998 engine 1275 head
Discussion
MR2Mike said:Exactly what I'm currently building. I think Cooperman has a 12G295 up for grabs.
The large ports on a 1275 head will tend to lose some low/mid range torque over a standard head. A nicely ported 12G295 is a better bet for a 998, though obviously not as easy to get hold of.
selbymsport said:
1275 head works well on a 998 pocketed block don't forget to use a 1300 head gasket, also a mild cam like the MG metro CAM6648 will give it a huge power band
The MG Cam is great for the price (i.e. free with an engine) but I have nothing but praise for the Kent 266 which gives the power advatages of the MG item but with excellent idle quality and bottom end power.
The best 998 engine I ever built for a road Mini had twin 1.5" SU's, a 276 cam, a 295 ported head with larger valves, 10.4:1 C.R., a lightened flywheel, bored to +0.020" with Hepolite flat-top pistons, a re-worked inlet manifold, a small-bore lcb exhaust manifold and a Maniflow centre exit twin box exhaust. It gave about 78 bhp at the flywheel and was very driveable across the range.
Nice, but not a patch on a 1275 lump, in my humble opinion. However, much nicer than a 970 'S' engine.
Nice, but not a patch on a 1275 lump, in my humble opinion. However, much nicer than a 970 'S' engine.
Surely putting a lopey cam on a 998 will make it behave more like a 970 torque wise anyway???
I would expect that a 12G940 on a small bore would not have sufficient gas velocity to give any sort of resemblance to a 295 head. I also don't like the valve shrouding aspect. If you are after a power from a screamer, I would go for the big head otherwise stick with the smaller ports and go for more lift.
(But my knowledge of tuned 998's is pretty theoretical so it could be miles out)
The first mini I drove was a 998 and it was ace. That was running a 295 and had only a bit less welly than my old MG Metro'd mini. Having said, the MG Engine did light the tyres up a treat (145/10's) yeehaaa!
I would expect that a 12G940 on a small bore would not have sufficient gas velocity to give any sort of resemblance to a 295 head. I also don't like the valve shrouding aspect. If you are after a power from a screamer, I would go for the big head otherwise stick with the smaller ports and go for more lift.
(But my knowledge of tuned 998's is pretty theoretical so it could be miles out)
The first mini I drove was a 998 and it was ace. That was running a 295 and had only a bit less welly than my old MG Metro'd mini. Having said, the MG Engine did light the tyres up a treat (145/10's) yeehaaa!
The biggest problem fitting a 12G940 head to a 998 engine is that you have to machine valve clearence pockets in the block. The other problem is to achieve a high comp ratio at least .100" needs to be machined from the head. Also there are many configurations of the 12G940 and the actual port design is much improved on later heads, the metro turbo version is good but has bigger water jackets and one has to be carefull not to machine off too much material as the head face will distort. If you would like to go down the route of fitting a 12940 head you will find the result is pretty good, the gasket to use is the metro turbo BK450 (Payen). The Mini 7 race series use this basic configuration and with only a single carb and standard MG Metro cam the engines achieve 85+ hp DIN at approx 7500 rpm with excellent tractability although fuel consmption can be a bit on the high side for a road engine.
selbymsport said:
The biggest problem fitting a 12G940 head to a 998 engine is that you have to machine valve clearence pockets in the block. The other problem is to achieve a high comp ratio at least .100" needs to be machined from the head.
I find this a bit strange as the 12G940 has the smallest combustion chambers of any A or A+ series heads (and the 12G295 the largest!). If anything the combustion chamber may need grinding to to reduce CR to sane levels.
MR2Mike said:
The biggest problem fitting a 12G940 head to a 998 engine is that you have to machine valve clearence pockets in the block. The other problem is to achieve a high comp ratio at least .100" needs to be machined from the head.
I find this a bit strange as the 12G940 has the smallest combustion chambers of any A or A+ series heads (and the 12G295 the largest!). If anything the combustion chamber may need grinding to to reduce CR to sane levels.
To get a 10.5:1 comp ratio you need 27cc unswept volume 5.7 in the piston + .3cc ring land .6cc pocket 4cc BK450 16.4 cyl head this means taking off .100 ish iff the head. On a 1 litre engine 10.5:1 it will run ok on low octane unleaded ( provided you have exhaust valve seat inserts. Skimming down 12G 295 heads to achieve high comp ratios is a problem as you can break through the oilway this is solveable but a pain as one has to mod the oil supply route to the rocker shaft. Also all A series heads flow best at approx .190-.205 head depth and to acheive this dished pistons are used.
With the 295 heaqd I use flat top pistons and machine the block so that the piston crown comes about 0.005" proud of the block top. Even with the 295 head properly gas flowed I can machine enough off the head to get to 10.5 : 1 c.r. I can't find the calcs for the last one I did, but from memory we had to machine about 0.065" off, which was OK as the depth of the oil way was 0.120 from the head mating surface.
The biggest improvement with the 295 head is bigger valves and the improvements to the throat rather than the combustion chamber itself. Bigger valves and a bit of 'de-masking' gives an excellent result.
For a race car, however, where evey bhp coiunts, especially in a 998,the mod mentioned to the oliway may well be necessary. I've never done this, although I've seen heads where it has.
The biggest improvement with the 295 head is bigger valves and the improvements to the throat rather than the combustion chamber itself. Bigger valves and a bit of 'de-masking' gives an excellent result.
For a race car, however, where evey bhp coiunts, especially in a 998,the mod mentioned to the oliway may well be necessary. I've never done this, although I've seen heads where it has.
miniman said:
It's been a while since I've rebuilt one, but aren't the 998 A+ pistons flat topped as standard?
They were certainly flat top on my block.
If you have flat top pistons (later engines) just move the 5.7 into the head. I suggested the dished piston configuration as its without doubt the best combination on the 998 engine for high torque/power output. We have won 7 out of 10 races this year in the M7 challenge series so I'm sure it works.
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