Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Theory on low comp pistons for turbo application - is my theory right?

EFI Tuning Fundamentals

Forum Posts



Tech Articles

Discussion and questions related to the course EFI Tuning Fundamentals

= Resolved threads



Firstly now getting into the training package I bought off the back of one of your webinars the other week. Must say I'm loving it, keep up the great work.

So onto my question. Having just watched the "Pressure volume cycle" tutorial below are my thoughts and I wonder if I'm thinking about this correctly, grabbing the wrong end of the stick or just over simplifying it added to the fact the pressure graph of low against high CR pistons was using a theoretical equal pressure at combustion?

I have always wondered why use low compression pistons on turbo engines. People have always said to me its so you can get more air in? This however did not make sense to me as its about the total amount of compressed air at TDC and surely you can use a 9.35:1 piston and just not put as much boost in as on a 8:1 piston and get the same pressure result at TDC? (This is of course putting aside piston strength etc as I'm assuming you can make a 10:1 / 11:1 etc piston as strong as they need to be if manufactured correctly)

In watching this tutorial though (as stated above) my thoughts now tell me that it is more to do with the pressure to push the turbo round. So with a 9.35:1 CR piston you would have less cylinder pressure at BDC than with a 8:1 piston and therefor not have as much available pressure to spin the turbo.

Are my new thoughts correct. Or am I barking up the wrong tree?

Thanks in advance,


For a given bore and stroke, the volume of displacement is the same. However, if you have lower compression pistons, this means the compressed volume must be larger (in order to have the lower compression ratio). Therefore, there is more volume available for the compressed mixture and you can put more molecules into that volume -- thus you have the potential for more power -- but you have to run more boost to get it.

I would guess it is primarily down to charge density and, to a lesser extent, the heat of compression.

In the first, while the mechanical compression ratio is the same, and the dynamic will be similar, because the charge the engine is drawing into the cylinder is already partly compressed the final, fully compressed mixture will be proportionally denser. This may cause the charge to create, more pressure around TDC than is desireable from a mechanical perspective - head gasket sealing, head 'lifting', piston or connecting rod failure, etc.

The other thing is as a gas is compressed it is heated (principle behind how diesels work), the more it's compressed the greater the heating and by reducing the peak compression there is a reduced chance of auto-ignition. With forced induction there is also the initial compressing by the super/turbo-charger so it is already starting from a compromised temperature - this is another reason why it's important to reduce the charge with a charge cooler as far as practical.

There is also the problem of the fuel's characteristics, 'octane' is only part of it.

Hmmm, I wonder if anyone has tried running a high mechanical, high boost spark ignition 'petrol' engine on diesel fuel instead of petrol, or used a blend? The problem with diesel used to be it was relatively slow burning and hard to ignite, but race diesels are already turning over 7k rpm, so a smaller engine, with less distance for the flame front to cover, could potentially wind up much higher?

Thanks for the replies guys. That makes sense about the larger combustion area (but retaining the same swept area for engine size) for more particles and also about the heat build up when actually compressing the particles.

Is there anything in what my thought process was though of needing higher pressure at the end of the combustion cycle to drive the turbo when the exhaust cycle begins. I'm sure I heard somewhere exhaust manifold pressure should equal inlet pressure on a turbo system given that you have to drive the turbo. I guess if you can have less pressure in the exhaust manifold but achieve the same intake pressure that would be good as it means the exhaust process of the engine is more efficient?

Very informative picture

Attached Files
  • Screenshot_20200307_224920_com.whatsapp.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

So from that graph if its correct you might as well run 9.35:1 CR pistons if your running anything up to and including 1 bar boost. Anything over that and you need to lower the compression ratio.

It is not as much as you need to but something to think about considering the goal. If you are building a drag race motor with a lot of boost it's one thing but if you want to get an efficient daily car with maximum throttle response it is another ball game. It all depends on what you want to get at the end of the day.

Not too sure what the graph is intended to represent*, as there are many, many factors - not least the fuel characteristics.

*It is 3:30 am here, though ;-)

The higher the CR the greater work can be extracted from the charge. If the engine isn't significantly knock limited a higher CR will make more power from the same air/fuel. However, often engines are significantly knock limited and you find a very complex system of compromise between compressor/turbine/IC and engine efficiency variance at different engine speed and boost as well as width of power band.

It is a common misconception. The top fuel dragsters are using 6.5 CR and making tons of power. The reason is that increasing CR 1 point up will give you from 1.5 to 4 percent more power whilst decreasing CR every 0.1 would give you 3 percent more power due to increased volume of air available (on forced induction applications). Moreover, increasing the CR improves the thermal efficiency but hurts volumetric and combustion efficiency. So there are quite a few things affecting the best CR for particular application -piston type, engine stroke, combustion chamber type - it is not simply "the bigger the better" thing.

Ah, same principle as NA - increasing the nominal CR can be beneficial but if taken too far then the other compromises required to run that CR actually give a net decrease in torque/power. The graph being representative that sometimes more is less, and vice versa :-)

As with everything, one has to have a grasp of the overall package, how it all works together, and what one expects from it - got one thing wrong and it compromises the whole thing.