Thread: Connecting rods

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Hello again guys/gals.

Well I have desided, after much debate, lol, that I will build a 383 small block for a truck. You guys have been instrumental in that decision. I think this would be a better place to start off at then some the exotic 350 I had planned lol. When I was searching for cranks (I chose Eagle) they recommend a 5.700" or longer connecting rod for their crank. Now I heard (thank you John Lingenfelter) that the longer it is the better, becuase you don't increase the stress on the trust side of the piston. Is that correct? Then something to the effect that with a shorter rod you form more acute angles on the crank journals causing uneeded stress on the piston and wrist pin. Did I get all that right? Hope I did it was a mouth full. lol.

Thanks,

-Chris

Whoops, here it is.

Chevrolet 400 Forged 4340 Steel Crankshaft
400 Main Journals, No Spacer Bearings Required
3.750'' Stroke (should I go longer since its going to be in a truck?)
5.700'' or Longer Rod Length
2.100'' Rod Journal
(do all 400 cranks have the 2.65 main journal diameter?)
2-Piece Rear Seal
Internal Balance
1900 Bobweight

Originally Posted by dhemi1

Hello again guys/gals.

Well I have desided, after much debate, lol, that I will build a 383 small block for a truck. You guys have been instrumental in that decision. I think this would be a better place to start off at then some the exotic 350 I had planned lol. When I was searching for cranks (I chose Eagle) they recommend a 5.700" or longer connecting rod for their crank. Now I heard (thank you John Lingenfelter) that the longer it is the better, becuase you don't increase the stress on the trust side of the piston. Is that correct? Then something to the effect that with a shorter rod you form more acute angles on the crank journals causing uneeded stress on the piston and wrist pin. Did I get all that right? Hope I did it was a mouth full. lol.

Thanks,

-Chris

Whoops, here it is.

Chevrolet 400 Forged 4340 Steel Crankshaft
400 Main Journals, No Spacer Bearings Required
3.750'' Stroke (should I go longer since its going to be in a truck?)
5.700'' or Longer Rod Length
2.100'' Rod Journal
(do all 400 cranks have the 2.65 main journal diameter?)
2-Piece Rear Seal
Internal Balance
1900 Bobweight

You can use the 350 main gearing dia.
By talking about the rod ratio/rod length you will get many ideas and thoughts what to do.

Thanks for lead in Erik.......yes, you will get a ton attitudes and ideas for the length of rod discussion. Not only side load is reduced a stress worries about harmonics and load factors, you increase piston dwell at the top and bottom of the stroke. This increases combustion qualities on the burn cycle and scavanging factors at the exhaust cycle. Lingenfelter is no dummy and his knowledge comes from experience and years of doing this sort of thing. My opinion; go as long as of of a rod as you can stand to afford. One point of curiosity, bob weight seems a bit high for my taste. What pistons are you running and rods, etc? I like 1650 to 1725 on my SB rotating assemblies.

i would not put any longer rod that you can not get a shelf piston for .if you have deep pocks then go for it but to a point the longer not worth the added money the 1900 is eagle target weight .not the real mass get out them big drill bits

Originally Posted by nitrowarrior

Thanks for lead in Erik.......yes, you will get a ton attitudes and ideas for the length of rod discussion. Not only side load is reduced a stress worries about harmonics and load factors, you increase piston dwell at the top and bottom of the stroke. This increases combustion qualities on the burn cycle and scavanging factors at the exhaust cycle. Lingenfelter is no dummy and his knowledge comes from experience and years of doing this sort of thing. My opinion; go as long as of of a rod as you can stand to afford. One point of curiosity, bob weight seems a bit high for my taste. What pistons are you running and rods, etc? I like 1650 to 1725 on my SB rotating assemblies.

On dirt mod. engines that are in the 7,900 to 8,200 rpm area I like to run a 6 inch Eagle with the ARP 2000 bolts.
I use SRP piston #140348.
This combo. seems to live along time for us.

I'm confused Pat. This comes from using bricks for pistons like the old TRW's. There are so many available options to reduce weight at a reasonable cost. I ask this in all honesty without trying to argue or doubt your judgement, do you feel his bob weight is good? Seriously, I desire info from other builders as the next guy. Shelf pistons brings up a good point, I have found the "Elites" to be as affordable as the shelf styles. Help me out here Pat, I have a different attitude about this and would like to know more from others.

no i think that is what the crank will support for a bobweight. i would say it is nice to be light but the crank has a counter weight that will support this much bob weight. all them china cranks you almost all the time you have to drill the hell out of them .i do not think he got the real bob weigth? on the pistons for small blocks i like the mahle power packs but i do not use alot of pistons for the small blocks alot of low buck jobs like cast pistons or hyper . have use more big block pistons je ross srp diamond and speed pro hyper kb trw and other alot of the time i am just happy that i can bore a block out for new pistons that have not been shot out of a canon into a brickwall any one feel my pain sitting with a file and a sharp pick working on a old set of ash trays

you don't design a engine around a rod ratio.... you build the right piston and then put a rod into it.. your wasting your time worrying about the differents between the 6-5.7. i would put you worry were it belongs the piston.. if it worrys you that much off set the wrist pins with custom ones. but i can tell you from 1000s of hrs on the pump it doesn't do chit for power or how durable it is...

Originally Posted by 500caddy

you don't design a engine around a rod ratio.... you build the right piston and then put a rod into it.. your wasting your time worrying about the differents between the 6-5.7. i would put you worry were it belongs the piston.. if it worrys you that much off set the wrist pins with custom ones. but i can tell you from 1000s of hrs on the pump it doesn't do chit for power or how durable it is...

yes i do that . i here guys say run a long rod run the pin in the oil ring pack and we will run a shirts short pistons short ring pack they say the piston will be more stable ? i have ia very hard time with this concept? for a longer rod what the hell seal up the hole the rod

yes sir, if you run a power adder your just asking for a big oil mess with a short piston.. the top ring isn't far enough from the top to put any power adder to it.. it least .220 i like .250 to the first ring..but what do i know.. like most the big time engine builders say what is rod ratio ??? reher thinks the rod is just there to connect the crank to the piston... i couldn't agree more.

HHmmm..... interesting attitudes from both Caddy and Pat. I like that Pat mention the ill-equipped foriegn cranks and the effort to lighten and balance. On the other hand, not worrying about the correct geometry has red flags going off. Let me present a question of good measure, why would you not consider rod to stroke ratios, dwell time, combustion burn time, and flame travel created by a combo when building a strong runner and reliable powerplant? Another thought would be, why should I not or any other builder not be concerened with a reciprocating mass of a dimension that can achieve more effort by lightening the assembly and transferring te work load to the ground? Good food for thought for those who wish to build a stout power plant.

yes i have that tattoo on me.. they did a test for circle track mag about 10 years back at the end of all the testing not much was found out. they said that they talk to reher and said the rod is used to hook up the piston to the rod .... i have run the 6.375 rod with a 4.375 crank they work i have a friend that runs a 5.000 crank with a 6.635 rod in the 10.200 block OK he scares the hell out of me

whos to say what the correct geometry is??? i have found no ill effect of a socalled short rod ratio and never any differents in power. i have heard dwell intake charge etc etc. it's just not there this is my finding of yrs of building engines and dynoing for every last hp. this isn't just me but many many pro engine builders same findings. give sonny or grumpy, david reher a call and get the same answer. there's just nothing there. it's one of those myths that got out of hand and everyone thinks there is some magic number.. what is the socalled magic number ???? because i and a bunch of us can't find it..

well that all come with a price any for a street car or a drag car well why would you go past a shelf pistons. and the cam will help out with the other i see all this stuff for max $$ and build i do like ligth pistons and mass but there are limits if i was building offshore or nas car ? well thats ends less money .i have seen big block with stock rods and 250 long never seen any thing and 454 with 400 long past stock the only time i like a longer rod is for more counter weigth so the counter weight all in the engine not in the damper or wheel but i have had some real runners 496 with stock short rods

Originally Posted by 500caddy

whos to say what the correct geometry is??? i have found no ill effect of a socalled short rod ratio and never any differents in power. i have heard dwell intake charge etc etc. it's just not there this is my finding of yrs of building engines and dynoing for every last hp. this isn't just me but many many pro engine builders same findings. give sonny or grumpy, david reher a call and get the same answer. there's just nothing there. it's one of those myths that got out of hand and everyone thinks there is some magic number.. what is the socalled magic number ???? because i and a bunch of us can't find it..

It may be a "myth" in an engine that has to live for 6 seconds at a time in a drag car.
In any of our IMCA mod. or USMTS engines with the longer 6 or 6.125 inch rod we have found our piston skirts have less scuffing along with our piston rings sealing better and lasting longer.
Again I think it is very hard to compare a 6 second drag engine with a circle track engine that turns 20 to 25 laps on a 1/2 mile track at 8,000 rpm's.