I honestly think you are going to have trouble running that on anything but some sort of hi-octane brew. It'd probably even want higher octane than the highest likely pump fuel with toluene added to 30%. My calcs came out around 13.6 as well. With a 'stock' thickness head gasket in there, it'd be up around 15.6, by which time you'd want to give serious thought to running methanol.

I haven't cut apart a datto a series head. I have tinkered with minis a while back, which had a similar shaped chamber, and coincidentally around 21cc. Got them modified to lower the comp ratio - and about the most they would go out to was around 28-29cc (28 for mine). They ran dished pistons, so the comp came out a little more friendly.

If you ran 28cc chambers, and a 2mm gasket you'd be at around 11.25:!, or about 12.5:! with a 'std' thickness gasket and you'd pick up some useful quench as well. Being a modest bore size (compared to big pushrod V8s etc) and an alloy head, with the quench gap more optimal - and (assumption on my part) a fairly hairy cam, with late intake closing, you might get away with that on pump fuel, or pump fuel with a little toluene in there.

Of course there is another option worth a thought - water injection. It can be setup affordably, and that big initial purchase/install out of the way, it's dirt cheap to re-fill. It'd take some experimenting to find how much water to use. I've read some NACA papers (predecessor to NASA, all aircraft research/test/reports and papers) which involve the use of water injection, tested mostly on military aircraft engines (including testing to complete destruction).. Anyway, on big high output supercharged engines, they tested adding water from anywhere from around 10% extra water (vs the amount of fuel going through per minunte) to equal amounts and above (*i.e. for ever lb of fuel per minute, they added a further lb of water) out at 1:1 the anti-detonant ability was extreme, but they also got to the point where there was water getting past the rings and contaminating the oil - so at that level one would have to change the oil each race. Obviously you don't have to go that far with water supply rate - anything from 10-25% is more typical on road cars. Another interesting thing from the plane testing was that since rich a/f mixtures are less prone to detonation or preignition due to the cooling effect of extra fuel (which ends up not getting fully burnt) well, they could lean the mixtures back closer to stoich (still richer than stoich mind you, but more like 12.5 or something rather than 10:1), but use extra water rather than extra fuel and get the same power output and safety. Water of course is practically free, so being able to use it rather than a massively rich mixture has some incentives.

Sorry for the detour this post took :)

I will find out 2nite i hope, see im running A14 pistons wich im going 2 have machined down to flat tops. Because of the 82mm stroke of the A15 crank. Will take measurements.

Sorry Clive, i cant remember. Its around .200" from memory. That could have been for turbo applications tho. The minimum depends on piston material and bore size also.
A question for an engine builder..

It would certainly be the best way to drop the comp. Leave a squish band around the edge of the piston. you wouldnt have to take much off.

If you could take 2mm off the top, leaving an 8mm squish band would drop your comp ratio to 11.6:1

so you would be removing 61.5mm diam x 2mm deep material.


Edit. this is presuming pistons are flush with with the deck. You might be able to get a little bit out of the combustion chamber also.

What crown thickness is acceptable?

I did the math too, and that seems right. What about deck clearance? Do the pistons come up flush to the deck?

I wouldnt touch the head. what is the crown thickness on the pistons? you maybe able to machine a slight dish in them. Its not a very big job in the right hands.

okay lets try : my gx head is currently : 21 cc on in the combustion chamber.

bore will be 60thou so 77.5mm
Stroke 82 mm A15 crank.
(77.5 * 77.5 * 82 * 3.14(pi))/1000 = 1546cc

so 1546/4 to get to a single cylinder would be 386.622 cc

Im running flat-tops so there is no gain except for the small pockets for the valves.

Headgasket if its 2mm would be (if i work on 78mm bore)
78 * 78 * 2mm * 3.14 = 38.207 devided by 4 = 9.55cc

So a single cylinder = 386.622cc
Head gasket = 9.55cc
head = 21cc

Compresion ratio = 386.622 + (9.55 + 21) / (9.55 + 21)
= 13.6 : 1 about.

Is this right ?

So if it is im considering to have the head metal-sprayed back to std spec...

You can get custom one off cometic head gaskets in mormal thickness but Im sure they come in other thickness options. Supposedly much better than copper.

Hey Clive,

I am in the same situation as you. My GX head has been milled so much [to run methanol] that I now need to drop the compression ratio.

For me the best, cheapest option is a thick head gasket, the other option is if I run a different head with a different manifold with different carbs.....this all costs extra money though.

Just out of curiosity, what is your compression ratio at the moment? Also, what is the volume of the combustion chamber [cc]. Reason I ask is; you may want to consider 'opening' up the combustion chambers to a larger capacity, that is what I am considering, instead of the thick gasket [and all the sealing issues].

Other option is using a high octane fuel. Do you guys have ethanol blend fuels in SAfrica?

Cheers,
Chris

So you are saying, i wont be making my setup work with n 2mm copper gasket and comp-r of 11:1 ? :( :(

I've tried hylomar and vht copper spray with good results. Got even better with a version of 'three-bond' (can't remember which off the top of my head, but the tube is likely still in the shed since around 2001/2 when I actually used it). I think that 'hondabond' might be repackaged threebond, it anecdotally works as well and might be easier to find in some places but likely not cheap.

Within reason, it's not a great way to reduce the comp ratio, because it increases the quench gap to a such a distance that it no longer 'works' at preventing detonation. Don't get me wrong, if there's no other way, then it's 'the go' but using a really thick head gasket to drop the comp ratio - if you could hypothetically run tight quench clearances and use a dished piston (dish to mirror the chamber shape) you might be safe with a cr of 9.5:1 and a given fuel and boost, but with a thick head gasket and no quench, you might have to go down to 8.5:1 to be safe - quench/flame propogation is a big deal to say the least.

Having said that, if it's detonating or pre-igniting, running the thick gasket and lower than 'optimal' comp ratio is still going to work miles better than destroying and engine!