Because with the lower CR you have more scope to run considerably more power before the CR is a cause of detonation ?? If your goal is only to run 300bhp then I agree totally with keeping the CR as near stock as possible as it improves off boost driveability/response.
Turbo questions...
- Thread starter Siliconslave
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Because with the lower CR you have more scope to run considerably more power before the CR is a cause of detonation ?? If your goal is only to run 300bhp then I agree totally with keeping the CR as near stock as possible as it improves off boost driveability/response.
Incorrect A/F mixture / lack of stoich is what causes detonation. High/normal CR and forced induction, when properly managed with a decent ECU does not lead to detonation.
Even above it, you aren't going to gain anything by lowering the CR. Only lose.
*n
Because, depending on the particular engine and combustion chamber design etc, you are more likely to have detonation problems with higher compression ratios. Why do you think production turbo cars have lower CR's than the normaly aspirated one?
You can use a high CR with limited boost, and it offers many advantages in terms off boost performance and economy, but to make decent power you need to get lots of air in and this is where high CR's cause problems.
Only if you can't get enough fuel in to maintain stoich. Lowering the CR also effects the squish area...Not good.
Twenty five years ago when we had dizzys and mechanical injection, the only safe way to do forced induction was to lower the CR.
Now we have standalone management, electronic ignition and injection control, eight-dimensional mapping capabilities...
There is NO reason to lower CR.
*n
Lowering CR and cranking up the boost to make big bhp figures makes as much sense today as using a dozen Pentium 2-based computers to render CGI for films. Yes, it will get there eventually but it will take a long time and you'll get more errors.
*n
*n
Soldato
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I've seen the natural compression of the engine reduced, and displacement increased by adding a thin slice of block and another head gasket, longer head-bolts required.
Actually...Hang on...
Doing the above wouldn't effect the displacement at all. Swept volume would remain the same...Only the volume of the combustion chamber would increase.
Your cousin doesn't drive an M3 to Manchester does he...?
*n
Doing the above wouldn't effect the displacement at all. Swept volume would remain the same...Only the volume of the combustion chamber would increase.
Your cousin doesn't drive an M3 to Manchester does he...?
*n
You dont want to maintain stoich, you want it richer to keep cylinder temperatures down if you are going to use turbos and plenty of boost.
Having a high CR and using lots of boost potentially causes detonation because compressing the mixture in the chamber causes the gasses to heat up and potentially if compressed enough will ingnite
Should have said 'to make sure it doesn't drop below stoich'.
If you are at the stage where you are getting premature ignition, you are going to be running serious boost...To the point where water injection and radical inlet design is pretty much essential.
*n
I guess it depends on what we are talking about when we say serious boost. On an NA car running standard CR of say 10.5:1 then 10 psi is pretty high boost most NA turbo kits Ive seen are normally set for 7psi, running 1 bar isnt going to be likely. On an OE turbo car when most are running 8.5:1 to 9:1 then 1 to 1.5 bar is no sweat.
If CR isnt the problem then why cant you run the NA at 1.5 bar and the OE turbo car can ??
AFAIK a lower CR and high boost car doesnt have to be a slug off boost, as you can advance the ignition off boost and also play with cam timing if you have pulleys.
Only if you do something horrible like bodge a decompression plate or two head gaskets in. No-one who is serious about turbocharging an engine would do that. It's entirely possible to lower CR whilst maintaining or even improving squish.
In that case you should be trying to re-educate the likes of Ford, GM, Toyota etc.
Accurate fueling and ignition certainly gives more margin to work in, and for light pressure application you are right, there is usually no need to lower CR, but you are never going to make really big power with high CR's if you are running pump fuel.
Nah its all good, Im only going on what I have read. If Im wrong I'll be happy because I would have learnt something as I am actually interested in tuning/engines etc.
edit: Oh aswell as Corkey Bell's book, I also enjoyed reading Forced Induction Performance Tuning by A. Graham Bell
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Can't remember all the calculations and formulas off the top of my head but very basically and very very roughly, if you take a standard engine at approx 7:1 compression ratio and give it 15 psi (1 Bar) of boost you will get an effective compression ratio of around 14:1.
So basically if you start with 10:1 to 12:1 high compression NA engine, and go put a big turbo on it running 1 bar pressure, you will get an effective ratio of 20:1 to 24:1 which is then making your nice petrol engine think it is a diesel with compression ignition.
This often results in a big mess to clean up.
So basically if you start with 10:1 to 12:1 high compression NA engine, and go put a big turbo on it running 1 bar pressure, you will get an effective ratio of 20:1 to 24:1 which is then making your nice petrol engine think it is a diesel with compression ignition.
This often results in a big mess to clean up.
Soldato
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Good point.
Misuse of terms aside; it was done to run a higher manifold pressure, reduce the natural compression of the engine to be more like an engine that's designed for a turbo.