FAQ |
Members List |
Social Groups |
Calendar |
Search |
Today's Posts |
|
#1
|
|||
|
|||
6.7 vs. 6.8 rod with 4.5 stroke crank?
Had a trusted builder tell me that they had trouble w/ the longer rod and the longer stroke. Issues w/ piston rock and scuffing the piston skirt. I want to do a street 4.5 stroke and their seems to be two main kits - SD and Pacific come w/ the 6.7 and SGI/butler comes w/ the longer rod.
Opinions?? |
#2
|
||||
|
||||
for the most power the current thinking is a 1.2ish" compression height piston. i have a current prostock piston that i am playing with that comes in at exactly 1.2"ch.
if you are going for reliability go with the shorter rod with a longer skirt. Last edited by J.C.you; 12-09-2010 at 01:26 PM. |
#3
|
||||
|
||||
pistons
here are a few shots of the ps piston. there appears to be a good bit of loading on the skirt.
Last edited by J.C.you; 12-09-2010 at 05:25 PM. |
#4
|
||||
|
||||
C/FX NSS
At least a 6.7 rod or maybe a 6.6 for that stroke.
__________________
505 KRE HP head 2x4 HP 1000s 60' 1.33, 5.78 @ 120.88MPH in 1/8 9.11@149.8MPH in 1/4 |
#5
|
||||
|
||||
Isn't having a 1.4888 rod/stroke ratio kind of scary? 6.7/4.5 If you had good rods what would be a safe rpm? Idle to 4000-5000??? Even the 6.8/4.5 only yields 1.5111 ratio. I guess it's not to far off from the SBC 5.7/3.75 383 engines coming in at 1.52 I suppose.
Factory rod length 6.625/4.5 = 1.4722 fwiw
__________________
Good luck to the new owner of the Ventura II! Sold the car after 13+ years. Look for it on the Hot Rod Power Tour in the future as it's currently being re-configured as a Pro-Touring ride! |
#6
|
||||
|
||||
Quote:
That's the old pontiac caveman thinking. Langer has (or use to) a 6.6 rod, and runs it to 9000 rpm.
__________________
Boobs & bacon |
#7
|
||||
|
||||
6.7 rod for ease of install & parts
|
#8
|
|||
|
|||
For a street engine looking to get some ring life we use the 6.7 ,1.250-1.275 CH.the longer piston seems to help ring life,on Allpontiac GrandAm we used a 6.8 rod 1.150CH .We dont think the .100 difference means much however if it was a NOS engine we would run the shorter rod.NHRA Pro -Stocks are at 1.69-1.71 rod ratio Bill C.
__________________
Checkered Flag Machine & Ceralli Competition Engines Racing engines and induction development http://www.checkeredflagmachine.net/ |
#9
|
||||
|
||||
__________________
http://www.youtube.com/watch?v=t9eWgLd0q-U |
#10
|
||||
|
||||
i have had my 542 with 4.5 stroke over 8k many passes with a 6.7 rod.
|
#11
|
||||
|
||||
My 535 has seen 7800+ a bunch of times with a 4.5 stroke and a 6.7 rod
__________________
69 'Blackbird' 535 CID IA II SD Wide Port E-head Pro Systems Dominator |
#12
|
||||
|
||||
i was thinking about this also. considering a 6.635 rod if it allowed a little more skirt and kept the pin out of the rings. but, really not sure if such a small amount would make any difference.
is there any special consideration needed in regards to cylinder wall thickness? |
#13
|
||||
|
||||
I run a 6.635" bbc rod on my 4.5" crank for the blower combo. If I did it again I'd go to a 6.5" and lose the rail supports altogether.
__________________
The secret to happiness is not getting what you want but rather, wanting what you have. |
#14
|
||||
|
||||
What a mess it makes.
__________________
Nothing is impossible if you break it down into possibilities. It's all a matter of "Want to". |
#15
|
||||
|
||||
With the short rods you need to check to see how far out of the bottom of the bore the piston is coming.
|
#16
|
|||
|
|||
On my 541 i have 6.565 rods.
|
#17
|
|||
|
|||
4.5 stroke,4.310bore,6.8 rod,4 seasons,minimum of 200 passes per season,...800-1000runs with no issues.
__________________
Jeff Verdi 2016 Spring Fling Million Winner 2016 CARS Million Winner |
#18
|
||||
|
||||
Quote:
he's definately not a stuffed shirt!!! We've all read about the "side loading" issue. We've all read, been pretty much educated that the longest rod, is best. BUT, after having taken in all the real world experiences I've read here, a long rod isn't all that necesary in drag racing, we need to get over it!!! Smokey Yunick used the longest rods possible, BUT his engine was turning 7000-9000 RPM's for 2-3 hours, and he needed every bit of help possible, vastly different from a quarter mile blast.... A lot of folks have made a lot of mistakes, and been humble enough to talk about them here, that we can all learn from. None of my junk will be pushing any manumantal boundries to worry about, but, I'll still listen, hopefully for my own good. Thanks Jeff
__________________
1977 Black Trans Am 180 HP Auto, essentially base model T/A. I'm the original owner, purchased May 7, 1977. Shut it off Shut it off Buddy, I just shut your Prius down... |
#19
|
||||
|
||||
What you want to do with a long stroke crank is get the pin as high as you can as long as it stays out of the oil ring pac. Then you can run a short skirt, keeping your length near equal above and below the pin area and control weight and stability at rpm.
|
#20
|
||||
|
||||
I run 4.5 stroke with a rod shorter than 6.7" at 8000 RPM.
|
Reply |
|
|