Compression Ratio For Turbo Engines

[DaveEllen]

OK,

Can someone cleverer than me (not difficult :P ) confirm/contradict the following.

When using forced induction (super/turbo charging) over .5 bar its advisable to lower the compression ratio of the engine to prevent detonation/help ensure engine longevity ?

methods of achieving this are thicker head gaskets or different pistons.

Thanks

Dave

[legendswraith]

OK,

Can someone cleverer than me (not difficult  :P ) confirm/contradict the following.

When using forced induction (super/turbo charging) over .5 bar its advisable to lower the compression ratio of the engine to prevent detonation/help ensure engine longevity ?

methods of achieving this are thicker head gaskets or different pistons.

Thanks

Dave

←

The traditional compression ratio for a four cylinder engine is 8.5 to 1. You can go a little higher with multi-point injection. Most non-turbo cars have 9 to 1 or higher compression ratios. You can get away with about 9 to 1 turboed but beyond that, and the boost you add with the turbocharger will cause pre-detonation and blow your engine. Also, the higher the compression ratio, the less boost you can pump into the engine. So you will need to add low compression pistons ideally to get more boost

the fuel system delivery will have to be upgraded. You'll need higher flow rate injectors, a higher pressure fuel pump and sub-pump, and a rising rate fuel pressure regulator. Without the additional fuel, the engine will run too lean under the boost of the turbo, and will detonate, and blow.

It was a bit of a cut and paste exercise but hope it helps

[ColinBarber]

The traditional compression ratio for a four cylinder engine is 8.5 to 1. You can go a little higher with multi-point injection. Most non-turbo cars have 9 to 1 or higher compression ratios.

These days it is more common to see 10:1, 10.5:1 or 11:1 for NA engines.

[DaveEllen]

whats the compression ratio for a standard IS ?

[ColinBarber]

old engine so it's 10:1

[DaveEllen]

Many thanks

[DaveEllen]

OK,

Just checked old PM's n stuff

Forged pistons that I have are 9.5.1 compression ratio

From the comments above I believe this to be too high.

Any ideas on what safe boost level I can run ?

[tdiplc]

OK,

Just checked old PM's n stuff

Forged pistons that I have are 9.5.1 compression ratio

From the comments above I believe this to be too high.

Any ideas on what safe boost level I can run ?

←

9.5:1 is not too high for the originally intended application, which was 0.8 to 1 bar with the supercharger.

This engine is exceptionally tolerant of elevated c/r levels due to the high combustion efficiency factor that it exhibits. We have seen 0.8 bar with a totally standard engine with effective knock control.

Having moved the goal posts to incorporate a turbocharger instead of a supercharger after we designed the package may require redesigning some of it, as a turbocharger creates higher cylinder temperatures than a supercharger due to the higher exhaust back pressure, which will make it less tolerant of ignition advance.

Purely as an educated guess, I would say that you can run 1 bar with a decent turbocharger system (meaning one that has been designed properly) provided that you use a decent engine management system and you provide sufficient fuel delivery capacity, and you have it all calibrated properly. You may even be able to use more boost but as we haven't been involved with the package that you have ended up with, I guess you are in a "suck it and see" environment.

For your information, we did recently did a 580hp 4 cylinder 4G63 engine with 9.8:1 c/r running 1.5 bar boost.

[DaveEllen]

OK,

Just checked old PM's n stuff

Forged pistons that I have are 9.5.1 compression ratio

From the comments above I believe this to be too high.

Any ideas on what safe boost level I can run ?

←

9.5:1 is not too high for the originally intended application, which was 0.8 to 1 bar with the supercharger.

This engine is exceptionally tolerant of elevated c/r levels due to the high combustion efficiency factor that it exhibits. We have seen 0.8 bar with a totally standard engine with effective knock control.

Having moved the goal posts to incorporate a turbocharger instead of a supercharger after we designed the package may require redesigning some of it, as a turbocharger creates higher cylinder temperatures than a supercharger due to the higher exhaust back pressure, which will make it less tolerant of ignition advance.

Purely as an educated guess, I would say that you can run 1 bar with a decent turbocharger system (meaning one that has been designed properly) provided that you use a decent engine management system and you provide sufficient fuel delivery capacity, and you have it all calibrated properly. You may even be able to use more boost but as we haven't been involved with the package that you have ended up with, I guess you are in a "suck it and see" environment.

For your information, we did recently did a 580hp 4 cylinder 4G63 engine with 9.8:1 c/r running 1.5 bar boost.

←

Thanks for this I'm somewhat reassurred.

I'll be using the HKS FCON-SZ ecu and have sorted the fuel delivery side out(in theory)

full steam ahead then

[legendswraith]

just out of interest (because i have exhausts on the brain at the minute) if you increased the diameter of the exhaust piping and/or silencer to provide a greater area this would reduce the back pressure generated on the engine therefore you would be able to add more boost in regard to the ignition but what is the ideal backpressurethe exhaust should be generating ?

[ColinBarber]

In an perfect world you would want zero back pressure so the engine doesn't have to do any work to expell the gases, this however is not possible and trying to achieve low pressures results in other problems.

Increasing the diameter would reduce pressure but it will also decrease the exhaust gas speed which can reduce power/torque. The mapping of an engine ECU is based on a certain amount of pressure so changing the exhaust may not give great benefits unless other work is also done.

[legendswraith]

i understand wehre you are coming from but a reduction in pressure gives an increase in velocity if through a duct so it would be possible to increase exhaust exit speed if you ducted part of the exhaust so if you went from a larger diameter pipe to a smaller using a convergent duct you would gain an increase in velocity of the exit gasses but a reduction in temperature and pressure (the opposite is true in a divergent duct)

i think i am going to stop here for a bit as i am giving myself a headache

[tdiplc]

Gas glow dynamics is a fairly complex subject (of which I have a reasonable amount of experience).

Exhaust manifold design is even more complex if pulse tuning and conversion angles are to be considered, but there is generally a compromise between gas speed and back pressure. Generally the smaller the diameter the higher the speed and pressure, and visa versa. What's needed is the middle ground where gas speed can be maintained with minimal pressure loss.

If you know what you are doing you can design a nice manifold, but sadly most of the manufacturers are clueless.

[legendswraith]

Gas glow dynamics is a fairly complex subject (of which I have a reasonable amount of experience).

Exhaust manifold design is even more complex if pulse tuning and conversion angles are to be considered, but there is generally a compromise between gas speed and back pressure. Generally the smaller the diameter the higher the speed and pressure, and visa versa. What's needed is the middle ground where gas speed can be maintained with minimal pressure loss.

If you know what you are doing you can design a nice manifold, but sadly most of the manufacturers are clueless.

←

i am just wondering if you could remove a section of the exhaust and put an ejector pump style duct into an exhaust system that would give a good rise in the exit speed for the gasses whilst dropping the pressure at the engine without too much hassle although this is an intellectual pursuit rather than a practical one

you too can have fun with thermal dynamics

[tdiplc]

Gas glow dynamics is a fairly complex subject (of which I have a reasonable amount of experience).

Exhaust manifold design is even more complex if pulse tuning and conversion angles are to be considered, but there is generally a compromise between gas speed and back pressure. Generally the smaller the diameter the higher the speed and pressure, and visa versa. What's needed is the middle ground where gas speed can be maintained with minimal pressure loss.

If you know what you are doing you can design a nice manifold, but sadly most of the manufacturers are clueless.

←

i am just wondering if you could remove a section of the exhaust and put an ejector pump style duct into an exhaust system that would give a good rise in the exit speed for the gasses whilst dropping the pressure at the engine without too much hassle although this is an intellectual pursuit rather than a practical one

you too can have fun with thermal dynamics

←

Got something better than that -something that creates a depression in the manifold to "suck" the gas out May offer it as an option to our new manifold.

[legendswraith]

sounds similar to using venturi effect but you would need a very free flowing exhaust otherwise you would end up with a higher point of static pressure behind it but i gather you got it worked out and obviously don't want to give the game away because if i knew something the other people in my sector didn't know i wouldn't tell them either :D

[RoadRash]

N/A engines mostly run 10:1 up to 13:1 (maximum, but no factory car uses this, but it's possible).

For a turbocharged engine I'd prefer 8.5:1 or 9:1, there I don't see a problem to blow it up to 1-1.5bar boost. :winky: