Rod ratio -stroker cranks

[bn69stang]

when shopping for stroker kits for say a 428 block , the popular crank is the 4.25 stroke with the 6.7 chevy based rod and i was wondering what the rod ratio would be compared to 4.375 stroke and the 6.7 inch rod ? . Seems like everyone uses the 4.25 inch crank even with most of the 427 block builds . Cubic inchs and h beam rods with better bearing surface and forged pistons is why i want build my other block , all NEW parts and just want to get the most of this build .. Bud

[blykins]

It's easy to figure out, just divide the rod length by the stroke. 

6.700/4.250 = 1.58
6.700/4.375 = 1.53

You're not going to notice any difference between the two.  Rod/stroke ratio is a subject where you'll get 1000 opinions and none of them will be right....LOL

You will find a guy with dyno results showing that a certain rod works better with a certain head, then you'll find another guy that has tried the exact same combo and showed that another rod works better.  Each engine is different.

If it were a race motor, I'd pick the shortest, lightest piston that was feasible, then find something to connect the piston to the crank. 

For a street engine, I wouldn't really put any effort in worrying about rod/stroke ratios.  With the 4.375" crank, you're not going to be able to play too much with rod lengths because the piston will get pretty short, and it will already require support rails with a 6.700" rod.  With the 4.250" crank, you can play with a 6.800" rod, but it's hard to find pistons readily available with that compression height.

[bn69stang]

With that said you probably build more 4.25/ 6.7 combos on say a 4.16 or 4.17 inch bore ? , the rod ratio is not that far apart though . Have you built and dyno d any 428 blocks with the 4.375 stroke ?  Bud

[jayb]

For what its worth, I am a firm believer that rod ratio is not a particularly important parameter for most performance engines.  If you are running in a class where cubic inches are limited and/or stroke is limited, there may be some benefit to certain rod ratio and head combinations.  For all other applications, cubic inches will trump the effect of rod ratios.  As an extreme example, my 585" SOHC has a 4.6" stroke and a 6.625" rod, making for a rod ratio of 1.44, which is extremely low in anybody's book.  That engine made 960 HP and revved to the limiter (7600) faster and more aggressively than any other engine I've ever had.  On the track I had to set the shift light at 6800 RPM in order to make the 1-2 shift before the engine hit the limiter. 

Bottom line - everything you hear about long strokes and small rod ratios not revving or making top end power is BS.  Go for the cubes - Jay

[bn69stang]

yeah i remember you saying that cubic inches rule the street , and i just wanted to get your opinions before buying any parts , this is again a street car with an occasional strip run . But i want a very healthy  BIG BLOCK , and not sure at this point if it will have 1 carb or 2 or maybe a victor jr intake and a multiport EFI setup , and leaning towards a hyd roller so that this 463 or 477 c i motor will be a strong and reliable performer . And im looking forward to not having a motor with 40 year old rods in it .. Thanks Bud

[Barry_R]

The ratio is darn near meaningless in the context of other things.
But the 4.25 stroke does have a few distinct advantages over the 4.375 that make it worth considering.

First is cost on many builds - I can get the pistons for 390 based kits for a lot less money these days - pistons and rings are less than just the pistons for other combinations.  Add in the extra cost for the crank and the 4.375 will cost about $800 more minimum

Second is packaging.  The 4.250 is a drop in on most applications - assembles like a stock build.  The 4.375 can tag the block in some builds, and the cam in others.  Nothing that happens all the time - but another thing to closely check.

Third is the oil rail deal.  The 4.250 does not pierce the oil groove - so zero issues there.  But the 4.375 will require the spacer every time.  That adds drag, and compromises oil consumption.  I know that a million hot rod small blocks are built that way - so it will work OK, but absolutely zero OE builds will do this and I view it as something to avoid on street applications when possible.  Just a risk with no reward. 

[bn69stang]

Thanks for the feed back Barry , 800.00 for 14 cubic inches , i could spend that on the roller cam and lifters thru your shop , along with the other parts . I appreciate all the feedback you guys , and i want this to be a straight forward build with no surprises or difficulties .

[bn69stang]

And given this will be an high altitude build , my elevation is 5500 ft above sea level and there is not lot of air to fill the cylinders so the need to assemble the engine that will work , make great hp and torque .. Bud

[runthatjunk]

n/m

[jayb]

Barry is referring to the pin height in the piston.  When you go to a 4.375" crank and a 6.700" rod, the pin location moves up in the piston, and actually breaches the lower ring land for the oil ring.  So, after you install the rod and pin on the piston, you have gaps between the lower oil ring lands on each side, where the pin had to slide through.  The way that is addressed is for the piston manufacturer to supply a steel rail that fits on the lower oil ring land, making it a continuous support over the gap in the land where the pin went through.  So, your oil rings go between the normal top oil ring land, and the steel rail that is sitting on the bottom oil ring land.

[blykins]

You'd be surprised on how many aftermarket pistons have that feature.  Small block stuff, all the way up to 4.300" and 4.500" stroke BBF stuff.  I've never had any issue with a piston being set up that way, knock on aluminum.  Was talking to one of the writers of Modified Mustangs & Fords the other day, and he has a 347 SBF with way over 100k on the clock that doesn't use any oil. 

It all depends on what other components you go with on how big the price differential is.  If you use premium parts and compare apples to apples, the price on going bigger isn't all that bad.  If you go with Probe/Icon/Race-Tec pistons, a cast 4.250" Scat crank, and I-beam rods, the price of a 4.375" rotating assembly is going to be a good bit higher.  However, if you go with a premium 4.250" setup, with Diamond pistons, a forged Scat crank, etc, etc., then the price isn't going to be much different at all, and you may end up with a cheaper setup overall. 

Figure out which route you're going to go, do some shopping, and then you can make a more educated decision. 

[Joe-jdc]

This is one area where I have to go with the longest rod possible theory.  I personally always try to use the longest rod available for a certain stroke, and I have always had good success with that process in building engines.  There are a lot of variables in any engine equation, but torque production always increases with the longest rod for a given stroke, and hopefully everyone knows by now that more torque for a given displacement moves the vehicle easier.  Over the years I have seen all the theories, heard all the arguments, and from my own personal experience from building 4 cylinder Volvos to Boss 429s to 635Pro Stock Chevies, I would rather have the longest rod possible in any performance build.  Yes the short rods work, and require different camshafts, but for maximum durability, and torque AND horsepower production, I will take the longest rod possible to fit the combination EVERY time as the best power producer.   Basically, in the long run every build is a compromise, but if I have a choice, I will use the longest rod available for a given stoke as my first choice.  PS (I do have a degree in automotive/and a degree in Aircraft maintenance as well as 6 years instructor/professor experience.  I have heard/read all the theories)  Joe-JDC.

[machoneman]

Long ago, a very wise engine builder of mine (Jerry Baker @ Pro Motor Engineering formerly of Des Plaines, IL, now doing stock car engines in NC state) said it this way. Only when one has optimized near 100% of a specific full-race-only engine package, come and see me about the miniscule gain a longer rod can bring.  And yes, they more than dabbled in very high HP drag engines with long rods for sure.

His point was far more effort should go into other aspects of optimizing an engine's mix of parts, flow optimization, C/R, etc. He also agreed (sorry...it was a Chevy heavy shop, me with a BBC at the time, and his place with numerous NHRA award winning C thru E/MP damn near 10,0000 rpm SBC's!) that the old Smokey Yunick book claim of a near 1.9 -1 'ideal' rod ratio lead many SBC fans far astray. They all seemed to miss the point that for old, flow-limited SBC OEM iron castings in small CID engines, yes, a high-rod ratio did work the heads marginally better.....but all bets were off for larger, later and/or heavily ported heads in larger (i.e. >302CID) engines not limited by many rules.  Certain cat lovers seem to always miss this critical point...LOL!       

[bn69stang]

well again thanks for all the input , i think i have pretty much decided to use the 4.25 / 6.7 rod kit and go from on the rest of the build .. Bud

[Barry_R]

... and you may end up with a cheaper setup overall. 

I rarely come out and disagree with you - but no way can that statement be true.  The absolute best you can get is "equal" but the odds are generally against achieving even that.

The 4.250 stroke pistons are lower cost catalog parts even from Diamond unless you are doing a high compression deal or working on an odd bore size or ring pack.  Best case is equal but never cheaper.  The rail spacers will ad a couple bucks but that's pretty insignificant.

Scat does not offer a 4.375 steel crank (other than $$$ billet) so we are locked into RPM.  They charge the same exact price for 4.250 as for 4.375 - both iterations require work before running them - so again the best possible case is equal.  Of course - since Scat does sell superior 4.125 and 4.250 steel cranks for a bit more money I would go to those at any opportunity - but that gets away from the apples to apples comparison.