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Slick
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 Posted: Wed Mar 26, 2008 2:41 pm Post subject: |
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The most accurate way to calculate effective compression is a combination of inputs & formula.
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Answer: The compression ratio listed for any engine combo is always the static compression ratio, which is fixed via component selection and machining practice when the engine is built. As you've surmised, it assumes the cylinder is full of air and fuel at bottom dead center (BDC) and that all the mixture is compressed into the top dead center (TDC) combustion cavity. But an engine really doesn't begin making serious compression until the intake valve closes and seals the air/fuel mix in the cylinder. That's why engines with identical static compression ratios can have significantly different cranking pressures, as seen with a common compression gauge. Although part of this is due to displacement differences (a larger-cubic-inch engine is a bigger pump), the main influence is camshaft design. By changing certain cam parameters, it's possible to bleed off cylinder pressure on the bottom end, decreasing fuel octane sensitivity, even though its static compression ratio remains unchanged. The actual cylinder pressure an engine sees is often referred to as dynamic compression, because (unlike the static built-in compression ratio) it changes dynamically according to camshaft variations. The most important of these variations is the intake closing point, because it extends beyond BDC into the compression stroke. Closing the intake later aids top-end power at the expense of low-end torque. Down low, where the engine is most likely to detonate, the late intake closure bleeds off cylinder pressure, effectively dropping the dynamic compression ratio.
Rod length and crankshaft stroke also play into the dynamic compression equation because they alter the piston's position within the cylinder at the intake-valve closure point. How all this comes together is explained by KB Pistons' John Erb: "Consider a 10:1 Chevy with a 3.48-inch stroke, 6-inch center-to-center rods, and a very hot race cam that closes the intake at 90 degrees after bottom dead center (ABDC). This engine will think it is running with 6.17:1 compression and will be happy with 80-octane fuel. As a general rule, the best available pump gas will work with an 8.0:1 dynamic compression ratio. To get 8.0:1 with the preceding rod, stroke, and cam intake closing event, you would need about a 13.2:1 static ratio."
Erb's 8.0:1 dynamic guideline is for classic, old-school, all-iron engines. An advanced, high-tech engine can go a point or so higher-but even with the latest high-tech electronically managed engines, actually shooting for a 10:1 dynamic ratio is way too high. Calculating the relationship between piston position in the bore and intake closing point and then determining the effective dynamic compression ratio requires some pretty serious math. 
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sly
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| Posted: Wed Mar 26, 2008 4:55 pm Post subject: |
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Indubitably!
So if the dynamic comp is hard to calculate, is it in any way related to the cylinder pressure as measured with a compression tester? |
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twr7cx
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| Posted: Wed Mar 26, 2008 5:35 pm Post subject: |
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| Pauls been very happy with his lane and the blower. I regret not copying him instead of going to CAPA. |
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badcooky
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| Posted: Wed Mar 26, 2008 5:46 pm Post subject: |
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| Halve your static and then add your boost and you have your dynamic it's not so hard. |
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xcabbi
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| Posted: Wed Mar 26, 2008 7:10 pm Post subject: |
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| Dynamic compression ratio is a total b**ch to workout as it does not factor in head flow rates. You've got to be a freaking genius with fluid dynamics to have dynamic compression ratio totally worked out. |
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sly
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| Posted: Wed Mar 26, 2008 7:47 pm Post subject: |
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| LOL Steady ED! It's nowhere near that simple. More like a cloverleaf intersection perhaps... |
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Slick
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| Posted: Wed Mar 26, 2008 9:11 pm Post subject: |
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| twr7cx wrote: | | Pauls been very happy with his lane and the blower. I regret not copying him instead of going to CAPA. |
We're going to post up a group buy notice in the near future. For 4k plus, you can't go wrong with a full kit that has everything you need. by the way, how much has the capa cost you so far to this date?
As for Paul's setup? he sure knows his s**t and theres no doubt about it.
Have you notice something in the air today, mid 20s all season & suddenly drops to a crispy 15 degrees. oh the throttle response was really noticeable. felt like I had a imaginary charger under the hood! LOL |
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Redford
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| Posted: Wed Mar 26, 2008 9:13 pm Post subject: |
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i love pauls m90 setup also, it looks great and must be alot of fun to drive.
i was comparing capa specs for the M90 & the MP112, and i was wondering if MP112 would run ok on the i6 cos the only problem i can think of is that if you wanted to use a low running pressure it would need to be underdriven. |
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Slick
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| Posted: Wed Mar 26, 2008 9:19 pm Post subject: |
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I was thinking of one time of running a M112 but my welding skills is only limited to arc... which was outdated 18 years ago! lol
I've got some rough drawings of the freestanding manifold & bracket. it sits on the exhaust side with the outlet pointing towards the battery tray{intercooler} |
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EL XR8
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| Posted: Wed Mar 26, 2008 10:14 pm Post subject: |
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| So aren't these Raptor blowers, basically just another Powerdyne? |
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xfpaul
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| Posted: Wed Mar 26, 2008 10:40 pm Post subject: |
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Effective compression is defined by the following formula:
E = C((B / 14.7) + 1)
Where E= Effective Compression, B= boost psi, and C= Static compression. Also remember that 14.7 is equal to 1 bar of boost.
Let's do an example. Let's say we have a au2 bone stock with 9.6:1 static compression who slapped on a M90 Eaton blower and is now boosting 7psi. That takes care of our variables. Let's do the math.
E = C((B / 14.7) + 1)
E = 9.6((7 / 14.7) + 1)
E = 9.6((.476xx) + 1)
E = 9.6(1.476xx)
E = 14.16
formula is the true compression ratio using 9.6 starting reference but should be mulitplyed by .8 for 80% for a true starting figure for the N/A motor
E = C((B / 14.7) + 1)
E = 7.68((7 / 14.7) + 1)
E = 7.68((.476xx) + 1)
E = 7.68(1.476xx)
E = 11.33568:1 |
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Slick
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| Posted: Wed Mar 26, 2008 11:43 pm Post subject: |
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Thats what I'm talking bot. so to work out the effective compression at 9.6:1, the formula would look like this: ((boost psi/ 14.7 is normal atmospheric pressure ) +1) X motor compression= effective compression.
eg: ((10 psi/ 14.7)+1) X 9.6:1 = effective compression of 16.13:1 |
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Dansedgli
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| Posted: Thu Mar 27, 2008 5:25 am Post subject: |
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| EL XR8 wrote: | | So aren't these Raptor blowers, basically just another Powerdyne? |
Yes. |
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sly
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| Posted: Thu Mar 27, 2008 6:55 am Post subject: |
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| EL XR8 wrote: | | So aren't these Raptor blowers, basically just another Powerdyne? |
Is a Vortech a Powerdyne? No, it's a Vortech.
Is a Paxton a Powerdyne? No, it's a Paxton.
Is a Procharger a Powerdyne? No, it's a Procharger.
Is a Raptor a Powerdyne? No, it's a Raptor.
Perhaps you could try to phrase a question that more precisely outlines what you want to know? |
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xfpaul
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| Posted: Thu Mar 27, 2008 6:56 am Post subject: |
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| LOL like ya math Slick, think i better put diesel in it next fill up |
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sly
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| Posted: Thu Mar 27, 2008 6:59 am Post subject: |
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| xfpaul wrote: | | formula is the true compression ratio using 9.6 starting reference but should be mulitplyed by .8 for 80% for a true starting figure for the N/A motor |
Sounds like the Effective Compression is based on the Dynamic Compression then rather than static compression. |
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Slick
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| Posted: Thu Mar 27, 2008 7:06 am Post subject: |
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| xfpaul wrote: | | LOL like ya math Slick, think i better put diesel in it next fill up |
LOL, Thats the hear say version for dummies and the easiest way to calculate how much boost you can run safely on a stock engine.  |
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xfpaul
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| Posted: Thu Mar 27, 2008 7:09 am Post subject: |
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| yes, the .8 is what allows for all the other variables valve over lap, flow rate through manfold , past valve etc , a forced induction device (didnt say which type) is suposed to get you as close to 100 % volume efect as pos but in real world not possible |
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xfpaul
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| Posted: Thu Mar 27, 2008 7:24 am Post subject: |
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| if nothing else we have people thinking, 10psi on a 7.5:1 engine is nothing, 7psi on a 10:1 engine is acctually lots, what fuel will they be running , if its an alchol motor go for 30 psi , if ya want to run pump gas 98 , they better do some math |
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