Shane661
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posted October 05, 2010 01:48 PM
 Edited By: Shane661 on 5 Oct 2010 21:49
- Calculating Compression, Static and Dynamic -
I have some ZX-14 cylinder heads and pistons laying around here. I am interested in a few things:
1) How do I calculate the static compression ratio?
2) How do I calculate the dynamic compression ratio?
3) How does cam timing affect dynamic compression?
Anyway, I have ADD at best...so I would like to avoid reading any engineering tomes or reports. I'm trying to get a grip on this in a language that I can understand. 
Shane
  
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smokinZX14
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posted October 05, 2010 04:28 PM
Start here... http://www.golenengineservice.com/html/calculators.html
Read that and then i'll tell you how to get Dome CC's if needed for pop up pistions ..
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Tampa Bay , FL .. Brocks Performance Dealer ..
Gen 2 ZX14R Best ET 8.43 , Best MPH 164.95
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whitehendrix
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posted October 05, 2010 06:03 PM
well, the math is all ther in the link.
dyn. comp is a function of how much pressure is generated in the combustion chamber. that is related to how much fuel is in there.. more intake cam duration/lift, more fuel gets in.. more fuel to burn.. more pressure.
in the most absolutely basic, simple terms
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dougmeyer
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posted October 05, 2010 08:56 PM
That's a good calculator. You will need to calc. the dome volume, and there are a couple ways to do that. I'll let Smokin' give you his method.
WH, your definition of "dynamic compression" is a little vague, so let me try. The calculated compression is a simple arithmetic comparison between two volumes, one the volume of the entire space between the top of the piston as the inside of the chamber at BDC and the other the volume of that same space at TDC.
The dynamic compression is the difference between the gas pressure in that space at TDC and the atmospheric pressure outside that space. The only effect that fuel has on that pressure is that it is an incompressible liquid trapped in the space at TDC and therefore reduces that volume and increases the pressure. If you took a standard day with 14.7 psi air pressure and had the piston at the bottom with the intake valve open, you'd have that whole space filled with air at 14.7 psi. If you could close the inlet valve like a switch and move the piston to the top you might find the pressure in the combustion chamber to be around say 200psi. The ratio between those two pressures is 13.6:1 Interesting.. no? But let's say that when you crank the engine you actually get 230 psi., a 15:6:1 ratio. Where is that extra pressure coming from? Well, it would take more air in there than just the static atmospheric pressure could supply wouldn't it? It would take a DYNAMIC event, such as a column of air moving at some velocity through a port with inertia that keeps it flowing even as the piston reverses it's direction and before the inlet valve is fully closed. It would also take an increase in temperature, such as that which occurs when you compress a gas, which is of course what is happening in the cylinder as you reduce its volume by a factor of about 14:1 The dynamic compression ratio is a moving target. This is exactly what density altitude is all about, isn't it?
Colder air is heavier, more mass, more inertia, more static pressure with the same TDC volume giving a higher cranking pressure, a higher dynamic C/R and ultimately more power.
Take a while and think about how cam timing works into this concept.
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smokinZX14
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posted October 06, 2010 11:04 AM
Edited By: smokinZX14 on 6 Oct 2010 19:36
Getting the volume on domed pistons that have been radically altered, either by profiling or fly cutting, will require a direct measurement. To do that, place the piston down in the bore a precisely measured amount (usually .500 inch), and add grease to the ring land area to seal the piston to the bore. Now measure the total cc's it takes to fill the cylinder to the deck with a burette. Compare the measured volume (cc's) to the mathematical volume of the cylinder at .500-inch depth. The difference is the dome volume or clearance. In this case, a 4.350-inch bore at .500 inch down has a volume computed mathematically of 121.7 cc. With the piston deck set at .500 inch down, we measured 114 cc of fluid to fill it. The missing 7.7 cc is the net dome volume.
Now cc the head and add cc's for the head gasket .. In the case of this motor , we have 84 cc chamber and 9 cc for the head gasket for a total of 93 cc ... Remove the 7.7 CC's from the 93 CC's and you have your numbers ..Flat top motors are easier .. All you need to know is how far down the hole the piston is a TDC , head gasket cc , head cc and valve pocket cc ..As you can see a domed piston add an extra step ..
In the motor above .. 4.350 bore , 3.750 stroke , 7.7 cc dome .9 cc head gasket , 86.7 CC head we end up with 11.9455 CR ..
Change nothing but the stroke to 4.150 from 3.750 stroke and you need up with 13.0123 CR
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Smokin Performance Cycles..
Tampa Bay , FL .. Brocks Performance Dealer ..
Gen 2 ZX14R Best ET 8.43 , Best MPH 164.95
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SteddyTeddy
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posted October 06, 2010 12:07 PM
I have an old copy of Desktop Dyno. All I have to do is take a few measurements and let the program do the rest.
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Y2KZX12R
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posted October 06, 2010 12:27 PM
Hey doug, nice to see you post.
One thing most people forget to take into account is the reduced dia. of the piston above the top ring. Its effect is small but its real. It will lower the static CR.
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dougmeyer
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posted October 06, 2010 12:32 PM
Smokiin',
Perfect explanantion. But Jim's comment is quite relevant, especially on big bore engines. You need to watch the grease above the top ring. I use a stiff brush to remove it.
Thanks Jim!
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2000redrocket
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posted October 06, 2010 02:30 PM
smoken it sounds like you are working on a mopar.
one of my friends built a engine based on dynamic compression to use pump gas.
it makes sence the static was lower and relying on sort of big duration numbers for the cylinder fill on top end. well he did not like it. it was very prone to the weather changes compaired to his 14.5 to 1 stroker 360 and needed a lot of spark advance. he did like only needing 93 pump gas, but that is the only thing he liked. it is now around 14 to 1 again.
shane from what i have been around the higher the static the less it will fall off to temp changes (i think humidity will be the same fall off cause it displaces the o2) and a lower compression engine with a lot of cam will not make as much low end torque and be peeky.
do i have this correct jim , doug? this is what we bumped into over the years anyeway. oh and do not retard a cam 8 deg and think is will make power. (on a V8 any way)
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whitehendrix
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posted October 06, 2010 10:00 PM
sorry i was a little vague on that! wanted to be as dry and simple as possible. i figured someone with a better way to explain the fundamentals would come thru..
thank you for the backup!
Smokin.. curious as to how you set the piston off the deck.. do you use a caliper or depth gauge off the deck and set the piston itself to that, of the top ring?
i would imagine the piston's top, at the lowest point?
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Duct tape is the handyman's secret
weapon. The black is for formal
occasions. - ZRXDean
ill never own a busa unless
someone gives me one.... and then
ill sell it -KZScott
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dougmeyer
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posted October 06, 2010 10:28 PM
Lower the top surface, directly above the pin down from the deck an arbitrary, yet sufficient amount to keep the dome lower than the deck surface. Use a caliper.
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smokinZX14
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posted October 07, 2010 06:41 AM
quote:
Lower the top surface, directly above the pin down from the deck an arbitrary, yet sufficient amount to keep the dome lower than the deck surface. Use a caliper.
Or TDC piston .. Deck to piston at the side of the piston , rock piston in bore and split the difference ...
____________
Smokin Performance Cycles..
Tampa Bay , FL .. Brocks Performance Dealer ..
Gen 2 ZX14R Best ET 8.43 , Best MPH 164.95
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whitehendrix
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posted October 07, 2010 09:42 PM
like this?
____________
Duct tape is the handyman's secret
weapon. The black is for formal
occasions. - ZRXDean
ill never own a busa unless
someone gives me one.... and then
ill sell it -KZScott
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entropy
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posted October 08, 2010 01:09 AM
Edited By: entropy on 8 Oct 2010 09:10
i wouldn't stress too much about all those measurements... nope
I fill the chamber with alcohol at TDC on the mocked up motor, measure the cc's with a burette.
Results are reproducible to 0.1 - .2 cc's, = about a tenth of a point of the CR.
We gotta ask ourselves what are we deretmining CR for anyway?
I use it only to determine "state of the build", to pick fuel, and set AF with that fuel.
The higher the CR, the lower i set AF.
My ZX12 ranges between CR 15.4 - 16.4:1 depending on Quench and how deep the valves are sunk, at that CR i can use VP C-44 fuel (98 octane).
with CR=16:1 on the 12, I target AF around 12:1; burned a hole in a piston at 13:1, bad ju-ju.
My Busa is 17.5-17.6:1 so VP Q16 is needed (116octane).
Your results may be different.
just one demented TFA person's viewpoint.
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whitehendrix
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posted October 08, 2010 01:13 AM
Edited By: entropy on 8 Oct 2010 09:26
karl, you bolt the head down and fill the chamber with alcohol thru the plug hole i take it?
grease the piston like previously mentioned?
i would assume an old head gasket?
yes, yes, and yes.
after you fill the chamber, shake the damn bubble outa the chamber.
with v lil technique yr result ar VERY reproducible.
____________
Duct tape is the handyman's secret
weapon. The black is for formal
occasions. - ZRXDean
ill never own a busa unless
someone gives me one.... and then
ill sell it -KZScott
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dougmeyer
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posted October 08, 2010 09:28 AM
I don't trust it on an initial "dry" build, but it's OK on a rebuild. You can't see the leak and you can't see the bubble.
But that's just me, and I've had work with some fairly weird looking chambers.
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entropy
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posted October 08, 2010 11:14 AM
quote:
I don't trust it on an initial "dry" build, but it's OK on a rebuild. You can't see the leak and you can't see the bubble.
But that's just me, and I've had work with some fairly weird looking chambers.
Weird combustion chambers are beyond my experience, but I do the "fill thru the plug hole" on every build and results are very, very consistent.
The 1st time i calculated CR i did it by the method above, measuring everything.
Too much trouble, especially when you have valves sticking above the head surface. And getting the bubbles out of the chamber when i cc-d the chamber was a pain.
The issue with bubbles is not a big deal, I do 2 cyls at the same time, look for inconsistency. Shake the motor to get bubbles out.
different strokes for different folks.
BTW: I fired up the 12 today, sounds GREAT!!!
2 weeks until the Texas Mile, WOO-HOO!!!
Doug's coming to Texas!!!!
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entropy
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posted October 08, 2010 11:18 AM
whitey,
yup, grease the top of the cyl then move the pistons to TDC, wipe away grease.
The grease above the rings does give a slightly higher than real CR, but i don't worry about it.
Important to get the motor level...
i use an old HG, but i have also used a new one with exactly the same results.
make sure yr valves are tight.
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