Su carb v hs carb the difference?

[Ljdavek]

Hi I’m Dave a total newbie to the piston heads ... I have a 1988 mini 998cc and it’s fitted with the su4 carb and original air intake... Well I’d like to get a little more power out of it so was thinking of a K&N air cone filter and maybe change the carb to a HS one it already has a stainless steel exhaust strait though with a single 2” approx back pipe... could anyone advise please..

[dojo]

The exhaust is prob too big for the amount of power you have in the engine - so will be costing you power, but sound good!
Unless its running super hot that is! I have 1"75 on my 1293.

You could get a better air filter like you say to help it breath.

I'm not sure what the best Carb is for that size engine? I use a HIFF44 on mine - maybe a HIFF38?

Presume as you have a 2" exhaust you have a LCB manifold?

You could also pick up a 1275 head and put that on to get some more power, be pretty effective

[Crosswise]

I think you're a little confused on the carb. Your car will have an SU HS4. SU are the manufacturer, Skinner Union, HS is the model, it is reasonably easy to identify as it will have a separate float bowl connected to the jet by a tube. Finally, the 4 refers to how many eights of an inch over one inch the diameter of the throat is, ie yours is 1 1/2".

A good upgrade for a mostly standard 998 would be a SU HIF38 (38mm throat) They were fitted to all 998 Metros in the late 80s. A HIF44 would be too big and you'd lose some low down torque without any real gains. Stage one kits are widely available, they contain a replacement inlet and exhaust manifold, a larger bore exhaust, cone filter and a different needle for the carb. The inlet manifold will be some kind of copy of the MG Metro item, they flow really well. The exhaust does not need to be 2", I run that on my 1293 turbo with 160hp and it's not restrictive, it would be completely excessive on a standard 998.

In my opinion, it's not worth doing more than that to a 998, spend any more and you may as well upgrade to a 1275. But for the sake of completeness, a rough idea of what a good road tune 998 would involve is as follows.
12g295 cylinder head, either skimmed or replace the pistons with flat top, bring CR upto 10.5:1
Kent 256 cam, or similar.
Twin HS2 carbs
Maniflow 1 3/4" twin box system with 3 to 1or LCB manifold

That would give you a really broad torque increase all through the rev range and would be perfect on the road.

[dojo]

Crosswise said:

I think you're a little confused on the carb. Your car will have an SU HS4. SU are the manufacturer, Skinner Union, HS is the model, it is reasonably easy to identify as it will have a separate float bowl connected to the jet by a tube. Finally, the 4 refers to how many eights of an inch over one inch the diameter of the throat is, ie yours is 1 1/2".

A good upgrade for a mostly standard 998 would be a SU HIF38 (38mm throat) They were fitted to all 998 Metros in the late 80s. A HIF44 would be too big and you'd lose some low down torque without any real gains. Stage one kits are widely available, they contain a replacement inlet and exhaust manifold, a larger bore exhaust, cone filter and a different needle for the carb. The inlet manifold will be some kind of copy of the MG Metro item, they flow really well. The exhaust does not need to be 2", I run that on my 1293 turbo with 160hp and it's not restrictive, it would be completely excessive on a standard 998.

In my opinion, it's not worth doing more than that to a 998, spend any more and you may as well upgrade to a 1275. But for the sake of completeness, a rough idea of what a good road tune 998 would involve is as follows.
12g295 cylinder head, either skimmed or replace the pistons with flat top, bring CR upto 10.5:1
Kent 256 cam, or similar.
Twin HS2 carbs
Maniflow 1 3/4" twin box system with 3 to 1or LCB manifold

That would give you a really broad torque increase all through the rev range and would be perfect on the road.

What would your engine be running be without the turbo?
How have you got there in terms of BHP - I'd be interested know mods to do get towards a 100bhp without a turbo

Cheers

[Crosswise]

dojo said:

What would your engine be running be without the turbo?
How have you got there in terms of BHP - I'd be interested know mods to do get towards a 100bhp without a turbo

Cheers

If I simply removed the turbo it and corrected the fueling and ignition timing to suit, it would run nowhere near 100hp. The simple reason is it has a CR of 8.0:1, everything about it is designed for forced induction. Having previously had a tuned MG Metro engine with a big valve head and Kent 286 cam, I'd seriously recommend turbocharging to get good power from an A series. The MG engine wouldn't have made 100hp, and it was horrible to drive most of the time, running a cam with that much overlap is no fun at all unless you drive everywhere above 4000rpm.

It probably comes down to how much you want to spend really, If you want a 1275 that's nice to drive with plenty of torque, it won't cost too much, but neither will it make 100hp. If you do go down the route of forced induction, it won't be cheap and it's a lot of work, but you'll easily make 100hp and still have a very driveable car.

If you do just want 100hp from a NA 1275, my MG Metro engine wasn't far off the spec you'd need, but it couldn't rev high enough as the internals were all standard and the head would need more work so it could flow better. So basically, a gas flowed cylinder head with the biggest valves you can reasonably fit, high compression forged pistons, lighted and balanced crankshaft and flywheel, HIF44 carb on MG Metro inlet, LCB Manifold with 2" exhaust. Kent 286 cam or something thereabouts, 1.5:1 roller rockers and electronic ignition. That would give you 100hp.