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5.7 vs 6.0 Rods

General engine tech -- Drag Racing to Circle Track

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5.7 vs 6.0 Rods

Postby petiepye » Thu Oct 15, 2009 8:10 pm

I have a good 010 350 block and I plan to build a drag race only bracket engine. Planning the bottom end, and when considering cranks, the rod length comes up.
What are the pros and cons of the different rod lengths?
All opinions and explanations will be appreciated.
Thanks in advance, Pete.
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Postby bigjoe1 » Thu Oct 15, 2009 8:16 pm

Here is a true story-- I build quite a few 383 Chevy engines, I usually use 6 inch rods because I like the light pistons. But, once when there were no 6 inch rods available, I put my regular 383 combo togeter with the shorter 5.7 rods. The engine was down everywhere about 15 to 20 HP, and 15 to 20 pounds of torque. That was the only time I used the 5.7 rods, and after that, I will never use them again.


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Postby petiepye » Thu Oct 15, 2009 8:26 pm

Joe, that is a powerful recommendation. I forgot to mention that, indeed, I am building a 383.
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Postby 1989TransAm » Thu Oct 15, 2009 8:30 pm

I was going to say with the long rods you get a lighter rotating assembly. However Joe's story is very interesting and compelling for the longer rods.
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hey

Postby levisnteeshirt » Thu Oct 15, 2009 10:08 pm

Big Joe , fan since 1979 ( '79 PHR 355 in Nova running 10.60's first read )

First , whats ur opinion of using a 6.125 rod in a 355 using a 383 piston for a 6.0 rod ?? ,

read one of your articles about a 406 , comparison between 5.565 rod and 5.7 ,, 5.565 down about 15 hp , then you put a dominator on it and got the difference almost back ,,,
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Postby bigjoe1 » Thu Oct 15, 2009 11:07 pm

Once the engine is together and running, you cant really tell any differance. I have done a few of those things like 6.125 rods, and it NEVER paid off. It is not worth the cost or trouble, unless you allready have the parts to use.Ihave had racers tell me things like their big speed secret was short rods, but I have never seen it happen. Again, I just like the lighter pistons


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Postby randy331 » Thu Oct 15, 2009 11:32 pm

As long as the rod is long enough to get the piston close to the head at TDC, yet short enough that it doesn't hit the head at TDC, or fail to keep the piston conected to the crank, and damage the head,(where the focus should have been) then the length is perfect.

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Postby Sprinter99 » Fri Oct 16, 2009 12:06 am

Over the last few years i've had 5.85 ,6.0 and 6.125 rods in 355s and the longest rod made the MOST power on the dyno but was SLOWEST on the track, the 5.85 made LESS peak power than both the other two but was MUCH faster off the corners than the 6.125. A big example of we dont race dynos.
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Postby trmnatr » Fri Oct 16, 2009 12:35 am

Sprinter99 wrote:Over the last few years i've had 5.85 ,6.0 and 6.125 rods in 355s and the longest rod made the MOST power on the dyno but was SLOWEST on the track, the 5.85 made LESS peak power than both the other two but was MUCH faster off the corners than the 6.125. A big example of we dont race dynos.


On a circle track car or street car i could see that
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Postby maxracesoftware » Fri Oct 16, 2009 2:49 am

bigjoe1 wrote:Here is a true story-- I build quite a few 383 Chevy engines, I usually use 6 inch rods because I like the light pistons. But, once when there were no 6 inch rods available, I put my regular 383 combo togeter with the shorter 5.7 rods. The engine was down everywhere about 15 to 20 HP, and 15 to 20 pounds of torque. That was the only time I used the 5.7 rods, and after that, I will never use them again.


JOE SHERMAN RACING ENGINES



another true story->

around 1991 to 1993+ time period
i Ported different pairs of SBC #041x castings 165 CC Legal limit
with varying Port velocities (epoxy shapes)
for a SS/I 4-speed Camaro

unknown to me,
they used a 6.250 C-C rod with the 1st pair i Ported.
The Pitot Probe velocity was fast , and i was experimenting with
how fast you could push port speeds in various locations.
Port CC's were between 160 to 162 cc well safe of 165 Limit

the SS/I Car ran as fast as 9.69 ET around 18 years ago
and always between low 9.70's to low 9.80's w/6.250 Rod
set the SS/I Record like it was

since he was very fast for its time,
they decided to make it 100% Legal switching to 5.7" Rods C-C
..from that point it slowed down a solid Tenth ET or more
it became a high 9.80's to high 9.90's Car

i ported another pair with barely any epoxy in Ports
slowing velocity as much as i could,
kept Port volume on ragged edge of 165 CC limit ( 164 to 165)
with a Class A Burette
ran a few high 9.70's to low 9.80's

after 2years, NHRA reset Record...slower,
so he set the SS/I Record , but to a slower ET

i think he reset the record a 3rd time in SS/I later on

won Class at Indy 1993

its interesting that the 2009 SS/I Record is only a 9.93 :)

never again did it run any 9.69 or any low 9.70's !
like it did with 6.250 C-C Rod
this was approx 575 HP @ 7300 w/6.250
556's HP @ 7200-7300 with 5.7 Rods

down the DragStrip, since this was a 4-speed manual Trans
with 5.83 gears 3.08 1st
the RPM/SEC acceleration rates were approx->
1st Gear= 4300 RPM/SEC Avg
2nd Gear= 2000 RPM/SEC Avg
3rd Gear= 1070 RPM/SEC Avg
4th Gear= 560 RPM/SEC Avg

as you can see, the RPM/SEC rate is very fast,
coupled with small port volume head that was edge of too fast,
seemed to respond a lot to Rod Length changes.

--------------------------

if that were a 2-spd Powerglide Car, the avg RPM/SEC would have been
a bunch overall slower and not tax the Rod Length / Head CC combination


likewise if there were no 165 Cc rule and
you just built the same CID shortblock with a modern head
the Rod length effects becomes less
to where some say Rod Length doesn't matter at all !

then you look at Smokey's Era where you used
old Double-Camel_Hump Heads and old #292 Turbo Heads
with relatively small port volumes and possible very hi , too fast
local velocities causing CHOKE situations,
then you can see why Smokey preached long rods all day long !
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Postby LoneRanger » Fri Oct 16, 2009 6:19 am

a few years ago we used longer rods in our Ford straight six stock class circle track car, it was a good season with with consistant 1st, 2nd, 3thd, placings & one of the leading teams in points in our local area, the engine ran very strong, however the next season they changed the rules in regards to using different length rods etc & it was a slow start but made gains in other areas later trying various exhaust, manifold & cam changes
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Postby CNC BLOCKS » Fri Oct 16, 2009 12:50 pm

Sprinter99 wrote:Over the last few years i've had 5.85 ,6.0 and 6.125 rods in 355s and the longest rod made the MOST power on the dyno but was SLOWEST on the track, the 5.85 made LESS peak power than both the other two but was MUCH faster off the corners than the 6.125. A big example of we dont race dynos.


Hopefully your not using the same cam for a short and long rod engines!
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Postby Old School » Fri Oct 16, 2009 1:36 pm

CNC BLOCKS wrote:
Sprinter99 wrote:Over the last few years i've had 5.85 ,6.0 and 6.125 rods in 355s and the longest rod made the MOST power on the dyno but was SLOWEST on the track, the 5.85 made LESS peak power than both the other two but was MUCH faster off the corners than the 6.125. A big example of we dont race dynos.


Hopefully your not using the same cam for a short and long rod engines!


What are the differences in the cams you use for a 5.7 rod compared to the 6 inch rod?
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ROD LENGTH

Postby jmwsb2 » Fri Oct 16, 2009 1:41 pm

I am a fan of Big Joe.s also but I remember when he did a comparison of a stock length of a 400 ci small block and a 5.7 length rod. If my memory is correct the 5.56 length rd made more torque and power than the 5.7 rod until 6250 rpmon the dyno.

I also built a 6 inch rod 406 sbc and the car slowed down about 3 tenths.I discussed this with Bo of BLP and he stated had built a 6 inch rod sbc and was unable to get it to run as fast as the 5.7 tha it replaced. He stated tried for a year to get the combination to equal the 5.7 but eventually put 5.7's in the engine and picked up the time. This was also in a fairly heavy car. I questioned him in what would happen if I changed various components. His answer was that I would wind up taking the engine out.

After replacing the rear gears, coverter, cam carb and a lot of tuning and trying to blow it up I wound up giving the eng. to my son for his dragster and it picked up time over his 5.7 rod eng.

I built another 5.7 rod eng for my car and gained back the 3 tenths I had lost. This was many years ago.

I now have an engine with 6.2 rods and the other with 6.125. both are very close in horsepower.

I think you have to look at all aspects of the package, not just one aspect. We also don't race the dyno even though it is a great tuning instrument.

John
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Postby CNC BLOCKS » Fri Oct 16, 2009 2:11 pm

Old School wrote:What are the differences in the cams you use for a 5.7 rod compared to the 6 inch rod?

It depends on the combination but rememeber that going from a 5.7 to 6.250 the engines is going to breath differently!!! And to optimize you can't run the same cam for both rod lenths.
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