Rod Length?
12-19-2002 | 07:50 AM
#21
Platinum Member
Joined: Sep 2001
Posts: 5,769
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From: tn
Long rod motors reduce piston wear and offer measurable power increases (especially at high rpm).
They do not in any way increase displacement. For this reason, a stroker motor will make more power than a nonstroker that uses a longer rod - displacement rules (especially at under 6000 rpm).
Timing - assuming that you are fairly well matched with your engine components, start around 28 degrees and do a pull to get a plug reading. Adjust your fuel mix if necessary and do a baseline run with known trim angles, etc.. Do it in two directions. Bump timing 2 degrees and run again - if you pick up rpm bump 2 more and run again. Keep doing this until you see no additional rpm - then back off 2 degrees...
If you have a b@stard set of engine components (huge cam with low NA compression and a 2 barrel carb on a homemade intake) then nobody can help you.
They do not in any way increase displacement. For this reason, a stroker motor will make more power than a nonstroker that uses a longer rod - displacement rules (especially at under 6000 rpm).
Timing - assuming that you are fairly well matched with your engine components, start around 28 degrees and do a pull to get a plug reading. Adjust your fuel mix if necessary and do a baseline run with known trim angles, etc.. Do it in two directions. Bump timing 2 degrees and run again - if you pick up rpm bump 2 more and run again. Keep doing this until you see no additional rpm - then back off 2 degrees...
If you have a b@stard set of engine components (huge cam with low NA compression and a 2 barrel carb on a homemade intake) then nobody can help you.
12-19-2002 | 08:04 AM
#22
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Posts: n/a
Originally posted by formulafastech
This is all well and good, in a straight line. But a crank isn't straight. When the crank is at the bottom of its stroke, the rod/piston that is on the shorter stroke is higher in the cylinder. The rod/piston that is on the longer stroke is lower. Then when the crank gets to the top of its stroke, the opposite is true. [/B]
This is all well and good, in a straight line. But a crank isn't straight. When the crank is at the bottom of its stroke, the rod/piston that is on the shorter stroke is higher in the cylinder. The rod/piston that is on the longer stroke is lower. Then when the crank gets to the top of its stroke, the opposite is true. [/B]
Last edited by ursus; 12-19-2002 at 02:57 PM.
12-19-2002 | 08:15 AM
#23
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Joined: Oct 2000
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From: Western Michigan
Originally posted by ursus
Using a standard 9.8" deck block and a 4.25 " stroke builders have basically 2 choices, 6.135" rods and 1.52" comp height pistons or 6.385 rods and 1.27".....The 6.385" rod however only leaves 1.27" deck height which is not much room for rings and they sometimes have sealing/reliability problems, this is why when building a 540cid it is preferable to use a tall block, 10.2 or more....
Using a standard 9.8" deck block and a 4.25 " stroke builders have basically 2 choices, 6.135" rods and 1.52" comp height pistons or 6.385 rods and 1.27".....The 6.385" rod however only leaves 1.27" deck height which is not much room for rings and they sometimes have sealing/reliability problems, this is why when building a 540cid it is preferable to use a tall block, 10.2 or more....
12-19-2002 | 08:33 AM
#24
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Hmm,
Looking thru some tech articles on this I am getting some conflicting information. Everyone here is stating that above the 350 SBC ratio of 1.64 is good (if I am reading this right?). However, look at the following numbers. All the motors that make good torque like what we all need in boats have in the 1.5 to 1.6 ratio range.
327 SBC = 1.75
350 SBC = 1.64
400 SBC = 1.45 With standard Rod
400 SBC = 1.52 With 5.7 (350) Rod
427 BBC = 1.63
454 BBC = 1.53
502 BBC = 1.53
That being said, the motor I am currently gathering parts to put together will be either a 421 CI SBC, 3.875 stroke - 4.155 bore - 5.85 Rod. For a ratio of 1.51. Or a 430 CI SBC, 4.00 stroke - 4.155 bore - 5.85 Rod. For a ratio 1.46. If I understand correctly, this should be a good low rpm torquey motor.
Your comments and suggestions please?
Tee
P.S. Here is a link that is pretty good. http://victorylibrary.com/mopar/rod-tech-c.htm
Looking thru some tech articles on this I am getting some conflicting information. Everyone here is stating that above the 350 SBC ratio of 1.64 is good (if I am reading this right?). However, look at the following numbers. All the motors that make good torque like what we all need in boats have in the 1.5 to 1.6 ratio range.
327 SBC = 1.75
350 SBC = 1.64
400 SBC = 1.45 With standard Rod
400 SBC = 1.52 With 5.7 (350) Rod
427 BBC = 1.63
454 BBC = 1.53
502 BBC = 1.53
That being said, the motor I am currently gathering parts to put together will be either a 421 CI SBC, 3.875 stroke - 4.155 bore - 5.85 Rod. For a ratio of 1.51. Or a 430 CI SBC, 4.00 stroke - 4.155 bore - 5.85 Rod. For a ratio 1.46. If I understand correctly, this should be a good low rpm torquey motor.
Your comments and suggestions please?
Tee
P.S. Here is a link that is pretty good. http://victorylibrary.com/mopar/rod-tech-c.htm
12-19-2002 | 10:01 AM
#25
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Joined: Feb 2001
Posts: 1,519
Likes: 31
I'm convinced that long rods are better. When Mercury built their Class 1 engine a few years ago (max of 500 CID allowed), they didn't use a 502 combination, R/S = 1.53. They went to 4.6 bore and used a 427 crank, 3.75 stroke, for 498 CID. Assuming they used a 6.385 rod in a standard deck that's a R/S = 1.70. If they used a tall deck with 6.785 rods the R/S = 1.81. That was a high revving, powerful engine making almost 900 HP.
But as was said above, if you're going to the expense of tall decks and custom rods, you're going to talk yourself into 4.75 stroke. That's what makes a 632, and I am also convinced that at normal RPM the extra 134 CID will far outweigh the disadvantages of the lower R/S ratio.
Good thread!
But as was said above, if you're going to the expense of tall decks and custom rods, you're going to talk yourself into 4.75 stroke. That's what makes a 632, and I am also convinced that at normal RPM the extra 134 CID will far outweigh the disadvantages of the lower R/S ratio.
Good thread!
12-19-2002 | 02:20 PM
#26
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Ursus makes a good point about not comprimising the ring pack...short skirts w/big bores...forged pistons w/loose clearances...hot combustion temps w/cold block ---Stability of the piston within the bore is critical. Longer rods are best with taller decks in boats.
12-19-2002 | 07:08 PM
#27
Charter Member #601
Joined: Dec 2000
Posts: 3,276
Likes: 5
From: Holland, Mi USA.
mcollinstn
You mean i have to scrap the two barrel.. was hoping to get some milage out of it.. oh well.
Seriously.. I am running 565" with an 8" rod.. I have not had the opportunity to run tests to dial in the combination. I started with 32 degrees and it hasnt melted down yet.. But I am still dialing in the EFI and this year I got close until I found the prop would hardly turn due to a broken bearing race/cage on the lower vertical shaft.. Another Imco part that gave up. The intake is kinda homemade.. Guy by the name of Hogan built it.. he's on the left coast.
I do thank you for your suggestion on the timing .. I will run that test when the water gets soft again in the spring..
I love this thread!! I built this motor on the log rod theory, but I have not had much support. If all else fails I could shorten the rod by an inch and add and inch to the crank.. lets see 4.600 x 5.25..... someone do the math please. I am gettin excited here.
Dick
You mean i have to scrap the two barrel.. was hoping to get some milage out of it.. oh well.
Seriously.. I am running 565" with an 8" rod.. I have not had the opportunity to run tests to dial in the combination. I started with 32 degrees and it hasnt melted down yet.. But I am still dialing in the EFI and this year I got close until I found the prop would hardly turn due to a broken bearing race/cage on the lower vertical shaft.. Another Imco part that gave up. The intake is kinda homemade.. Guy by the name of Hogan built it.. he's on the left coast.
I do thank you for your suggestion on the timing .. I will run that test when the water gets soft again in the spring..
I love this thread!! I built this motor on the log rod theory, but I have not had much support. If all else fails I could shorten the rod by an inch and add and inch to the crank.. lets see 4.600 x 5.25..... someone do the math please. I am gettin excited here.
Dick
12-19-2002 | 10:52 PM
#29
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Joined: Oct 2000
Posts: 4,541
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From: Western Michigan
Originally posted by liquid lounge
---Stability of the piston within the bore is critical. Longer rods are best with taller decks in boats.
---Stability of the piston within the bore is critical. Longer rods are best with taller decks in boats.
