Big Boyz Compression Calculator Accuracy?

[robertg]

How accurate is the Big Boys compression calculator? My 120R has the heads milled 0.030", 0.040 head gasket and T-Man 662-2 cams. My compression reads about 205-208 psi on two different gauges. The calculator says I should be at about 195 psi. I was thinking my gear drive cams may be off one tooth, because if I change the intake valve closing from 51 to 40, the number for my compression is correct.

I opened up my cam chest last night and the marks are lined up. My other option is to degree the cams.

I want to change to T-Man 590 cams eventually, but I'm afraid my compression would be too high.

[BUBBIE]

I know that certain Cams do differently on Compression THAN according to BigBoyz calculator... My example is SE/CVO 255 cams. More compression is there than the chart shows.

ALSO I'm at 4400 foot elevation and When I go Down lower elevation, my compression come UP... (? have you figured your elevation IN)

I have 200# at my 4400 foot elevation. when I go to Lower elevation, I CAN really feel the Power of the higher compression... 

That is My case for the Calculators... Having the More compression Sometimes can be NOT a good thing.

Their Charts will get you close, Using your measurements/MATH would be needed in some cases. Might that be more accurate?

Sure Nice to have FREE USE of Bigboyz calculators though...

signed....BUBBIE

[MikeL]

Why did you use a .040 head gasket? Do you have the cc measurement of the cylinder head combustion chamber. Do you have the measurement of the piston height in reference to the top of the cylinder (piston deck height). Are you using forged, cast, hyper, pistons.There could be a lot of carbon buildup in the combustion chamber which could cause the ccp to be higher also..........


                                                                                      MIKE

[Ohio HD]

If your measurements are accurate, my experience has been the calculators are pretty darn close. I do mean measured, not estimated.

[BVHOG]

The calculator is to be used as a guide and is very accurate, what can change results are things like camshaft specs that are measured at .053 and not telling the entire story about the rest of the lobe.  Also an accurate measurement of deck height and valve relief is required to get any consistent accuracy.

[BUBBIE]

I used the Guide to see what your Compression should be and I can't get it as LOW as you did... 215 or so is what I get... SEA-Level

Go back and enter your information to see...

HOW did you enter the Shaved Head?

signed....BUBBIE

209.8/210.7...... Not taking it the .030 off the heads? The .030 shave will give you More ccc using the Bigboyz chart...

Are You sure the Heads were Shaved? I sure can't get it to work at Your Checked Compression... Looks like using the .040 head gasket and NO shaved heads comes out to your Pressures you checked and came out with...

[panic]

The math is pretty good, but (as said) different cams will have different effects that only a Cam Doctor read-out will show.
Because it's guessing at how much reversion takes place (how long after BDC does back-flow stop?), there are 2 more variables that are not entered (and difficult to quantify):
1. displacement vs. intake port cross-sectional area (large motor with small port will test higher than it predicts)
2. starter motor speed, which is also affected by battery size, condition, oil viscosity, and engine temperature

The calculators also use a fixed value for the ratio of variable heats of gasses, and it's a guess. The actual number in a specific engine varies between about 1.25 and 1.40. 

[Admiral Akbar]

The calculator is to be used as a guide and is very accurate, what can change results are things like camshaft specs that are measured at .053 and not telling the entire story about the rest of the lobe.  Also an accurate measurement of deck height and valve relief is required to get any consistent accuracy.

I suspect that you can "and isn't very acccurate"

It should mainly be used as a guideline..


to OP..

What are the head, piston, deck?   I would degree the cams.   IIRC the pinion gear is 17 teeth.. If you were off a tooth you'd be off 21 degrees and not 11.

[Matt C]

The calculator itself is very accurate. "inaccuracies" can be attributed to flawed data being input.
Garbage in = garbage out, it's only as accurate as the data you put into it.

[kd]

MVA heads are advertised as 95 cc. What cc are the heads after the .030 cut? Head cc really effects the CR.

[Admiral Akbar]

The calculator itself is very accurate. "inaccuracies" can be attributed to flawed data being input.
Garbage in = garbage out, it's only as accurate as the data you put into it.

It's very precise but the accuracy is only pretty good.. Toss up the equations and we can go through them..

[robertg]

I have gear cams, so the crank sprocket has 31 teeth. My heads were 95cc before I milled them 0.030", so I'm doing a calculated guess at 90-91cc dome. Head gasket is 0.040" and 4.100" bore, deck is 0.002" above, piston dome is -1.5 cc, altitude is 750 ft. Intake close is 51 ABDC.

When I enter this I get I get 191.7 psi. If I change my intake close to 40 I get 205.4, which agrees with my gauge.

[kd]

There was a thread here that estimated 7 cc per .010. I believe. I'm not too sure though. The MVA head may be different due to the chamber shape. Maybe one of the porters can confirm.

[robertg]

There was a thread here that estimated 7 cc per .010. I believe. I'm not too sure though. The MVA head may be different due to the chamber shape. Maybe one of the porters can confirm.

I get 6.36 cc when 0.030" is taken off a round combustion chamber with a diameter of 4.060". Since the chamber is not round I rounded it down to 5cc. Someone correct me if I'm wrong.

I did cc the heads when they were new and they were 95cc. I changed the head gaskets a month ago because they were leaking. I cleaned the carbon off the pistons then.

[PoorUB]

There was a thread here that estimated 7 cc per .010. I believe. I'm not too sure though. The MVA head may be different due to the chamber shape. Maybe one of the porters can confirm.

Pretty sure it was roughly 1cc per.007" removed.

[kd]

I remember the 7 

[Admiral Akbar]

I have gear cams, so the crank sprocket has 31 teeth.

Oops got the tooth count wrong. 

[Matt C]

The calculator itself is very accurate. "inaccuracies" can be attributed to flawed data being input.
Garbage in = garbage out, it's only as accurate as the data you put into it.

It's very precise but the accuracy is only pretty good.. Toss up the equations and we can go through them..

I don't need to go through them. You can if ya want to.

[HD/Wrench]

Well once cut it should have been CC'd.  The amount removed can vary from machine to machine.  What RPM was used to machine it, what tool, etc tooling deflection is normal.  So run it at 1600 RPM or 2200 and you will see that there is a difference based on tooling.. Compare the pinnacle fly cutter basic rl8 tool.. TOO many way's for the cut to vary.. And as for the program no I do not believe that its the end all be all use it to get a idea of where it should be.. Bob already stated the why on the cam design.. numbers at .053 are the tip of  the issue.

[Admiral Akbar]

The calculator itself is very accurate. "inaccuracies" can be attributed to flawed data being input.
Garbage in = garbage out, it's only as accurate as the data you put into it.

It's very precise but the accuracy is only pretty good.. Toss up the equations and we can go through them..

I don't need to go through them. You can if ya want to.

I have.. It's a bunch of fudge factors..

[Don D]

MVA heads are advertised as 95 cc. What cc are the heads after the .030 cut? Head cc really effects the CR.

90cc nominal

[Matt C]

The calculator itself is very accurate. "inaccuracies" can be attributed to flawed data being input.
Garbage in = garbage out, it's only as accurate as the data you put into it.

It's very precise but the accuracy is only pretty good.. Toss up the equations and we can go through them..

I don't need to go through them. You can if ya want to.

I have.. It's a bunch of fudge factors..

"fudge factors" are a practical way of dealing with series' of complex mathematical calculations.
We don't have all the data (the polynomial that describes the lobe) for one, or the will and
patience (in my case) to make attempts to prove the "fudge factors" wrong.

And quite honestly, I'm not sure I'd even remember how to set that up and solve the damn thing.
So I'll use the BB calculator and call it close enough for government work. After all, this ain't the
f--king space shuttle.

Practical experience and dyno testing is good enough for me these days... thanks& have a nice night

[panic]

The math begins by calculating the trapped volume (at IVC), then the corrected compression ratio from that. Where "SE" is the effective stroke, "S" is the nominal (full) stroke, "R" is the rod length, "A" is the crank-shaft angle in degrees ABDC from 0 to 90°, and "SQRT" (or "SQR" ) is the square root function:
SE = (S ÷ 2) + R + ((S ÷ 2) × cosA) - SQRT ((R2) - ((S ÷ 2) × sinA)2)
The result is raised to (roughly) its 1.25 exponent (I use a conservative value here), then multiplied by atmospheric pressure in psi (14.7 for sea level, less for altitude).

[Admiral Akbar]

The math begins by calculating the trapped volume (at IVC), then the corrected compression ratio from that. Where "SE" is the effective stroke, "S" is the nominal (full) stroke, "R" is the rod length, "A" is the crank-shaft angle in degrees ABDC from 0 to 90°, and "SQRT" (or "SQR" ) is the square root function:
SE = (S ÷ 2) + R + ((S ÷ 2) × cosA) - SQRT ((R2) - ((S ÷ 2) × sinA)2)
The result is raised to (roughly) its 1.25 exponent (I use a conservative value here), then multiplied by atmospheric pressure in psi (14.7 for sea level, less for altitude).

Very good but you forgot to subtract 14.7 psi from the result to get gauge pressure not absolute.. I use 1.188 tho as that appears close to what RB racing uses on their calculator.

[Matt C]

Actual IV Effective "close" on a running engine is likely to be something greater than zero.

Trapped volume (static) calculations are one thing and very useful, however, the effective pressures when
the motor is running is an entirely different thing.  It's like trying to hit a moving target.

That's all I'm trying say.