Connecting rod to stroke ratio

Howard Jones

Gold Supporter
#1
Another thread on 331 verse 347 and their comparison to standard 302 stroke/bore dimensions opened up the question of rod to stroke ratio.

The question was asked (by Jac so I know this is a good question) which was the best ratio: 302= 1.696, 331=1.661, 347= 1.588

My guess is the 302...………………..

So what do you think?

I found these:

Rod to Stroke Ratio - Tech - Honda Tuning Magazine

Connecting Rod vs. Stroke Analysis: panic Tech Paper No. 1

Importance of Rod Ratio? - Speed Talk

https://www.hemmings.com/magazine/hcc/2009/06/The-Mechanical-Advantage/1827793.html
 
#2
My thinking which is likely to be wrong is.......


I would go with the 1.588 as it gets the piston accelerating down the bore quickest and creates the most vacuum which gets the momentum on the column of air above the valve moving the best, resulting in the most power.


this will probably have a negative effect on engine longevity


Ryan
 
#4
i get that JP, however as most classes these days seem to have either a capacity limit or a rev limit or both, what would you choose?

that's what i based my decision on.

I understand that it may be geometrically impossible to get some of the ratios into a standard 8.2" deck block, i didn't think that was the point of the question.
 
#7
I know very little.....in fact, probably nothing....about this. But where there's a capacity limit (as there is in most classes) it seems building revs with short stroke is one good option.

Ferrari F1 must be onto something......
 

Attachments

#9
Wow Jac Mac

That gives you a real short pin height - .875 if my calcs are about right for an 8.2 deck
That would put the oil ring a fair way down the pin area
Does it work ok with normal support rails???


Is that rod an aftermarket Holden Rod ???



regards

KB
 

Jac Mac

Active Member
#10
Wow Jac Mac

That gives you a real short pin height - .875 if my calcs are about right for an 8.2 deck
That would put the oil ring a fair way down the pin area
Does it work ok with normal support rails???No, slightly shorter pin & spirolocs, alloy button @ each end with groove cut for oil ring and drilled to allow any oil that migrates to pin bore to drain back to pan, sounds complex, but quite simple once first one is made. Both comp rings are above pin. I had pistons made by Denco in NZ ( Ross Blanks ) and cut valve notchs & made pin/button setup myself.


Is that rod an aftermarket Holden Rod ???Yes, Scat makes em.



regards

KB
In text above, from comments being made on this thread most seem to have it back to front, eg living with whats available rather than getting the max rod length able to be fitted and then work out cam, head after you arrive at max rod length. You can notice a difference even if you fit 289 rods ( 5.155" ) to a 302
 
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John

Active Member
#11
Jac, i had a 387 built many moons ago for my XC , and from memory they uses a Valiant rod, not 100% but pretty sure, and the pistons were a weisco special.
with an EP2 cam in it she made just over 400 at the wheels and revved like a bumble bee on steroids. was the by far the quickest Cleveland i'd ever owned for sure and i'd had plenty.
john
 

Jac Mac

Active Member
#12
Jac, i had a 387 built many moons ago for my XC , and from memory they uses a Valiant rod, not 100% but pretty sure, and the pistons were a weisco special.
with an EP2 cam in it she made just over 400 at the wheels and revved like a bumble bee on steroids. was the by far the quickest Cleveland i'd ever owned for sure and i'd had plenty.
john
Were they 6cyl hemi rods, they will be 6.00" long and 2.00" rod journal, so offset grind the 351 crank to get the ~ 387 cu in with around 1.59 rod ratio. by the same token you can get the AU 302 rod @ 6.025 " and some short pistons from OZ to change the 351 w & c to a longer rod ratio around 1.7. Lots of options out there for Fords, many that are not catered for directly by the industry, but with some careful part swapping you can get really good combos.
 

John

Active Member
#13
mate i think your right on the money there, 265 hemi rods sounds like them and yep the crank was off set ground, i did still have the build sheet on file till a couple of months ago when my hard drive decided enough was enough and bloody lost everything,
it was built by Bob Matic in Sydney back in 1997.
cheers John
 
#14
I have head that a 363 CI SBF can be built with a aftermarket block, Dart I think.
This uses a bigger bore and relative shorter rods? What do you guys think of this combo. The block seams to address the web strength issue of stock FMC small blocks. And I like the overall CI size of 360ish.

Thanks for the knowledge guys. I have learned quite a bit.

The 363 - The Hottest Ford Stroker - Hot Rod Network

363 CUBIC INCH BOSS SHORT BLOCK| Part Details for M-6009-363 | Ford Performance Parts

AMS RACING DART SHP SBF FORD 363 CI STROKER SHORT BLOCK
 
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Jac Mac

Active Member
#15
Not many proof readers @ the Hot Rod Network! No reason other than $$$ and lack of talent in that 363 build which could have been done with pistons/rods as I described in posts 8 & 10 above.
 
#16
I cant seem to find any evidence of back to back testing where the ratio was changed and an appreciable increase or decrease in performance was noticed.
 

Jac Mac

Active Member
#19
I cant seem to find any evidence of back to back testing where the ratio was changed and an appreciable increase or decrease in performance was noticed.
Its one of those situations where if you change the rod ratio you also change several other factors at the same time and it becomes one of those situations where what caused what.
 
#20
Its fascinating, but its not helping me progress my chassis right now so I am going to have to shelve it for a while and come back to it when I need to investigate further.


thanks Jac, I think, my head hurts now.


cheers


Ryan
 
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