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Author
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Topic: Rod lenght ratio
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V-8
Gearhead Posts: 297
From: TURKEY/Istanbul
Registered: Dec 2003
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posted 01-05-2004 06:06 PM
   
As you know stock 351w rod lenght 5.956.I want use 6.200 lenght Lunati sportsman h beam rods with Lunati special order pistons.There any advantages of using 6.200 lenght rods?
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V8 Thumper
Gearhead Posts: 4377
From: Arizona
Registered: Dec 2001
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posted 01-05-2004 06:09 PM
Absolutely! The longer the rod, the more 'leverage' on the crankshaft. Piston dwell increases, piston speed decreases, sideloading decreases, it's a win-win deal  ------------------
1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9" All Blue Oval, no blue bottle
http://mustangsandmore.50megs.com/V8Thumper.html
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V-8
Gearhead Posts: 297
From: TURKEY/Istanbul
Registered: Dec 2003
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posted 01-05-2004 06:36 PM
quote:
Originally posted by V8 Thumper:
piston speed decreases
[/B]
Yes.Because of the piston speed decreases the engine doesn't want more intake runner volume so they can work properly with TFS tw heads?
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V8 Thumper
Gearhead Posts: 4377
From: Arizona
Registered: Dec 2001
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posted 01-05-2004 06:43 PM
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1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9" All Blue Oval, no blue bottle
http://mustangsandmore.50megs.com/V8Thumper.html
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V-8
Gearhead Posts: 297
From: TURKEY/Istanbul
Registered: Dec 2003
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posted 01-05-2004 06:50 PM
I forget to ask the most important question:Can 6.200 lenght rods work properly with standard crank?
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Moneymaker
Administrator Posts: 25883
From: Lyons, IL, USA
Registered: May 99
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posted 01-05-2004 06:53 PM
quote:
Originally posted by V-8:
I forget to ask the most important question:Can 6.200 lenght rods work properly with standard crank?
You must do more math before you consider such an undertaking.
There is more to it than just a new aftermarket custom length rod and custom pistons.
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Alex Denysenko
Co-Administrator and Moderator NHRA/IHRA/SRA member and licensed Superstock driver
MCA member# 53321
NHRA and IHRA SS/LA & SS/MA National Record Holder '00,'01,'02,&'03
First SS/MA in the TENS!
IHRA division 5 Superstock Champion
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28 The Barry of BarrysGrrl Quote #1: "I never met a magazine mechanic I liked."
Quote #2: "Make sure brain is in gear before engaging mouth!"
Quote #3: "If you can't run with the big dogs, stay on the porch!"
www.moneymakerracing.com
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Mpcoluv
Gearhead Posts: 1278
From: Charlotte NC usa
Registered: Apr 2001
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posted 01-05-2004 06:56 PM
Rod length/ratio doesn't really matter that much. In fact the local dirt track guys dislike the long rod motors because they say that they "feel" lazy off the corners, even when they maky good power on the chassis dyno.
I would spend the money on other modifications. BTW the Pete Sauracker article on the long rod 351 is total BS.
Note that I didn't say that long rods did nothing, I just said that they don't do much.
Any rod ration over 1.5 or so is fine for strokers. OEMs tend to have a longer rod to stroke ration, around 1.7 or so. The stock 351W has a rod ratio of 1.69 or so.
Long rods are a lot like other "tricks" like chamber/piston coatings, gapless rings, etc... you don't see a lot for your money.
If you have an unlimited budget, then by all means try it but the gains will be small.
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RonnieT
Gearhead Posts: 827
From: Port Allen, La. 70767
Registered: Jun 99
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posted 01-05-2004 09:52 PM
I do not understand how the length of the rod will affect piston speed. The piston is still going to move the same distance in a 3.5" stroke motor at a given RPM no matter what the rod length is. Thinking about it I can see that it may change the acceleration and decelleration times, but if the piston speed is lower in one area of the stroke it has to be higher in another area to cover the same distance in the same amount of time. ------------------
Ronnie
69 mach1 351W-4V engineless at the time!
70 Torino GT 351C-4v with a "shaker"
Mustangs and More Member #23
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Buster
Gearhead Posts: 1437
From: Hurricane alley
Registered: May 2002
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posted 01-05-2004 10:56 PM
Mpcoluv, I respectfully disagree. I?ve build long rod 351W engines in the past using 351M rods and IMHO using 351M rod in a W is worth around 20 hp, enough hp that you can really feel the difference, kinda like adding headers to an engine. I also believe that a long rod engine works better at a higher RPM. Therefore it?s not a setup that would be a toque monster off the corners. That's one difference between the circle track guys and drag racing guys... in a drag car its in its power band more so then a circle track car. If I remember correctly the 351M rod is 6.580? .380? longer then the 6.200? rod, that?s quite a bit longer. V8, if you?re going to build a long rod engine setup, I would support the 351M rod combo over the 6.200? rod setup... I don?t think the 6.200? is long enough to see a hp difference. I think you will notice more power using the 6.580? rod over the 6.200? rod. The 6.200? is not a lot over the stock rod ratio. I for one REALLY like long rod engines (not that I?m really that obvious about it) and would HIGHLY recommend using the 6.580? rod combo over the 6.200? rod, which is a 1.88:1 rod ratio vs 1.77:1. Ronnie, long rod engine piston speeds are slower at the top and bottom of the stroke and faster in the center. The shorter rod engine is closer to the same speed from top to bottom, pulling away from the top and bottom faster then a long rod engine. In long rod engine the pistons hangs at the top and bottom of the stroke more so then a shorter rod engine. Because of the longer dwell time, the air/fuel mixture has more time to burn at a higher cylinder pressure. As you can tell... I really like long rod setups, lol. =) Of course this is just my .02 Alex, please correct me if I?ve made an error here... I?ve had a really long day at my new job today....btw, did I tell anyone that I've got a new job?
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Moneymaker
Administrator Posts: 25883
From: Lyons, IL, USA
Registered: May 99
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posted 01-06-2004 12:13 AM
A long rod with the right stroke and piston will make a ton of power over a very narrow power band.
Long rod engines never make power over a broad range. The six inch chebbie rod was developed for NASCAR racing on the long tracks where the engines stay in the 8-9k RPM range all day.
Hot lick in the late 70's through mid 80's was a 3.625 stroke and a 6 inch rod.
366 inch deal was SOP with the GM teams.------------------
Alex Denysenko
Co-Administrator and Moderator NHRA/IHRA/SRA member and licensed Superstock driver
MCA member# 53321
NHRA and IHRA SS/LA & SS/MA National Record Holder '00,'01,'02,&'03
First SS/MA in the TENS!
IHRA division 5 Superstock Champion
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28 The Barry of BarrysGrrl Quote #1: "I never met a magazine mechanic I liked."
Quote #2: "Make sure brain is in gear before engaging mouth!"
Quote #3: "If you can't run with the big dogs, stay on the porch!"
www.moneymakerracing.com
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Buster
Gearhead Posts: 1437
From: Hurricane alley
Registered: May 2002
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posted 01-06-2004 09:28 PM
Alex, how would you classify a narrow power band... another words, what's the RPM spread for a long rod engine? I've never seen a dyno sheet on a long rod engine before.
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Moneymaker
Administrator Posts: 25883
From: Lyons, IL, USA
Registered: May 99
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posted 01-06-2004 11:07 PM
1500-2000 RPM spread.------------------
Alex Denysenko
Co-Administrator and Moderator NHRA/IHRA/SRA member and licensed Superstock driver
MCA member# 53321
NHRA and IHRA SS/LA & SS/MA National Record Holder '00,'01,'02,&'03
First SS/MA in the TENS!
IHRA division 5 Superstock Champion
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28 The Barry of BarrysGrrl Quote #1: "I never met a magazine mechanic I liked."
Quote #2: "Make sure brain is in gear before engaging mouth!"
Quote #3: "If you can't run with the big dogs, stay on the porch!"
www.moneymakerracing.com
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FORDPOWER94
Journeyman Posts: 1
From: ONTARIO, NY USA
Registered: May 2004
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posted 05-15-2004 11:48 AM
I just finished putting together a 351w long rod engine for a circle track car using 400m rods. My question is what would be a good starting point for total ingition timing? With my old motor, which had stock length rods, I ran 36 degrees of total timing. I've heard the long rod engines work better with less timing. Is this true? Any ideas?
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Buster
Gearhead Posts: 1437
From: Hurricane alley
Registered: May 2002
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posted 05-15-2004 02:44 PM
My experience with the long rod setup is as much as 42* total timing, more timing and not less. Longer rod engines have more dwell time and move slower towards the top and bottom. Therefore, the piston is in a different position in the bore then a shorter rod engine at a given crank position. 42* in a long rod engine is closer to 36* in a shorter rod engine with regards to the actual piston location in the bore.
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blackford
Journeyman Posts: 33
From: Corona, Ca
Registered: Jan 2004
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posted 05-16-2004 01:26 AM
Years ago I took a look at things mathematically using Excel to see what the difference really was. The change in piston motion was very small on paper when I was comparing some popular long rod setups to the stock steups. I'm talking about a maximum of .010 - .013 difference in piston position in the bore for rod lengths that were .5 longer than stock. This maximum position difference occurs at 90 and 270 degrees. As you approach TDC or BDC the difference gets smaller. For instance, .006 difference at 45 degrees for a long rod 302 and a stock rod 302. Very similiar results for long rod 351s and others. Imagine how much power has been released by the time the rod gets to 45 degrees or even 60 degrees for that matter and then consider how little the difference in piston position is for a long rod engine. It took a rediculously long rod (8-10") to really see the dwell increase and the piston acceleration/deceleration increase.This was an excerpt from the post: "I used an oblique triangle to model the crank/rod/piston relationship. The 3 sides were made up of the rod (a), the crank throw (b), and the distance from the crank centerline to the center of the rod small end (c). The 3 simultaneous equations for this when the rod length, crank throw, and crank angle are known are: sin B = (b*sinA)/a , C = 180 - (A + B) , and c = (a*sin C)/sin A. A,B, and C are the angles opposite sides a,b, and c respectively." My conclusion was that the rod length results in very little change of piston position thru its entire 360 degrees of motion for rods that are less than an inch longer than stock. The increase in power is primarily a result of reduced friction caused by the shorter piston skirt and the improved rod angle. I have the excel file still if anyone is interested. The results will surprise you.
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'65 "Black" ford FB, 329 with H beam 289 rods, fully prepped 351w heads, 282S cam, C-4, 3.5 9" posi. Many Suspension and steering mods. 4 year project so far. Restomod estimated completion in May.
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rustang@home
Journeyman Posts: 94
From: Clarion, PA
Registered: Jan 2004
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posted 05-16-2004 06:32 AM
It seems like current thinking is to use long rods for engines with restrictive intake systems (head, intake, restrictor plate, cam), and short rods for motors with high flowing intake systems. Circle Track did a test one time (that could be the Peter Saurracker article?)with two different rod lengths, and found the more restrictive the breathing, the long rod had a more dramatic affect. What I've heard is the Nascar guys still stuff as long a rod as possible in their restrictor plate motors, for the other tracks not as big.However, in Pro Stock, where airflow is huge, it seems like they're going with shorter and shorter deck blocks, which would have to mean shorter rods. When I was putting my 3.5" stroke Clevor together, the engine builder, who's a Ford guy and probably knows Cleveland stuff as well as anyone, talked me out of the 6.200 rod and going with a 6.125. He said it would "pick up between gear changes better" So...that's my .02! 
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