What is the difference between these two cylinder heads?
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Re: What is the difference between these two cylinder heads?
I wouldn't be so sure Phil......you won't hear the detonation (much worse than pinking) ......but it could be there. Fortunately your use of the engine is in 'short bursts' of power - so it may not be a problem. Are you allowed to use Tetraboost - or must it be straight pump fuel?
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philthehill
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Re: What is the difference between these two cylinder heads?
bmc
Most runs are well less than 2 minutes so yes short sharp bursts of power.
Only straight pump fuel unfortunately.
Most runs are well less than 2 minutes so yes short sharp bursts of power.
Only straight pump fuel unfortunately.
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daveaseries
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Re: What is the difference between these two cylinder heads?
On the whole, I agree with everything said here, but a couple of things may be worth mentioning. . .
On today's petrol, the ideal compression ratio for a road going engine is generally considered to be 10:1 or thereabouts.
So for an A+ 1litre your combustion chamber volume wants to be 24cc, or for an older engine - 948 or 998, 22cc or maybe 21cc.
This is why a 12G940 goes particularly well on the older engines as it gives much bigger valves and ports and automatically ups the compression all in one hit, with it's 21cc chambers. . .
1098 is a bit of a grey area for me. Certainly, a 12G295 in completely stock trim, unskimmed, makes a marked difference, even with standard carb and manifolding - I've tried it. But pistons may need skimming to correct the compression if using a 12G940 - it's not a good idea to recess the valves as this impedes gas flow and can affect the rocker ratio.
I did calculate, once, that a 12G295 would need as much as an 80thou (2mm) skim to correct the compression when used on a lesser engine (than the 1098 Midget engine it came off) , but it was such a long time ago I can't remember what the exact application was. This really is where you have to get your burette out and do the sums. And by the way, whatever you are doing, it is often worth decking the block to the pistons before you go anywhere, especially if your combo needs the compression raising.
Most importantly, do not try using a 940 on any sub 1275 engine (save the 1071 and 970 of course) unless you take into account the valve overlap with the block. Any A+ 1litre, or any engine with anything other than a standard cam, will need pockets machining into the block face so that the valves do not contact. A standard, pre A+ engine will be fine as the cam does not have enough lift. . .
And don't think you will pick up a 940 on the cheap - prices are catching up with the 295 and 206, and they are all getting rarer. In fact, in many respects, it's better value just to build a 1380 and have done with it. . . .
D
On today's petrol, the ideal compression ratio for a road going engine is generally considered to be 10:1 or thereabouts.
So for an A+ 1litre your combustion chamber volume wants to be 24cc, or for an older engine - 948 or 998, 22cc or maybe 21cc.
This is why a 12G940 goes particularly well on the older engines as it gives much bigger valves and ports and automatically ups the compression all in one hit, with it's 21cc chambers. . .
1098 is a bit of a grey area for me. Certainly, a 12G295 in completely stock trim, unskimmed, makes a marked difference, even with standard carb and manifolding - I've tried it. But pistons may need skimming to correct the compression if using a 12G940 - it's not a good idea to recess the valves as this impedes gas flow and can affect the rocker ratio.
I did calculate, once, that a 12G295 would need as much as an 80thou (2mm) skim to correct the compression when used on a lesser engine (than the 1098 Midget engine it came off) , but it was such a long time ago I can't remember what the exact application was. This really is where you have to get your burette out and do the sums. And by the way, whatever you are doing, it is often worth decking the block to the pistons before you go anywhere, especially if your combo needs the compression raising.
Most importantly, do not try using a 940 on any sub 1275 engine (save the 1071 and 970 of course) unless you take into account the valve overlap with the block. Any A+ 1litre, or any engine with anything other than a standard cam, will need pockets machining into the block face so that the valves do not contact. A standard, pre A+ engine will be fine as the cam does not have enough lift. . .
And don't think you will pick up a 940 on the cheap - prices are catching up with the 295 and 206, and they are all getting rarer. In fact, in many respects, it's better value just to build a 1380 and have done with it. . . .
D
Re: What is the difference between these two cylinder heads?
In theory -all above is correct. However for a 'cheap and easy' power lift - recessing the ex valves is quick and inexpensive, and the advantage over the old head is so enormous, the amount lost in the recesssing (and KC says it HELPS 
) is tiny by comparison. And yes - an 80 thou skim is the norm for a 295 on older engines with dish pistons. I see standard 940 heads selling for ~ £50 and all the 206/295 heads seem to be in excess of £100 these days - some are WELL in excess ! 
) is tiny by comparison. And yes - an 80 thou skim is the norm for a 295 on older engines with dish pistons. I see standard 940 heads selling for ~ £50 and all the 206/295 heads seem to be in excess of £100 these days - some are WELL in excess !