COMPRESSION HEIGHT & LONGER RODS
Thread Starter
Burning Brakes
Joined: Aug 2000
Posts: 1,211
Likes: 6
COMPRESSION HEIGHT & LONGER RODS
I WAS LOOKING AT THE DIFFERENT COMPRESSION HEIGHTS OF VARIOUS ROD LENGHTH COMBINATIONS AND STARTED NOTICING THAT THE LONGER THE ROD THE SHORTER THE COMPRESSION HEIGHT. IN MY EXAMPLE FOR A BB W/BORE=4.28, STROKE=4.25 AND WITH A ROD=6.385 THE COMPRESSION HEIGHT FELL BELOW 1.3".
I REMEMBER A DISCUSSION A WHILE BACK CONCERNING ROD LENGHTHS & STROKER MOTORS AND SEEM TO RECALL SOME CONCERN ABOUT HAVING THE COMPRESSION HEIGHT FALL TOO LOW. WHAT IS THE DRAWBACK OF HAVING A LOWER COMPRESSION HEIGHT?
THANKS, MARK :flag :chevy
I REMEMBER A DISCUSSION A WHILE BACK CONCERNING ROD LENGHTHS & STROKER MOTORS AND SEEM TO RECALL SOME CONCERN ABOUT HAVING THE COMPRESSION HEIGHT FALL TOO LOW. WHAT IS THE DRAWBACK OF HAVING A LOWER COMPRESSION HEIGHT?
THANKS, MARK :flag :chevy
Drifting
Joined: Oct 2000
Posts: 1,825
Likes: 56
From: Shawano WI
Re: COMPRESSION HEIGHT & LONGER RODS (marky mark)
If you run too long of a rod and have to go with a short compression height then the wrist pin gets into the oil rings requiring you to use a oil rail ring support. This is all right on a race motor that is taken apart frequently but can cause some longevity problems on a street motor.
To calculate compression height the formula is :
Deck height - rod length - 1/2 of the stroke = compression height
9.800 - 6.385 - 2.125 = 1.29
Then you have to take 1/2 the diameter of the wrist pin (generally .990 on a big block) and subtract it from your compression height. 1.290 - .495 = .795
That give you the space you would have to fit your ring package into before you would need an oil ring support rail.
[Modified by Pete79L82, 2:43 PM 5/13/2003]
To calculate compression height the formula is :
Deck height - rod length - 1/2 of the stroke = compression height
9.800 - 6.385 - 2.125 = 1.29
Then you have to take 1/2 the diameter of the wrist pin (generally .990 on a big block) and subtract it from your compression height. 1.290 - .495 = .795
That give you the space you would have to fit your ring package into before you would need an oil ring support rail.
[Modified by Pete79L82, 2:43 PM 5/13/2003]
Thread Starter
Burning Brakes
Joined: Aug 2000
Posts: 1,211
Likes: 6
Re: COMPRESSION HEIGHT & LONGER RODS (Pete79L82)
Thanks Pete,
As a follow up question concerning stroker motors. How low is too low in regards to "rod ratio". The 425 crank w/ 6.385 rods comes in a bit below the standard 454 rod ratio. And well below the standard 427 ratio. Is the effect mainly in the lack of a higher rev range?
In general, does a 425 BB stroker motor have a shorter life span all things being equal??
Regards, Mark
As a follow up question concerning stroker motors. How low is too low in regards to "rod ratio". The 425 crank w/ 6.385 rods comes in a bit below the standard 454 rod ratio. And well below the standard 427 ratio. Is the effect mainly in the lack of a higher rev range?
In general, does a 425 BB stroker motor have a shorter life span all things being equal??
Regards, Mark
Drifting
Joined: Oct 2000
Posts: 1,825
Likes: 56
From: Shawano WI
Re: COMPRESSION HEIGHT & LONGER RODS (marky mark)
Thanks Pete,
As a follow up question concerning stroker motors. How low is too low in regards to "rod ratio". The 425 crank w/ 6.385 rods comes in a bit below the standard 454 rod ratio. And well below the standard 427 ratio. Is the effect mainly in the lack of a higher rev range?
In general, does a 425 BB stroker motor have a shorter life span all things being equal??
Regards, Mark
As a follow up question concerning stroker motors. How low is too low in regards to "rod ratio". The 425 crank w/ 6.385 rods comes in a bit below the standard 454 rod ratio. And well below the standard 427 ratio. Is the effect mainly in the lack of a higher rev range?
In general, does a 425 BB stroker motor have a shorter life span all things being equal??
Regards, Mark
There is a great debate going on with rod ratios. Rehr-Morrisom claims that a longer rod will not do anything for performance in a motor, others will say a longer rod motor will move the RPM band up 100 RPM for ever .100 longer rod you run (I agree with this), others will say a longer rod will only help you at high RPM and all have good data to support there claims.
A 1.5 rod ratio will not hurt you at all. I have had circle track 780HP 434 CI small blocks run a full season with a 1.5 rod ratio. As long as you use quality parts there is no reason a long stroke/ low rod ratio motor would have any less life to it.
Just because you increase the stroke does not mean that you lower the RPM range. The RPM will be limited to the quality of parts you use. Warren Johnson says with his 500 CI Pro Stock motors that he launches off the line at 10,000 RPM. But the only way to have a motor do that is with the very best of parts. A longer stroke motor increases the piston speed dramaticaly, putting a lot of stress on the rods and pistons. So on a street motor your choice of rods and pistons will be the determening factor as to your max RPM.
Pete
