I would like to build my new shortblock 350 assembly with 6" rods to increase my "rod ratio". I read the article http://www.airflowresearch.com/Articles/A3-P1 and that peaked my interest. I don't want to go as radical on the bottom end as they did but if just going from stock 5.7" rods to 6" rods would help with my power without sacrificing streetablity then great. I think I will go with JE pistons. Haven't decided on other bottom end hardware.

I am wanting to use a later model 350 block that will have the one-piece rear seal. Anything different about the crank for 6" rods vs. 5.7" rods like there is with the piston.

Does the one-piece rear seal engines have an externally balanced flywheel/flexplate. If I have the rotating assembly balanced do I have the flexplate balanced with it. Any other advice in this balancing area would be appreciated.

Next, Should I by a more expensive harmonic balancer like a Fluid dampner or the like. This engine may turn as much as 6500 rpm with a LT4 hot cam.

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91 Camaro RS 5.0L Black Convertible 700R4
** Added so far **
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***Future 330hp Vortec Crate with LT4 hotcam and 454 TBI

 

350's are internally balanced. The cranks are the same for 6.0, and 5.7 rods, just the pistons are different. Usually longer rods help produce more torque b/c the piston dwells at TDC longer. If you have this balanced bring all the parts related to the rotating *** . to the shop doing the work. This means the flywheel/flexplate, pistons, rods, and the balancer, and yes I would get an aftermarket one. IMHO, the stock pc. doesn't do a suffecient job past 4500rpm.



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Joshua Johnston
1991 Z-28
350, T-5, K&N, Ported Vortec heads, Edelbrock RPM, Holley 750 D.P., HEI, 11.07:1 CR, Comp Cams Roller-.510"/.520"-282*/288* dur., Shorty Headers, Dual 2.5 Exhaust, Dynomax Bullet Mufflers, T&R Motorsports custom air intake, Bald tires

 

IMHO, don't waste your money. It's been a discussion on here a few times and there are many more headaches that come with using 6" rods then there are benefits.

There has been numerous tests done by some of the best engine designers/builders in the world. They all come up with the same results, usually. Expect about a 1-2% increase in maximum output. That's it. There are other variables that are responsible for the results. One is more rotating mass. I.E., more weight.

You can do it if you are building a 1500+HP engine. Then you would see a 15-20HP gain. But on a 400HP engine, then you MIGHT see 4hp. I would say that the gains would be more relative to torque, due to the leverage increase. But again, very slim for all the trouble that's involved.

BTW, a 1 pc. rear main seal engine does use a external balanced flywheel/flexplate. It still uses the same harmonic damper that the 2 pc. rear main engine uses though. But it's not something that you need to worry about anyway because only the right flywheel/flexplate will bolt up to each of the crankshafts anyway.

AJ

 

using the 6 inch rod is and has been the way to go for quite some time. any bit time professional engine builder uses them for that reason. they do make crank's specific for 5.7 and 6 inch rods, but you can use 6 inch rods on pretty much any 350 crank without any problem. one piece rear main seal engines internally balance in the front and externally balance in the rear. just make sure to specify when you order your flywheel/flexplate and it won't be a problem. the engine shop doing the machine work won't have a problem balancing it. pretty much all the chevy small blocks i've done are one piece rear seal and they work well. the fluidamper is a great idea for higher rpm usage. remember also that 6 inch rods allow for higher compression so up to 11 to 1 with cast iron heads and 12.0 with aluminum heads can be ran on pump gas with fuel injection.

 

Changing the rod length shouldn't change the compression ratio at all. All you're doing is lengthening the rod, not the stroke. To use a longer rod, the wrist pin is positioned higher up in the piston to accomidate the extra length. The piston is still in the same position at TDC as it is at BDC. The only time the compression ratio will change is if you use pistons designed for 5.7" rods but then the piston may go higher than the deck height. It's just like using 350 rods in a 400. Special pistons are used and the wrist pin is moved into the oil control ring area.

When building any engine with aftermarket internal parts, everything should be balanced. Since the 350 is internally balanced, the flexplate/flywheel and harmonic balancer are already neutral balanced. Changing the rods or pistons will change the internal weight and must be balanced with the crank to neutral balance everything again.

An aftermarket balancer is a good idea on any engine but the engine should still be balanced.

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who said anything about it changing compression? i stated that using longer rods allow for more compression as they have less chance of detonation. it is one of the advantages of using longer rods.

 

I understood you Zippy. I thought that was one of the advantages of running the longer rods.

I am going to check with some of the local engine shops and see if any of them have experience with this type of engine and check out their equipment.

I doubt this engine will reach 400hp but I want it as close as possible.

So, with hp figure in mind. Should I run hyper pistons or forged. I don't intend to run any power adders like nos or supercharger. Also, recommendations on the rods. Crank is the stock crank good for 400 hp or does it need to be forged as well. I know forged is better but is it overkill. I don't want to have $5000 in an engine that would have run exactly the same for $3500. I will have assembly balanced and rev limiter set at 6500rpm.