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Re: port velocity?
[Re: mopar dave]
#1622178
05/23/14 12:41 PM
05/23/14 12:41 PM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
yes,just when I thought I was understanding. ok both stroke and rod length can effect port velocity.
can you tell me what rod length would work best in a sb with 4" stroke and using an indy 360-1 head with 2.87 [Email]csa@13.5:1[/Email] compression?
Your basically stuck with certain length rods.. you
need to fill the space from the crank to the piston
and based on stroke and deck height your in the 6.125
length with some minor length changes due to pin
height in the piston
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Re: port velocity?
[Re: polyspheric]
#1622179
05/23/14 02:46 PM
05/23/14 02:46 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
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Kalispell Mt.
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I think the real big difference is frictional loads, at low RPM the flow theory stuff probably wins out but add in high RPM long stroke and friction begins to win out. Look at the rod angle in a 400 SB chevy at 1/2 stroke and see how it would be forcing the piston into the wall then look at a 340 under the same condition and the visual difference is tremendous. It is very easy to see how the friction goes through the roof with a bad RS ratio at high RPM. Still getting the displacement and airflow up is gonna trump a bad RS ratio as proven by the big bad mountain motors.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: port velocity?
[Re: mopar dave]
#1622180
05/23/14 02:53 PM
05/23/14 02:53 PM
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Joined: Jan 2003
Posts: 30,989
Oregon
AndyF
I Win
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I Win
Joined: Jan 2003
Posts: 30,989
Oregon
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Quote:
yes,just when I thought I was understanding. ok both stroke and rod length can effect port velocity.
can you tell me what rod length would work best in a sb with 4" stroke and using an indy 360-1 head with 2.87 [Email]csa@13.5:1[/Email] compression?
You can run PipeMax with different rod lengths to see what the results are. My guess is that you will not see much if any difference. Maybe a few HP max. PipeMax might not be a perfect simulation tool, but it is very good for the price.
What you will see in PipeMax is that with a 4 inch stroke and those heads you will need a ton of valve lift to make power. PipeMax will probably recommend something in the .800 lift range for max power. Run it and see what it says.
Mopar guys tend to leave a lot of power on the table since valve lift is expense with stock type Mopar heads. You really have to switch over to a Jesel type setup before you can run the big lift necessary for big power. That is what I've learned over the years from running lots of dyno pulls and dyno simulations.
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Re: port velocity?
[Re: Sport440]
#1622182
05/23/14 03:55 PM
05/23/14 03:55 PM
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Joined: Jan 2003
Posts: 16,376
dogdays
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I Live Here
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Of course I was assuming he was talking about piston velocity.
There are quite a few wrong ideas floating around this post, though.
For those who are interested in piston dynamics, I suggest you look up online work by Bowling.
R.
Last edited by dogdays; 05/23/14 04:03 PM.
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Re: port velocity?
[Re: mopar dave]
#1622184
05/24/14 12:08 PM
05/24/14 12:08 PM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
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Posts: 52,972
Romeo MI
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Quote:
when I run my numbers thru pipemax shorter rods always make more power. don't see any differences in port velocity if i'm reading that right.
You could already be at the choke point and thats why
you arent seeing port velocity changing
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Re: port velocity?
[Re: dogdays]
#1622186
05/26/14 11:51 AM
05/26/14 11:51 AM
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Joined: Feb 2004
Posts: 6,257
acworth / N. georgia - south e...
cheapstreetdustr
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master
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Posts: 6,257
acworth / N. georgia - south e...
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Quote:
I knew that Calculus would come in handy some day.
The TRUE FACT of the matter is that at TDC and BDC, for that instant where the piston is at the maximum distance either up or down, velocity is ZERO.
Acceleration is maximum as (it moves away from )at those points, and as those ancient Greeks would have figured out, eventually, the acceleration at TDC is greater than at BDC. Put another way, the piston displacement vs. time graph is not a perfect sine wave.
We're mostly worried about the piston action at TDC. That's where the power is made or the intake charge is trying to get in.this is called "DWELL" and it effects alot of things especially chamber burn/shape and port efficiency and length.
Because a longer rod causes lesser acceleration at the top of the stroke, the piston spends more degrees very close to TDC with a long rod than a short one.again this is called "dwell" and it has positive and negative attributes..they have to be factored..and there are some advantages
For years and years the prevailing theory was to lengthen the rod to get that "perfect" ratio of rod to stroke, 1.8 was one number thrown around. And at 9500 rpm, the theory seems to work, because that's how NASCAR engines are built, and they'd kill their Grandma for 10 extra hp. Rod lengths of around 6.2 to 6.3, combined with strokes around 3.3, compute out to roughly 1.9. So it works for them.
But we're for the most part not talking about building NASCAR engines.the Kaase short stroke motor was an exercise in this direction
One of the most interesting things about the Enginemasters competition is that the winners for the last several years have had REALLY Short R/S ratios. This year the winning engine and the runner up's stroke was something like 4.7, rod length 6.658 for R/S of 1.4 or so. This is a hp/cubic inch competition, averaged over an rpm range picked to more closely represent street cars, so if there were huge disadvantages with extra side loading on the pistons, you'd think they wouldn't be doing something as "stupid". A few years ago Jon Kaase built a short rod ford 400 and won. This is in a block with deck height of 10.29 or so inches. He used 4" stroke and 6", IIRC, rods. R/S of 1.5.
Also a few years ago, it was reported that he had built a mountain motor with extremely long rods to see if it would work better and he found no power advantage over his normal R/S of 1.38. dynos are not race cars or race tracks..the answer to this sinareo was they ran better in the car down the track..vrs dyno...to a degree
I have to feel that the advantage is in the quicker piston acceleration away from TDC. Heads are flowing better and better. Maybe the quicker acceleration away from TDC gets the mixture flowing faster sooner for more cylinder filling. That's my best explanation at this point.not necessarily...aftermarket heads designed for this YES but not if your using stock/architecture heads
I have officially stepped away from the "long rod" religion. as long as you know the cause and effect you build it to suite your needs
R.
365" Iron J heads,,3480lbs best 1.39 60ft on SS springs.10.54,124 mph ...6.67 1/8th et.average 60fts 1.46 w/ small cam &.063 no2 pill tagged & insured
[image][/image]
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Re: port velocity?
[Re: mopar dave]
#1622187
05/26/14 04:02 PM
05/26/14 04:02 PM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
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Posts: 52,972
Romeo MI
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Quote:
thanks mike. pipemax should come with instruction on how to read all data.
I cant remember at the moment but you can click on
different points to get a quick/short explanation
but also mine came with a book(green paper cover booklet)
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Re: port velocity?
[Re: cheapstreetdustr]
#1622188
05/26/14 11:16 PM
05/26/14 11:16 PM
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Joined: Oct 2007
Posts: 4,205
New York
polyspheric
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master
Joined: Oct 2007
Posts: 4,205
New York
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Yes, it's called "dwell" but this is very misleading.
The piston actually spends this amount of time at TDC: none whatever. Yes, none; a theoretical "instant".
The apparent (observed) lack of motion is due to 2 factors (of which #1 is not always present).
1. Offset piston pins make the highest piston position not occur at 0° (and also changes the stroke length).
2. The total of pin to piston bore fit, and pin to rod eye fit are stacked up (all on the bottom), and then reverse (all on top) after every end-of-travel event (TDC & BDC), and piston motion at these points is small enough that for the first degree or two of rotation it's swallowed by the clearances.
Boffin Emeritus
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Re: port velocity?
[Re: polyspheric]
#1622189
05/27/14 01:02 AM
05/27/14 01:02 AM
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Joined: Feb 2004
Posts: 6,257
acworth / N. georgia - south e...
cheapstreetdustr
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master
Joined: Feb 2004
Posts: 6,257
acworth / N. georgia - south e...
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the important part to "Dwell" is piston top architecture and cumbustion chamber architecture and how it relates to burn pattern and efficiency.
also...as mentioned...how it pulls air through the intake tract..
short rods need larger, shorter intake runners..
thats why chevies have big short intake runner.
the longer rod creates a longer draw on port velocity in the intake tract.
the heads are built this way based on the total package..and long rod ratio is part of the reason factory mopars have longer intake runners and not massively huge ports or valves..
its two different philosophy's ...both have there necessities.. and or attributes..
explaining them, and understanding them is more important than just stating one is better than the other.. it would be like claiming a long socket extension is always better than a short socket extension... they both have their place.
some of those short stroke Kasse motors did not produce an advantage once employed in a drag race vehicle.
thats why tuners soon realize just because it makes a higher number on a dyno.. it doesnt always correlate to faster in the car on the track....you cant race on dyno's
and we havent even mentioned cam shaft lobe lift rates. or valve events..and the differences
due to crankshaft degree's of rotation just before or just after tdc.. short rod to long rod.
and or piston dwell..
degrees of crankshaft rotation...
365" Iron J heads,,3480lbs best 1.39 60ft on SS springs.10.54,124 mph ...6.67 1/8th et.average 60fts 1.46 w/ small cam &.063 no2 pill tagged & insured
[image][/image]
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Re: port velocity?
[Re: AndyF]
#1622192
05/27/14 07:41 PM
05/27/14 07:41 PM
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Joined: Feb 2004
Posts: 6,257
acworth / N. georgia - south e...
cheapstreetdustr
master
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master
Joined: Feb 2004
Posts: 6,257
acworth / N. georgia - south e...
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a non mathematical or scientific way to look at Port velocity/. in strickly visual terms..think of port velocity like a rope..you want a nice tight rope.. . but keep all the slack out of the rope gives you best velocity.
another way to visually understand it..is a garden hose turned on just a little bit... with an open end on the hose..hold your thumb over the end of the hose.. let it build pressure too keep it spraying... hold it too tight it backs up.
remove too much thum and just trickles out.
365" Iron J heads,,3480lbs best 1.39 60ft on SS springs.10.54,124 mph ...6.67 1/8th et.average 60fts 1.46 w/ small cam &.063 no2 pill tagged & insured
[image][/image]
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