1098 low compression Vs. high compression

[youngcamper]

Hi All,

I've got a little bit of a conundrum for you, basically I've been getting a little bit fed up with my low compression 1098 I currently have fitted to my 4 door saloon, it's currently running with a 12G940 head and HS2 carb. Sadly it doesn't feel quite as spritely as any of the other cars in our fleet ( all with HC 1098 engines totally standard). the engine it's self is well tuned up but a little tired and consumes a bit of oil. So I've finally plumped to rebuilt my spare HC 1098, now the question is: am I likely to expect any increase in performance by fitting a bog standard HC 1098 ? or will I need to mildly tune it ( 12G940 and HS4 ) to see any increase in performance ?

Cheers,

Will

[liammonty]

To be honest, it's impossible to say without knowing how your particular car goes. I've a LC 1098 in a 1968 2 door, and a HC 1098 in a 1968 Trav, and the Trav is a bit quicker, though to be honest, the LC car flies, and is bog standard. The engine's in good order though. My point is that a good LC engine can go quite a bit better than a tired HC. If you're rebuilding yours, I should think it will be fine kept as standard. My Trav (HC) happily pulls a 3.9:1 diff, and it' has bags of torque. I suspect if you rebuild yours well, to standard spec, it will perform really well compared to the tired modified engine you've got at the moment.

[mike.perry]

The only difference is the dished pistons

[youngcamper]

The only difference is the dished pistons

Yep I know that, although it supposedly looses the engine 5bhp too...

[liammonty]

Thinking about it, the compression of your engine if it's got a 12G940 head fitted will be around the same as, or a little higher than a standard HC engine anyway, as the head chambers are around 5cc smaller than on the 12G202 head (which would more than compensate for the dish in the pistons). It sounds as though it's not the CR that's the issue here- just that your engine is knackered! You're right- standard HC is meant to be 48 bhp, vs 43 bhp for the LC.

[bmcecosse]

The real problem is the hopeless tiny twin carbs - and the general state of the engine. Go HC with the 940 head (sink the valves slightly i you need to) and a good HS4 (or HIF38) carb on a good alloy inlet. You won't be disappointed.

[youngcamper]

The real problem is the hopeless tiny twin carbs - and the general state of the engine. Go HC with the 940 head (sink the valves slightly i you need to) and a good HS4 (or HIF38) carb on a good alloy inlet. You won't be disappointed.

I only have a single HS2 . Just as I thought, I'll go 12G940 and HS4 . Not to worry BMC valves are already sunk

Cheers,

Will

[bmcecosse]

Ahh - sorry - thought I read carbS It's still the reason for lack of GO. That carb is very much the limiting factor for power on the 1098 engine (and twins are no better because each piston still sees only ONE small carb..). As you are doing a build - you should take the chance or a better cam - MG Metro or 256 or 266 will work well....... Just watch the total lift if you go 255/266 and check for ex valve clearance.

[youngcamper]

Thanks for the advice BMC, I think I'll probably leave the camshaft stanard, as I don't want to go too far from a stock engine that I can revert to if I don't like the setup. I've got a HS4 already which is ready to go, I'll saw off the exhaust section of a stock manifold and then fit the HS4 to a separate inlet.

[smithskids]

Hi, I have a 1098 with a piper 270 cam and it runs well with the bigger HIF38 carb like BMC says. It doesn't come onto the cam until about 2750 rpm but it revs well. This type of cam runs better in the 1275 eng.
Cheers.

[bmcecosse]

270 is too much for a 1098 - simply because it must not be revved over 6000, and so loses some at low revs but can't use the high revs capability. The standard AEA 630 cam works well - and 'lugs' nicely from low revs even with the 940 head - and against the mumblings of the doomsters who witter on about 'low gas speed' etc. It works fine - but an MG Metro cam would be better I reckon - since it was specifically designed to make the most of the A series engine UP TO 6000 rpm in the Metro.

[youngcamper]

270 is too much for a 1098 - simply because it must not be revved over 6000, and so loses some at low revs but can't use the high revs capability. The standard AEA 630 cam works well - and 'lugs' nicely from low revs even with the 940 head - and against the mumblings of the doomsters who witter on about 'low gas speed' etc. It works fine - but an MG Metro cam would be better I reckon - since it was specifically designed to make the most of the A series engine UP TO 6000 rpm in the Metro.

ohh interesting stuff, I may have a look at that then. I'll also be reading vizards bible tonight...

[MarkyB]

Don't get carried away with HP numbers, the Americans have a saying that covers it " You buy horsepower but drive torque"
An engine with grunt is much nicer to drive than one where you have to rev the life out of it to get some oomph.

[youngcamper]

Don't get carried away with HP numbers, the Americans have a saying that covers it " You buy horsepower but drive torque"
An engine with grunt is much nicer to drive than one where you have to rev the life out of it to get some oomph.

exactly why i don't like the LC engine I've got, it requires a fair bit more revving that the standard HC engines in my brothers 2 door and Dads van which just seem to purr along

[rayofleamington]

smaller carbs are fine if you want it to purr and have best all round driveability - bigger carbs if you want ultimate horsepower...
IMHO, (based on experience).

[youngcamper]

smaller carbs are fine if you want it to purr and have best all round driveability - bigger carbs if you want ultimate horsepower...
IMHO, (based on experience).

Interesting, I'll take that on board, I guess there is only one way to find out which I will prefer. I can always change it back later...

[bmcecosse]

The real beauty of the SU carb is that it self adjusts the choke area depending on the air flow demand from the engine.....So a larger carb works just as well at low air flows (within reason of course) - yet is able to pass large air flow when required. The rate of flow is set by the strength of the spring above the piston. The rate of change of that flow is set by the viscosity of the oil in the piston damper. This is the huge advantage the very clever SU design has over most other carb designs.