[GTOLou]

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??

[J.C.you]

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.

[J.C.you]

here are a few shots of the ps piston. there appears to be a good bit of loading on the skirt.

[bigchief468]

At least a 6.7 rod or maybe a 6.6 for that stroke.

[71 Ventura II]

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

[J.C.you]

i have had my 542 with 4.5 stroke over 8k many passes with a 6.7 rod.

[BADDTA]

Originally Posted by 71 Ventura II

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

That's the old pontiac caveman thinking. Langer has (or use to) a 6.6 rod, and runs it to 9000 rpm.

[PONTIAC DUDE]

6.7 rod for ease of install & parts

[cfmcnc]

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.

[Matt Meaney]

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?

[Torment]

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.

[1qikta]

What a mess it makes.

[slowbird]

With the short rods you need to check to see how far out of the bottom of the bore the piston is coming.

[Hillard]

Originally Posted by PONTIAC DUDE

6.7 rod for ease of install & parts

x2.

[greggutknecht]

On my 541 i have 6.565 rods.

[Jeff V]

4.5 stroke,4.310bore,6.8 rod,4 seasons,minimum of 200 passes per season,...800-1000runs with no issues.

[badassgoat]

Originally Posted by J.C.you

i have had my 542 with 4.5 stroke over 8k many passes with a 6.7 rod.

My 535 has seen 7800+ a bunch of times with a 4.5 stroke and a 6.7 rod

[Cliff R]

After watching Jay Delaigle's "professionally" assembled short block wear itself out in short order, I would run a shorter rod with more piston skirt and a lower pin location. Jay's engine was the first 4.5" stroked Pontiac engine I actually had some association with. The high pin location and long rod deal skuffed the cylinder walls up pretty hard. It also didn't stay sealed up very long.

Personally, after building a few SBC 400's with 5.56" rods and 3.75" strokes, and watching them make fantastic power without the first issue anyplace in the piston/cylinder wall/ ring pack area, I lost interest in trying to get long rods into any of these engines.

We read about "side loading", and other issues associtated with using short rods. After having one of these 400's last 10 seasons of drag racing and it was in perfect shape when we tore it down for inspection, I'm not afraid of using short rods in a Pontiac engine with the longer stroke crankshafts.

Shorter rods also reduce reciprocating and spinning weight. Whether they bring any high rpm limitations into the "recipe" seems to be a matter of opinion, rather than fact. I've also seen folks run the piss out of BBC's with 4" strokes and 6.135" rods for years, without the first hint of a problem in that area.......Cliff

[77 TRASHCAN]

Originally Posted by slowbird

With the short rods you need to check to see how far out of the bottom of the bore the piston is coming.

I remember reading, this was one of the things Jon Kaase was concerned with when he built a 508 Pontiac for Engine Masters, a few years back. Man, the engine had one-off everything. His bore/stroke was a little unusual, trying to keep the stroke as short as possible, and use the 4.310 bore, because of the great choice of rings available in that size...Would'a won the deal, a piece of gasket plugged up a jet. I got to talk to him at an IHRA event in Tulsa in 2009, thoroughly enjoyed talking to him, it was COOL...
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

[PONTIAC DUDE]

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.