flak monkey
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| posted on 26/8/09 at 11:56 AM |
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Boosted compression ratios and bypass valves
OK, so I have been in talks with a couple of companies and it seems the duratec will take 11psi on std internals all day long. However this is using
the otherwise std inlet/exhaust etc and is only making 240bhp or there abouts.
It looks like with the addition of throttle bodies and forged rods and pistons 320bhp and 220lbft is easily achievable and this is what I am aiming
at. I am just waiting to hear back about the spec of this engine, i.e if it is running std CR or lowered with higher boost.
Given that people are successfully running 11psi on std internals suggests to me that the Duratec is relatively knock resistant in its design. So what
would you do? Would you lower the CR by 1 point (taking it to 9:1) and run higher boost, or would you leave it at the std CR (10:1) and run a lower
boost? At the end of the day surely both are going to end up at almost the same power output as the boosted CR is going to be nearly the same (approx
18.4:1)?
What would you do?
Second thing, byass valves. I have decided to go for the simple method of sizing the pulley correctly to give me max boost at max rpm. So I need a
simple bypass valve, seems like the bosch ones are common and the best is the one from the Saab 9000 turbo. I am not bothered about making lots of
noise dumping excess boost to the atmosphere so would I be better to recirculate the dumped air? (assuming that this is already hot and putting it
through again is going to heat it up some more) or would it be better to dump it to atmosphere through a quiet dump valve?
Choices choices.
[Edited on 26/8/09 by flak monkey]
Sera
http://www.motosera.com
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nick205
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| posted on 26/8/09 at 12:15 PM |
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I guess it dependsto some extent how much money you want to plough into it. As you have the stock motor already I'd go for stock + lower boost
and see how it performs.
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alistairolsen
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| posted on 26/8/09 at 12:21 PM |
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a boost figure is useless on its own.
For instance take a turbo engine. If you want to run 11psi at 3krpm like most standard 2l turbo oe engines do by fitting a small turbo and a flatline
boost control (do not ever allow more than 11psi) then you need a lowish cr to prevent massive Peak Cylinder Pressure.
If you do what the honda boys do, and fit a larger turbo and a proper boost controler to prevent the midrange spike and just use the boost to fill in
at the top end, then you can effectively run massively more boost pressure (20+psi) on std internals because they dynamic cr and the VE are lower at
those revs.
Your supercharger will start to make boost early, unlike a huge turbo, bu the boost comines in linearly, unlike the small turbo, so is more akin to
the second example above. I suspect if you got for your 11psi at at max rpm you will find that you only have half that or less at 3k, and hence you
10:1 cr should be fine.
Always run as high a cr as possible to keep the response!
My Build Thread
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MikeRJ
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| posted on 26/8/09 at 12:21 PM |
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quote:
Originally posted by flak monkey
Would you lower the CR by 1 point (taking it to 9:1) and run higher boost, or would you leave it at the std CR (10:1) and run a lower boost? At the
end of the day surely both are going to end up at almost the same power output as the boosted CR is going to be nearly the same (approx 18.4:1)?
It doesn't work like that; to end up with the same dynamic CR when you start with a lower static CR, it means you are compressing more gas, so
you get more power.
Lower static CR means off boost performance and economy suffer more however.
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BenB
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| posted on 26/8/09 at 05:10 PM |
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Aha! The holy grail of superchargers- good low rpm boost without terrible inefficiency at the top end. Not best to start making boost only to vent it
as your parasitic losses will go up hugely and blowers only like going round so fast....
Try looking into Lysholm superchargers. They offer a good compromise.
Then again do you want low-end power? Too much power low down in the rpm range can cause issues in a light car. A centrifugal blower makes little
power at low RPM but comes on strong as you go up the rev range.
I suppose it depends very much on the application.
But generally with belt driven superchargers (where you have to take parasitic losses into account) you don't generally want to be making boost
and then venting.... (except on rapid throttle closure when the BOV is necessary to avoid bending your butterfly).
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beaver34
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| posted on 26/8/09 at 06:52 PM |
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i spoke to richard at omex in length about charging my duratec, and he wouldnt really advice i ran it on standard internals if it were used in anger
on track etc..... the odd blast on road said it would prob be o.k, but if you speak to jamsport they say its fine etc.....
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