Hey guys,

I'm starting to gather parts for another blown small block project and am trying to find out the pros and cons of using a 5.7" vs. a 6.0" rod in a 383 (4.030 x 3.75 ). Any info would be appreciated... power, durability, price, H vs. I-beam, brand, whatever... Thanks in advance!

Originally posted by 10sec69:
Hey guys,

I'm starting to gather parts for another blown small block project and am trying to find out the pros and cons of using a 5.7" vs. a 6.0" rod in a 383 (4.030 x 3.75 ). Any info would be appreciated... power, durability, price, H vs. I-beam, brand, whatever... Thanks in advance! I like a 6" rod in 383's because it makes life easier when balancing internally which is always my choice if it is reasonably possible.
Some don't like the ring suppoort deal but we have a bunch of them running around in 383's, 400, & 496's with absoulty no problems.
As far as performance advantages,,,,,my opinion is it is a subject that gets beat to death for no good reason.
There are very good reasons and arguments for both long & short rods & again, my opinion but if you are looking for power it ain't in the rod length :D
For a rod, either will work just fine.
My preference is Oliver for a high end rod & feel Scat products are the best of the less expensive cranks & rods.
Great quality control when it comes to sizing etc.
Been using their stuff for about 12 years now with no problems that are worth mentioning which is not what I have seen with some of the other brands.
They have both an H beam & a newer 7/16 capscrew I beam that are real nice,

We have pretty good pricing on either. You can email me about them if you like.

Probably the biggest performance advantage to 6" rods is the pistons are lighter. In my 350 the pistons are about 40-50 grams lighter than 5.7 pistons. I'm using Eagle SIR rods that weight 583 grams, which is about the same as 5.7 rods. So the overall weight of the reciprocating assembly is lighter. Power gains are probably negligible, although there may be some. The engine should rev a bit quicker.

I had considered building mine with 6" rods but for a budget engine, there seems to be a whole lot greater selection of pistons available for 5.7" rod motors. I believe there is less blcok clearancing needed with the 5.7's also. I am using the Scat 7/16" capscrew rods that have been machined for cam clearance, and am using a regular base circle cam and have plenty of rod-cam clearance. I have also heard that the rod-cam clearances can get pretty tight with the longer rods, but don't know this for a fact.

Thanks for the responses. Everything you guys mentioned backs up what I've seen so far.

Originally posted by travis g:
I had considered building mine with 6" rods but for a budget engine, there seems to be a whole lot greater selection of pistons available for 5.7" rod motors. I believe there is less blcok clearancing needed with the 5.7's also. I am using the Scat 7/16" capscrew rods that have been machined for cam clearance, and am using a regular base circle cam and have plenty of rod-cam clearance. I have also heard that the rod-cam clearances can get pretty tight with the longer rods, but don't know this for a fact. ==
Travis,
The block clearancing is the same for both length rods if they are the same style as is the cam deal.
All you are doing is moving the pin up in the piston & the angles don't change enough to make any difference at the pin rod end & there are a ton of pistons for either rod ;)

I got 6" rods for my assembly. I figured if I'm buying a rotating assembly anyhow I'd go for the 6" rods.

It was after the fact that I found out that the pistons are lighter and it was so much easier for the machinist to internally balance it.

Originally posted by cjlandry:
I got 6" rods for my assembly. I figured if I'm buying a rotating assembly anyhow I'd go for the 6" rods.

It was after the fact that I found out that the pistons are lighter and it was so much easier for the machinist to internally balance it. =
The piston weight helps but the major deal is they can make the counterweights bigger on the crank before the bottom of the piston smacks them at BDC. ;)

The strange thing with my combo, Mike, is that I bought a Scat Crank for external balancing, along with an external balance damper and flexplate.

My machine shop found it easier to balance it internally using an internal balance damper and flexplate. I wonder if I got an internal balance crank in an external balance box.

Definitely worked out better for me.

Hi Chad,

Interesting, do you remember your total bobweight & the part number of the crank?
Did they have to add any weight at all?
It all depends on your parts. The 6" pistons are pretty lite, especially flat tops & those rods are pretty light on the top half, lighter than the 3/8 capscrew rod on the pin end even though the total is a bit higher.

This is just an assumtion but a 6" rod would raise compression since the piston's travel is closer to the head right????

The SCR doesn't change since the top of the piston at TDC remains the same. Ths distance from the pin to the top of the pistons is moved to compensate for the extra rod length.

Originally posted by Wolfplace:
Hi Chad,

Interesting, do you remember your total bobweight & the part number of the crank?
Did they have to add any weight at all?
It all depends on your parts. The 6" pistons are pretty lite, especially flat tops & those rods are pretty light on the top half, lighter than the 3/8 capscrew rod on the pin end even though the total is a bit higher. I didn't ask for the bobweight. Weight had to be removed from the crank counterweights for the balance, none had to be added. The pistons are dished to keep the CR streetable.

The crank isn't installed yet. I remember the "Scat" stamp on it. Would the part number be stamped somewhere on it as well? I'll check it when I bring it into my shop (it's in storage right now).

My Scat 9000 crank, Eagle 6" SIR rods and KB D-dish postons almost went internal. It was close, but external ended up being a bit easier. Would have needed to add malory metal to the crank to get internal balance and that gets expensive.

As far as rod length. Hard to say, we have three 383s and only mine has 6" rods. The other two are 5.7". Mine revs much quicker everywhere in the range(600rpm to 6500rpm) than the other two engines. The short rod engines do seem to have just a bit more right around idle though, I mean between 800 and 1500rpm. Over that, it is no contest, my motor makes much more torque. I wouldn't call them rules, but so far those are my observations.

That's my rotating assembly, Rad Racer, except I have the Scat rods instead of the SIR's.

Glad to hear that you're happy with the combo.

What heads and cam are you running?

I noticed that there are more low compression pistons available for the 5.7" rods. I think maybe because the decreased compression height with a 6" rod piston gives you less room to make a deep dish.

So if you have a small combustion chamber and need a big dish (like over 20cc) to get the CR you want for a blower motor, you will find more pistons for the 5.7" rod. If you already have a decent sized combustion chamber and can get the compression ratio you need with a fairly moderate dome then go with the 6" rod (I've seen a few piston selections with -18cc dish for 6" rods)

Just my opinion.

Chad,,,, & Jim because you started this post & are probably getting more info than you wanted, at least about balancing :D
The numbers will be 9- for cast & 4- for forged.
the next number is the main size like 350 or 400.
Next is stroke like 3750 & then rod length like 5700 or 6000.
Like 4-350-3750-5700 is a forged 350 main size 3.750 stroke for a 5.7 rod.
They have a new 7- series that are custom forged basicly any stroke.
If you used forged pistons it's very possible it was the 5.7 rod crank as the balance is real close to neutral with the 6" rod.
It could go either way depending on piston weight etc.
With KB's it has usually necessary to add weight in the ones I have done.
The newer 5.7 cranks are supposed to have more counterweight in them but I don't know this for fact.

Rad,
If your engine runs that much better I can guarantee it is because of something else you have done better with your particular combo & not because of rod length.
In theory, a short rod engine will make more torque all else being equal because it pushes harder on the crank sooner but again, what we are talking about is extremely small & if you change cam timing to suit the rod length you can pretty much build the same power & torque with either combo.
Again, in my opinion rod length is just not much of an issue in anything we are discussing here.
If it connects the crank & piston it will work.

Jim,
Something that hasn't been mentioned but you may want to consider is the 5.7 rod will allow you to move the rings down which for a blown engine gives you a more durable piston top in the event of any detonation.
On the other hand, the 6" rod tends to load the cylinder wall a little less because of the rod angle difference
See what I mean about arguing either case :rolleyes:
If it were my choice I would use the 6" rod.

Originally posted by Eric68:
I noticed that there are more low compression pistons available for the 5.7" rods. I think maybe because the decreased compression height with a 6" rod piston gives you less room to make a deep dish.

So if you have a small combustion chamber and need a big dish (like over 20cc) to get the CR you want for a blower motor, you will find more pistons for the 5.7" rod. If you already have a decent sized combustion chamber and can get the compression ratio you need with a fairly moderate dome then go with the 6" rod (I've seen a few piston selections with -18cc dish for 6" rods)

Just my opinion. Thanks again guys. Eric, you're right about trying to get the C.R. down on a 6" rod. I haven't picked out the new heads yet but I could easily go with a 76 cc chamber (vs. 64-67 cc) that would allow me to keep the C.R. down.

Decisions, decisions!!!

Cool, then. If you can get 72-76cc heads then don't even think about going with anything but a 6" rod . . . graemlins/beers.gif

Compromise....run a 5.850" rod! graemlins/thumbsup.gif
Seriously...it gets the ring placement up above the wrist pin.

I agree Wolfplace, I think it is that I have WP Sportsman II heads and they are both running stockers. ;)

For a street engine I wouldn't build another 6 incher, took too much work on the block to make it clear anyway.

I don't like the oil ring in my piston pin either. But, it never smokes.

I'm running the 383 in a 68 Nova with Sportsman II heads and an old Crane "3/4 Race cam"(lol) the 274/274 on a 106LSA. I really like the car now. Took some time with the carb(had to drill out the IFRs), but now I love it. graemlins/hurray.gif My insurance starts tomorrow! graemlins/hurray.gif