Short rod taller piston vs. longer rod shorter piston?
#1132749
12/12/11 08:57 PM
12/12/11 08:57 PM
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Hey all, Thinking of a 3.79" stroke smallblock and now have myself wondering what is better- longer rod with a shorter CH on the piston, or a shorter rod with a much higher CH on the piston I.E. a chevy 5.7" or 6.000" rod with piston compression heights of a respective 2.05" and 1.705" assuming a stock 9.6" deck height with the 3.79" stroke...This would be compared to using a longer chevy style rod like a 6.250" rod with a piston that has a 1.45" compression height What are the implications for each setup in terms of longevity? overall performance and piston dwell? side loading? piston rock? etc.? I have heard many ideas, but interested to hear what people think about this particular combo Just trying to learn something here and possibly decide on what route my build might take....this will be a street/strip motor that sees a good 3-4k street miles a year and atleast 50 passes at the track, and lots more on the street...Mild nitrous use in the future as well All opinions are welcome since this is all new to me
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: LSP]
#1132751
12/12/11 09:25 PM
12/12/11 09:25 PM
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Quote:
I'd go with the 6.250" rod, as shelf pistons are available, and cylinder wall loading will be slightly less. The rock won't be an issue, and the negligible difference in dwell doesn't add up to anything.
I wouldnt be using shelf pistons anyways, so the availability isn't an issue... the shelf pistons that are available I am not too keen on using due to their position in the bores and my overall compression I would end up with.
what would change between each setup- With the shorter rod length and taller pistons, how much more cylinder wall loading would there be?
As far as the theory of dwell, I have no idea of how real the effects of longer or shorter dwell would play out on an engine like this
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: Streetwize]
#1132755
12/12/11 10:05 PM
12/12/11 10:05 PM
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: mshred]
#1132756
12/12/11 10:05 PM
12/12/11 10:05 PM
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LSP
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Quote:
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10-4, I lost a cylinder wall on a 340 block earlier this year, so I would put the longest practical rod in it to minimize cylinder side loading, and get a better piston design at the same time. the longer compression height stuff can get top heavy.
I just put together a X block with the 3.790" crank, and used a 6.300" long rod.
I appreciate the input! you have a pm
What was the rod ratio in that 340 you were running?
The one I put together for my dad was a odd deal. When the 340 block cracked, we had a 9.250" deck X block lying around and decided to use it. The 6.300" Arrow rods and K1 3.790" crank from the 340 block were used, netting a 1.66 rod ratio, along with a 1.055" compression height custom piston.
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: mshred]
#1132757
12/12/11 10:11 PM
12/12/11 10:11 PM
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Streetwize
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Mshed,
I say generally because there's less of an RPM drop between gears in a 4 speed and less torque (slip and heat) is absorbed through the transmission with a manual....resulting in more net power to the wheels. A 4" arm helps offset that some but a long arm stroker can sometimes have trouble putting all that additional torque to the ground through the middle. By moving the rpm upstairs the shorter arm/somewhat higher revving motor (evertything else being equal and you've got the valvetrain and airflow to carry it) can put it to better use upstairs.
Last edited by Streetwize; 12/12/11 10:14 PM.
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: Streetwize]
#1132758
12/12/11 10:13 PM
12/12/11 10:13 PM
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Quote:
Mshed,
I say generally because there's less of an RPM drop between gears in a 4 speed and less torque is absorbed with a manual giving you more net power to the wheels. A 4" arm helps offset that some but a long arm stroker can sometimes have trouble putting all that additional torque to the ground through the middle. By moving hterpm upstairs the shorter arm/somewhat higher revving motor can put it to better use upstairs.
I never thought of it like that...thanks for breaking it down for me
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: mshred]
#1132759
12/12/11 10:34 PM
12/12/11 10:34 PM
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I run a 3.79 in the race car... its a 9" deck R3 block with 6.1 chevy Eagle H-beams with CP pistons and with that combo the oil ring is up in the wrist pin bore... no big deal there, it has a bridge... I normally turn it to 8200 but have been as high as 9600 but it didnt make power past 8300... with the heads and cam I need to spin it... its only 405ci
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: MR_P_BODY]
#1132760
12/12/11 10:48 PM
12/12/11 10:48 PM
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Al_Alguire
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Get the piston made to suit your intended purposes, get the stroke you are after and wherever the rod length falls run it. Dont get all caught up in the rod/stroke BS. Have the piston built to withstand the HP and cylinder pressures you inted and get a rod made to fit the stroke with that piston.
"I am not ashamed to confess I am ignorant of what I do not know."
"It's never wrong to do the right thing"
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: MR_P_BODY]
#1132761
12/12/11 10:59 PM
12/12/11 10:59 PM
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Quote:
I run a 3.79 in the race car... its a 9" deck R3 block with 6.1 chevy Eagle H-beams with CP pistons and with that combo the oil ring is up in the wrist pin bore... no big deal there, it has a bridge... I normally turn it to 8200 but have been as high as 9600 but it didnt make power past 8300... with the heads and cam I need to spin it... its only 405ci
with nitrous use though isn't it a better idea to keep the oil ring out of the pin area? Since I plan on using nitrous, should I think about that
Is there a general idea of at what compression height the oil ring groove comes in line with the pin bore?
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: Al_Alguire]
#1132762
12/12/11 11:00 PM
12/12/11 11:00 PM
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Quote:
Get the piston made to suit your intended purposes, get the stroke you are after and wherever the rod length falls run it. Dont get all caught up in the rod/stroke BS. Have the piston built to withstand the HP and cylinder pressures you inted and get a rod made to fit the stroke with that piston.
well with what I outlined, do you think a taller or shorter piston would work good for what I am doing? Im not sure what the best design is for what I am doing. Im thinking that having the oil ring in the pin bore wouldn't be most ideal if I plan on spraying it, but im not sure if im right about that or not
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: mshred]
#1132763
12/13/11 12:02 AM
12/13/11 12:02 AM
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id run a shorter rod with NOS youll end up with more meat on the piston top,ringlands in a better location,more piston stability too.if its gonna see alot of street miles that also makes a difference.most of these long rod combos are drag only so they dont have concerns regarding wear during street crusing.i have a short rod sb that was bulit for spray and runs pretty good on motor also.6"rod 4"stroke indy 230s... 9.7s @3400lbs on 275/60DRs no NOS passes yet and its a true street car.
Last edited by KOS; 12/13/11 12:08 AM.
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: KOS]
#1132764
12/13/11 12:42 AM
12/13/11 12:42 AM
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Brian Hafliger
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There will still be plenty of piston available for a thick crown using a 6.25 rod and 3.79 stroke.
Brian Hafliger
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: KOS]
#1132765
12/13/11 12:56 AM
12/13/11 12:56 AM
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Quote:
id run a shorter rod with NOS youll end up with more meat on the piston top,ringlands in a better location,more piston stability too.if its gonna see alot of street miles that also makes a difference.most of these long rod combos are drag only so they dont have concerns regarding wear during street crusing.i have a short rod sb that was bulit for spray and runs pretty good on motor also.6"rod 4"stroke indy 230s... 9.7s @3400lbs on 275/60DRs no NOS passes yet and its a true street car.
so you are saying long rod combos would increase or decrease wear on a street motor?
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: Brian Hafliger]
#1132766
12/13/11 12:59 AM
12/13/11 12:59 AM
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Quote:
There will still be plenty of piston available for a thick crown using a 6.25 rod and 3.79 stroke.
thanks for the info Brian!
The class I was thinking of running just became N/A only and max cubic inch of 385ci, so this 3.79" arm stuff just got that much more appealing
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: mshred]
#1132767
12/13/11 02:11 AM
12/13/11 02:11 AM
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id run a shorter rod with NOS youll end up with more meat on the piston top,ringlands in a better location,more piston stability too.if its gonna see alot of street miles that also makes a difference.most of these long rod combos are drag only so they dont have concerns regarding wear during street crusing.i have a short rod sb that was bulit for spray and runs pretty good on motor also.6"rod 4"stroke indy 230s... 9.7s @3400lbs on 275/60DRs no NOS passes yet and its a true street car.
so you are saying long rod combos would increase or decrease wear on a street motor?
long rod is great if your comp height is 1.5" or more.its the piston that the issue not the rod.the piston gets skimpy,likes to rock around when comp height is short, rings are in the pin area also.the crank you have most likely has a mopar rod journal size so i would run a stock lenght H-beam or 6"if their available.
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Re: Short rod taller piston vs. longer rod shorter piston?
[Re: KOS]
#1132768
12/13/11 02:17 AM
12/13/11 02:17 AM
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id run a shorter rod with NOS youll end up with more meat on the piston top,ringlands in a better location,more piston stability too.if its gonna see alot of street miles that also makes a difference.most of these long rod combos are drag only so they dont have concerns regarding wear during street crusing.i have a short rod sb that was bulit for spray and runs pretty good on motor also.6"rod 4"stroke indy 230s... 9.7s @3400lbs on 275/60DRs no NOS passes yet and its a true street car.
so you are saying long rod combos would increase or decrease wear on a street motor?
long rod is great if your comp height is 1.5" or more.its the piston that the issue not the rod.the piston gets skimpy,likes to rock around when comp height is short, rings are in the pin area also.the crank you have most likely has a mopar rod journal size so i would run a stock lenght H-beam or 6"if their available.
The crank would be chevy 2.1" rod journals so the next size down would be a 6.00" chevy rod. That would result in a piston with a compression height of 1.705" assuming a 9.6" deck height. Would the oil ring still be in the pin area?
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