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  #16 (permalink)  
Old 09-15-2010, 10:49 AM
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I took a half hour to develop a spreadsheet to show the negligible affect rod ratio has on piston speed and acceleration. Here's the results for 4 common rod lengths on 5 available stroke lengths (i didn't mess with the odd ball stuff, even though some catalogs carry it). Speed is in fps, and acceleration is in fps^2.

3.00"
Rod Piston Speed Piston Acceleration
5.565 MAX: 81.35273184 62631.66938
AVERAGE: 49.86149584 173.5369695

5.7 MAX: 81.2217284 62316.82548
AVERAGE: 49.86149584 172.664379

6 MAX: 80.96341524 61667.90252
AVERAGE: 49.86149584 170.8658751

6.125 MAX: 80.86394849 61416.28119
AVERAGE: 49.86149584 170.1684967

3.25"
Rod Piston Speed Piston Acceleration
5.565 MAX: 88.65638092 69051.08981
AVERAGE: 54.0166205 191.3244677

5.7 MAX: 88.49083235 68681.57823
AVERAGE: 54.0166205 190.3003829

6 MAX: 88.16175085 67919.98111
AVERAGE: 54.0166205 188.1896332

6.125 MAX: 88.03616929 67624.67049
AVERAGE: 54.0166205 187.3711803

3.48"
Rod Piston Speed Piston Acceleration
5.565 MAX: 95.48435977 75120.04824
AVERAGE: 57.83933518 208.1409136

5.7 MAX: 95.28467493 74696.37768
AVERAGE: 57.83933518 206.9667492

6 MAX: 94.87737602 73823.15516
AVERAGE: 57.83933518 204.5466697

6.125 MAX: 94.72824965 73484.56204
AVERAGE: 57.83933518 203.6082729

3.75"
Rod Piston Speed Piston Acceleration
5.565 MAX: 103.6482577 82443.91909
AVERAGE: 62.32686981 228.4346153

5.7 MAX: 103.3976067 81951.94411
AVERAGE: 62.32686981 227.0711802

6 MAX: 102.8877172 80937.94192
AVERAGE: 62.32686981 224.260994

6.125 MAX: 102.7023432 80544.76193
AVERAGE: 62.32686981 223.1713318

4.00"
Rod Piston Speed Piston Acceleration
5.565 MAX: 111.3603822 89417.34464
AVERAGE: 66.48199446 247.7572713

5.7 MAX: 111.0535025 88857.57277
AVERAGE: 66.48199446 246.2059797

6 MAX: 110.4374599 87703.83746
AVERAGE: 66.48199446 243.0086019

6.125 MAX: 110.2102344 87256.47658
AVERAGE: 66.48199446 241.7688047

Sorry the data isn't cleaner, this forum doesn't import spreadsheet data too well.

If anyone wants the spreadsheet I can email it to you, just PM me. The spreadsheet has the speed and acceleration for each degree of crank rotation, but it is FAR too much to post on this forum, for each of the 20 combinations there is 3 columns of data (position, speed, and acceleration) with 360 rows each (each degree of rotation).

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  #17 (permalink)  
Old 09-15-2010, 04:04 PM
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Makes me smile and reminds me of the movie title "Much a'do About Nothing" when it comes to angles, degrees, and dwell time.
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Old 09-16-2010, 10:06 AM
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"I am sorry to bump this old thread. I like it and would you guys to read it."

...please god...not again...
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Old 12-11-2013, 07:31 PM
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This is an old but interesting thread. I found this in my research for my next build, a 6.125 rod 327. Ill be using a 350 piston designed for 6 inch rod. Also I have late model block, small journal crank and spacer bearings with Scat H beam rods. I think I have the math right!!.
Go ahead, flame me!!
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Old 12-11-2013, 08:18 PM
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how high are you going to rev this engine?
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Old 12-11-2013, 09:14 PM
How fast is fast enough?
 
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You can go with a 3" stroke and 6.25" rods too if you want. You can even get crazier than that but it becomes more expensive and there really no merit.

Keep in mind a 383 is almost never limited by the short block but by the valve train when the same attention is given to each. Oil control is another big issue with high rpm. Reducing the stroke helps very little and costs you displacement.

That being said I'm assembling a 302 chevy, but only to be different there's no logic behind it.
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Old 12-12-2013, 05:35 AM
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We hope to turn this 7800-8000, thats the idea anyway. Again its going in a Jersey Skiff so RPM is the key. The 6.125 rods and the 350 pistons are reasonably priced too, not much more than regular aftermarket stuff.
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Old 12-12-2013, 06:59 AM
How fast is fast enough?
 
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Quote:
Originally Posted by Richiehd View Post
We hope to turn this 7800-8000, thats the idea anyway. Again its going in a Jersey Skiff so RPM is the key. The 6.125 rods and the 350 pistons are reasonably priced too, not much more than regular aftermarket stuff.
a 350 can handle those rpm's, its not the stroke that needs adjusting, its the valvetrain.

Since you're looking at prices I don't know if 8,000 is going to be practical for you, it's not like chasing 7,000 rpm, if you do hit it I'm sure all of us would love to see pictures and videos.



BTW, a 434 sbc at 6,000 rpm displaces the same as a 327 at 8,000 rpm- but its a lot cheaper to build and maintain.
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Old 12-12-2013, 07:14 AM
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4.185 bore and 3 inch stroke,245 cc heads with shaft rockers,rev kit,and a trunk full of springs."Precision" balance,high volume oiling system,(probably a blue printed dry sump),vacuum pump,,,,
just build a short stroke nascar engine and detune it to run 8k
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Old 12-12-2013, 07:36 AM
How fast is fast enough?
 
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Stock block and crank and worried about the cost of rods and pistons. This engine will never see 8000 rpm. Maybe 7000.
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Old 12-13-2013, 03:49 PM
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Quote:
Originally Posted by jacobm99 View Post
I've read somewhere that the stroke to rod ratio in a 327 is closer to what the the perfect ratio is for optimum performance.
Not sure exactly how that works... It's just something I read a coupld years ago
By now you've read the arguments. As you can see from Turbolover's calcualtions the impact on piston speeds and dwell times is not large. This is backed up with countless dyno runs where the affect on power isn't outside the "noise" level. "Noise" being a term for random fluctions within a run and between runs with common equipent and configurations.

However, if the Chevy 400 small block is anything of an indicator very short rods appear to greatly affect cylinder-wall life as these engines all suffered from high wear rates compared to other SBC's. But you cannot divorce the fact that the 400 siamized the cylinders so how much that change can be attributed to excess bore wear can't be teased out of the data.

Still most of us recommend the use of the longest rod practical. Certainly Chrysler has taken this subject to the extreme with consistantly good result at the track.

Bogie
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Old 12-13-2013, 04:49 PM
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Quote:
Originally Posted by Richiehd View Post
This is an old but interesting thread. I found this in my research for my next build, a 6.125 rod 327. Ill be using a 350 piston designed for 6 inch rod. Also I have late model block, small journal crank and spacer bearings with Scat H beam rods. I think I have the math right!!.
Go ahead, flame me!!

327 with a 6.25" rod and a 383 piston is a bolt in with no clearancing needed on the rods, cam or block. Also, there alot of budget minded 383 pistons to choose from. In my opinion, if you are looking to increase you rod to stroke ratio and are planning on buying a set of 6.125" rods, then you might as well buy 6.25's if you a running a 327 crank.

I don't have much experience comparing a 6" vs 6.125" vs 6.25", but I can tell you that my son's 327 with 6.25" rods revs to the moon in a hurry.

I'm sure others will offer their opinions, but take mine for what it's worth - FREE

Good luck - Jim
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  #28 (permalink)  
Old 12-16-2013, 11:05 AM
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stroke to rod ratio

Google stroke to rod ratio and you get dozens of articles. The one I liked the most was http://www.strokerengine.com/RodStroke.html. There is alot of information to read and in the end any rod length to stroke that is above 1.55:1 ratio or better will spin an engine 6000 rpms with no problems. For a comparison look at the 400 chevy compared to the 350 chevy. The 400 with a 1.48:1 ratio will spin 5500 rpms without any harsh feeling from the engine. The 350 with a rod ratio of 1.64 will spin over 6000 easily with no harsh feeling coming from the engine. But you have to keep an eye on the com pression height with the long rods or you will be using oil ring spacers and you will have more piston rock with the shorter skirts on the pistons. Read the articles they explain it better. Man, I gotta start looking at the dates of these post!

Last edited by cdminter59; 12-16-2013 at 11:16 AM.
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  #29 (permalink)  
Old 12-16-2013, 05:46 PM
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So does anybody need a set of nice used 6.125 rods that i have?
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