A "pro's" view on rod length.
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A "pro's" view on rod length.
A sidebar in Chevy High Performance magazine, by Darin Morgan, Reher-Morrison's R&D cylinder head specialist.
Rod Length
Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engine where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables.
The result was zero difference in average power and a zero difference in the shape of the horsepower curves. However, I'm not going to say there's absolutley nothing to rod ratio, and there are some pitfalls in going above and below a certain point. At anything below 1.55:1 ratio, rod angularity is such that it will increase side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes.
Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of you induction package to increase velocity.
Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can without getting into the oil ring. Whatever connects the two is your rod length.
Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engine where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables.
The result was zero difference in average power and a zero difference in the shape of the horsepower curves. However, I'm not going to say there's absolutley nothing to rod ratio, and there are some pitfalls in going above and below a certain point. At anything below 1.55:1 ratio, rod angularity is such that it will increase side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes.
Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of you induction package to increase velocity.
Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can without getting into the oil ring. Whatever connects the two is your rod length.
Last edited by Injuneer; Jun 24, 2006 at 03:20 PM.
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Re: A "pro's" view on rod length.
Rods are a good thing to connect the piston to the crank, Huh.
He left out the part about port flow influence with different lengths, That's a whole different book.
Todays factory blocks limit your choice on RR. If ya had a 10.5" tall SBC ya could really try some high RR's.
He left out the part about port flow influence with different lengths, That's a whole different book.
Todays factory blocks limit your choice on RR. If ya had a 10.5" tall SBC ya could really try some high RR's.
Last edited by 1racerdude; Jun 26, 2006 at 10:59 PM.
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Re: A "pro's" view on rod length.
Originally Posted by SStrokerAce
dude, i'd rather have a 8.2-8.7 deck SBC.
Not for a 4" stroke Hell that's a 5.1 rod in the 8.2 and a RR of 1.27. Don't know if they make rods that short except for lawn mowers.
Ya could get a set of TiT rods made for around 4,000 dollars.
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Re: A "pro's" view on rod length.
Originally Posted by 1racerdude
Not for a 4" stroke Hell that's a 5.1 rod in the 8.2 and a RR of 1.27. Don't know if they make rods that short except for lawn mowers.
Ya could get a set of TiT rods made for around 4,000 dollars.
Ya could get a set of TiT rods made for around 4,000 dollars.
Bret
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Re: A "pro's" view on rod length.
Originally Posted by SStrokerAce
I was thinking more for strokes under 3.5 ;-) and something like a Honda Rod.
Bret
Bret
That small won't be any fun,think LARGE.
That' a 1.52 RR still to much side load and ya may loose a bunch from the piston running away from the combustion and the crank angle isn't much.
Probably put a hell of a pull on the intake though.
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Re: A "pro's" view on rod length.
Correct me if I'm wrong but the engines being run in F1 right now make <300 lb/ft of torque. That could just be some internet BS as the gentleman in a former post was refering to. But with an F-Body weighing in generally over 3000lbs, would this not be a problem?
I guess what I'm asking is whether or not stroke has an effect (direct or indirect) on torque, and being as though torque is "real" power, I could see it as being the more sought after number.
Just some thoughts.
I guess what I'm asking is whether or not stroke has an effect (direct or indirect) on torque, and being as though torque is "real" power, I could see it as being the more sought after number.
Just some thoughts.
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Re: A "pro's" view on rod length.
Originally Posted by jerminator96
Correct me if I'm wrong but the engines being run in F1 right now make <300 lb/ft of torque. That could just be some internet BS as the gentleman in a former post was refering to. But with an F-Body weighing in generally over 3000lbs, would this not be a problem?
I guess what I'm asking is whether or not stroke has an effect (direct or indirect) on torque, and being as though torque is "real" power, I could see it as being the more sought after number.
Just some thoughts.
I guess what I'm asking is whether or not stroke has an effect (direct or indirect) on torque, and being as though torque is "real" power, I could see it as being the more sought after number.
Just some thoughts.
There is NO replacement for displacement.
The F1 cars have a packaging problem and they are fighting weight.
If built right a stroker will deliver much more TQ and HP.
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Re: A "pro's" view on rod length.
Originally Posted by 1racerdude
That small won't be any fun,think LARGE.
That' a 1.52 RR still to much side load and ya may loose a bunch from the piston running away from the combustion and the crank angle isn't much.
Probably put a hell of a pull on the intake though.
That' a 1.52 RR still to much side load and ya may loose a bunch from the piston running away from the combustion and the crank angle isn't much.
Probably put a hell of a pull on the intake though.
Bret
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Re: A "pro's" view on rod length.
Originally Posted by jerminator96
Correct me if I'm wrong but the engines being run in F1 right now make <300 lb/ft of torque. That could just be some internet BS as the gentleman in a former post was refering to. But with an F-Body weighing in generally over 3000lbs, would this not be a problem?
I guess what I'm asking is whether or not stroke has an effect (direct or indirect) on torque, and being as though torque is "real" power, I could see it as being the more sought after number.
Just some thoughts.
I guess what I'm asking is whether or not stroke has an effect (direct or indirect) on torque, and being as though torque is "real" power, I could see it as being the more sought after number.
Just some thoughts.
Yep, the 146 cubic inch F1 engines are probably well under 300 lb-ft at max torque which is around 12-14000 rpm. At power peak (about 19000 rpm) they make about 200 lb ft. Of course that's 720 or so horsepower, and since they are turning the engine about 3 times as fast as an LT1, they get to use more gear to multiply the torque. 3 x 3.23 =9.69. Imagine 9.70s in your 180+ mph Camaro. Anyway that 200 lb-ft @ 19,000 is like 600 lb-ft @ 6300. That would move your F-body fairly well.
Compare engines by torque produced per cubic inch, especially at hp peak rpm.
The main effect stroke has on torque production is the displacement it causes. A 4 in bore, 4 inch stroke V8 (402) makes more torque than a 4 x 3 (302) because it has 33% more displacement, not because it has a 33% longer stroke. If you used a 4.62 bore and a 3 inch stroke (402) you could get about the same torque as the 4 x 4. Well probably more because the larger bore allows better breathing.
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Re: A "pro's" view on rod length.
Originally Posted by SStrokerAce
A 5.4" Rod is enough with the right piston, I just wanna lower the weight of the rod and shorten the pushrods the right way. Not to mention a shorter deck height would be better for pacakging and weight. For those who like to turn corners.
Bret
Bret
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