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Tue Jun 09, 2020 4:21 pm

Anyone have a link to a known good one?
I’ve tried 4 different ones and gotten 4 different results so I don’t know which to believe. They’re all over the chart from 5.4:1 to 16.8:1. My static is 8.7 so 5.4 would be a bummer fo sho.

Tue Jun 09, 2020 4:29 pm

United Engine and Machine.

Tue Jun 09, 2020 4:29 pm

Dynamic compression as you know will change with overlap and how the cam is degreed. That being said, to get the CORRECT answer you will need the correct data input. How close do you want to be? If you know the correct information and all engine specs, you can PM them to me and I can run it threw one of my programs.

Cheers

Tue Jun 09, 2020 6:02 pm

IMHO they are bs.... especialy on a vw with these rocker ratios we have. if you put the cam lift&duration from our vw's in a v8 you have a pretty wold camshaft... but what has changed??? rocker ratio is the main thing. IR induction is the other. I wont use those chart's. if you have somany that say somany different things....how do you choose?? ask bubba??how did bubba choose??? asked mikey?

Tue Jun 09, 2020 6:51 pm

Most of the internet V-8 DCR calculators are totally wrong for a VW engine.

Tue Jun 09, 2020 6:54 pm

Try this one,http://www.wallaceracing.com/dynamic-cr.php

Tue Jun 09, 2020 8:39 pm

Performancetrends.com is available for a few hundred dollar's. The price is reasonable. Or you can spend thousands and they comes with in house training and yearly seminars.

Wed Jun 10, 2020 8:57 am

This is what I use: I aim for a conservative 7.5 usually on 91 octane. My current build is at 8.1. https://uempistons.com/p-27-compression-ratio-calculator.html

Wed Jun 10, 2020 9:28 am

At least the UEM calculator gets my static correct and says my dynamic is 7.6.
That's a lot closer to what I believe is reality than the 5.4 or the 16.8.

Wed Jun 10, 2020 10:39 am

my 2028 cc had 10.4 cr with about 80,000 miles in that configuration when I sold the car. it should get another 80,000 without a problem if the new owner does not 1 fuck with it,2 changes oil/filterevery 2 years with synthetic 5-20 oil,3 dosent let somebody else fuck with it. I havent a clue what the dcr was and dont care. CB 2250 cam. your results may varry, alterations may not be allowed in all states and provinces.. some discollaring may occur. as well as stains in your shorts. IMHO tomany people leave power and torque in the build room and end up with less than what they want and what the engine is reliabily capable of. as well as a unhappy dirty engine.

Wed Jun 10, 2020 11:20 am

https://uempistons.com/p-27-compression-ratio-calculator.html

Wed Jun 10, 2020 11:39 am

I know from the numbers that I’m leaving some hidden potential. Maybe this winter or maybe this August (since it has no A/C) I’ll deck the case to tighten up the deck to .050 instead of the current .081 and fly cut the heads to give me 54cc from 60cc
That’ll bump it to 10.06 static and 8.6 dynamic. At least according to this calculator.
In the meantime, I’m looking into putting it on a Dyno to see what I have now and I’ll do it again after for a comparison, but first I need to hammer on PLX to get the $500.00 worth of Bluetooth gauges working for more than 3 seconds so I can verify/dial in the tune.

Wed Jun 10, 2020 1:15 pm

jeffrey8164 wrote: . . . That’ll bump it to 10.06 static and 8.6 dynamic. At least according to this calculator. . .
I don't use any online calculators. I use a piece of paper and a pencil and some basic high school trigonometry. (Let me know if you're old-school like me and you want me to post up the formulas).

As a general rule, I believe 7.5 to 1 DCR to be the minimum for any efficient running engine. 8.0 to 1 DCR is better, but will require premium fuel. And 8.5 to 1 DCR is the absolute limit if you still want to use pump gas.

And just for comparison, my static compression is 8.0 to 1, my Intake Closing Angle @ 0.050" is 30 degrees ABDC, my rods are 137mm, and my crank is 76mm. This gives me a DCR of about 7.65 to 1.

Definitely get that deck shaved down!

Wed Jun 10, 2020 1:35 pm

That’s funny. I won an award in High School from my Trig teacher. Unfortunately, my friends and I skipped out on the awards ceremony so some detention was in order :? .
Now I barely remember any of it.

Thu Jun 11, 2020 8:16 am

Floating VW wrote: (Let me know if you're old-school like me and you want me to post up the formulas).

I'd love to see those forumlas (formulae?) to understand the theory and what's going on.

Thu Jun 11, 2020 10:08 am

stewartsoda wrote: Floating VW wrote: (Let me know if you're old-school like me and you want me to post up the formulas).

I'd love to see those forumlas (formulae?) to understand the theory and what's going on.
No problem. First, you need to know your crankshaft stroke, rod length and the Intake Closing Angle @ 0.050" of your cam. I prefer to measure the cam directly to get this information, but to get a ballpark number, the formula for this is:

Intake Closing Angle (ICA) = (Duration ÷ 2) + Lobe Center Angle ‒ 180°

Be sure to use duration @ 0.050" for that formula, not advertised duration.

Next, you need to calculate the dynamic stroke of your crankshaft. Here are the formulae:

1. You will need to know your crankshaft stroke (CS), your rod length (RL) and your intake closing angle (ICA).

2. Calculate Horizontal Rod Displacement (HRD): HRD =1/2CS × (sine ICA)

3. Calculate Rod Distance Below Crank Center Line (RD): RD = 1/2CS × (cosine ICA)

4. Calculate Piston Rise from Rod Distance Below Crank Center Line (PR1): PR1 = √(RL² ‒ HRD²)

5. Calculate Piston Rise from Crank Center Line (PR2): PR2 = PR1 ‒ RD

6. Calculate Dynamic Stroke (DS): DS = CS – [(PR2 + 1/2CS) – RL]

Once you know the dynamic stoke, calculating your dynamic compression ratio is the same as calculating your static compression ratio, with the only difference being that you use the value for dynamic stroke in place of the actual stroke of the crankshaft.

Thu Jun 11, 2020 3:05 pm

I once made an exel chart that calculated that stuff, for every degree for 720 degrees full rotaton. Don't use it now, though.

What I do is use the Wallace calculators, using the known baseline of the stock engine. 8.5:1 C/R , with the stock timing, gives a DCR which will probably need premium gas. Play with that, give it more and more C/R, and close the intake later and later, trying to keep a reasonable DCR. thereare a lot more variables, but it's a starting point.

Thu Jun 11, 2020 5:39 pm

jeffrey8164 wrote: At least the UEM calculator gets my static correct and says my dynamic is 7.6.
That's a lot closer to what I believe is reality than the 5.4 or the 16.8.

Like Dave mentioned, calculators will give you info, but the watercooled heads of a V8 will have different running characteristics than an Aircooled VW.

Good running engines running on 93 octane I have seen built, or driven in a customer's vehicle had around 7.5 dynamic on the UEM calculator. These were from small engines below 9.0:1 static when using a small cam to a large engine at 11.5:1 when using a large cam. I've also driven a customer assembled engine with a dynamic of 8.0 on the UEM calculator that pinged like a bitch.

Bugguy1967, is the 8.1 the engine you are assembling, or one that is already running?

Thu Jun 11, 2020 6:07 pm

Maybe I’m good where I am then.

Thu Jun 11, 2020 6:15 pm

Floating VW wrote: stewartsoda wrote: Floating VW wrote: (Let me know if you're old-school like me and you want me to post up the formulas).

I'd love to see those forumlas (formulae?) to understand the theory and what's going on.
No problem. First, you need to know your crankshaft stroke, rod length and the Intake Closing Angle @ 0.050" of your cam. I prefer to measure the cam directly to get this information, but to get a ballpark number, the formula for this is:

Intake Closing Angle (ICA) = (Duration ÷ 2) + Lobe Center Angle ‒ 180°

Be sure to use duration @ 0.050" for that formula, not advertised duration.

Next, you need to calculate the dynamic stroke of your crankshaft. Here are the formulae:

1. You will need to know your crankshaft stroke (CS), your rod length (RL) and your intake closing angle (ICA).

2. Calculate Horizontal Rod Displacement (HRD): HRD =1/2CS × (sine ICA)

3. Calculate Rod Distance Below Crank Center Line (RD): RD = 1/2CS × (cosine ICA)

4. Calculate Piston Rise from Rod Distance Below Crank Center Line (PR1): PR1 = √(RL² ‒ HRD²)

5. Calculate Piston Rise from Crank Center Line (PR2): PR2 = PR1 ‒ RD

6. Calculate Dynamic Stroke (DS): DS = CS – [(PR2 + 1/2CS) – RL]

Once you know the dynamic stoke, calculating your dynamic compression ratio is the same as calculating your static compression ratio, with the only difference being that you use the value for dynamic stroke in place of the actual stroke of the crankshaft.

Thank You so much!
Now I finally understand dynamic compression ratio.