Somebody want to explain calculating compression ratios?
Turbo Mini Forums

Since they're so important can anybody explain this?

With an example would be good and how to go about getting the ideal CR.

What chamber sizes are possible?

Thanks, just want to make sure my thinking is correct.

36cc's have been seen in heads before, but there is not much chamber shape left.

Team Racing

1st things 1st... the CR most people talk about is static Compression Ratio, this is easily defined...

cylinder volume x combustion chamber volume
CR = --------------------------------------------------------
combustion chamber volume

Cylinder volume is simple, bore and stroke. Combustion chamber volume is a touch more complex as you need to take into account all the volumes that exists, piston dish, ring land volume, head gasket thickness volume, deck clearance and actual combustion chamber volume in head.

If you want to change the CR you have several options, the easiest of this is to carve out the cylinder head so its volume is bigger. The problem is that this can only get you so far, there simply isn't that much material in the head. i don't have figures for what the standard volume is but your going to be lucky to get more than 32cc in total apperently. So the next easiest solution is to use low comp pistons, these either have a bigger dish, a lower height or a combination of both.

For most turbo cars running basic controls, like ours do with dizzys and carbs, your looking at an optimum CR of 8:1.

One last point to note is the effects of dropping the CR on performance, economy and emmisions... As you drop the CR so the off-boost performance worsens. Basically, the engine will feel flat off boost, this tends to give the classic real kick in the back that many turbo drivers love! Economy is also badly hit, which is why BL stuck with a high CR for the Metro as they where already limiting the amount of power they wanted.

Any good?

Alex

AlexF

Whilst Static CR is what is discussed here, in plenty of detail by Alex, Dynamic CR is also worth a quick mention - and why Super high lift cams are not the best for a turbo.

As you know from nat-asp race engines, these are super lumpy 'off cam' then spring to life, transforming the engine over 4000RPM. This is when they start to become dynamically efficient - making shed loads of power. However - they still have a lower dynamic CR than a 'soft' cam - hence why the CR has to be increased on a nat-asp engine to get the full benefit.

Throw the same super-dooper long duration cam into a turbo, and you straight away have a cam that has an off boost CR even lower than if it had the factory cam in there. A low CR off boost prevents the turbo spooling up as quickly as it would with a higher CR, so off boost performance is reduced - and this isn't even taking into account the cam is not even running efficiently - ie is "off cam" at low RPM.
Only when boost begins to be generated, and the dynamic CR increases as a result of the increased cylinder filling by virtue of the forced induction, does the cam start to perform - but the CR will still be lower than if a factory cam was in there.
Trying to optimise the static CR around this, to get the best out of the camshaft and boost pressure is also very difficult to judge. There is a lot of mathematics behind this - or plain simple dyno testing to achieve the optimum.

What this simply means, is that you may well find that a 300 degree cam allows you to run a lot more boost before the limit of compression is reached, but it is a fine balance between where the point of high boost and high duration cams converge, and then become inneficient - or put simply, the power band narrows, and narrows to the point that a lot of power is made, but a lot higher up the rev range... Exactly what is not needed for a road engine...

If any of you win the lottery and want a genuine 275hp A-Series engine, buy a Dyno, pay my wages, and i'll get you your power! With unlimited funds, this IS possible, of that i'm sure. Just don't ask me to find a gearbox that will cope with it!

D

Edited by turbodave16v on 12th Nov, 2003.

Dave is dead right :)

Another point is the reason why modern turbo cars in competeion are incresingly moiving towards high CRs to get round restrictors... At low RPMs turbos are similar to NA cars, they respond to the same tuning tricks and have similar volumetric efficiencies... but as you go up the rev range (and hence add boost) the VE of a turbo engine goes through the roof, where as on a NA car is remains static (mostly)!! A restrictor limits the amount of air that can physically be drawn into the engine, so at high rpms the VE is dragged down.

On the subject of cam timing, very little attension seems to have been focused on the turbo A-series... I expect theres alot of modern thinking that could be applied to this area.

Dave, shall we go on to explaining the Miller effect? LOL

alex

AlexF