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

[hrdtail78]

"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

How do I figure ductile iron cylinders into that?

[wfolarry]

I think he did an excellent job no matter how you measure it. 

[Scramjet]

I thought the expansion rate formulas might be close enough to "ballpark" the cylinder expansion.  It seems to me that the cast iron sleeve, steel cylinder studs and head bolt compression would work to constrain the expansion.  It would be my guess that the cylinder expansion is closer to the low side of the estimates relative to the calculated compression - hot.

Hardtail, the expansion rate of iron is similar to steel, about 6x10-6/deg/in.

Max, that looks about right but the heads and tops of the cylinders are hotter.  The rods and crank would stay around oil temperature but the cylinders would be around 300 degrees F.  I would guess a slightly higher number.  Maybe:

Rods=7.667"       Crank throw= 2.1875" (for 4.375 stroke)       Total rod and crank= 9.8545"

Rods and Crank: 240*F x 9.8545" x .000006 = .0102"
Cylinders and Cases: 300*F x 9.8545" x .0000125 = .0369

Calculated expansion hot = .0227"      (Edited)

Remember, "no mathematical model is perfect some some models are useful"

My hat is off to Springer for the spreadsheet.  I have found it very useful.  Thanks Springer.  When you put your work "out there" it is very easy for others to second guess your work.  I appreciate the sharing.

B

[jsachs1]

Big help in my business. 
Thanks,
John

[springer-]

I appreciate the positive comments, Thanks.

I have some updating to do to the calculators, adding some cams to the cam comparator and a few changes to the drive line calculator.  I am open to suggestions that anyone might have for any of the calculators, so please feel free to post them.  Also if anyone has any additional cam specs that I might add, I would appreciate the info.

Thanks

[GoFast.....]

I also use the cam compairing one. Just used it last night, The altitude thing is what I really like because I am at 5600 ft but wonder sometimes how it can be accurate. Also it still seems everyone is all over the place on cylinder expansion, I'm like, well which number do I feel like throwing in there today.

[Admiral Akbar]

"Rods and Crank: 240*F x 9.8545" x .000006 = .0102"
Cylinders and Cases: 300*F x 9.8545" x .0000125 = .0369

Calculated expansion hot = .0227"      (Edited)"

I would probably consider this a worse case difference.. I'd bet that difference between the Rod/ Crank and cylinders cases is less then say 10/20 degrees untill you get to the upper 1/2 of the cylinder. There you could see 50 degrees.  Crank and rods may actually be hotter since the only way they can loose heat is through the oil. Cases/cylinders have outside air..  Heat is energy. It has to flow somewhere. It just don't disappear... (Unless you want to get into a discussion on entropy..  )

Still, notice that the total expansion is limited to 37 thou on the cases and cylinders.. It's a safest worst case..  Maximum

[Scramjet]

Your comment brings to mind the following discussion on heat transfer:

Dr. Schambaugh, of the University of Oklahoma School of Chemical Engineering, Final Exam question for May of 1997. Dr. Schambaugh is known for asking questions such as, "why do airplanes fly?" on his final exams. His one and only final exam question in May 1997 for his Momentum, Heat and Mass Transfer II class was: "Is hell exothermic or endothermic? Support your answer with proof."

Most of the students wrote proofs of their beliefs using Boyle's Law or some variant. One student, however, wrote the following:

"First, We postulate that if souls exist, then they must have some mass. If they do, then a mole of souls can also have a mass. So, at what rate are souls moving into hell and at what rate are souls leaving? I think we can safely assume that once a soul gets to hell, it will not leave.

Therefore, no souls are leaving. As for souls entering hell, let's look at the different religions that exist in the world today. Some of these religions state that if you are not a member of their religion, then you will go to hell. Since there are more than one of these religions and people do not belong to more than one religion, we can project that all people and souls go to hell. With birth and death rates as they are, we can expect the number of souls in hell to increase exponentially.

Now, we look at the rate of change in volume in hell. Boyle's Law states that in order for the temperature and pressure in hell to stay the same, the ratio of the mass of souls and volume needs to stay constant. Two options exist:

If hell is expanding at a slower rate than the rate at which souls enter hell, then the temperature and pressure in hell will increase until all hell breaks loose.  If hell is expanding at a rate faster than the increase of souls in hell, then the temperature and pressure will drop until hell freezes over.

So which is it? If we accept the quote given to me by Theresa Manyan during Freshman year, "that it will be a cold night in hell before I sleep with you" and take into account the fact that I still have NOT succeeded in having sexual relations with her, then Option 2 cannot be true...Thus, hell is exothermic."

The student, Tim Graham, got the only A.

[Admiral Akbar]

 

[fuzznut5197]

Also if anyone has any additional cam specs that I might add, I would appreciate the info.
Thanks

OK 

Can you put the 4 evo Ultima cams on there?

BTW, the S&S 640 is 25/60 not 26/60. I know, typo... 

[skyhook]

bob wood has a couple of new cams that aren't on your calculator...408-44 and 9b-6

[GoFast.....]

Springer could you post your deck height article with the pictures for us here and any other stuff thats helpful

[PanHeadRed]

Springer, I have a suggestion, how bout dropping CCP to whole numbers and CR to a maximum of 2 places to the right of the decimal? CR in fractions (1/4 max resolution) would be even better. 

[springer-]

OK 

Can you put the 4 evo Ultima cams on there?

BTW, the S&S 640 is 25/60 not 26/60. I know, typo... 

Done and done, thanks for the recommendation.

bob wood has a couple of new cams that aren't on your calculator...408-44 and 9b-6

Also done, thanks
I added many cams from Wood and found some of the listed specs on his site seem to be incorrect.  I will contact him to confirm but for time being I used the listed specs.  The cams you mentioned are not the cams in question.

Springer, I have a suggestion, how bout dropping CCP to whole numbers and CR to a maximum of 2 places to the right of the decimal? CR in fractions (1/4 max resolution) would be even better. 

I kept the CCP at 1 decimal place too give indication to what whole number it is closest too.  CR is 2 places now. thanks.  I also changed the CC measurements to 1 place.  Displaying fractions was not an option.


I also made some additional changes based on the discussion about hot compression and cylinder expansion.  This has been a bit of a thorn and as mentioned was only kept for the play factor.  Well it's gone now since it had little or no real use.  I also changed the piston dome volumes.  I have indicated a range of values rather than a value.  As a side note, only actual measured values should be used for accuracy.

Additionally on the Gear Ratio Calculator I added a belt size calculator and added 09 transmission data.

Thanks for all the suggestion and feel free to keep them coming.

[Scramjet]

Don't know that it matters but:

The 2009 SE catalog now shows the stock 2007-2008 EFI cam at 30* intake close.

B

[springer-]

Don't know that it matters but:

The 2009 SE catalog now shows the stock 2007-2008 EFI cam at 30* intake close.

B

Good catch, fixed.  The Cam Comparator had the correct info but the Compression Calculator didn't.  HD specs are one of the hardest to get correct, they publish conflicting information and wrong specs all the time.  When creating the gear ratio and drive line calculator I found most of HD specs on the gear ratios for the 07-08 models to be incorrect.  After theorizing they were wrong we took apart an 07 transmission and counted the teeth on each gear to get the correct information.

Andrews cam specs are all over the place too.  I found up to 3 different cam timing specs for the same cams on Andrews website.  I had to call them to clarify what was correct, it was a mess.

Thanks

[Billy]

springer, thanks for a very useful tool, I use it often. I do notice a discrepancy between the specs you have in the TC compression and displacement calculator and camshaft comparator with the specs for the T-Man 525C he has listed on his website.

http://www.tmanperformance.com/cams.htm
   
   

[GoFast.....]

Springer I think the little changes are great and thanks so much for having something like this, but I disagree with those who say aluminum expansion is not an issue with compression and if they want to put zero on the line thats fine but we have no line now.

[GT]

nice work i like the vin calulcator thing

[ejk_dyna]

<<but I disagree with those who say aluminum expansion is not an issue with compression and if they want to put zero on the line thats fine but we have no line now.>>

no matter what expansion rate you use .040, .020 etc ,the compression calculator calculates a LOWER "hot" compression rate and as a result lower cranking compression hot versus cold.  this value will be wrong 100% of the time because hot cranking compression will be higher than cold 100% of the time because the expansion of the combustion chamber mixture more than offsets the cylinder expansion.

so why is that you feel it is important to input this expansion number?

[springer-]

Billy, thanks.  I corrected that as well added additional Tman cams.  Those correction will be up this weekend.

GoFast..... Too many variables that effect the "hot" compression and really it doesn't have a practical purpose.  I agree with ejk_dyna

GT, thanks.  The VIN Analyzer has been a work in progress for some time.  I just decided to put it out there for some feedback.  Still working on the Twin Cam portion.