So I'm toying with the idea of building a 350 with 6" rods. Its pretty well known that the longer dwell time of the piston gives various benefits. The crank assembly kits from Speedway are affordable as are individual parts. So, the question is: what camshaft profile makes best use of the longer rod motor? Same question for the induction. It seems a natural for a TPI setup, but that's OT. Let's stick to normally aspirated. What manifold, carb CFM. Etc. It would seem that keeping charge velocity up would be a challenge. The old Hot Rod article with a 400 block, 300 Ford 6 rods and 327 crank didn't really go into cam theory. Sent from my SM-G965U using The H.A.M.B. mobile app
You are on the right track for top end performance only......if that's what your goal is. For lower RPM ranges (where most street rods operate), the shorter rod shows better performance on a dyno.
@hotrodart I recall the article the OP is referring to, as I recall the engine made a lot of torque early and didn’t fall off much. I think the article was along the lines of “the 350 Chevy should have made”? Or similar. It also was done on regular gas as well.
We did one with 6-1/8” rods and 400 piston specs for the pin height as I remember. All parts off the shelf. HP or not I use the longest rod I can. I still have a set of Elgin 6-1/8” rods for a street engine not strong enough for the racing we do now.
I would like to know too, planning a poor mans sbc 302 build, using 6" rods and machining .030 off the tops of flat top 350 pistons.
Check out recent "Engine Masters" on Motor Trend TV. David Frieburger & Co. did dyno tests using a BBC with Long & short rod combos. Their tests showed that long rods made more power at the low end, short rods more power on top. Although, the differences were minimal, just a few HP different. Street motor, not enough to bother with, all out race motor, yes.
I always thought the long rods caused less side loading on the cyl walls and piston skirts. JMO Lippy
Side loading was mentioned on the Engine masters episode. As 42merc said, the gains were very minimal, the engine really did not care that there was a longer rod as far as HP and torque.
I am building a 350 Chevy, with 40 over flat top Carrillo pistons. Not sure of the rod brand but they are 6 in. I have a set of World products cast iron heads with 1.94 - 1.50 valves. I have a Performer intake ( dual plane ) and The cam I have is a solid flat tappet and I do not know the manufacturer or the specs. This stuff came out of a round track car and ran off a 7200 chip. I hope to have in together this spring. I will post how it does , but I look for it to run good.
As 42merc said, the typical performance difference is not that great. But if you are in a situation where winning and losing is differentiated in small numbers it's worth it. But as I said before I take calculated risks in race engines that get routine maintenance, not so on street engines. My favorite street rod engine is a 5.7" rod 406. The 5.7" rod keeps the wrist pin out of the oil ring. On a 6" rod you need to run a ring bridge.
Unknown and mis-matched parts (I e circle track and dual plane intake). Seems like a poor choice from the back of my phone. Best of luck.
TADad...Your typical all iron flat tappet dirt track engine makes its power at 6200 but will turn over over 7000 and hit the mandatory 7200 chip. A dual plane manifold will bring the torque down and tend to limit the rpm depending on your rear gear. Unless it came off a 1/2 mile it probably ran 6.20" to 6.50's in the rear which is why it could see the 7200. The cam may be a little much depending on the rockers but most tracks have a .600" max lift and 1.5 rocker normally don't make that. You can only do so much with a iron cam. It would be nice to see what the valve spring pressure is... I'm guessing 120 to 130. I'd run Hot Rod oil with zinc. It's obvious not a IMCA modified which runs alcohol and high compression even as a claimer so I would guess a super or pro stock as usually street stocks cannot have aftermarket heads. My guess is you will enjoy it.
I keep forgetting this is a gasoline deal. With the correct camshaft on nitro piston dwell can be your friend. Sorry. It will run good on gas either way. LOL
I can't wait, I know it will be fun ! I am in the process of getting everything ready to go to the machine shop. The valve spring pressure is concerning because they are pretty big, this is going in front of a manual shift Turbo 400 with a brake along with a 4200 stall 10in. Turbo Action convertor ( yeah that's a ,little loose, but they will tighten it up for $150.00 )29in. tall tires, along with a Moser 60 on Super Stock springs with a 4:10 and a spool, you know this is gonna be enjoyable ! Everybody's definition of a Hot Rod is different !
The Jalopy Journal
