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Re: port velocity?
[Re: mopar dave]
#1622139
05/19/14 01:23 PM
05/19/14 01:23 PM
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Kalispell Mt.
HotRodDave
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If there is not enough velocity then the fuel falls out of the air and does not burn properly. Too much and it can slam the fuel into the outside of the curves and seperate it that way. I don't know what numbers to shoot for I just tend to stick with what works.
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]
#1622142
05/20/14 10:10 AM
05/20/14 10:10 AM
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Romeo MI
MR_P_BODY
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Quote:
ok, so is there a way to pick the ideal bore,stroke,rod length and port cross section using the math for port velocity?
I dont know what that would be but I'm sure there
is... I just look at my header program in the calculate
HP and it shows the cross section needed and what RPM..
its based on the stroke you enter
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Re: port velocity?
[Re: MR_P_BODY]
#1622146
05/20/14 04:12 PM
05/20/14 04:12 PM
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Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Quote:
300 fps seems to be the magic number for most to shoot for
Average? Peak? Measured where?
Without some context, that 300 fps value doesn't mean much.
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Re: port velocity?
[Re: AndyF]
#1622148
05/20/14 05:00 PM
05/20/14 05:00 PM
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Joined: Apr 2007
Posts: 2,026
Trumbull,CT.
jim sciortino
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Like most other particulars, it is design specific.
For example.....
Try to up the port velocity over a bad short turn and see what happens.
Last edited by jim sciortino; 05/20/14 06:17 PM.
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Re: port velocity?
[Re: BradH]
#1622149
05/20/14 06:30 PM
05/20/14 06:30 PM
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Joined: Jun 2003
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Romeo MI
MR_P_BODY
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Quote:
Quote:
300 fps seems to be the magic number for most to shoot for
Average? Peak? Measured where?
Without some context, that 300 fps value doesn't mean much.
I will assume its the average of the port... I get
the data from PipeMax which is a pretty damn good
program for a ton of info
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Re: port velocity?
[Re: MR_P_BODY]
#1622150
05/20/14 08:19 PM
05/20/14 08:19 PM
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Portage,michigan
B3422W5
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My guess is that unless a person is building a high end motor like pro stock or a " class" type motor this stuff is best left in the hands of an accomplished head porter who is armed with the basics of a combo and it's intended use. The longer the stroke with a given head the lower rpm a motor needs to be spun to make peak horsepower, and peak torque will come in sooner as well. I would plan the combo around how much you plan to invest in valve train components, and how long you want the motor to live. Those will dictate how big an arm can be used with the intended head. All the above assumes the OP isn't building a pro stocker or a trick " class" car  
69 Dart GTS A4 Silver All steel, flat factory hood, 3360race weight
418 BPE factory replacement headed stroker, 565 lift solid cam
Best so far, 10.40 @127 1/4
1.41 best 60 foot
6.60 at 103.90 1/8
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Re: port velocity?
[Re: AndyF]
#1622155
05/21/14 01:21 AM
05/21/14 01:21 AM
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Joined: Jan 2007
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Mt Morris Michigan
mopar dave
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360 small block with indy 360-1 heads. so you think a 3.79 stroke would be a good combo/most efficient with these heads? better than a 4" stroke?
Last edited by mopar dave; 05/21/14 01:23 AM.
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Re: port velocity?
[Re: mopar dave]
#1622156
05/21/14 01:38 AM
05/21/14 01:38 AM
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Joined: Jun 2003
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Romeo MI
MR_P_BODY
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Dave if you have PipeMax dont you put your engine combos through it to see what you get... biggest thing is getting the volumetric efficiency correct
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Re: port velocity?
[Re: mopar dave]
#1622159
05/21/14 05:35 PM
05/21/14 05:35 PM
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Joined: Oct 2007
Posts: 4,206
New York
polyspheric
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The speeds quoted are, of course, averages, and not an accurate picture of port conditions even in the (theoretical) range of maximum piston-derived vacuum (somewhere near both the ICL and the highest piston velocity - typically in the mid-70s depending on the rod ratio).
With a strong exhaust pulse, the vacuum across the chamber during overlap is stronger than piston vacuum. The static conditions in the chamber between high and low compression (small vs. large chamber volume) and high and low rod ratio are fairly different. In relative terms, cross-chamber flow is "lazy" with a really long rod (like n=2) and low compression. This effect is almost harmless if the port is too small (vs. displacement), and very annoying if the port is too big. The reverse condition (short rod, high compression) is significantly less affected by this factor.
Boffin Emeritus
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Re: port velocity?
[Re: mopar dave]
#1622161
05/22/14 01:24 PM
05/22/14 01:24 PM
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polyspheric
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Not quite.
As the stroke is reduced, or the rod is lengthened, or both, piston motion around TDC slows down. Piston motion around BDC speeds up.
Longer stroke, shorter rod etc. = reverse.
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Re: port velocity?
[Re: HotRodDave]
#1622167
05/22/14 05:40 PM
05/22/14 05:40 PM
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polyspheric
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Sorry, "yes and no" is simply wrong.
A longer stroke (with no other changes) always increases piston speed around TDC and decreases it around BDC.
There is no opinion on this, it's plane geometry, and it was settled 2,500 years ago by Euclid and Pythagoras.
So, when is the piston speed the same at TDC and BDC?
When the stroke is zero, or the rod length is infinite.
A 4.50" stroke with 7.10" rods (n=1.578) will have a velocity curve similar to a 4.25" stroke with stock 440 rods: faster than a stock 440 @ TDC, slower @ BDC.
Unless the heads are very good, this is the opposite of what you want to keep the VE as high as possible. Where practical, the rod ratio should go up when displacement is increased, regardless of how this is achieved.
For the stock 440 rod ratio with 4.50" stroke, the rod must be 8.11" long - which is why it's not done.
Boffin Emeritus
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Re: port velocity?
[Re: polyspheric]
#1622168
05/22/14 06:30 PM
05/22/14 06:30 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
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evidently my measurement set up must not be very accurate then, old dial indicator on the piston top? Should be correct down to a couple thou.
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: HotRodDave]
#1622170
05/22/14 08:46 PM
05/22/14 08:46 PM
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Joined: Jan 2003
Posts: 16,376
dogdays
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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 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.
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.
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.
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.
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.
I have officially stepped away from the "long rod" religion.
R.
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Re: port velocity?
[Re: Sport440]
#1622173
05/22/14 11:42 PM
05/22/14 11:42 PM
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Joined: Oct 2007
Posts: 4,206
New York
polyspheric
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Port velocity may drop off, and even reverse, without actually tracking piston motion.
The entire rod ratio thingy is a conundrum. The difference in piston motion and position among all the rod ratios you would reasonably use in an engine (from roughly n=1.4 to 2.1) is almost imperceptible, yet the results are not merely obvious but wildly disproportionate to any calculations.
How can such a small change give such drastic results? Yet, they've been proven by tests for many decades.
Example: Harley-Davidson significantly increased the rod length (but not the stroke) in 1937 to improve peak power in a low RPM, low compression engine with small ports and mild cam.
There is a similar and converse effect @ BDC: a short rod covers a longer piston travel path (in inches) during the same amount of crank rotation (in degrees).
What this does it capture a larger percentage of the full stroke length with the same intake valve closing point. This (not overlap, or LSA, or duration) is what changes DCR with rod ratio changes: shorter rod, longer stroke, or both always has slightly higher CCP.
Last edited by polyspheric; 05/22/14 11:43 PM.
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Re: port velocity?
[Re: mopar dave]
#1622175
05/23/14 11:44 AM
05/23/14 11:44 AM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
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Quote:
ok, good info guys. it looks like the rod length and not so much the stroke is what we need to be concerned with to keep good port velocity?
when its stated short rod long rod, what are you considering short and long?
I use 6.123 sb rod. would these be considered long or short? thanks
I use the 6.125 chevy rods in basically all my builds
and I call them mid to long.. the ford SB rods are
short.... if you went to a 6.2 I would call them long
in a SBM... but thats JMO
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Re: port velocity?
[Re: mopar dave]
#1622176
05/23/14 11:44 AM
05/23/14 11:44 AM
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New York
polyspheric
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The effect on piston motion is exactly the same for identical changes in rod length and stroke length measured as percent.
However, rod length has no direct effect on gas speed - stroke does.
For many engines, maximum rod length is limited by deck height - you can't get your rod ratio back unless you use a smaller stroke increase.
Get the idea - there are no simple answers?
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Re: port velocity?
[Re: polyspheric]
#1622177
05/23/14 11:56 AM
05/23/14 11:56 AM
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Mt Morris Michigan
mopar dave
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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 csa@13.5:1 compression?
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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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Romeo MI
MR_P_BODY
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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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Kalispell Mt.
HotRodDave
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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: 30,992
Oregon
AndyF
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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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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
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Romeo MI
MR_P_BODY
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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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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
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Romeo MI
MR_P_BODY
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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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polyspheric
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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.
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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
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acworth / N. georgia - south e...
cheapstreetdustr
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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
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acworth / N. georgia - south e...
cheapstreetdustr
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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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