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Old 05-24-2006, 04:04 PM   #1
Shabib67
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Default Cobb Rods

I was on Cobbs website and saw there connecting rods. They have two versions the standard sti length and the longer rod length. They perfer the longer rods. Is there any benefit or reason for running longer rods? Also who makes there rods?
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Old 05-24-2006, 04:08 PM   #2
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Those would be a couple of great questions in the email you should send them.
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Old 05-24-2006, 06:37 PM   #3
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Herewegoagain,
Don't be mean because you don't know the answer or just don't want to take the time to explain it! Be nice, Just don't post!

Shabib67,
The reson for running longer rods is to decrease the rod angle between the piston and the crank there by reducing both side load on the piston wall and resistance to crank rotation. By keeping the rod more inline with the axis the piston travles you transfer more power to the crank and have a more reliable engine.

Wayne
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Old 05-24-2006, 06:54 PM   #4
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Quote:
Originally Posted by Shabib67
Also who makes there rods?
They wouldn't tell me and I have them in my motor. My mechanic thinks maybe Carillo, but that is pure speculation based on looking at them from the bottom end. According to him, they did not appear to be either Pauter or Eagle.
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Old 05-24-2006, 07:28 PM   #5
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maybe crower??
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Old 05-24-2006, 07:28 PM   #6
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would these rods be safe if i wanted to spin my car to 8k.
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Old 05-24-2006, 10:17 PM   #7
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yes. pauter, carrilo, eagle. all should be alot stronger than stock. the thing you need to worry about is valve float, not your rods. its all in the heads.

~Josh~
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Old 05-24-2006, 10:51 PM   #8
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Quote:
Originally Posted by Wayne_Coots
Herewegoagain,
Don't be mean because you don't know the answer or just don't want to take the time to explain it! Be nice, Just don't post!

Shabib67,
The reson for running longer rods is to decrease the rod angle between the piston and the crank there by reducing both side load on the piston wall and resistance to crank rotation. By keeping the rod more inline with the axis the piston travles you transfer more power to the crank and have a more reliable engine.

Wayne
Hmmm, I thought the reason for using a longer rod was to allow the piston to be at the top of the stroke for a longer period of time and thus transfer more energy from the combustion gases.
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Old 05-24-2006, 11:12 PM   #9
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well im defintley going to build my heads.
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Old 05-25-2006, 12:34 AM   #10
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Quote:
Originally Posted by Wagun
Hmmm, I thought the reason for using a longer rod was to allow the piston to be at the top of the stroke for a longer period of time and thus transfer more energy from the combustion gases.
You both sort of said the same thing, just a little differently. More time at TDC should transfer more power and does stress the pistons and cylinders less.

We'll see once I get it on the dyno with the same tune as before.
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Old 05-25-2006, 12:36 AM   #11
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Quote:
Originally Posted by Shabib67
would these rods be safe if i wanted to spin my car to 8k.
I think the rods will be among the least of your potential problems at an extended 8K redline.
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Old 05-25-2006, 02:04 AM   #12
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The reason that you use a long short piston combo is to reduce rotating mass. The other advantage is a longer rod will make you rode to stock ratio closer to the optimal ratio.
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Old 05-25-2006, 02:59 AM   #13
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Quote:
Originally Posted by Wayne_Coots
Herewegoagain,
Don't be mean because you don't know the answer or just don't want to take the time to explain it! Be nice, Just don't post!

Shabib67,
The reson for running longer rods is to decrease the rod angle between the piston and the crank there by reducing both side load on the piston wall and resistance to crank rotation. By keeping the rod more inline with the axis the piston travles you transfer more power to the crank and have a more reliable engine.

Wayne
This is correct. Our piston pin bores are offset as well, so this will also help reduce the rod angle and is great when matched up with our long rods.

Using a long rod shortens bottom dead center dwell time, and top dead center dwell is increased (the piston stays at bottom dead center for a shorter period of time and stays at top dead center for a longer period of time.)

Advantages
-less rotational friction
-using a longer rod allows you to use a shorter and lighter piston
-less wear on the cylinder walls


-J
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Old 05-25-2006, 03:13 AM   #14
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Quote:
Originally Posted by Shabib67
would these rods be safe if i wanted to spin my car to 8k.
Long rods would be safer to spin to higher RPM's than a shorter rod.
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Old 05-25-2006, 05:41 AM   #15
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i know where the cobb internals are made and who makes them. ill say that its none of the aforementioned companies, but its very good stuff. friend of friend who does alot of porsche stuff as well makes them.

longer rod is created through juggling math. math = good.

8K on a 2500cc? i duno. theres a reason stock redline is far away from 8K on a ej257.

a 2 liter ej207 inthe other hand has oem redline of 8500....
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Old 05-25-2006, 06:46 AM   #16
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Quote:
Originally Posted by Jeremy@Cobb
Using a long rod shortens bottom dead center dwell time, and top dead center dwell is increased (the piston stays at bottom dead center for a shorter period of time and stays at top dead center for a longer period of time.)
-J
can you explain this more....i dont see why it will have a shorter dwell time at the bottom of the cylinder vs. the top. does it have to do w/ offset pins or?





what the original poster should know while a longer rod is 'always' better, it does create a longer time at TDC, thus inducing spark knock (detonation) and that becomes a bit more of a problem. Nothing that a tuner wont be able to control though. it also increases piston speed from tdc to bdc, basically, the engine really wants to suck (on the intake stroke)
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Old 05-25-2006, 02:20 PM   #17
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Quote:
Originally Posted by modaddict
can you explain this more....i dont see why it will have a shorter dwell time at the bottom of the cylinder vs. the top. does it have to do w/ offset pins or?





what the original poster should know while a longer rod is 'always' better, it does create a longer time at TDC, thus inducing spark knock (detonation) and that becomes a bit more of a problem. Nothing that a tuner wont be able to control though. it also increases piston speed from tdc to bdc, basically, the engine really wants to suck (on the intake stroke)
You can see detonation from tuning, but you will not see detonation from a long rod and the piston at TDC longer. The Ign. Timing requirements change with a long rod.
If the flame front travels to fast you could see detonation, timing should be corrected and thus no problems.
Like any custom engine (not origional design that was tuned for), changes made with engine effiency(more airflow from ported heads/cam lift, or a tighter quench area) or compression (static compression from a piston, or cam duration)..tuning is necessary.

Longer dwell time you get with a longer rod allows the intake charge has a longer time to enter the combustion chamber and exhaust has more time to exit thus producing more torque over a wider RPM range.

Longer rod engines have less dwell at BDC because with a longer rod during the compression stroke the piston travels from BDC to BTDC faster, but travels slower from 90deg. BTDC and TDC. This is compared to a shorter rod.

I found a picture that illustrates a long rod vs short rod
Long Rod on the Left _______ Shorter rod on the right
The pistons in the bore is in the same position.




For the newbs
BDC - bottom dead center
TDC- top dead center
BTDC- before top dead center
ATDC- after top dead center
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Old 05-25-2006, 02:25 PM   #18
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you "squaring off" jeremy ;-)
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Old 05-25-2006, 03:53 PM   #19
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Not trying to start a debate or anything, just trying to inform in a way everyone can understand, whether somone knows a little or alot about engines and component design.
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Old 05-25-2006, 03:57 PM   #20
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Does anyone know how much longer the Cobb rods are? I am looking for a set that is 2mm longer than factory.
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Old 05-25-2006, 04:24 PM   #21
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They look like Manley rods.
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Old 05-25-2006, 04:48 PM   #22
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Quote:
Originally Posted by Jeremy@Cobb
You can see detonation from tuning, but you will not see detonation from a long rod and the piston at TDC longer. The Ign. Timing requirements change with a long rod.
If the flame front travels to fast you could see detonation, timing should be corrected and thus no problems.
Like any custom engine (not origional design that was tuned for), changes made with engine effiency(more airflow from ported heads/cam lift, or a tighter quench area) or compression (static compression from a piston, or cam duration)..tuning is necessary.

Longer dwell time you get with a longer rod allows the intake charge has a longer time to enter the combustion chamber and exhaust has more time to exit thus producing more torque over a wider RPM range.

Longer rod engines have less dwell at BDC because with a longer rod during the compression stroke the piston travels from BDC to BTDC faster, but travels slower from 90deg. BTDC and TDC. This is compared to a shorter rod.

For the newbs
BDC - bottom dead center
TDC- top dead center
BTDC- before top dead center
ATDC- after top dead center
speaking about torque...also with the longer rod, isnt it correct in saying at 90 deg. the rod is more straight pushing on the crank vs. a shorter rod. So your not waisting as much angle pushing down on the crank during the compression stroke. Basically, when it goes boom inside the cylinder, the flamefront exploding, its pushing more at 90 deg.


Thanks jeremy for your help. its nice to see you cobb guys on here. and im a proud ap owner....soon to be protuned.
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Old 05-25-2006, 04:49 PM   #23
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Quote:
Originally Posted by modaddict
speaking about torque...also with the longer rod, isnt it correct in saying at 90 deg. the rod is more straight pushing on the crank vs. a shorter rod. So your not waisting as much angle pushing down on the crank during the compression stroke. Basically, when it goes boom inside the cylinder, the flamefront exploding, its pushing more at 90 deg.


Thanks jeremy for your help. its nice to see you cobb guys on here. and im a proud ap owner....soon to be protuned.

Yes you are correct you have less angle pushing on the crank.
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Old 05-25-2006, 05:09 PM   #24
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off topic but what is the ETA on Cobb Cams. I wanna get some DPR, Cosworth or Kelford cams but in one of the last post you were talking how the Cobb cams are gonna be truly designed for the USDM AVCS heads. So i think i will hold out.
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Old 05-25-2006, 05:30 PM   #25
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I cant say for sure, but I am working on a daily basis now with these cams and refining a few small things in the design to make these cams optimal. Around the same time we will be releasing cam gear tools for a really reasonable cost compared to the factory tools. These tools will have a better fit to the gears and will make things alot easier for you guys and me!
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