Detonation and Compression Ratio
Turbo Mini Forums

I've been looking at the factors causing detonation in a turbocharged engine. Principally to try to decide on a compression ratio for my K100 Turbo.

Essentially the main factor causing detonation is heat, not pressure. Boost pressure generated inefficiently causes heat, but it is the heat that causes detonation. I read somewhere that 300 degrees C is about the limit for normal pump fuel.

Hence if you can have the boost pressure without the heat, then you can run higher compression ratios, which will then produce more power more efficiently.

We need to understand the thermodynamics of compression. The temperature rise due to compression is represented by the following formula:



T = Temperature (absolute)
P = Pressure (absolute)
n = Gas constant (1.4 for air)

You can assume T1 and P1 occuring at the start of the compression stroke and T2 and P2 ocurring at ignition.

I'm ignoring the efficiency of the compression for the purpose of this explanantion, but T2 will be higher due to the isentropic efficiency of the process.

P2/P1 is effectively the compression ratio.

Hence for a given compression ratio, the temperature rise within the cylinder is the same regardless of the initial value of P1, whether it is atmospheric or boost.

What does this mean for us?

Well, it means that we could run much higher compression ratios provided we run the most efficient turbos and efficient intercoolers correctly mounted to get good airflow. Afterall, if we could get an intercooler that was 100% efficient there would be no need to drop the compression ratio at all.

I'm going to try 10:1 at 10psi on my K100 and then see how it goes from there.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

i think you are on the borderline on pump fuel ther paul ....... be intresting to hear how you got on

even retarding ignition compounds det as you know

We need to consider the dynamic compression ratio rather than the static compression ratio, and that is a factor of the cam being used

At 10:1 i would think you were running in det limitation, rather than MBT

Edited by Sprocket on 23rd Dec, 2007.

You both appear to have missed the point. You've read 10:1 and 10 psi and sucked your teeth.

The point is about the temperature of the inlet charge.

If you plug some numbers into the equation above you get some interesting results.

Take a T3 at 72% effy, an intercooler at 80% effy and an 8:1 dynamic compression ratio then the inlet charge temperature would be 40 degrees C and the temperature after compression would be 294 degrees C.

Take a GT17 at 78% effy, an intercooler at 90% effy and 8:1 etc, the inlet charge temperature would be 32 degrees C and after compression 279 degrees.

Calculating it back, you could run 8.75:1 and still get 294 degrees.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

What about the ambient air temp having a direct effect on both inlet charge temp and intercooler efficiency

If you say that 300 degrees is the limit of pump fuel and you run chamber temperatures of 279 or 294, what happens when the ambient air temps rise, the engine temp rises and a combination of them all, too close for comfort

I still think you would be running the engine in det limitation and that is not its most efficent.

Full engine management gives you control over the ignition, but retarding because of det limitation is not where you really want to be, you are wasting torque. Sometimes you have no choice, but thats usualy in low rpm high load areas.

Edited by Sprocket on 23rd Dec, 2007.

OK, I agree.

I've used an ambient of 25 Degrees C in my calcs.

I think that what the calcs show is that if 8:1 with a T3 and small intercooler was OK, then 8.75:1 would also be OK with a GT17 and a better intercooler.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I also found this, which is a quite an interesting read, looks like there is more to it than just increasing the compression ratio.
http://www.modularfords.com/forums/showthread.php?t=51059

i think 9.4: is as high as you should go on static compression ratios, whatever that works out at in dynamic on a standard MG cam.

Increasing the boost, with the GT17 and a very large intercooler with correctly mapped ignition, should a reasonable place to start. after all, 9.4:1 is where people back off from when increasing the boost.

Theres always water injection and nitrous or co2 intercooler spraying for the hotter periods of the year

Or low boost High CR

Edited by Sprocket on 23rd Dec, 2007.

Hi,
Interesting points raised here, as I've just discovered the MPi lump is 10:1.
I've got a GT17, a cossie IC out in the fresh air and fully mapped fuel and ignition.
How do I work out if I'm better sticking with 10:1 or dropping to 9.4?
I'm not looking for anything more than a std g/box could cope with.
I'm looking to keep as much part throttle economy as poss, which goes against the dropping CR.
Cheers,
Gavin :)

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If you want to limit the torque output to say 100 ftlbs then you will only want to run 6-7 psi boost, in which case you maybe OK.

I say maybe because you have not said how you are going to accuratley control the fueling, because no one has managed to do that yet on a turbocharged five port.

Edited by Paul S on 23rd Dec, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I may not have this right but if an intercooler has the efficiencey of 80%, that means it can drop charge temps to within 20% of ambient air temp right? Where does that leave chargecoolers efficiency wise? Could they be really usefull if space isn't too tight?

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Hi,
Paul .. Glad I said mapped and not accurately!
I'm not sure on that point yet, I'm thinking of going TBi as at least I won't be any worse off than using an SU.
I'm hoping that will suffice as I'm not going for lots of boost...

Jay ... Depends on the efficiency of the charge cooler TBH
Cheers,
Gavin :)

Edited by Bat on 23rd Dec, 2007.

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Edited by Jay#2 on 23rd Dec, 2007.

Hi,
Thats going to depend on the efficiency of the rad cooling the water and the chargecooler itself.
However I would imagine it is possible as some large AC systems use water as the transfer medium, rather than refrigerant.
Just remember you can't get rid of heat, just move it somewhere else
Cheers,
Gavin :)

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Refrigeration is all about latent heat.

Heat transfer is all about surface area.

Very interesting thought Paul

Ive been thinking the same, but without all the mathematics

Its something id like to experiment with in the future

Edited by BENROSS on 23rd Dec, 2007.

Hi,
Going by that graph I might just squeeze 6 psi in there!
Cheers,
Gavin :)

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On 23rd of Dec, 2007 at 05:41pm BENROSS said:

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Are those graphs for static or dynamic CR

If they are for static, they would apear nonsensical, the dynamic CR and the end result is dependant on the cam of choice. Id be tempted to say that the only acurate way to draw a graph like that for it to be of any use to everyone is by using the dynamic CR, that is the end result.

Not doubting the information, its in print and most probibly authered by some one FAR more knowledgeble than me

Edited by Sprocket on 23rd Dec, 2007.

I'm sure that it is intended for static compression and will have been drawn with a large safety margin.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Hi,
You'd need a large safety margin if you're going to use it on a 5 port head!
Cheers,
Gavin :)

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I think TD clarified before that those graphs are for US octane ratings.

yes that’s true jay, here in the uk you can add 3 points to that

paul, these are static its a guide and no more! and .... its not too far off the Mark!!!

if you look outside the box you dont know whats in the box because your looking outside .................

Anyway paul, i hear what your saying buddy, its hard to push the envelope a little without using Octane.. which i do and many others on here

theres too many Parameters, ambient temp, intercooler efficiency, exhaust tail pipe temps etc .....these all equate in the long term and would take hours and hours to sort on one particular set up alone but.. I respect the envelope your pushing paul

Hi,
When you say add 3 points to that, do you mean our 95 is the same as 92 on that graph or is it 98 on the graph?
Cheers,
Gavin :)

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