Short stroke engine
Turbo Mini Forums

As Roger says - low cr big boost is so late 90's! if you look at the trend of cr ratio in boosted engines as electronic fuel/ignition control has developed you will see the cr's rising and boost levels not reducing that much - the key is in fuel/spark control and charge temp control. For a clockwork ignition system and a carb I too would run low cr to avoid det. For FI/mapped igntion wit hgood boost control you will be able to run higher cr's. I'll let you know how the SC supercharged motor copes on a moderate cr - I'm hoping to have it together in Sep time - if it spits its dummy out you'll know about it.

as an extreme example take the Toyota engined supercharged Exige car - take a guess at the cr.......9:1, no, 10:1 no, 11:1 no, 11.5:1 - yes, and this is up to 16psi in the GT3 car!

-no fancy fuels - just good control.

JK

If Carling made Mini engines
it would probably be like this one!

i think good control is correct,, but a big part of the toyota engines cr is the variable cam timing .

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Just looking at this again for myself

based on using the 1.6mm SC gasket

73.5mm pistons and 9:1 CR target piston dish requirements are approx.

8v 13.8cc
16v 17.8cc

1293/+20/71.12 pistons and 9:1 CR target

8v 11.1cc
16v 15.1cc

So as you can see, assuming that your running full management and can provide a coolant system to keep up with the alloy head, there isn't too much of a problem ..... although it's a challenge to do a 16v 1380.

Every day is a school day ...........

How fast and how expensive ...... the same question...

Hi - its nothing to do with the variable valve timing I'm afraid - the "soft" cams are 25oish deg duration and run till 5000rpm, then the high lift circa 280ish take over - the cars are making full boost at less than 3500rpm - so cylinder pressures are very high from very early in the rpm range.

Look for info on Exige S cars

for reference the GT3 is running cams over 300 degs duration (no variable timing) at this compression - it is all down to fuel/spark/inlet temp control.

Roger is bang on with the cr/piston volumes for my 1293 supercharged 16v engine.

JK

If Carling made Mini engines
it would probably be like this one!

Hi john,
doesn't this one fuelled with e85?

ah thats good info john thanks ,

i think that my point was not that the duration is long or short per se , but that the lca/lobe shape etc, could be varied at the design point to create a more det resistant chamber during running by giving a choice of profiles as the rpm rises and so avoiding the drawback of a system with a compromise situation caused by only one set of timing .

the 300 degree cam will create far lower trapping and cylinder fill at the lower rpm 's and this will allow a far greater cr than rd durations will .

with ref to high boost and high cr .
the saab engines are running a fairly substantial cr ,and sometimes up to 19 psi boost ,
this is my theory:

any cylinder on any engine can only take so much pressure at a certain temp before the fuel will spontaineously light up wether due to preignition or detonation . ..run it cooler and you increase your pressure tolerance ,run it higher pressure and you decrease your pressure tolerance.the faster you can get the burn to take place the higher the tolerance for both.

if your cylinder is at the point of enough pressure and temp to det ,no more boost or temp can be tolerated ,so , usually at max ve ,say 4500,,you may find that 9 to 1 ,and 15 psi , is your max for 97 octane fuel with an engine head temp of 95 degrees c .... but that same engine ,at 5500 ,with a lower ve ,will take 22 psi before it gets to the same cyl pressure and temp that would create det .
and the same engine at 7k rpm ,would take 28 psi to create the same detcon, what ill call detcon 5.
now at 3500 the engine may have a situation where although its ve is lower than 4500 ,its duration of flame travel is slow enough to create a detcon 3 ,where you may or may not get a pocket of flammable gas go off before the flame front gets there .
in an engine with enough control ,the boost pressure can be constantly varied as the cylinder conditions change ,for example , if the cyl is cold ,a flash of much higher boost can be used ,also if the air is lower pressure colder damper etc ,all can allow a higher boost ,and vica versa . another thing is if the engine is running at a higher altitude ,more boost can be used ..all these situations are dancing around this magic combination of heat and pressure.
in my opinion ,no more total power can be got from an engine with a mapped ignition or injection ,provided it has no individual cyl control ,than one could get from a carb and a dizzy when running steady state at a stabilzed rpm ,., but what can be achieved is a far greater ability to cater for transients and area undr the curve ve ,as opposed to max bhp ,where the engine will exist for a tiny amount of time compaired to lower down the rpm range ,,this is where the real gains are to be had with ecu control .
on the other hand ,with individual cyl control ,its different ,and i feel that more can be gained everywhere .this is particularly true with head temp increasing and the need for responsive timing knock related .

another thing that afects the temp of the cyl is the amount of ex gas that gets left in the cyl or even driven up the inlet ,by poor cam design ,but that another subject ,and iv waffled on enough to glaze eyes i feel lol
regards robert .

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Do those Exiges run a restrictor?
Alex

AlexF