21253 equivalents higher compression compared to the ones in my engine (1993 italian job 1275)

[racingbob]

why not give it a compression test with my standard mg metro head on was

around 195, my mates with the lower compression was about 165


the mg metro head and cam or 266 will make it go nice