How Accurate is the Big Boyz Compression Calculator

[GoFast.....]

I have heard diffrent things about how Accurate it is. One Issue could be the cylinder expansion rate .020 to .040
http://bigboyzheadporting.com/TwinCamComp.htm

[RoadKingKohn]

I have heard diffrent things about how Accurate it is. One Issue could be the cylinder expansion rate .020 to .040
http://bigboyzheadporting.com/TwinCamComp.htm

It is pretty good.  Remember, you are not splitting atoms.

[Admiral Akbar]

"One Issue could be the cylinder expansion rate .020 to .040"


Don't worry about it.. Hot you'll get 5-10 psi higher than cold. Expansion rate is not a factor. Cylinder and head heat keeping the volume from cooling increases pressure more than the lower CR causes it to drop.. The 20-40 expansion is also off IMO. It is usually less. Max

[NightTrain67]

I have found it to be pretty accurate with its number estimations. 

[ejk_dyna]

<<One Issue could be the cylinder expansion rate .020 to .040">>

don't even worry about those numbers.  they only factor into hot calculated cr which is bogus anyway.  the hot calculations says compression goes down because of parts expansion...but it actually goes up because of even greater mixture expansion.

[jsachs1]

Some listed piston dome volume's are not accurate.  Otherwise good.
John

[GoFast.....]

Some listed piston dome volume's are not accurate.  Otherwise good.
John

john what do you feel is a safe cylinder expansion number

[jsachs1]

I always figure .020"-.040" on aluiminum.
John

[GoFast.....]

I always figure .020"-.040" on aluiminum.
John

so do you split the diffrence 0.30

[ejk_dyna]

<<Some listed piston dome volume's are not accurate.   Otherwise good.>>

BBoyz did extensive testing of dome volumes using several different methods and independently came up with exactly the same volumes as Doug at HQ....so there you go.

[ejk_dyna]

<<BBoyz did extensive testing of dome volumes using several different methods and independently came up with exactly the same volumes as Doug at HQ....so there you go.>>

JUST KIDDING JOHN!!!   

...actually i think they just pulled them from the spreadsheet i originally posted at old htt.

[ejk_dyna]

<<so do you split the diffrence 0.30>>

what does it matter?  it does not change your cranking compression number which is what you use the calculator for.

[tireater]

If the calculator says 192...and I measure 198 CCP...Is that close enough or did I put a wrong number in..?

[Don D]

This tool is a guide only and is usually fairly close. There are some variables that are not known with respect to the cam. If the cam is slightly advanced?? or retarded?? and what happens and how quickly (degrees of crank rotation) does it take from 0 to .053 lift the number we use as intake closing. Once again my point it is just a guide not absolute, and if the numbers obtained are in the ballpark then all is good. The calculator is good for planning a build. When comparing compression, CCP, what about the length and diameter of the hose for the gauge, or.... Just too many variables to trust the numbers exactly but when all the stats say you should have 170# and you have 200# that's a different story and remember Jeffd and his cam tming error (defective tw26).

[Scramjet]

***  In engineering they say "No mathematical model is perfect but some models are useful"  ***

In my experience the Excel calculator is as good as it can get without knowing all the variables.  Some of the variables you can measure, come you can't measure.  On one build it was right on.  Calculator said 185 CCP, actual measured 185 CCP.  On the other build it was way off (and I still haven't figured out why).  The calculator said about 190 CCP.  Actual measured 200 CCP.  This is on a 2007 103" BB, FT cast piston, Cometic HG and SE251 cam.

It is nothing but a math formula.

B

[jsachs1]

ejk,
You made my heart palpitate,and my hair stand up !!!!!!!!!!!!!!!!!!!!!!! whew
John 

[skyhook]

too funny ed, we need more comic relief!!!!!

[HD/Wrench]

Also what valve size was used for the program?? If you are checking CCP valve size will play a part in that.  It is just a tool, like a dyno nothng more than a tool & numbers vary. 

Your 251 cam did you ever put a degree wheel on it??? We have run into cams that where off, creating more or less ccp than we had seen in the past.  After going back and checking we found that cam was ground off from given specs.  Not like with a evo where you can move it +/- a small amount. You are rather stuck with it.

[ejk_dyna]

<<ejk,
You made my heart palpitate,and my hair stand up !!!!!!!!!!!!!!!!!!!!!!! whew
John>>

I have an improved version of the spreadsheet calculator that does altitude corrections and has newer cam info on it.  I am going to put the new dome volumes for all the pistons after you and Red sort it out.  Will post it here after that.

[springer-]

The calculator is intended for estimates only though I feel it is accurate.  The values for the HC piston domes are approximate based on reverse engineering when I was writing the calculator, IIRC.  All values used should be actual measured values, values in RED are approximate and for example purposes only.  As pointed out above, it is nothing but a math formula, it is based on commonly available engine math formulas.

The "hot" corrected compression does not take into account the expansion of gasses based on heat.  The term "hot" is far to vague to base any accurate calculations on.  What is "hot"?  Is it 150 deg F?  Is it 230 deg F?  Is it 500 deg F?  Even if I picked a temp or offered an input temp field, different running conditions vary the temp and would not produce a meaningful number.  So I decided not to take into consideration the expansion of gasses and base it solely on the expansion of aluminum/cylinders.  The hot compression uses the variable entered in the cylinder expansion field.  I did not measure cylinder expansion myself.  I have seen a few conflicting reports to how much the cylinders expand.  Typically ranging from .017" to .035".  I left it open so users can choose whatever they want for an expansion value.  There is so much more to heat expansion that I could not really take it all into consideration and therefor kept it very basic.

I thought about removing the hot compression info all together as it has no real value to the typical user, IMO.  I chose to leave it for the "play" factor and by request.

The adjustment for altitude is based on actual measured data from a member of another forum that collects data working with a drag racer.  The complications of accurately calculating how the cold cranking compression is effected by altitude is extremely complex.  This method was much easier and so far has proven to be fairly accurate but as they say "your mileage may vary".

[GoFast.....]

Springer. You are talking like you are the one resonsiable for it and if you are I want to say thank you for doing this, I have found it to be of great help, My only queation is if the top is .035 why do you have it at .040

[Scramjet]

I have used cooefficient of linear expansion of metals often in gear manufacturing.

Aluminum is about 12x10-6 and steel is about 6x10-6 per degree per inch.

It seems to me that you can use the steel expansion rate for the rods and crank.  Then use aluminum expansion rate for the cylinders/cases and come up with a reasonable assumption.  The only thing i don't know is the distance from the centerline of the crank to the top of the cylinders.  It should work unless I am missing something.

The calculation would be the delta of 6x10-6 per degree per inch times the distance from the crank centerline to the top of the cylinders.  One would have to make some assumption on the temperature as the cases are around 230 degree F oil temp.  The cylinders would have a gradient temp from case temp to head temp.

B

[springer-]

Springer. You are talking like you are the one resonsiable for it and if you are I want to say thank you for doing this, I have found it to be of great help, My only queation is if the top is .035 why do you have it at .040

I'm glad you have found it helpful, that is the reason it is there.

.035" isn't the "top".  .020"-.040" seems to be in the generally accepted range.  I have seen some say the engine grows as much as .060".  Since I did not measure it myself, I did not want to lock in (or out as the case may be) any specific values.  Any value can be used.

[springer-]

I have used cooefficient of linear expansion of metals often in gear manufacturing.

Aluminum is about 12x10-6 and steel is about 6x10-6 per degree per inch.

It seems to me that you can use the steel expansion rate for the rods and crank.  Then use aluminum expansion rate for the cylinders/cases and come up with a reasonable assumption.  The only thing i don't know is the distance from the centerline of the crank to the top of the cylinders.  It should work unless I am missing something.

The calculation would be the delta of 6x10-6 per degree per inch times the distance from the crank centerline to the top of the cylinders.  One would have to make some assumption on the temperature as the cases are around 230 degree F oil temp.  The cylinders would have a gradient temp from case temp to head temp.

B

It gets complicated quickly.  Don't forget the aluminum pistons attached to the steel rods expand differently as well.  Are they forged or cast pistons, again different rates of expansion.  The cylinders are steel lined with cast aluminum around them.  And there are steel cylinder studs clamping the heads and cylinders down that expand too.

I am sure a very elaborate formula could take all these things into account but why?  It just isn't that critical for the purpose of building an antique, 45 degree, air cooled, pushrod based engine in my opinion.

[Admiral Akbar]

"Aluminum is about 12x10-6 and steel is about 6x10-6 per degree per inch."

Ok this means that Aluminum grow  6x10-6 per degree per inch more then steel.. Lets assume the top of the piston and the cylinder grow at the same rate.. You got 2 inches of flywheel and 7.7 inches of rod or 9.7 inches..  Ambient is 70 so lets say the motor is 270 gives us a nice 200 F increase..

200 X 6x10-6 X 9.7 = 0.01164

Max