2 replies to this topic

[Patriot]

Hi,

 

Some advice would be greatly appreciated before I go out and spend money on a new 12G940 head.

 

I am just finishing the build of a new engine, but I am proposing on using the original 12G940 head.

The head has 35.9mm & 29mm valves (A+ Head) but it has no porting.

 

Everyone seems to indicate that the head is the most important upgrade.

 

My question: – is it worth replacing the standard head & would I be restricting the power by keeping the original head.

 

Engine Spec:

1310cc (1275 +40)

Balanced Conrods, Crank & Flywheel

Kent 266 Cam

Twin 1” ¼ HS2 (K&N Filters)

RC40 Twin Box & LCB Manifold.

3.9 Final Drive Diff

 

 

Many Thanks

 

 

[IainStallard]

If you upgrade the head, the carbs may also be worth upgrading to twin 1 1/2" or an HIF44 as they may end up restricting the power at high revs

[Cooperman]

There is no need to buy another head, but the one you have needs to be gas-flowed properly. Even if you buy another head casting it will need gas flowing, so you might as well get your existing one done well.

The valve sizes should be 35.6 mm inlet and 29.5 mm exhaust and the compression ratio needs to be around 10.3:1.

Unless the head is well gas-flowed you will not see anything like the improvement you could be as the rest of the specification is good.

Those carbs are good for up to around 80 bhp, but to get a bit more a pair of HS4's might be better.

Unless you are going to be fitting a straight-cut close ratio gearbox, a 3.9:1 FDR is a bit low for normal use. With the 3.9 1st gear will be very low using a standard gearbox and in top gear you will only be doing 14 mph per 1000 rpm, which means that at 4000 rpm you will be doing 56 mph. I have a 3.9 in my rally 'S', but that runs a SC CR 'box with a 286 cam and over 110 bhp. Best FDR for your engine spec will be a 3.44:1 as the torque band will be wide enough.