Just wondering because a few of my friends were talking and it was brought up that its better to have shorter rod on a blower car.. is this true? Should I be ok with 6" rods in a 383??

 

As long as your pistons are designed for blower use, the 6" will be better. Gives you a better rod/stroke ratio, and less side loading (friction loses) on the block.

 

I'm running 6" rods with good success

You'll be fine

 

I thought the 5.7" rods would be better becauase it allows for more material on top of the piston. Even if the piston is a good blower unit, wouldn't the extra material uptop still help?

 

Country,
I went with 6". I have seen people say to go 5.7 for FI for the reason of being a shorter rod, I guess the idea of shorter less likely to bend. But then for 6", the wrist pin sets higher which gives more stability for the piston (as texluch stated). As for thickness on the top of the piston I guess it will depend on the manufacture. I am sure there are other ideas on this.

 

I got 5.7 in mine, because bending a rod is the last thing I want...

but you do have a valid point texlurch, and luke

 

So what would be better then for FI?

 

the shorter rod allows for a longer piston I mean the distance from ring land to pin.

problem being the longer the rod the less room to get a good dish and thick enough crown for forced induction.

on the 396 I anm building now used a custom wisco and on there recomendation a 5.7 rod, I did get an offset piston pin to compensate for the shorter rod.

the offset pin reduces angle of attack they have been doing it for years in imports.

I now have a 383 with 5.7 rods no issues so far and the pistons didnt need a backup for the oil ring bettor for reduced oil consumption.

John

 

The gain (or even worse, the NEED) for a longer rod is overdramatized. In a FI setup, especially for street use, where some miles will be put on engine, durability should (IMO), even though most time of operation will be spent off boost, be main priority. To gain an extra couple of hp., at the expense of using a piston with compromised durability, is a poor tradeoff. Another one of those traps fallen into, feeling a need to mimic use of race engine components on the street.

As far as less side loading on piston/walls with use of longer rods are concerned, remember the sbc was not originally put in consumer hands yesterday. To put in perspective, in the last 50 years, hundreds of millions of miles have been logged by the sbc with unquestioned durability. This would include hundreds of thousands of miles, in competitive race only environments, also with oem length rods, as well. EDIT: How much less do you need?

 

Here is some info to think about.....

Yes the small block has been around since '55, but the original 265 small block was designed with a 3" stroke & a 5.7 rod.
By comparison the same r/s ratio would put a 7.125 rod in a 383.
If the 5.7" rod x 3.75" stroke ratio was good, they would have put 4.5" rods in the first small block & made the engine shorter & lighter.

The LS1 has a 6.125 rod, wonder why?

The longer rod will not have a bending problem as stated above.

The long rods will allow a lighter piston/rod ***'y than a shorter rod adding durability by lessening reciprocating/rotating loads. This allows higher RPM & longer engine life..... not to mention possibly a few HP.

Yes the piston will require an oil ring spacer, but it is not that big of a deal. People act like this is a horrible thing. If you have severe loads on the oil ring, you have other problems. The side loads are on the skirt side of the piston anyway.

Could the oil spacer hurt the oil consumption that much? The looser clearances coupled with the forged piston required for the blower will have much more effect on the oil consumption. The oil ring is only unsupported for approx. .75" above each wrist pin boss.

If the "weaker" 6" rod piston, due to the tighter ring pack, the oil ring spacer & less room for dome thickness is a problem, you are likely planning on a motor well over 900hp. If building a 900hp motor, oil consumption, long life, etc. means nothing it will have to be rebuilt with some frequency. Strength will be your only issue. I have quite a few 6" rod 383 blower motors that are powerful & reliable with this setup.

 

Hi Lonnie.

Wonder why? Actually, you answered that question above. What is the deck hgt. dimension of the LSx block? The LSx series of engine has a longer rod, for the same reason the std. sbc has a shorter rod. The sbc (as you noted) was designed around the 5.7 rod, just as the LSx series engine was designed around a longer rod.

By comparison the same r/s ratio would put a 7.125 rod in a 383.
If the 5.7" rod x 3.75" stroke ratio was good, they would have put
4.5" rods in the first small block & made the engine shorter & lighter.

Lonnie, you and I both see the foolishness in that reasoning. I'm didn't state a longer rod in the sbc was bad, or that it was not good. The longer rod has it's advantages. For sake of arguement, as your above comment apparently was for also, what will the piston look like, when fitted into the 9.025" block, and bolted to a 7.125" rod? That was the point of my reasoning above. I"m not stating, (and you have proven that with your experiences) the combo will not work, or not survive comfortably, in some FI applications. To me, it just makes little sense, to jeopardize piston durability, in exchange for the opportunity, to install a little longer rod, and maybe a few hp. It makes little sense sense to me, for guys to emulate the race crowd, with a desire for a longer rod, to possibly gain a few hp. they will probably not even feel, on the street. From what I have observed on this forum, with the tuning in general, and A\F ratio management in particular, it would net a whoooole lot more torque/hp., concentrating on the 'tune' of the assembly, than attempt to 'squeeze' a few hp. out of a longer rod. That basically, is the core of my reasoning here.

The longer rod will not have a bending problem as stated above.

I trust, you do not believe I also, was using that agruement, as a concern.
If by chance you do.... Geez Lonnie, give me a break here, ok?

The long rods will allow a lighter piston/rod ***'y than a shorter rod adding durability by lessening reciprocating/rotating loads. This allows higher RPM & longer engine life..... not to mention possibly a few HP.

That didn't appear to stop the 302, with the short rod from twisting over 7000, and live a long and healthy street life. Long, heavy piston and all.

Yes the piston will require an oil ring spacer, but it is not that big of a deal. People act like this is a horrible thing. If you have severe loads on the oil ring, you have other problems. The side loads are on the skirt side of the piston anyway.

Could the oil spacer hurt the oil consumption that much? The looser clearances coupled with the forged piston required for the blower will have much more effect on the oil consumption. The oil ring is only unsupported for approx. .75" above each wrist pin boss.

No point in commenting further on that.

If the "weaker" 6" rod piston, due to the tighter ring pack, the oil ring spacer & less room for dome thickness is a problem, you are likely planning on a motor well over 900hp. If building a 900hp motor, oil consumption, long life, etc. means nothing it will have to be rebuilt with some frequency. Strength will be your only issue.

Having capped it with the last sentence, I'll agree with that paragraph, Lonnie.
But, and thinking along the same line here as above, when assembling little more than a medium output FI application, why be concerned with that couple of extra hp? I gotta agree with Mr. Spock on this one. Take care.

 

This has to be one of the most over-discussed issues on the board. But FWIW for forced induction I prefer a 5.7" rod witha stroke up to 3.75" (383) and a 5.85" would be a good choice for a 3.875" stroke (396). For NA, 6" or longer is a good choice. But a wide variety of combos will work fine.

Rich

 

Arnie,
Not trying to cause an argument, just stating some thoughts & trying to help.
You state some good facts & it sounds as you are well informed with automotive knowledge.

Unfortunately these web sites are way too full of comments without factual or real world experience. There are far too many "A friend of my cousins neighbor said...." or "I read on xyx forum that it was bad". I will only post on my real world experiences. I also come here to learn, but it is hard as you have to weed out the BS from the fact. For example... How it is impossible to remove the heads in a 4th gen if it has head studs. I've read this many times & also have done this "impossible" task many times. They come right off over the studs. Go figure.

The original post was asking if the 6" rods would be OK to have in his motor. So as not to sway the person in the original post to think they were bad, I gave some reasons why they were OK to have. I use them & will continue to do so.

I have seen the supposedly weak 6" rod SRP pistons survive in 800+HP cars that blew head gaskets (more thn once) with head studs & 20# boost. The pistons survived the severe detonation nicely, so I do not question their reliability. Anything can be broken, but this shows some strong qualities.

The bending issue was directed to a post above yours.... not you directly or to criticize anyone for that matter. Just trying to keep another "rumor" from being started here. I was hoping this would prevent a new advanced topic from being "Will 6" rods bend?"

Your comparisons on the LS1 are dead on.... Just thought I would show longer rods are viewed as a "good" thing.

The 302 was a 3" stroke motor as well... like the original '55 design. It revved well above 7k as I had one of these. A 383 has a similar average piston speed at 5600rpm as the 302 did at 7k. Keep in mind the internal loads are 56% higher on a comparable 383 at the same RPM with its 25% longer stroke because internal loads increase with the square of the speed. When I'm spinning my 383 to 7K, it feels like 8750 on the mentioned 302 to the piston. The lighter components do definitely help in this situation to reduce some of the inertial loads on the rotating assembly.

I know that deeply dished pistons are not possible with 6" rods & that may be a problem with building a very low compression motor.

For reference, who here "has" a motor w/ 6" rods that were a problem? Let us know what happened to them or what the problems were. If you have a preference, that is fine, just remember in order to like Budweiser better than Coors, you have to have tried both. No heresay please.... only actual experiences.

 

the only time a 6 inch rod has a disadvantage over a 5.7 is in a forced induction motor when using a large dish, since this is a forced induction forum thats probably the application. If you call the tech lines of any of the piston manufactorers they will all tell you the same thing. If they are making the pistons they should know what works best.


BTW doesnt a shorter rod make a little more torque.

John

 

In >99% of the combos out there, any effect of rod length on performance or longevity is going to be lost in the noise of the myriad of other variables that have a much larger effect. The only really relevant factors are the potential for a lighter piston/rod combo which favors a long rod for a high revving NA setup and the need for thick ring lands and a deep dish on pistons for a blower car, which favors the shorter rod.

All the rest is purely theoretical interest. There ARE measureable effects on hp, torque, emissions, peak and average cylinder pressures, detonation resistance, rpm capability, bore wear, etc., etc. but the effect are going to be completely trivial except for discussion purposes.

Rich