I'm interested to see what cr could be used.
Had A14 balanced at the beginning of the year and at he sametime bored it out to 60thou over. The engineering company checked the cr and wasn't feeling comfortable with running more than 10.8 static cr.
We ran the motor with 10.8 static on a 268 cam, bit even with the 280 we put in then they would not raise the cr. They actually had to lower it to het to 10.8.
The motor never pulled the way I expected, but is way more lively.
So the other day I decided to have the timing set on the dyno.
And to my suprice it's actually making maginally less than previously. Which made me think that the cr is way too low for the setup I've got.
Thinking of taking the motor apart again after the season closed to raise the cr proper.

BIG CALL; as jmac explained earlier, heaps of things to take into consideration & heaps of testing.(very engine differs)

So what would be the max safe dynamic CR that could be run on pump gas?

as jmac said; its all about "cam" & combo, ive run 13;1comp on pump fuel 98(huge cam)ran fine. static comp is a starting point,which is used to work out dynamic comp. dynamic comp is what you need to work out (using cam specs).

Umm the head was machined down to all buggery. Very small chambers.

Yeah flat tops. If I did another engine build I'd be using dome tops. The dynamic comp ratio was okay though and I didn't really consider that when I wrote this. But yeah either way it needed a fat shim so I put a 2mm one in until I could afford another head.

You can also use flat faced valves and super thin head gaskets to bump your compression ratio. You could even fill/reshape the combustion chamber if you wanted to.

Been ages but how did you achieve such a high compression ratio? Milling the head? Flat top pistons? Or both?

Hmm thanks for the response. A little late though!

The head in concern is now sitting on eBay.

E85 rocks.

e85 isn't as corrosive as straight methanol. There are some hose alternatives that can handle it. It might be worth an experiment or two. We can all probably access a few litres of methylated spirits (about 95% ethanol, and the rest methanol or somewhere around that ratio, and I wonder if the methanol is added purely to make it unsafe to drink) from any hardware type store. Maybe it'd be worth getting a few litres and putting various carb internals and or hoses into jars full of methylated spirits and leaving them for a month or so and seeing what starts to fall to pieces and what doesn't.

FWIW, there are a couple of people now running e85 in street/strip cars though perhaps not long enough for longevity of components feedback (though they are afaik all running methanol spec fuel systems - injectors and the like). Without question it the jury is 'in' - it allows a lot more boost and makes more power than regular high octane pump fuel. Or more compression safely on the NA engines (and I suspect that even 12.5:1 with a modest road cam, let alone race cam would be safe and even more would be justified)

Frankly the only drawback to e85 is that it takes more of it to make each hp. So fuel consumption goes up (I dunno about 40% roughly). If the car isn't the main a to b getaround, and there is a local source for the e85, then it can be a goer potentially. There's probably less people around who would have direct experience with carb setup for e85, but it's essentially the same basic trend, so their method of closing in on the right spec for gasoline based fuels would apply, just perhaps take a little longer to get it spot on, But certainly doable.

11:1 with 98 octane
and forget E85 unless your going to adress the corrosion issues
E85 eats normal rubber fuel line

And as mentioned... engine specs with cam details will determine your actual dynamic compression

short version if i can :

no such thing as 'X comp ratio is safe, Y is not' -

you have to take into account

cam duration (bigger duration, later intake closing, safer to run more static compression'
And the head material 'alloy = good, cast iron = less comp to be safe'
and the bore size - bigger bore (all else being equal) more likely to see detonation. So the dattos (relative to some big dinosaur pushrod engines) are good on that front.
chamber and piston design - generally dattos have decent quench if you're running a closed chamber version of the head, and pistons of either flat top or a dish shape mirroring the chamber shape - both = safer to run higher compression
rpm range - believe it or not, the slower it turns, the more chance bad combustion events can happen.
timing - just how well is the timing suited to teh combo.
Intake air temp (huge factor) and coolant temp (less of a factor than most realise, but if it soars, you're stuffed, but running it ultra cool won't protect as much as it hurts it when it's too high)
port shape/cross section/entry angle. If it's got good velocity and swirl, it'll suspend the fuel droplets, even mixture distribution, good clean burn, less residual burn going on as the exhaust valve opens, so the exhaust runs cooler, less chance of pre-ignition or detonation.
exhaust valve/seat thickness. Sufficient contact area to conduct heat away from ex valve whilst seated.

So all those things affect it BIG TIME. Just saying 10.5:1 is silly - I can think or more than a few street sensible motors with less than that much comp ratio that would _never_ survive on high octane pump fuel, and conversely I know of engines safe as houses with 12.5:1 (though that is approaching the upper limit..

What should probably be honestly discussed here is that the DYNO PROCESS itself is actually one that can destroy an engine. For a couple of reasons. Firstly, it's impossible to get enough air through there to get full radiator cooling (imo) so you get a heat buildup (and intake temps will climb too) - whereas in real life, for anything except tractor pull competition you are moving darn quick, darn soon, with plenty of airflow to stop coolant and underbonnet temps soaring. On a related note you're also usually only on the power for 10-12 seconds (maybe 15) before you're at the end of the dragstrip or the straight, and are braking. If the dyno run takes longer, you get more heat soak (basically the rate at which heat in the cylinder is conducted away to the cooling system has some lag time and the chamber surface temp can rise, even if the coolant temp looks more or less ok, or a little high but stll safe).

Essentially you have to be frank about the nature of use of the engine. If you're only on the throttle for a short time, then more comp etc can be safe. If it's top speed runs you do, or perhaps hillclimbs, or tractor pulling, you need to be more modest with the comp ratio (certain classes of speedway are also fairly brutal - slower speeds, but full throttle a lot, and often clogged cooling systems when mud blocks airflow through the core etc) and some dyno testing is about as bad as it gets for stressing an engine. I know of one mini specialist who point blank willl not put any mini on a chassis dyno, because it's so likely to damage the engine, they instead do datalogging of a/f ratio and use an accelerometer (basically a g-tech) and even the ego sensor/reader log which can plot rpm vs time, so they can see how quick it spins through each gear.

No offence to the dyno operator, but if they only asked about compression ratio, and didn't look at it in the context of the cam, head, bore, usage - basically all the stuff mentioned above, it might be best not to go there. On the other hand, there is one thing in their favour - they are open and honest and don't want to put a car through their dyno if they think it might damage it...