12G295 Vs 12G940 Head

[jaguar68]

Hi guys,

I have the following cylinder heads, an unleaded 12G295, skimmed to give a 9.5:1 compression ratio, a standard 1275 A+ head, and a 12G940 head which I believe may be a big valve head.

I intend to use the big valve 940 on an 1275cc A+ engine. However, which is the better head performance wise to use with a 1098cc engine, the unleaded 295 or the standard 940?

Would the 940 head need any modification, such as skimmimg and possibly sinking the exhaust valves, in order to fit a 1098?

Any thoughts?

Regards

John

[bmcecosse]

The 940 is by far the better head - and the 295 will sell for ~ £100 on ebay! Yes - you need to make sure the exhaust valves on the 940 have 320 thou clearance from face of valve to face of head - if not - sink them until there is that clearance. Don't worry about 'unleaded' - just set exhausts to 15 thou clearance - and check/reset every 3000 miles.

[Peetee]

As BMC says the 1098 would be better serviced by the 940. You would still see benefits from the 295,just not as good. The 295 is an excellent head to put on the 948. The engine is hugely improved in every respect with, in my experience at least, no compromises whatsoever. Conventional wisdom suggests that larger valves increase power at high revs but reduces mid range lugging ability (what you need for towing or climbing hills in a high gear, for example). It is quite possible that a large valve 940 on a 1098 would suffer this to some extent, but the benefits of additional power would probably outweigh this drawback.
Fit the smaller valve 940 and sell the other two. With the proceeds invest in an HIF38 carb, alloy inlet and large bore exhaust to really get the best from your chioce of head.

[jaguar68]

Hi Guys,

Thanks for the info. I'm probably going to build the engine with the 12G295 initially, using an HS4 1 1/2" carb on a Mini manifold, using a stainless exhaust. I have all of these bits to hand, from both the original engine in the car and from my '56 Splitscreen project. I have a HIF38 carb and manifold too, bought earlier, when the Splitscreen was going to be fitted with a tuned 1098cc engine before I sourced a 1275cc engine. I don't really want to buy another LCB exhaust system at this point in time, but it may be an option at a later date.

Once the cars are built, I will probably have a good purge of all of the duplicate spares I have littering the garage!!

Regards

John

[bmcecosse]

My engine was that spec - with a modified 295 head - it went well. Then I fitted the 940 head - it went very much better! I urge you to fit the 940 head - it's much better!

[jaguar68]

Ok, Bm, I'm convinced. Which carb would be better, the HIF38 or the HS4?

Cheers

John

[bmcecosse]

HIF 38 - because it's not prone to 'surging' - and it's newer! You want an AAA needle for it.

[GeorgeHurst]

I flogged my 295 head for just shy of 150 on eBay, so defo fit the 940 and use the proceeds of the 295 to fund carb, inlet and exhaust upgrades