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Determining best compression ratio in supercharged motor?

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Determining best compression ratio in supercharged motor?

Postby plovett » Sat Apr 07, 2007 3:50 pm

I'm pretty comfortable picking a static compression ratio for naturally aspirated motors based on cam timing, octane, gearing, vehicle weight, etc. There are lots of dynamic compression ratio calculators to help with this. In general I shoot for a DCR of about 8.5:1 on high octane pump gas.

Here's my question: How do you pick a static compression (and dynamic) ratio on a blower motor?

Sure, just like for N.A. motors there are rules of thumb. Something like: Use 8:1 to 9:1 static compression for low boost non-computer controlled carbureted blower motors. But that doesn't take cam timing into account.

Surely cam timing has just as big an affect on optimal static compression ratio on blower motors. If you have two blower motors, running say 8 lbs. max boost, carbureted, and non-intercooled. One has a 230/240 duration cam and the other has a 250/260 duration cam. They would "want" different static compression ratios, wouldn't they?

Is there a rule of thumb for dynamic compression ratio in blower motors that would allow me to pick the best static compression ratio?

Thanks for any thoughts,

paulie
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Postby Stan Weiss » Sat Apr 07, 2007 4:11 pm

When you calculate Dynamic Compression Ratio how do you adjust for the positive pressure in the intake track?
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Postby plovett » Sat Apr 07, 2007 4:15 pm

Stan Weiss wrote:When you calculate Dynamic Compression Ratio how do you adjust for the positive pressure in the intake track?


I suppose that's pretty much what I'm asking. I don't know how to do that. You said it much more succintly than me though. I can get to rambling. :roll:

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Postby Stan Weiss » Sun Apr 08, 2007 10:18 am

Do any of the calculators you use for dynamic CR estimate cranking pressure, what you would see on a compression test?
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Postby plovett » Sun Apr 08, 2007 12:24 pm

Stan Weiss wrote:Do any of the calculators you use for dynamic CR estimate cranking pressure, what you would see on a compression test?


Yes, the Wallace Racing DCR calculator incorporates boost pressure and estimates cranking pressure.

http://www.wallaceracing.com/dynamic-cr.php

Would you shoot for the same DCR as a naturally apirated motor?

Their calculator says my current N.A. motor has a DCR of 9.16:1 and 189 psi cranking compression. The cranking compression matches what I find on my motor very well. The DCR is higher than the calculator I like to use. The Wallace Racing DCR calculator uses 0.050" intake valve closing. The calculator I like to use takes the advertised intake valve closing point which I feel better reflects the true DCR. Using the other calculator and advertised duration figures my DCR comes out to about 8.5:1. Anyway that's beside the point.

So, if my current naturally aspirated motor runs fine on 92 octane pump gas with what their website says is 9.16:1 DCR, then I should a blower motor run fine on pump gas with the same DCR?

I'm sure there's more to it than that, but would that put me in the ballpark?

thanks,

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Postby plovett » Sun Apr 08, 2007 12:48 pm

plovett wrote:
Stan Weiss wrote:Do any of the calculators you use for dynamic CR estimate cranking pressure, what you would see on a compression test?


Yes, the Wallace Racing DCR calculator incorporates boost pressure and estimates cranking pressure.

http://www.wallaceracing.com/dynamic-cr.php

Would you shoot for the same DCR as a naturally apirated motor?

Their calculator says my current N.A. motor has a DCR of 9.16:1 and 189 psi cranking compression. The cranking compression matches what I find on my motor very well. The DCR is higher than the calculator I like to use. The Wallace Racing DCR calculator uses 0.050" intake valve closing. The calculator I like to use takes the advertised intake valve closing point which I feel better reflects the true DCR. Using the other calculator and advertised duration figures my DCR comes out to about 8.5:1. Anyway that's beside the point.

So, if my current naturally aspirated motor runs fine on 92 octane pump gas with what their website says is 9.16:1 DCR, then I should a blower motor run fine on pump gas with the same DCR?

I'm sure there's more to it than that, but would that put me in the ballpark?

thanks,

paulie


Okay, I played with the Wallace DCR calculator and the results don't change regardless of whether you input boost or not. Input 0 psi boost or 8 psi boost and the DCR and cranking compression doesn't change. I guess that makes sense as I don't think cranking compression wouldn't be affected by the addition of a blower. And DCR is based on static compression and intake valve closting point, not boost. So that doesn't help me. Hmmm.

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Postby sleeper sedan » Sun Apr 08, 2007 8:29 pm

Interesting subject (post)... I've got some thoughts, I'm certainly no expert however. Curious, what were some of the early FORD supercharged engine combinations from the '60's doing for compression/cam timing? Old tech. for sure but may be a point to draw a reference from?
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Postby F1Fever » Sun Apr 08, 2007 10:29 pm

I have put a lot of thought and research into this. I have concluded that the term "dynamic compression ratio" is rather misleading because by definition it's not dynamic, although in real life it is. Ah.. I'm not good with words and it's a deep subject. At any rate I have written a calculator that also takes RPM & boost into play as well as factoring the IVC using both the ADV & DUR@050" figures. In addition to calculating using those figures is recommends a Minimum octane...

http://www.hotroddersauctions.com/Dynam ... ulator.php
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Postby Stan Weiss » Sun Apr 08, 2007 10:46 pm

When you lower your static CR you also lower your dynamic CR if the cam is the sample and phasing has not changed. The cranking pressure that is calculated for a N/A engine is based on 0 PSIG when IVC. Because you are using forced indication when the IVC you will have positive PSIG. the amount of positive PSIG will cause the cranking pressure to be higher that the sample dynamic CR on a N/A engine. You need to balance this pressure to run on what octane you have chosen.
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Postby plovett » Mon Apr 09, 2007 11:58 am

F1Fever wrote:I have put a lot of thought and research into this. I have concluded that the term "dynamic compression ratio" is rather misleading because by definition it's not dynamic, although in real life it is. Ah.. I'm not good with words and it's a deep subject. At any rate I have written a calculator that also takes RPM & boost into play as well as factoring the IVC using both the ADV & DUR@050" figures. In addition to calculating using those figures is recommends a Minimum octane...

http://www.hotroddersauctions.com/Dynam ... ulator.php


Very cool. Thanks! I'll give it a try.

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Postby KennyM » Mon Apr 09, 2007 8:58 pm

To be sure the following don't get lost;
Static compression is a calculated "RATIO" between the cylinder volume plus the combustion chamber volume to 'just the combustion chamber volume'.
It never changes as a ratio unless it is physically changed.

The dynamic compression (not ratio) is a function of the cam timing, rpm and given that the throttle is wide open. Such as for compression pressure testing.
With forced induction, the dynamic compression is added to [in the presence of boost] and can be considered {as if} the CR is much higher. For example; a 9 to 1 motor with boost will have cylinder pressures (as if) the the CR were very high in the 13 to 1 range +/- depending on boost levels.
These 'effective' cylinder pressure ratios are able to be calculated within reason.
This is why hi octane fuel is reguired for boost application over an N/A application.
Cranking with a blower attached gives no more cylinder pressure than N/A because no boost is beng made under those conditions..
Depending on the combustion chamber design and cam timing, 9 to 1 or even a little more can be used for boosts as high as 15 psi.
The higher the cylinder pressured the more good parts are needed.
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Postby John Wallace » Tue Apr 10, 2007 11:22 am

http://www.wallaceracing.com/dynamic-cr.php

I fixed the boost input in the equation.
I 'fat-fingered' the variable. :oops:
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Postby plovett » Tue Apr 10, 2007 5:32 pm

John Wallace wrote:http://www.wallaceracing.com/dynamic-cr.php

I fixed the boost input in the equation.
I 'fat-fingered' the variable. :oops:


Thank you! Any chance you could show us the equations? I am curious.

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Postby plovett » Tue Apr 10, 2007 6:40 pm

sleeper sedan wrote:Interesting subject (post)... I've got some thoughts, I'm certainly no expert however. Curious, what were some of the early FORD supercharged engine combinations from the '60's doing for compression/cam timing? Old tech. for sure but may be a point to draw a reference from?


Hey Sleepersedan! I'm planning a possible centrifugally supercharged Ford FE in case you hadn't guessed.

Have you ever read about the McCulloch variable speed superchargers from the 50's? I think they would work well today. I think they used boost pressure against springs to move the drive belt back and forth on a variable diameter drive pulley limiting maximum boost. So at low rpm it has a high drive ratio generating high boost. At higher rpm as the boost increases it pushes harder against the springs lowering the drive ratio and limiting maximum boost. It's something like that anyway. This overcomes the main drawback of centrifugal blowers which is low boost at low rpm.

Check it out.

http://www.vs57.com/index.htm

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Postby plovett » Tue Apr 10, 2007 6:49 pm

F1Fever wrote:I have put a lot of thought and research into this. I have concluded that the term "dynamic compression ratio" is rather misleading because by definition it's not dynamic, although in real life it is. Ah.. I'm not good with words and it's a deep subject. At any rate I have written a calculator that also takes RPM & boost into play as well as factoring the IVC using both the ADV & DUR@050" figures. In addition to calculating using those figures is recommends a Minimum octane...

http://www.hotroddersauctions.com/Dynam ... ulator.php


I have a question about your calculator. When you're inputting volumetric efficiency for a blown motor do you input the naturally aspirated volumetric efficiency or the volumetric efficiency under boost?
Thanks.

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