Wild guessing, DCR, Carburetor size, engine HP formula.

[mathd]

Hey, here is some formula i found on the net, keep in mind they are approximate just for a WILD GUESS
(first one is by Larry Meaux) to find the DST(Dynamic Stroke) for calculing your DCR using the cam timing data.

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)

Now, if we use the formula with my engine.
a 383 4.030 bore, 3.75 stroke, 76cc head, a 41 ABDC intake valve closing, 5.7" rod lenght, 18cc piston
the give something like that
rd = 1.875 * (sine 41) = 1.23
rr = 1.875 * (cosine 41) = 1.41
pr1 = Square root (5.7 X 5.7) - (1.23 X 1.23) = 5.56
pr2 = 5.56 - 1.41 = 4.15
dst = 5.7 - ((4.15 + 1.875) - 5.7) = 3.425
If we plot 3.425 as the stroke value into the Compression Ratio Calculator it give a 7.65 DCR(without boost obviously)
You can plot the DST value instead of the true Stroke in the CR calculator (http://www.rbracing-rsr.com/compstaticcalc.html)

Lets say i want to assume max HP using some CFM value

Lets say the 882 head flow about 205 CFM at max intended intake valve lift.
A real short formula for non boost/power adder to have a wild guess at the hp/head flow ratio:
intake head flow x .25 x cylinder numbers = wild hp guess.
205 x .25 x 8 = 410 hp. < very wild guess/no max rpm.

Now lets get more realistic and take accound the VE% because its a boosted app, the MAX RPM and the engine C.I. displacement.
lets say i have a 383 with a max RPM of 6000 and a VE% of 1.32%
(C.I. displacement) x (max RPM) x .5 x VE% / 1728 = Engine max CFM flow
383ci x 6000 x .5x 1.32 / 1728= Engine CFM flow(877CFM)
now most head manufacturer assuve a 1.67 CFM per 1 HP(or 2 to 2.2 for stock/mild app).
so
877CFM / 1.67 = about 525 FlyWheel HP.
Assuming a 15% drivetrain loss
525 x .85 = 446.25 RWHP

Now,
If i want to average what carb CFM rating would good.
i have once again a 383 that run a max of 6000 rpm with a VE% or around 1.32
(C.i. displacement) x (max rpm) / 3456 x VE% = CFM rating
383 ci x 6000 / 3456 X 1.32VE% = 877 CFM

OR

(C.i. displacement) x (max rpm) / 3456 x (Boost PSI / 14.7) +1 = CFM rating
The last one seem less accurate but is from holley.


Now, keep in mind those are wild guess.

Feel free to Edit/delete my post.

 

[Indycars]

Re: Wild guessing, DCR, VE%, Carburetor size, engine HP form

Now if i needed to assume the VE% for my engine.
My static CR is 8.5 my DCR N/A is 7.65 thats a 0.90(90% VE%), now you can remove about 7% for heat/efficiency loss, that make about 83%VE.
You lost me at this point. I've never seen a approximation for Volumetric Efficiency (VE), by dividing the DCR by the SCR.

Are we talking about the same thing, to me VE is:
"Volumetric efficiency is the measurement of how close the actual volumetric flow rate is to the theoretical volumetric flow rate."


Feel free to Edit/delete my post.
Well that's not going to happen, we welcome discussions here.

 

[mathd]

Re: Wild guessing, DCR, VE%, Carburetor size, engine HP form

Now if i needed to assume the VE% for my engine.
My static CR is 8.5 my DCR N/A is 7.65 thats a 0.90(90% VE%), now you can remove about 7% for heat/efficiency loss, that make about 83%VE.
You lost me at this point. I've never seen a approximation for Volumetric Efficiency (VE), by dividing the DCR by the SCR.

Are we talking about the same thing, to me VE is:
"Volumetric efficiency is the measurement of how close the actual volumetric flow rate is to the theoretical volumetric flow rate."


Feel free to Edit/delete my post.
Well that's not going to happen, we welcome discussions here.

Yeah, this is not a formula i found on the net but it give a fairly good average of the value.
After plotting my engine data into Desktop Dyno 2000 and the Engine Analyser i checked the VE% and came up with the formula, i was impressed because the result where extremly close(enough for me to consider thats a wild guess at the actual value).. sure it do not take into account the head flow, carb, engine temp and lots more.. its why its only a wild guess.

In short i assume the VE% using the cam intake closing point data only(wich to me is what affect the VE% the most.), if someone has a good set of non-restrictive heads/intake/exhaust, it get really close to the actual value.
I can be wrong too, if i say something thats completly wrong just tell me i will remove and/or edit it from my post.

 

[Indycars]

Re: Wild guessing, DCR, VE%, Carburetor size, engine HP form

Yeah, this is not a formula i found on the net but it give a fairly good average of the value.

Did you mean to say "this is NOT a formula" ??? I'm assuming you meant to leave out the "NOT". Do you mean just the formula you quoted in the "Code" box or all formulas in your original post came from the Internet ???

After plotting my engine data into Desktop Dyno 2000 and the Engine Analyzer i checked the VE% and came up with the formula, i was impressed because the result where extremly close(enough for me to consider thats a wild guess at the actual value).. sure it do not take into account the head flow, carb, engine temp and lots more.. its why its only a wild guess.

In short i assume the VE% using the cam intake closing point data only(wich to me is what affect the VE% the most.), if someone has a good set of non-restrictive heads/intake/exhaust, it get really close to the actual value.
I can be wrong too, if i say something thats completly wrong just tell me i will remove and/or edit it from my post.

This is a formula you came up with? Have you tried it in several other engine combinations to see if it provides reasonable results??? What happens when you have a high SCR like 13:1 and a DCR of 8.5:1 , wouldn't this make the equation give you a much lower VE when it could be higher ??? Maybe it was just a coincidence that the formula gave you a reasonable number ???

 

[mathd]

Re: Wild guessing, DCR, VE%, Carburetor size, engine HP form

I meant that just the one formula on the code box is from myself and i dont think it is just a coincidence.
All the other formula from my original post are formula i got here and there from random forums/websites..
So far i have tested it with 2 -3 different engine spec and it SEEM to be really close for all of them.(keep in mind i have nothing to measure, all i can compare to is theorical value from Dyno 200 and Engine Analyzer.

If you had an engine with a 13:1 SCR and 8.5:1 DCR you probably have a poorly breathing cam(extremly late intake valve timing, you probably have the intake valve hit the piston at this point, or leaking valves/rings whatever) /cylinder not filling completly.
The translate into a low VE%, the formula assume, 0.65%

I dont know all the formula(like for air volume/density), but lets say at 13:1 you have 176.4PSi of air in the cylinder and at 8.5:1 you have about 110PSI of compressed air in the cylinder.
at 176 you surely have more air into the cylinder then at 110 psi.
Once again thats a 0.65% ratio.
EDIT: To me less air in the cylinder(low DCR) mean less air flow through the stroke/rpm range, the SCR theorically assume the cylinder filling completly while the DCR assume its value using the cam timing and how much air get into the cylinder, dosent it?
Sure its not the flow ratio, because its not measured over time(cfm, cubic foot perMINUTE), but with some thinking it probably can be made really accurate, assuming the rpm and other values.

Correct me if i am wrong.
My point was to have a easy way to have a wild guess at the actual value, not to have a 100% accurate result.

For a more accurate result someone could use this:
http://www.installuniversity.com/instal ... 012000.htm
But once again it need to have the engine running, with snesor.. not only paper/pen.

 

[Indycars]

Re: Wild guessing, DCR, VE%, Carburetor size, engine HP form

If you had an engine with a 13:1 SCR and 8.5:1 DCR you probably have a poorly breathing cam(extremly late intake valve timing, you probably have the intake valve hit the piston at this point, or leaking valves/rings whatever) /cylinder not filling completly.
The translate into a low VE%, the formula assume, 0.65%

Those numbers are not impossible to get in a drag race type engine, see graphics below. Later IVC is how you get better filling of the cylinder. This of course requires higher RPM, but I think you are assuming the RPM is what ever is required to obtain Max VE. If closing the intake valve earlier provided a higher VE, then stock engines would have the highest VE compared to a race engine. Which doesn't make sense.


I dont know all the formula(like for air volume/density), but lets say at 13:1 you have 176.4PSi of air in the cylinder and at 8.5:1 you have about 110PSI of compressed air in the cylinder.
at 176 you surely have more air into the cylinder then at 110 psi.
Once again thats a 0.65% ratio.

You don't have a higher volume of air at 176 psi, it's just squeezed into a smaller volume. It's the same engine how can it have two different amounts of air ???


EDIT: To me less air in the cylinder(low DCR) mean less air flow through the stroke/rpm range,

Lower DCR with a later IVC angle means more air in the cylinder at the right RPM, because it continues to flow into the cylinder even thou the piston is rising in the cylinder. If it meant less air, then it would be flowing back into the intake port.

I'm enjoying the discussion ! Hope you don't take any of this in a negative way.

 

[mathd]

Re: Wild guessing, DCR, VE%, Carburetor size, engine HP form

First of all

I'm enjoying the discussion ! Hope you don't take any of this in a negative way.

Yes i enjoy the discussion and am glad you do .

I was unsure about that, keep in mind, i am unsure if iam right or not, am just trying to find out if the formula can hold true or not .

Later IVC event ABDC means some air escape the cylinder while on the upgoing stroke before the intake valve close is what i was thinking,
In my mind that was meaning less air in the cynlinder, what is causing that lower DCR.
Now on a race engine usually you want a high DCR, or for a blown/turbo app a lower DCR.. Thats why the wider LSA and blower/turbo cams like above. But as boost go up so do the DCR, as do the VE% for your race engine.
A boosted app will have a low DCR, like 7.65:1 in my case, but with about 7 psi of boost(this is a random number i run no more then 5 psi, but thats another story) it go up about to 11.29:1, with 7 psi of boost according to my math.
If you use that calculator: http://www.rbracing-rsr.com/comprAdvHD.htm do the match twice, once without boost and once W/7psi boost.
bore: 4.03
stroke: 3.75
rod lenght: 5.7
static CR: 8.5
IVA: 41°
boost: 0 psi and then 7 psi
altitude: i have it at 1 for testing(sea level)
at 0 psi it display a 7.84 DCR thats close to my wild guess or 7.65,
at 7 psi it display 11.57, wich is also close to my measurement of 11.29 assuming the "7 psi Boosted " VE% (in my original post)
So that mean a over then 100% VE on a race engine with low DCR AND boost.

You don't have a higher volume of air at 176 psi, it's just squeezed into a smaller volume. It's the same engine how can it have two different amounts of air ???

Yes, but uncompressed thats a bigger volume of air(more uncompressed air through the engine for each stroke = more air volume through the engine each stroke"so probably more flow each stroke?"). Now VE% is for the flow through the engine or through the cylinder?, whatever it should have a direct really close effect on both.

Lower DCR with a later IVC angle means more air in the cylinder at the right RPM, because it continues to flow into the cylinder even thou the piston is rising in the cylinder. If it meant less air, then it would be flowing back into the intake port.

You have a good point there, i think that sum it really well
I really didnt think about it this way, time/rpm and many other factor where not in my mind when i made this quick/dirt wild guessing formula .
But still, when comparing with the rsr CR calculator and taking into account some boost it get close for a wild guess

 

[mathd]

Re: Wild guessing, DCR, VE%, Carburetor size, engine HP form

What inspired me this formula is a holley formula for the carb CFM sizing.
In the S/C instruction sheet, (i can upload if needed)

they say to use:
max rpm X c.I. displacement / 3456 X (boost psi / 14.7) + 1 = carb CFM requirement
Now take attention to the last part (boost psi / 14.7) + 1

I came around for another formula from weiand or holley, that say this:
max RPM X C.I. displacement / 3456 X VE% = carb CFM requirement
The last part is taking my attention..
One of the formula use the VE% value and the other appear to be a formula for the VE%.
now if we used the last part of the holley formula (boosted PSI / 14.7) + 1 for 7 psi of boost we assume: 1.47%VE(thats way innacurate if you plot this anywhere).
But
if i use that VE% to find my BOOSTED DCR, it will looks something like this:
1.475 x 7.65 DCR = about 11.29:1 DCR with 7 PSI of boost.
NOW, if i use that BOOSTED DCR with the unboosted SCR it give something like this:
11.29 / 8.5 = 1.32VE%(boosted)

Now if you play around with those value and the engine spec in the original post with the Cr calculator above, it yeld a close result.
But yeah , its still not really accurate anyway.
I edited the original post and removed that part.

 

[Indycars]

Well.......I don't know where to go from here. Is there a question or are we done?

 

[mathd]

Well, i have no more questions, i said everything i could think about.
Feel free to ask questions or post some engine related formula

 

[grumpyvette]

"Hey Grumpy, we all know which engine components determine vacuum at idle, but what determines vacuum at WOT during acceleration? "

USE THE CALCULATORS to match port size to intended rpm levels... but keep in mind valve lift and port flow limitations[/color]
http://www.wallaceracing.com/runnertorquecalc.php
http://www.wallaceracing.com/ca-calc.php
http://www.wallaceracing.com/area-under-curve.php
http://www.wallaceracing.com/chokepoint.php
http://www.wallaceracing.com/header_length.php
mostly intake port size, cam timing, header scavenging efficiency,compression, the cars drive train gearing ,engine rpm, displacement ,venturie cross sectional area and port air speeds
you don,t need to know any of that, just hook a vacuum gauge to the intake plenum, if it reads higher than 1.5" of vacuum in top gear going thru the lights at the end of the 1/4 mile , with your foot on the floor and the carburetor fully open,chances are very good that a slightly larger carb will help peak power

the chart above can be used as a rough guide to match cam duration at .050 lift and static compression in engines obviously other factors come into play so its only a rough guide