11 replies to this topic

[MrBounce]

This is only a "can it be done, and if so, how?" question that I am asking for a friend.

He has an 1100 engine for his Biota project. We don't know the origins of the engine, nor any details about its internals. We think it *may* have been modified but until it comes apart we have no way of knowing. Height of the engine is a major issue with this car - there's only about a couple of mm clearance for a small bore engine. The 1275 engine will not fit without a lot of unwanted extra work so in case he wants a bit more power, he wants to look at other options. He quite likes the idea of keeping the "original" engine, which looks like it's from an Austin 1100.

He also has a big valve ported 12G940 head (possibly a Minispeed item but he can't be certain) - the question is will it fit on the current block? I am aware that much careful measurement is required in case block pocketing is necessary for the smaller 940s but is the big-valve head too much for the little 1098? I am also a bit concerned about crank strength at higher revs.

It's all a bit pie in the sky at the moment as the car's a long way from the road but we're just looking at various potential ideas.

[mk1leg]

I would think it would fit as the bores are big enough

[nicklouse]

I would think it would fit as the bores are big enough

Nope the bores are the same as th 998 so there is a good chance that you would need to pocket the block.

Otherwise it is an option to get some more power but reworking a suitable small bore head might be a better option if the engine is not to be removed and reworked.

[tiger99]

As well as the valve clearance and block pocketing, I would suggest thinking about changing to a 998 crank and pistons (the rods are the same). The shorter stroke engine can safely be revved much higher and has far greater tuning potential. It is the strongest of the mainstream engines, and can have a very long life. (Things like the 970 S were stronger but are rare and expensive.) The 1098 always was somewhat weak. That did not matter in its original use in the 1100 rust buckets, frequently owned by middle-aged women.

 

The 1098 will give better low-down torque, which is simply a function of crankpin radius or stroke. But that is its only advantage.

[Cooperman]

In fact the last of the 998 cranks, the A+ version, is the strongest of the 850/997/998/1098 cranks.

I did have a Mini with a 1098 engine. It had a 731 cam, a 12G295 head and twin HS2 carbs, etc., and it gave splendid torque at mid-range and was a fine road car (back in the day). I didn't rev it much over 6000 rpm, so with the 731 it was fine.

In fact, a 1098 with a 266 or MG Metro cam would be great, especially with a 3.44:1 FDR, as it would only need to rev to 6000 rpm and a 1098 crank is good for that.

'Tiger' is absolutely right that if you want a really powerful small-bore engine then the 998 crank is the one to have, but for road use don't dismiss the 1098 ads it does give a lot of torque and that is more important than max. power. For road use it's mid-range torque that's needed not mega-power at 7000 rpm. Too many people concentrate of the max. power figure, which is really only important to racing cars, and end up 'over-camming' their engines where this max. power only comes along at un-sustainable revs.

For this reason a 295 head with slightly larger valves in a mildly gas-flowed head can be made to work extremely well. Just don't over-rev it or fit a cam that requires a lot of revs for max. power. Aim for best torque from, say, 2500 to 5000 rpm.

[tiger99]

And please don't forget to fit a torsional damper at the pulley end! Some people say that they are not necessary. I disagree, especially with the 1098. It may save its cost many times over if you unwittingly hit the crankshaft resonant frequency. AHU1878 is currently quite expensive, £113.10 from Somerford, but compare that to a smashed up crankshaft, rods, pistons, and probably block and gearbox internals...

[Cooperman]

The inability of certain A-series crankshafts to 'rev a bit' is over-stated when they are used in road cars.

 

The 850 and 1098 cranks are quite capable of taking 6200 rpm 'through the gears' and 5800-ish rpm sustained. With the 850 the original split-skirt pistons were as much a limiting factor as well as the inability of the standard cam to allow high revs.

 

It is once these engines are much modified that the problems can arise. If a cam giving max power at 6400 rpm (e.g. a 286) is fitted, it is necessary to rev to 6800 to make the cam work as designed and there will be a risk of breakage. With a road cam suited to that engine, i.e. one producing good mid-range torque, there will be no problem.

 

However, if used in any sort of competition engine the revs must be limited, a crank damper fitted and, ideally, the flywheel lightened and everything accurately balanced.

 

I built a 1098 engine for an historic frog-eye Sprite. It was bored to +0.060 and fitted with flat-top pistons. The cam was a 276 with twin HS4 carbs and 10:1 C.R. A fully gas-flowed 295 head with slightly larger inlet valves and an Aldon 'red' dizzy added, with the flywheel lightened plus an 'S' crank damper and Duplex vernier timing gears/chain. The entire bottom end was balanced. At 6000 rpm it produced 74 bhp and it would rev to 6400 through the gears on tests. Actually in a very light car like a Sprite Mk.1 it went very well and it didn't blow up, even when cruised on road sections at over 5000 rpm. But I think if we had taken it to over rpm it might have 'let go' in a big way!

[carbon]

Also if you're going to build a competition 1098 unit worth selecting a decent set of conrods. Avoid the ones with clamp bolt at small end, and might be worth investigating possibility of using 'push-fit' late A+ 998 rods. Use the best quality rod bolts you can get hold of.

[Cooperman]

Oh yes, never use rods with small-end clamp bolts on any sort of performance engine.

Also, if building an in-line unit for Sprite, Midget, Minor, etc., use the block with the solid web at the centre main.

I did have a Mini in the '60's into which I put a slightly modified MG 1100 engine. It even had a 3.76:1 FDR and I did use it fairly hard to & from work to keep the mileage down on my Cortina GT. It never blew up or broke down due to my revving the long stroke engine - maybe I was lucky!

[mailman]

Is the big valve 12g940 wasted on a mild 1098 build? Will the lack of engine speed mean the over sized valves would hamper performance rather than help?

[Turbo Phil]

Is the big valve 12g940 wasted on a mild 1098 build? Will the lack of engine speed mean the over sized valves would hamper performance rather than help?

Depend what you consider as “big valve”? But ye, in general the 1275 heads aren’t best suited to road going small bore motors. Even less so once oversize valves are fitted and port sizes increased.

Phil.

[mailman]

Standard ports, 35mm inlet and 29mm exhaust (might be 31mm exhaust but I have to dig the invoice out).