7 replies to this topic

[Paulb53]

Looking for some advice on building a decent race engine, want to keep it 1275cc (A+)
Is standard apart from a stage 1 kit (LCB, minisport intake, carb spring, jetting)

Anyone built something along these lines hoping for 80-100bhp N/A
Thinking high comp pistons, race cam, alternative carb options?

Cheers

[jaydee]

with a NA 1275 is very difficult to get over the 80 bph mark, unless you go for some expensive route like a 16v head.

Rally and racing are two different things, a racing cam wont have enough torque for rallying, a rally engine wont have the ultimate power. I think an SW10 cam will sit in the middle providing bags of power and a decent torque, but you'll have to rev it above 4k rpm otherwise will be simply slow.

You'll need a stronger bottom end (fully balanced, centre main strap) an a pair of HS4s.

As always, more detailed the description of what you're going to do with the engine, the more help we can provide on the forum.

[Paulb53]

Thanks for the reply, engine will be used mainly for a few track days and some classic rally's. I may even consider bore changes but really wanted to keep it 1275. Have a dyno print out with 59bhp from the previous owner, internally the engine is standard.

Cheers

[Cooperman]

For classic rallying (i.e. Historic Rallying), you will need around 90 to 100 bhp to be competitive. For a race engine you need to be looking at 120+ bhp.

As 'jaydee' says, the two are very different.

 

For the rally engine you need:

 

Good pistons, typically Karl Schmidt, Omega or similar. Capable of sustained 7000 rpm+ at around 11:1 CR

A rally cam. The 286 is what I always prefer.

A well gas-flowed big-valve head with 35.6 mm inlets and 30.5 mm exhaust valves.

A centre-main-bearing strap.

Good carburation, typically  twin HS4 SU's or a Weber 45DCOE (refer to the regulations if going with a Weber)

Lightweight flywheel,

Fully balanced

Duplex timing gears.

 

The gearbox will need to have Straight-Cut Close Ratio gears and a low final drive ratio, probably 3.9:1

 

A distributor with a customised advance curve will be needed (refer to the regulations and/or call Aldon Automotive for advice)

 

To carry the power to the wheels it is sensible to fit competition drive shafts which have the outer CV joint circlip groove deleted and a collar welded on instead.

 

There is a lot more to be considered, but that is the basics for a 90 to 100 bhp engine.

 

Now, a 120+ bhp race engine is a different matter and will involve a lot more.

 

In all cases, accurate build is essential.

[Paulb53]

I have a s/c box with 3.75 crown and pinion set up from MED, set of twin SUs, ST544 minisport race/rally cam which I may change, flywheel is 4.5kg, was just really piston and bore options. Would be interested to see some other 1275 rally / sprint engine builds and see what is commonly used.
Carb set up correctly can be the biggest gain in performance in my experience. Have been looking for a webber as I always preferred them.
Anyone got a big valve head and webber for sale?

[Cooperman]

My rally engine is: 11.1:1 CR, fully gas-flowed head with 35.6/30.5 mm valves, 286 cam, 1.5:1 roller-tip rockers, 1310 cc (+0.040"), twin H4 carbs on ally inlet manifold, Maniflow LCB with twin box 1.75" ID centre-exit system, lightweight steel flywheel, SCCR gearbox, 1:1 SC drop gears, 3.9:1 FDR, X-pin diff, comp drive-shafts, Aldon custom distributor, duplex timing gears with vernier sprocket, Karl Schmidt pistons, competition clutch, et. The build was done very accurately and everything is balanced and matched.

Power is strong from around 5000 to 7200 rpm, with peak at 6400 rpm.

[Earwax]

thread hijack - apologies;  Cooperman what sort of flow figures does a well ported head produce- either head alone or on a manifold also.  Cheers D

[Cooperman]

I can't quote flow figures without looking it up. I'll take a look later.