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Hi,

This interesting stuff.

Im not trying to be picky but, I really hope the piston speed is "0" at TDC and BDC. I have had a few that were not, and it wasn't pretty.

I think the instantaneous speed statement is a little different than I generally configure things in my mind (remember feeble kinda mind). To me rod length and created piston acceleration rate is very important to creating the the start of cyl filling. Have seen it have such a change in flow (and flame propagation) that it took some work to get the desired results.

I know the valve angles here are very important as well.
I dont really know how to address this mathematically.

Im really tied up today, and I got to run, but Paul and Shawn, you guys are doing a great job and this is very interesting.


Jeff
 
I wrote a program in MS Excel in College that would calculate piston position (inches below TDC) as a function of crankshaft angle (degrees past TDC). The user could input rod length and stroke and evaluate how these variables affected piston position.

With some help from my peers we took it a few steps further. We had MathCad calculate the derivative of the position formula. This gave an equation for piston velocity. The input variables were those listed above plus RPM. The instantaneous piston velocity was calculated as a function of crank angle in feet per minute.

Next, we calculated piston acceleration. The formula would output instantaneous acceleration as a function of crank angle, in feet per minute per minute.

I recall inputing several different rod lengths for a given stroke. The piston position as a function of crank angle does not change much. The instantaneous velocity changes is a bit less in a long rod motor compared to a short rod motor. The instantaneous acceleration was a bit less in a long rod motor compared to a short rod motor. The difference in velocity was greater than the difference in position, and the difference in acceleration was greater than the difference in velocity when rod length was varied.

Now the bad news is that I lost that program prior to graduation. I have since rewritten the position formula. Unfortunately, I am not smart enough to calculate the derivatives by hand, and don't have access to MathCad.
 
I've seen others that have done similar and this apparently has lead to the current internet buzz that rod length "doesn't matter". What they don't take into account is the interaction of cam timing and inlet velocity relative to piston speed.

Flow bench data is taken with the valve opening fixed and often without the intake manifold or carb attached so even that doesn't closely simulate the dynamics of a running engine where the air in the port has to stop when the valve is closed and accelerate when it opens.

We've already shown that bore diameter changes piston area which influences inlet air velocity for any minimum cross section. We've seen that the stroke variable influences MPS which impacts air velocity and inertial supercharging. You've confirmed that instaneous piston speed is effected by rod length. Maybe not as much as big stroke changes but it has to be considered.

A change of only 2-4 crank degrees can have a measureable effect on engine output. A typical multi position cam sprocket has only +4, 0 and -4 crank degrees of adjustment but the cam is turning half as fast as the crank.

Anybody that's changed a stock cam for an aftermarket cam knows that inlet valve opening and closing relative to crank degrees has a big influence on VE, torque and HP. Air velocity relates to VE which translates into torque combined with rpm, hp is derived from.

It can get very complicated and some variables may have an effect on operating parameters other than HP so the consequences may get overlooked
Fuel economy, width of torque curve, exhaust temps, driveability never seem to come up in discussions but an engine builder may notice these come out less than ideal after changes.

If you compare the modern Z06 7 liter and an L-88 7 liter engine and you can easily see the operating characteristics are very much different (and improved) thanks to engineering progress.

You can't point to any one thing that makes the whole difference. It's the combination of everything and the optimization of variables that some think "don't matter".
 
Discussion starter · #64 ·
I have asked a few of my engine/cylinder head friends and none can seem to come up with the source. One of them that does prostock heads told me "huh, guess i don't know, just know it works", which has kind of always been my take on it. Whether or not the thing came from some deep reasoning by a SAE engineer type or that someone said, wow, look here, this fits well, doesn't really matter to me. I just know that if i need something to get me in the ballpark, it comes very,very close. That being said, nothing is perfect. Here's another example of using it in practice that one of my sources shared, thought you might find it interesting-

Dart Pro-1 215CC Dyno Test + Flow Numbers
Pushrod Area Choke Testing

i just finished a series of Dyno Tests using Dart Pro-1 aluminum SBC
215 CC Heads

SF-600 FlowBench Data (Ported but unwelded, 1st series of Tests)
Manley Valves= 2.125" Int +.100 Long 1.600" Exh +.100 Long
4.125 Flow Fixture No-Pipe on Exhaust Port
Lift---Intake--Exhaust
.200--146.6--111.0
.300--215.5--161.3
.400--260.0--203.8
.450--279.0
.500--293.5--228.4
.550--300.8
.600--304.0--239.7
.650--307.0
.700--310.6--243.7
.750--315.8
.800--318.8--243.3
.850--320.8
Comments=> Speed FPS too high at pushrods



Engine Specs=> 4.165 Bore x 3.875 Stroke = 422.4 CID
GM "Bowtie" Intake Manifold max-ported + reworked plenum
with Moroso #65000 2 inch Dominator adapter
Dart Pro-1 215CC 2.125/1.600 max-ported but unwelded @ pushrods
HP-1250 Carb
C-16 Race Gas
MSD Distributor
Diamond Pistons 14:1 CR 224 Cranking psi
Cam Motion solid roller .776"/.743" Lift 284/300 Duration @.050"
112 Centers on 108 CL .025" lash across hot
Cam Motion Red Rockers 1.65/1.65 Ratios

RPM--Torque---HP--SF-901 Dyno Data @ 600 RPM/SEC
5500--551.0---577.1
5600--553.2---589.8
5700--556.5---603.9
5800--557.6---615.7
5900--558.8---627.7
6000--562.1---642.1
6100--560.6---651.1
6200--560.2---661.3
6300--558.1---669.4
6400--555.7---677.2
6500--550.4---681.2
6600--549.3---690.3
6700--547.7---698.7
6800--543.3---703.4
6900--531.6---698.4
7000--524.5---699.1
7100--509.9---689.3
7200--509.0---697.8
7300--500.5---695.6
7400--492.6---694.1
7500--481.5---687.6
7600--475.3---687.8

Avg=> TQ=535.9 HP=665.4 Fuel=260.2 Lbs from 5500-7600 RPM

Note Fuel= 260.2 Lbs. avg from 5500-7600


********************************************************


2nd Test Series Results
with Welded Heads at Pushrod area + slight more Short Turn rework
with widened Port's pushrod area
Pushrod area outside wall thickness = .040"
with Offset Lifters + Crane 1.65 Offset Rockers


SF-600 FlowBench Data
Manley Valves= 2.125" Int +.100 Long 1.600" Exh +.100 Long
4.125 Flow Fixture No-Pipe on Exhaust Port
Lift---Intake--Exhaust
.200--146.6--111.0
.300--215.5--161.3
.400--262.0--203.8
.450--278.0
.500--293.5--228.4
.550--302.0
.600--314.0--239.7
.650--317.5
.700--319.1--243.7
.750--321.4
.800--323.4--243.3
.850--325.4

Slowed down Speed FPS @ pushrod area to more acceptable level


RPM--Torque---HP--SF-901 Dyno Data @ 600 RPM/SEC
5500--555.0---581.2
5600--555.4---592.2
5700--560.2---607.9
5800--561.3---619.9
5900--565.6---635.3
6000--566.4---647.1
6100--570.6---662.7
6200--568.7---671.3
6300--567.3---680.5
6400--567.8---691.9
6500--563.1---696.8
6600--555.2---697.7
6700--554.1---707.3
6800--551.3---713.7
6900--538.8---707.9
7000--540.8---720.8
7100--527.9---713.7
7200--518.3---710.6
7300--517.3---719.0
7400--511.5---720.6
7500--503.8---719.4
7600--492.9---713.3

7700--489.9---717.5
7800--480.7---713.8

Avg=> TQ=546.1 HP=678.7 Fuel=258.5 Lbs from 5500-7600 RPM

Note Fuel= 258.5 Lbs. avg from 5500-7600



With AirSpeed @ Pushrod area slowed down to more acceptable level,
Engine has a better Torque + HP Curve..especially after RPM point
of Peak HP
and using about the same amount of Fuel


Note= my Dyno is on conservative side,
so 720 HP on my Dyno at 600 rpm/sec
could be 740 to as much as 770 Hp on other Dynos
especially at slower test rates.

likewise FlowBench is about 10-15+ CFM numbers on conservative side

shawn
 
Hi,
you sure you want know? Just kidding.

(Pi/4) = (3.141592654 / 4) = .785398163

360 * .785398163 = 282.7433388 or .003536777


the .00353 is just the 1/x reciprocal of 283.286119

its a combination of conversion constants into 1 constant = 282.286119
it converts inches into area and inches into feet
It seems that the minimum cross section area is tough information to come by if a person is cylinder head shopping.
I called both CFE and Air Flow Research; neither one could tell me the minimum CSA of their heads.
I have a set of CFE 350cc BMF heads on my 548 Chevy, and wanted to run the numbers through the formulas given.

Also, not to nit pick, but in the above quote there is three different numbers
282.7433388
283.286119
282.286119
I assume that the first two are different due to rounding and the third is a typo. Just wanting to confirm that they are basically the same number.
Brian
 
Discussion starter · #66 ·
I know Carl knows the min. cross of the BMF heads. I'm sure AFR could give you there's too. You probably just didn't talk to the right person, or they just didn't want to do the work to find out. The BMF head is cnc'd so all you need is the CAD of the port and you can find out in a few minutes. I'm not sure if you were talking about the cnc version of the AFR head or not. That 350 head on a 548 should run good, if you have enough compression and cam. Those are a very good cylinder head. I know at 14.5-1 , a healthy roller, and a ported single 4 with dom. you should be able to make 925hp+ on our dyno. That engine will spin nicely to about 8000. You'll need at least a 5500rpm converter to maximize the potential of the combo. Yes, the third shoud be 283., not 282.
shawn
 
I ran a 283 with stock bore and arias 12:1 piston my engine would twist 7000 all day long with stock rods and bolt. I would not recommend everyone try this but it worked for me my cam was a extrem 274 comp which was .501/.510 soild lift I was running a seat late model camel humps and a torker intake and 600dp.
 
2 equal engines in power, the one that accelerates to its powerband the fastest will always win.

If I can stress one thing is that an engine is a give and take receipe. If you are going to give it a lot of head then take camshaft away. If you don't have a lot of head then add camshaft.
 
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