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Calculations, dynamic CR, gasket
Posted: March 11th, 2008, 6:56 pm
by cherokee
With the recent addition of the stroker calculator (nice touch btw), I figured I'd actually sign up and ask a couple questions.
Does anyone know the math to calculate dynamic compression ratio and is willing to post an explanation and/or formula here? Actually the explanation in the other thread is great, I'm just looking for more math related to it.
Given the numbers from the varied head gaskets and pistons with the goal of getting a near or better than stock quench height the .032 gasket gives me a .002 better quench than stock. Which gasket is this and where can it be obtained? If anybody's interested in the numbers I plugged in: head vol 58, .030 overbore, 3.895 stroke, 0 deck, 0 plain, -17.5 dish, 5.875 rod, 1.592 piston height. Static compression ratio of 9.56 with unmachined pistons
Anybody else added their own stuff to their own calculator? I plugged in the math to give theoretical air flow and piston speed at varied rpms. The difference between stock and stroked air flow volume turned out to be a surprising percentage difference. When I set up a quick calculation to show the increase in size of a bored throttle body versus the increase in displacement the bored throttle body seemed a large percentage behind. Something on the order of 10+% increase from displacement versus <10% from the throttle body. Anybody have some opinions on this?
Short description of the intended vehicle and usage: 88 MJ restored for a fun DD.
Re: Calculations, dynamic CR, gasket
Posted: March 11th, 2008, 8:06 pm
by 1bolt
It would be interesting to see those percentages. I've always felt the TB is a weak link even bored out as far as it can be.
Dino's mod of using a Mustang TB is closer, but we have to remember that a stock 5.0 Mustang was putting out considerably less power than a Stroker... So even it may still be smaller than absolutely optimal. Probably not by much though.
Re: Calculations, dynamic CR, gasket
Posted: March 12th, 2008, 9:20 am
by Muad'Dib
Doing a quick google search i found this:
Calculating DCR: Calculating the DCR requires some basic information and several calculations. First off, the remaining stroke after the intake closes must be determined. This takes three inputs: intake valve closing point, rod length, and the actual crank stroke, plus a little trig. Here are the formulas: (See the bottom of the page for a way around doing all this math.)
Variables used:
RD = Rod horizontal Displacement in inches
ICA = advertised Intake Closing timing (Angle) in degrees ABDC
RR = Rod Distance in inches below crank CL
RL = Rod Length
PR1 = Piston Rise from RR in inches on crank CL.
PR2 = Piston Rise from crank CL
ST = STroke
1/2ST = one half the STroke
DST = Dynamic STroke length to use for DCR calcs
What's going on: First we need to find some of the above variables. We need to calculate RD and RR. Then, using these number, we find PR1 and PR2. Finally, we plug these number into a formula to find the Dynamic Stroke (DST).
Calcs:
RD = 1/2ST * (sine ICA)
RR = 1/2ST * (cosine ICA)
PR1 = sq root of ((RL*RL) - (RD*RD))
PR2 = PR1 - RR
DST = ST - ((PR2 + 1/2ST) - RL)
This result is what I call the Dynamic Stroke (DST), the distance remaining to TDC after the intake valve closes. This is the critical dimension needed to determine the Dynamic Compression Ratio. After calculating the DST, this dimension is used in place of the crankshaft stroke length for calculating the DCR. Most any CR calculator will work. Just enter the DST as the stroke and the result is the Dynamic CR. Of course, the more accurate the entries are the more accurate the results will be.
See this thread for more info (Thanks to Mr. J-Rod)
http://www.ls1tech.com/forums/showthread.php?t=102153
Hope that helps.
Re: Calculations, dynamic CR, gasket
Posted: March 12th, 2008, 9:53 am
by fedrusion
When comparing the TB's you need to make sure you are comparing the flow area and not just the diameter. This would make the bored TB approx 14.7% larger than stock since we are comparing the flow area.
Re: Calculations, dynamic CR, gasket
Posted: March 12th, 2008, 10:04 am
by Cheromaniac
1bolt wrote:Dino's mod of using a Mustang TB is closer, but we have to remember that a stock 5.0 Mustang was putting out considerably less power than a Stroker... So even it may still be smaller than absolutely optimal. Probably not by much though.
Believe me you won't want a TB bigger than 65mm. The 65mm TB that I used was from a '93 Cobra and it's more than big enough for a mild 4.6 stroker like mine. The throttle response is very sharp and makes street driving huge fun, but it's too oversensitive if you need to feather the throttle for rockcrawling. I'd say a 62mm TB is as big as you're gonna need.
Re: Calculations, dynamic CR, gasket
Posted: March 12th, 2008, 10:27 am
by Muad'Dib
Cheromaniac wrote:1bolt wrote:Dino's mod of using a Mustang TB is closer, but we have to remember that a stock 5.0 Mustang was putting out considerably less power than a Stroker... So even it may still be smaller than absolutely optimal. Probably not by much though.
Believe me you won't want a TB bigger than 65mm. The 65mm TB that I used was from a '93 Cobra and it's more than big enough for a mild 4.6 stroker like mine. The throttle response is very sharp and makes street driving huge fun, but it's too oversensitive if you need to feather the throttle for rockcrawling. I'd say a 62mm TB is as big as you're gonna need.
I had that problem when i installed my 61mm tb on my fairly stock RENIX. After i installed a TB spacer, the throttle was much better "rounded" if that makes since. I could lightly touch the throttle, and it wouldnt just want to put my ass in the seat like it did right after i installed the bored TB.
Re: Calculations, dynamic CR, gasket
Posted: March 12th, 2008, 6:55 pm
by oletshot
I had that problem when i installed my 61mm tb on my fairly stock RENIX. After i installed a TB spacer, the throttle was much better "rounded" if that makes since. I could lightly touch the throttle, and it wouldnt just want to put my ass in the seat like it did right after i installed the bored TB.
Thanks Muad'Dib, you just put another smile on my face. I guess I was wrong about the spacer.
Re: Calculations, dynamic CR, gasket
Posted: March 12th, 2008, 7:56 pm
by cherokee
Muad'Dib, THANKS! That's a great start and damn near exactly what I was looking for. I'm sure I'll come up with more questions about it soon.
Fedrusion, how are you figuring flow area? Related to the height of the throttle body or volume between the throttle plate and bottom of the body? We'll have to compare notes when we cross paths next. That 14.7% is above the volume increase of the strokers if I remember right.
Cheromaniac, this MJ will probably never be wheeled, so that relatively huge throttle body could actually be fun. I'll probably stick with a 62mm one until it can actually be road driven though.
On a side note I plugged in some numbers yesterday and realized that with the renix head volume, a .032 gasket, and the highest un-machined dish pistons that a 9.2ish static CR can be reached. Maybe that's the secret to the stock parts stroker that never pings?
Re: Calculations, dynamic CR, gasket
Posted: March 12th, 2008, 8:45 pm
by Shark
flow area? i imagine thats the surface area of the cylindrical wall of the TB. A = pie x diameter x height
Re: Calculations, dynamic CR, gasket
Posted: March 13th, 2008, 9:53 am
by fedrusion
Flow area is the area determined by A=d^2*pi/4 when you look at a planar section through the T-body at the point of the smallest diameter. This is directly related to flow by Q=V*A where V is Velocity and Q is flow rate. Since we really dont need to know what V is at this point only that more flow is required you can look at the ratio of area's for the two sizes being considered. This would be making the assumption that V is the same for both T-bodies. Ideally you want the smallest area to occur at the throttle plate in order to get the best throttle response and metering effect.
The length of the T-body doesn't come into play in this comparison. If you were looking at bodies with different lengths you would expect a slightly higher pressure drop on the longer one, but the length difference required to cause this would be several feet or more to make it noticeable.
I could have explained this better when I first mentioned it. And I am sure that this explanation is hardly perfect since the compressibility of air hasn't been touched on at all.
Any questions at all feel free to respond, add or PM me.
Re: Calculations, dynamic CR, gasket
Posted: March 14th, 2008, 8:13 pm
by cherokee
Question about the 'RD' value, should the intake close event be converted to something other than degrees before taking the sine value? I keep seeing a negative result.
Airflow at 3000rpm is consistently 14%+ over a stock 4.0l given most possible combinations (or at least according to my calculations). Percentage bore increase of a 62mm throttlebody is just under 7%. However using the flow area increase it becomes very close at 14%+.
Re: Calculations, dynamic CR, gasket
Posted: March 15th, 2008, 8:18 am
by fedrusion
If you are doning this calculation in excel, remember that excel calculates sin cos and tan in radians and not degrees. Convert degrees to rads by multiplying by pi/180 or 2pi/360. I made a quick calc sheet that I can send to you if you want.
Re: Calculations, dynamic CR, gasket
Posted: March 15th, 2008, 8:25 pm
by cherokee
Open Office actually, but it turns out it uses sine and cosine the same way. Used the RADIANS function to get the right input and it makes more sense now.
Re: Calculations, dynamic CR, gasket
Posted: March 17th, 2008, 5:00 pm
by cherokee
Anybody have some part numbers for the different thickness head gaskets?
This might not be the best location for this, but what's a good dynamic compression ratio to shoot for? I'm seeing about 7:1 dynamic, 9.2 static. I'd like to run 87 octane, but if that means a lot of extra machine costs I can live with higher.
Re: Calculations, dynamic CR, gasket
Posted: March 17th, 2008, 6:33 pm
by seanyb505
Detroit Gasket 671045 .045
Direct Connection p4529242 .043
Felpro 9076pt .052
SCE 679035 .050
SCE 679034 .043
SCE 67033 .032
Victor Reinz 5713 .045
Part numbers in bold, gasket thickness in italics. Taken from Yahoo strokers group database.
Edit: I just tried searching for SCE head gaskets and I could not find them for Jeeps.