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jamestwo Samba Member
Joined: November 01, 2004 Posts: 2203
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Posted: Thu Mar 08, 2007 10:30 am Post subject: |
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Are you seriuos? Think about your question, because if you don't understand that it takes more material to make a larger stroke, no one can explane it to you.
stealth67vw wrote: |
[quote="Stripped66]
1) short stroke crankshafts are lighter
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How is a shorter stroke crank lighter than a longer stroke crank? If the journal is 2.165 diameter on an 82 or a 69, weight wise how does the crank know it has been moved 13mm?[/quote] _________________ hEY, lOOK, i'M WRONG HALF THE TIME, AND THE OTHER HALF i'M NOT SURE WHAT THE HECK i'M TALKING ABOUT. MY POST ARE FOR MY OWN ENTERTAINMENT VALUE ONLY. |
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jamestwo Samba Member
Joined: November 01, 2004 Posts: 2203
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Posted: Thu Mar 08, 2007 10:35 am Post subject: |
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I would agree with everything you say, but want to be clear in sayng a longer stroke can make more torque, not because the stroke is longer, but because of a more favorable rod ratio.
Stripped66 wrote: |
krusher wrote: |
There has to be more to it than said here, if stroke to bore ratio bears no impact on the engine why do we have things like short strokers for high rpm like motorbikes and race cars?
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1) short stroke crankshafts are lighter
2) for a given RPM, a long-stroke crank will produce greater piston velocities and accelerations than a short-stroke crank...there's only so much force (via the accelerating mass of the piston) the connecting rods and piston/wrist pin can take at high RPMs
3) larger bores ultimately allow for better head flow and cylinder filling. Let's take two engines of similar displacement...you build a 69x87 combo and I'll build a 59.5x94 combo. I think I'll run a set of CNC-ported Comp Eliminators with 48mm intake valves and 40mm exhaust valves. What heads are you gonna run? Wait...doesn't matter...you've already lost.
The problem with any of these bore vs. stroke arguments are that there are so many factors contributing to the torque and HP an engine produces...displacement, head flow characteristics (not just flow, but velocity), VE across RPM, camshaft, induction, exhaust, etc. Too many people focus on "stroke" when really it's stroke's effect on displacement plus all of the other factors they changed. Even if the only thing a person changed was the stroke and nothing else, the increased displacement is going to improve VE across the RPM range, up to the point where the rest of the components max out.
If some of you folks are still stuck on the dogma of more stroke = more torque, go buy a Honda Their engines are all LONG-stroke and small-bore, but they have certainly demonstrated that there are (way) more factors to producing torque than just stroke. |
_________________ hEY, lOOK, i'M WRONG HALF THE TIME, AND THE OTHER HALF i'M NOT SURE WHAT THE HECK i'M TALKING ABOUT. MY POST ARE FOR MY OWN ENTERTAINMENT VALUE ONLY. |
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nsracing Samba Member
Joined: November 16, 2003 Posts: 9481 Location: NOVA
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Posted: Thu Mar 08, 2007 10:56 am Post subject: |
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Item #2 is correct? Are you sure Strippe?
Long stroke cranks have more degree dwell, so they take more time to swing that rod end around. Therefore, piston speeds will be much slower. Shorter strokes will have faster piston speed. Add a shorter rod and even more faster piston speeds.
Also, a smaller stroke crank will accelerate faster. The rotating masses are much closer to center of gravity and/or axis line. It will take less effort to spin a short stroke crank. If you spin with your arms out, you will spin faster if you tuck your arms in. |
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The Noof Samba Member
Joined: January 25, 2005 Posts: 4346
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Posted: Thu Mar 08, 2007 11:51 am Post subject: |
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nsracing wrote: |
Item #2 is correct? Are you sure Strippe?
Long stroke cranks have more degree dwell, so they take more time to swing that rod end around. Therefore, piston speeds will be much slower. Shorter strokes will have faster piston speed. Add a shorter rod and even more faster piston speeds.
Also, a smaller stroke crank will accelerate faster. The rotating masses are much closer to center of gravity and/or axis line. It will take less effort to spin a short stroke crank. If you spin with your arms out, you will spin faster if you tuck your arms in. |
Isn't it this way Nick:at 3000 rpm, the pistons in a stroker (lets say 82mm) have traveled quite a bit farther than the 69 mm stroke engine. In 60 seconds of running time, the strokers have gone a greater distance, and therefore the piston speed must be higher? |
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Stripped66 Samba Member
Joined: May 31, 2005 Posts: 3470 Location: Charleston, SC
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Posted: Thu Mar 08, 2007 12:22 pm Post subject: |
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The Noof wrote: |
nsracing wrote: |
Item #2 is correct? Are you sure Strippe?
Long stroke cranks have more degree dwell, so they take more time to swing that rod end around. Therefore, piston speeds will be much slower. Shorter strokes will have faster piston speed. Add a shorter rod and even more faster piston speeds.
Also, a smaller stroke crank will accelerate faster. The rotating masses are much closer to center of gravity and/or axis line. It will take less effort to spin a short stroke crank. If you spin with your arms out, you will spin faster if you tuck your arms in. |
Isn't it this way Nick:at 3000 rpm, the pistons in a stroker (lets say 82mm) have traveled quite a bit farther than the 69 mm stroke engine. In 60 seconds of running time, the strokers have gone a greater distance, and therefore the piston speed must be higher? |
Thank you, Noof. It's also tied into James' major point about rod-ratios. While a large stroke crank may have more dwell time, it's usually accompanied by a short rod-ratio, further increasing piston acceleration around TDC. To credit James, this is a good thing on a street engine, as greater acceleration after TDC helps pull in the intake charge, especially at low-RPMs. On HIGH rpm race engines (motorcycles, F1, etc)...not a good thing! _________________
66brm wrote: |
Bodacious wrote: |
Why not just make a custom set of wires with a Y splice in them. Then you could just run one distributor. |
I don't think electrickery works that way |
Last edited by Stripped66 on Thu Mar 08, 2007 12:40 pm; edited 1 time in total |
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Stripped66 Samba Member
Joined: May 31, 2005 Posts: 3470 Location: Charleston, SC
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Posted: Thu Mar 08, 2007 12:24 pm Post subject: |
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jamestwo wrote: |
Are you seriuos? Think about your question, because if you don't understand that it takes more material to make a larger stroke, no one can explane it to you.
stealth67vw wrote: |
[quote="Stripped66]
1) short stroke crankshafts are lighter
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How is a shorter stroke crank lighter than a longer stroke crank? If the journal is 2.165 diameter on an 82 or a 69, weight wise how does the crank know it has been moved 13mm? |
[/quote]
Thank you, James!
I suppose stealth67vw might be thinking about a Bergmann welded stroker crank where there really is no extra material supporting that journal hanging out in space _________________
66brm wrote: |
Bodacious wrote: |
Why not just make a custom set of wires with a Y splice in them. Then you could just run one distributor. |
I don't think electrickery works that way |
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Stripped66 Samba Member
Joined: May 31, 2005 Posts: 3470 Location: Charleston, SC
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Posted: Thu Mar 08, 2007 12:36 pm Post subject: |
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jamestwo wrote: |
I would agree with everything you say, but want to be clear in sayng a longer stroke can make more torque, not because the stroke is longer, but because of a more favorable rod ratio.
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That's very true...we generally get more favorable rod-ratios when stroking the VW engine...but that doesn't mean you can't get a good rod-ratio with the stock stroke: IIRC, Matt Davis is building an engine for a customer that is running a 4.7x" alu rod from Pauter...it will give them a 1.73 rod-ratio in a 1914 _________________
66brm wrote: |
Bodacious wrote: |
Why not just make a custom set of wires with a Y splice in them. Then you could just run one distributor. |
I don't think electrickery works that way |
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nsracing Samba Member
Joined: November 16, 2003 Posts: 9481 Location: NOVA
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Posted: Thu Mar 08, 2007 12:44 pm Post subject: |
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Noof... better look again.
We are NOT talking about distance traveled by the piston (stroke). We are talking about PISTON SPEED. You are confusing the two. They are not the same.
HOw quickly the piston moves in the cylinder are dictated by the stroke and rod ratio. HOw far down the piston can go is just STROKE. PISTON SPEED is something else. |
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matt davis Banned
Joined: December 01, 2003 Posts: 53 Location: Livermore, CA
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Posted: Thu Mar 08, 2007 1:16 pm Post subject: |
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nsracing wrote: |
Noof... better look again.
We are NOT talking about distance traveled by the piston (stroke). We are talking about PISTON SPEED. You are confusing the two. They are not the same.
HOw quickly the piston moves in the cylinder are dictated by the stroke and rod ratio. HOw far down the piston can go is just STROKE. PISTON SPEED is something else. |
Exactly.
Stripped... those are in my engine. 4.73" on a 69mm. _________________ www.mattdavisracing.com
1967 Sedan Vert
1967 Hardtop Sedan
1967 Ghia'tine Drag Car
1966 Hardtop Sedan
Livermore, CA |
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[email protected] Samba Member
Joined: August 03, 2002 Posts: 12785 Location: Salt Lake City, Utah
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Posted: Thu Mar 08, 2007 1:20 pm Post subject: |
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People wake up!
2 engines of the "same" displacement, one big bore/small stroke, the other small bore/big stroke.
While it is true the longer stroke engine has "more leverage" due to the longer stroke, IT HAS A SMALLER PISTON PUSHING ON IT. So, it's a wash. As a further argument, the stroker engine has valve shrouding problems due to the small bore the big bore engine does not have to deal with.
The end result is I don't feel it matters. Build the biggest BORE you can, and after you have done that if you have $ leftover, then stroke it. It's stupid to stroke a stock bore engine because it's the more expensive route to go, and the power difference is much smaller then boring first.
Many people do not understand that engine A and engine B will move the same amount of air at every point in crank rotation. In one the piston moves more BUT the bore is small so it doesn't move a lot of air. In the other the piston moves less but because of the big bore it moves the same air. Power is the same in both.
In fact, my experience has shown that GIVEN THE SAME DISPLACEMENT the big bore engine will make more power and torque, than the small bore stroker. But what do I know......
John
Aircooled.Net Inc. _________________ It's just advice, do whatever you want with it!
Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net
"Like" our Facebook page at
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and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net |
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The Noof Samba Member
Joined: January 25, 2005 Posts: 4346
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Posted: Thu Mar 08, 2007 1:21 pm Post subject: |
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Nick...please check out this link:
http://www.csgnetwork.com/pistonspeedcalc.html
Run the calculator with 82mm stroke, and then 69mm stroke, both at 6000 rpm and tell me what you get. |
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matt davis Banned
Joined: December 01, 2003 Posts: 53 Location: Livermore, CA
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Posted: Thu Mar 08, 2007 1:35 pm Post subject: |
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[email protected] wrote: |
People wake up!
Build the biggest BORE you can, and after you have done that if you have $ leftover, then stroke it. |
Which is why my street engine is 101x74 _________________ www.mattdavisracing.com
1967 Sedan Vert
1967 Hardtop Sedan
1967 Ghia'tine Drag Car
1966 Hardtop Sedan
Livermore, CA |
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metric autohaus Samba Member
Joined: July 06, 2005 Posts: 195 Location: Millersville, MD
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Posted: Thu Mar 08, 2007 2:13 pm Post subject: |
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but what no one has explained to this guy is that with the 94's, the material left on the head are alot thinner at the cylinder bolts, and the 94's will heat up quicker than a 90.5
if going for a long lasting motor, that will give you many years of good service, i say stroke it and go w/ the 90.5 less heat, a little more money, and the same power. you can always port and polish the heads to make a little extra power..... |
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matt davis Banned
Joined: December 01, 2003 Posts: 53 Location: Livermore, CA
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Posted: Thu Mar 08, 2007 2:15 pm Post subject: |
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I have YET in 12-13 years had a problem with 94's. _________________ www.mattdavisracing.com
1967 Sedan Vert
1967 Hardtop Sedan
1967 Ghia'tine Drag Car
1966 Hardtop Sedan
Livermore, CA |
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jamestwo Samba Member
Joined: November 01, 2004 Posts: 2203
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Posted: Thu Mar 08, 2007 2:29 pm Post subject: |
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Why run the thin 90.5 , when you can run the thick 92mm?
heath0203 wrote: |
but what no one has explained to this guy is that with the 94's, the material left on the head are alot thinner at the cylinder bolts, and the 94's will heat up quicker than a 90.5
if going for a long lasting motor, that will give you many years of good service, i say stroke it and go w/ the 90.5 less heat, a little more money, and the same power. you can always port and polish the heads to make a little extra power..... |
_________________ hEY, lOOK, i'M WRONG HALF THE TIME, AND THE OTHER HALF i'M NOT SURE WHAT THE HECK i'M TALKING ABOUT. MY POST ARE FOR MY OWN ENTERTAINMENT VALUE ONLY. |
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grimace007 Samba Member
Joined: August 30, 2006 Posts: 2673 Location: swampville, florida
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Posted: Thu Mar 08, 2007 2:37 pm Post subject: |
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word |
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grimace007 Samba Member
Joined: August 30, 2006 Posts: 2673 Location: swampville, florida
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Posted: Thu Mar 08, 2007 2:38 pm Post subject: |
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and on top of that if you THAT worried about how much material your takin from the heads isnt it possible to turn down the bases of the cyls? |
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sszuch Samba Member
Joined: February 17, 2007 Posts: 102 Location: Texas
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Posted: Thu Mar 08, 2007 2:43 pm Post subject: |
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Given the same rod ratio at 2500 RPM:
69 Stroke -
max piston velocity = 54 m/s
max piston acceleration = 3591 m/s^2
76 Stroke -
max piston velocity = 56 m/s
max piston acceleration = 4146 m/s^2
at about 110 (and 220) degrees of crank rotation (through 1 cycle) |
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The Noof Samba Member
Joined: January 25, 2005 Posts: 4346
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Posted: Thu Mar 08, 2007 2:47 pm Post subject: |
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I can't get any of the so called experts to concurwith those findings.Way too much misinformation in the world. |
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Stripped66 Samba Member
Joined: May 31, 2005 Posts: 3470 Location: Charleston, SC
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Posted: Thu Mar 08, 2007 3:23 pm Post subject: |
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matt davis wrote: |
nsracing wrote: |
Noof... better look again.
We are NOT talking about distance traveled by the piston (stroke). We are talking about PISTON SPEED. You are confusing the two. They are not the same.
HOw quickly the piston moves in the cylinder are dictated by the stroke and rod ratio. HOw far down the piston can go is just STROKE. PISTON SPEED is something else. |
Exactly.
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No, and no.
Speed = distance / time.
Average piston speed = stroke / time.
For example, at 3000 rpm, the piston travels the entire distance of its stroke in 0.01 seconds. Therefore, a 69mm stroke will have an average piston velocity of 6.9 m/s (6900mm/sec); an 82mm stroke will have an average piston velocity of 8.2 m/s.
Then there's peak velocities, such as those posted by sszuch:
sszuch wrote: |
Given the same rod ratio at 2500 RPM:
69 Stroke -
max piston velocity = 54 m/s
max piston acceleration = 3591 m/s^2
76 Stroke -
max piston velocity = 56 m/s
max piston acceleration = 4146 m/s^2
at about 110 (and 220) degrees of crank rotation (through 1 cycle) |
Longer strokes produce greater average piston velocities, greater peak piston velocities and greater piston accelerations. _________________
66brm wrote: |
Bodacious wrote: |
Why not just make a custom set of wires with a Y splice in them. Then you could just run one distributor. |
I don't think electrickery works that way |
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