Calculated and Dynamic C.R. calculator

[76zimmer]

To not hi-jack "77 Stroker engine build" thread anymore I'll move my discussion here..


First off the calculator:

http://www.kb-silvolite.com/calc.php?action=comp

And some discussion that went hi-jack..

Zim I didn't read that whole speal on that page because I have done hours of research on that years ago and wrote my own excel spreadsheet that calculates dynamic compression, but why would you want to use the number at .050"? Unless you piston is traveling fast enough that is starts to squeeze the air before the valve is all the way shut (not likely, but possible), you want to use the advertised bumbers. I use the number at .006" or .015". I do understand that at .050 is more standard, because companies vary where they measure advertised duration, but the piston is not really moving super fast relative to the valve during the last bit of the closing sequence. For example, take the KB pistons calc...

http://www.kb-silvolite.com/calc.php?action=comp2

they use the cam timing @ .050 plus 15 degrees. That is to allow a little more time for the valve to close all the way. But that's just an estimate since different profiles close at different rates, roller cams open and close much quicker than flat tappets, etc.

I did notice that he is a little more lenient in the numbers, I have always used 8.0 as the limit for pump gas. But if you use the timing at .050 your dynamic compression will be a little higher than advertised numbers.

Anyway, Zim, depending on what your cam card says, the numbers at .050 are usually stated somewhere, maybe not all of them. But if you know the numbers at .050 and the lobe centerlines you can figure it out. It's not just a common number though.

Heath, I'm asking about the .050 only because the calculator asks for it. I assumed it would need it for correct numbers....here is my cam spec card....

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=468&sb=0

I will have to look at my build sheet for all of the numbers required to complete the equations, but seems like I ended up with about 9.4 calculated CR.

 

[folkenheath]

Here is my numbers for my engine...(pasted from my spreadsheet for an example), if you give me your inputs I'll just plug them in. I need your piston, bore (461 I believe?) gasket # and cylinder head volume. Also, what did you set your quench at?

Number of Cylinders 8
Bore (in) 4.280
Stroke (in) 4.250
Connecting Rod Length (in) 6.135
Anticipated Redline (RPM) 6000
Gasket Volume (cc) 9.7
Piston Volume (cc) 12
Comb Chamber Volume (cc) 110
Deck Height (in) 9.800
Extra Quench Height (in) 0.005
Intake Closing(deg) 71
Boost Pressure (lbs/sq-in(psig)) 0
Volumetric Efficiency (%) 90

Outputs
Cylinder Volume (in3) 61.15
Swept Volume (in3) 6.64
Engine C.I.D. 489
Engine Liters 8.02
Bore/Stroke Ratio 1.01
Connecting Rod Ratio 1.44
Compression Height (in) 1.540
CFM Flow 764
Compression Ratio 10.20
Dynamic Compression Ratio 7.83

 

[folkenheath]

From looking at your cam card, your intake centerline is at 106. Your intake duration at .050 is 230, and adv is 268, so theoretically, the difference between the 2 is 38 degrees div by 2 is 19 degrees per side. So since your cam closes at 60 ABDC adv, the .050 would be ~19 degrees before that. But that is assuming similar opening and closing rates, which isn't always the case. Although it is the intake centerline, it still could be a few degrees different, depending on how they measured it. The true way would be to measure it on your engine with a degree wheel, but that should be close enough. Anyway, if I assume a 119 cc chamber and a 18 cc dome with .005 quench above a .040 compressed gasket, that puts you at 9.4:1 static, and 7.85 dynamic. But that's just a guess, we need your real piston and head numbers, adn quench. PM or call me if you want to Dave.

 

[76zimmer]

I'll post up what numbers I got on my build sheet Heath, but looking up the pistons, they are 23cc dome. and I think they were .007 in the hole.

These are the ones:

http://www.spiderautomotive.com/srp212135.html

 

[Chevy305]

ok so if this is true:

A DCR of 8.25:1 is the absolute max that most people can run on pump gas. At 8.45:1 with a the piston .025 down the hole, this motor with be especially prone to detonation. It will probably destroy itself in a short order.

Then how come I have a CCR of 9.45 and a DCR of 8.7 and my engine runs great on 93 right out of the pump?

 

[folkenheath]

Zim ,what about your heads? What is the chamber size?


Chevy305, this may be one of those cases where the numbers don't always add up to reality because of all the other variables, for years no one ever calculated DCR, and they didn't think twice about running 9:1 on pump gas with a small cam. However, there are many other factors that can help or hurt when it comes to detonation. One of the big ones they are talking about there is quench. If you're quench is closer to optimal (~.040 for your engne), you have a much less chance of detonation. Also, air temp, engine temp, cylinder head material, hot spots, sharp spots in the combustion chamber, spark plug heat range, engine timing, engine load, gearing, and on and on. Also, actual compression pressure (psi) is a more final indicator to go on as well, which takes into account almost everything in your engine, except things like timing and temperatures, loads, etc. Also, a difference of 9.4 to 8.7 seems pretty small, your cam would have to be pretty darn small for that, what are your numbers?

If you want to read some good info on quench go to KB pistons website and look in their tech articles, they have some good info there.

 

[76zimmer]

Here is my numbers for my engine...(pasted from my spreadsheet for an example), if you give me your inputs I'll just plug them in. I need your piston, bore (461 I believe?) gasket # and cylinder head volume. Also, what did you set your quench at?

Number of Cylinders 8
Bore (in) 4.280
Stroke (in) 4.250 ............4.00
Connecting Rod Length (in) 6.135
Anticipated Redline (RPM) 6000......5500
Gasket Volume (cc) 9.7...................039
Piston Volume (cc) 12...................23cc dome
Comb Chamber Volume (cc) 110....119
Deck Height (in) 9.800......cut .010
Extra Quench Height (in) 0.005.......Piston is .005 in the hole, and head gasket is .039 compressed (Felpro 17046)
Intake Closing(deg) 71............60ABDC
Boost Pressure (lbs/sq-in(psig)) 0
Volumetric Efficiency (%) 90

Outputs
Cylinder Volume (in3) 61.15
Swept Volume (in3) 6.64
Engine C.I.D. 489
Engine Liters 8.02
Bore/Stroke Ratio 1.01
Connecting Rod Ratio 1.44
Compression Height (in) 1.540
CFM Flow 764
Compression Ratio 10.20
Dynamic Compression Ratio 7.83

Heath, I believe these to be the numbers best I can remember....I'm at work and my build sheet is home...if they are different, I'll change them when I get home...
THX

 

[folkenheath]

Dave with those numbers I get 9.8 static and 8.1 dynamic. When you get the final numbers I'll give you the detailed outputs. I bet your fine, and why would you stop revving that puppy at 5500, a solid cam with 230/236 @ .050 and your going to stop it at 5500? I would say 6k easy with those solid lifters, that's probably a good number, no need to rev it too high or it's just hard on parts with no more power, but 5500, my grandma would of revved your Jeepster higher than that!

 

[76zimmer]

ooh, calling me out huh...I only have a cast crank and a 2 bolt.....my sh!t is weak.
My head guy suggested this cam...it had a better torque curve, than any of the hydrualics we were looking at....Voodoo's and other Comp Cams, and the torque is down to less than 500 by 5000 rpm anyway....no need to rev to 6K

 

[Chevy305]

Chevy305, this may be one of those cases where the numbers don't always add up to reality because of all the other variables, for years no one ever calculated DCR, and they didn't think twice about running 9:1 on pump gas with a small cam. However, there are many other factors that can help or hurt when it comes to detonation. One of the big ones they are talking about there is quench. If you're quench is closer to optimal (~.040 for your engne), you have a much less chance of detonation. Also, air temp, engine temp, cylinder head material, hot spots, sharp spots in the combustion chamber, spark plug heat range, engine timing, engine load, gearing, and on and on. Also, actual compression pressure (psi) is a more final indicator to go on as well, which takes into account almost everything in your engine, except things like timing and temperatures, loads, etc. Also, a difference of 9.4 to 8.7 seems pretty small, your cam would have to be pretty darn small for that, what are your numbers?

If you want to read some good info on quench go to KB pistons website and look in their tech articles, they have some good info there.

This is the cam I am running:

Lunati Voodoo hydrolic flat tappet - 262/268 and 219/227 at .050

http://www.lunatipower.com/Product.aspx?id=1983&gid=287

My 350 is bored .030 with 10cc dished pistons and 64cc Aluminum heads. I run 12* advance and 38* total timing.

I'm not sure what is meant by quench though... Is that piston clearance to the deck at tdc?

 

[4X4HIGH]

This is the cam I am running:

Lunati Voodoo hydrolic flat tappet - 262/268 and 219/227 at .050

http://www.lunatipower.com/Product.aspx?id=1983&gid=287

My 350 is bored .030 with 10cc dished pistons and 64cc Aluminum heads. I run 12* advance and 38* total timing.

I'm not sure what is meant by quench though... Is that piston clearance to the deck at tdc?

Quench is the distance from the top of the piston at TDC to the head gasket surface of the head, ideally .040" quench gives the best results to avoid detonation.

 

[folkenheath]

ooh, calling me out huh...I only have a cast crank and a 2 bolt.....my sh!t is weak.
My head guy suggested this cam...it had a better torque curve, than any of the hydrualics we were looking at....Voodoo's and other Comp Cams, and the torque is down to less than 500 by 5000 rpm anyway....no need to rev to 6K

Did you get the final numbers checked Dave?

And it's a big block, cast crank and 2 bolt main are no problem for 6K. I have a cast crank too buddy, and my regal has had a cast crank in it for 13 years, and I have sprayed 175 hp of N20 on top of that on more than one occasion, running mid 10s on a stock cast crank and rods probably isn't the best idea, but so far it has held up fine, and thats a small block. Besides, it's not only the power you are shifting at that matters, it's what it drops back down to after you shift. You want to keep that range centered in your best power curve.

This is the cam I am running:

Lunati Voodoo hydrolic flat tappet - 262/268 and 219/227 at .050

http://www.lunatipower.com/Product.aspx?id=1983&gid=287

My 350 is bored .030 with 10cc dished pistons and 64cc Aluminum heads. I run 12* advance and 38* total timing.

I'm not sure what is meant by quench though... Is that piston clearance to the deck at tdc?

Scott is right, and so are you, quench is the distance between the top of the piston and the head surface at TDC(including the gasket). It shrinks at higher RPM as stuff flexes from the stress, it kind of creates a shock wave of air to help get the combustion started more evenly. For your small block .040 is a good number to shoot for (including the gasket, which is probably exactly that or a .001" under.) I have been running my 388 small block at .040" for years, runs great, very efficient, and puts some good power down. On my big block I was going to fo .040 but my machinist recommended .045 for larger engines or with a little more RPM.

Read these...(they are interesting, I read them when I was still a teenager)

http://www.kb-silvolite.com/article.php?action=read&A_id=35

http://www.kb-silvolite.com/article.php?action=read&A_id=39

http://www.kb-silvolite.com/article.php?action=read&A_id=4

http://www.kb-silvolite.com/article.php?action=read&A_id=34

As for your engine, did you have the block decked at all or is it factory original height? Because if it's original you probably have an extra 015" - .020" or so above the piston before the gasket even.

When I say "extra quench", I am referring to any extra in addition to the gasket thickness, or, the piston top to the deck top at TDC.

If you look at your cam, the timing they show is @ .050(1.5+180+37.5=219). You could just add 15 degrees as the generic guess, or we could find out the advertised (closer to actual) closing point using the adv duration. 262-219=43 /2=21.5 +37.5=59 degrees

If I use those numbers with .015 extra quench, I get 9.4 CR and 7.9 DCR (if you decked your block to zero I get 9.8 and 8.1 DCR)at 59 degrees closing point. I would say that is pretty good. I don't see why they use the numbers at .050". You can't compress the air much until the valve is closed (unless piston speed is very high relative to the valve gap, which isn't the case near BDC), the advertised numbers (at .006, .010, .015 or whatever) are closer to actual closing than at .050", if you measured leakdown at .050" valve opening it would be pretty horrible. Now, at high RPM with high air speed then you may start to get some compression before the valve is shut all the way because you are cramming air in the cylinder so fast, but this is not when you are most susceptible to detonation anyway.

 

[shovelbill80]

.050 is only used as an industry "standard" for comparison purposes to measure duration......every company uses a different number to measure off the base circle so this keeps things even when choosing. use your cam's actual measurments to find what want to know. a degree wheel and dial indicator is better yet.

 

[folkenheath]

.050 is only used as an industry "standard" for comparison purposes to measure duration......every company uses a different number to measure off the base circle so this keeps things even when choosing. use your cam's actual measurments to find what want to know. a degree wheel and dial indicator is better yet.

Yeah I understand that, .050" is the standard for duration measurement, and I like to know both (.050 and adv) because it helps to determine how fast the valve is opening and closing (along with lift, assumnig you are aware of the different seat values). However, it's not a very good number to use for dynamic compression calculations, so I don't see why they use it for that is what I was trying to say. In fact, just to see what people are asking, I read the "original" link...(from the hijacked thread)

http://www.empirenet.com/pkelley2/DynamicCR.html

...and the guy even states he wants to use the actual closing point (closer to advertised), yet the calculator was asking for .050" closing point. I didn't download the calculator, but maybe it was converting with more information and it asked for the .050 because he assumed that's what the cam card would have. Anyway, thanks for the help clearing it up for everyone.

 

[shovelbill80]

duration can be adjusted by valve lash also......every combo is unique. you can't use .050 for figuring the exact #.

 

[folkenheath]

duration can be adjusted by valve lash also......every combo is unique. you can't use .050 for figuring the exact #.

That's a good point I agree (if you are using a mechanical cam), which even adds more question in why that persons calculator is asking for the .050 closing number.

I just use advertised because usually I am doing the calcs trying to select the parts, so I don't have them yet. Once you have the cam and pistons installed you can double check the closing point with a degree wheel but more than likely you won't be changing them at that point. We're not prostock racers looking for an extra 2 hp so it's usually good enough to not detonate on pump fuel at least.

 

[Chevy305]

Scott is right, and so are you, quench is the distance between the top of the piston and the head surface at TDC(including the gasket). It shrinks at higher RPM as stuff flexes from the stress, it kind of creates a shock wave of air to help get the combustion started more evenly. For your small block .040 is a good number to shoot for (including the gasket, which is probably exactly that or a .001" under.) I have been running my 388 small block at .040" for years, runs great, very efficient, and puts some good power down. On my big block I was going to fo .040 but my machinist recommended .045 for larger engines or with a little more RPM.

Read these...(they are interesting, I read them when I was still a teenager)

http://www.kb-silvolite.com/article.php?action=read&A_id=35

http://www.kb-silvolite.com/article.php?action=read&A_id=39

http://www.kb-silvolite.com/article.php?action=read&A_id=4

http://www.kb-silvolite.com/article.php?action=read&A_id=34

As for your engine, did you have the block decked at all or is it factory original height? Because if it's original you probably have an extra 015" - .020" or so above the piston before the gasket even.

When I say "extra quench", I am referring to any extra in addition to the gasket thickness, or, the piston top to the deck top at TDC.

If you look at your cam, the timing they show is @ .050(1.5+180+37.5=219). You could just add 15 degrees as the generic guess, or we could find out the advertised (closer to actual) closing point. 262-291=43/2=21.5+37.5=59 degrees

If I use those numbers with .015 extra quench, I get 9.4 CR and 7.9 DCR (if you decked your block to zero I get 9.8 and 8.1 DCR)at 59 degrees closing point. I would say that is pretty good. I don't see why they use the numbers at .050". You can't compress the air much until the valve is closed (unless piston speed is very high relative to the valve gap, which isn't the case near BDC), the advertised numbers (at .006, .010, .015 or whatever) are closer to actual closing than at .050", if you measured leakdown at .050" valve opening it would be pretty horrible. Now, at high RPM with high air speed then you may start to get some compression before the valve is shut all the way because you are cramming air in the cylinder so fast, but this is not when you are most susceptible to detonation anyway.

Good information in those articles, I wish I read those before building my engine, I could have set it up a little smarter.

I never did have my block decked. I checked the clearance when assembling and got .025 piston to deck clearance. My head gasket has a compressed thickness of .024. So that gives me a quench of .049. I guess that extra quench is what is saving me from detonation.

 

[folkenheath]

Good information in those articles, I wish I read those before building my engine, I could have set it up a little smarter.

I never did have my block decked. I checked the clearance when assembling and got .025 piston to deck clearance. My head gasket has a compressed thickness of .024. So that gives me a quench of .049. I guess that extra quench is what is saving me from detonation.

.049 is not really bad, in the range of .040 at least. And extra quench is not good, you want as close to optimal as you can. Without the close quench it doesn't have the same effect anymore. But I would say you are fine where you are at, it's good you chose the .024 gasket, that was a smart move, it put your quench at .049 instead of .064, which wouldn't of been very good quench. Also, I think your smaller chambers and dished pistons are good for some increase in efficiency, assuming the valves aren't shrouded and your pistons have the quench surface around the piston tops.

 

[shovelbill80]

That's a good point I agree (if you are using a mechanical cam), which even adds more question in why that persons calculator is asking for the .050 closing number.

I just use advertised because usually I am doing the calcs trying to select the parts, so I don't have them yet. Once you have the cam and pistons installed you can double check the closing point with a degree wheel but more than likely you won't be changing them at that point. We're not prostock racers looking for an extra 2 hp so it's usually good enough to not detonate on pump fuel at least.

bingo! my a55 dyno won't know the difference.

 

[76zimmer]

Did you get the final numbers checked Dave?

And it's a big block, cast crank and 2 bolt main are no problem for 6K. I have a cast crank too buddy, and my regal has had a cast crank in it for 13 years, and I have sprayed 175 hp of N20 on top of that on more than one occasion, running mid 10s on a stock cast crank and rods probably isn't the best idea, but so far it has held up fine, and thats a small block. Besides, it's not only the power you are shifting at that matters, it's what it drops back down to after you shift. You want to keep that range centered in your best power curve.



Scott is right, and so are you, quench is the distance between the top of the piston and the head surface at TDC(including the gasket). It shrinks at higher RPM as stuff flexes from the stress, it kind of creates a shock wave of air to help get the combustion started more evenly. For your small block .040 is a good number to shoot for (including the gasket, which is probably exactly that or a .001" under.) I have been running my 388 small block at .040" for years, runs great, very efficient, and puts some good power down. On my big block I was going to fo .040 but my machinist recommended .045 for larger engines or with a little more RPM.

Read these...(they are interesting, I read them when I was still a teenager)

http://www.kb-silvolite.com/article.php?action=read&A_id=35

http://www.kb-silvolite.com/article.php?action=read&A_id=39

http://www.kb-silvolite.com/article.php?action=read&A_id=4

http://www.kb-silvolite.com/article.php?action=read&A_id=34

As for your engine, did you have the block decked at all or is it factory original height? Because if it's original you probably have an extra 015" - .020" or so above the piston before the gasket even.

When I say "extra quench", I am referring to any extra in addition to the gasket thickness, or, the piston top to the deck top at TDC.

If you look at your cam, the timing they show is @ .050(1.5+180+37.5=219). You could just add 15 degrees as the generic guess, or we could find out the advertised (closer to actual) closing point using the adv duration. 262-219=43 /2=21.5 +37.5=59 degrees

If I use those numbers with .015 extra quench, I get 9.4 CR and 7.9 DCR (if you decked your block to zero I get 9.8 and 8.1 DCR)at 59 degrees closing point. I would say that is pretty good. I don't see why they use the numbers at .050". You can't compress the air much until the valve is closed (unless piston speed is very high relative to the valve gap, which isn't the case near BDC), the advertised numbers (at .006, .010, .015 or whatever) are closer to actual closing than at .050", if you measured leakdown at .050" valve opening it would be pretty horrible. Now, at high RPM with high air speed then you may start to get some compression before the valve is shut all the way because you are cramming air in the cylinder so fast, but this is not when you are most susceptible to detonation anyway.

Heath I added more info to my numbers above. The deck was cut .010 and a couple other things changed a little as well.

I guess I don't see a need for higher RPM (at this point) The pistons are forged for possible abuse in the future, but I'm thinking 5500 is what I'll shoot for if/when I get it that high for a redline/shiftpoint.

Thanks for checking this out Heath.