Hey guys I just got back from the machine shop and they had just finished balancing my rotating assenbly for my 383 and the bob weight was 1805 and I'm just wondiring if that's good or bad. What are some of you guys bob weights for forged 383s?
I have a Howards Tracksmart 4340 forged crank 3.75" stroke, 6" H beam rods and Mahle forged ft pistons & rings.

 

Not a perfect comparrison, but I just finished building one for a friend and the total bob weight was 1,745 with cast Scat 3.75 crank, forged 5.7" I beams, and mahle forged pistons.

 

Since bob weight is a function of the sum of the weights of the rod, piston, and all the other misc parts hanging off the journal, the comparison of bob weight used for balancing your crankshaft is meaningless, unless compared to an engine with identical parts. And even then, it would only prove that identical parts weigh the same.

 

So would this combo be safe to spin to 7000 rpm, the block has splayed billet caps and straps with ARP main studs?

 

RPM's of an engine is limited more by the valve train than the rotating assembly. Doesn't matter what your bob weight comes out to be as long as it is balanced properly (with some limitations).

Just FYI. All one piece rear main seal engines are designed from the factory to be externally balanced. Getting random parts put together to balance internally can be a large hassle sometimes.

 

Not it the old standard of a maximum of 4100 piston feet per minuteis still valid and I believe it is. At 7000 rpm, this combination will be travelling 4375 pfpm. At 6500 you'll be at a much safer 4063 pfpm. That's only true when using quality parts...like you appear to be using.

Some people believe you can push a forged crank to a higher pfpm speed and you probably can but I've no idea what that speed can be.

Remember, the longer the stroke, the less rigid the crankshaft. Be sure to use a high quality damper with this setup or you could be picking up pieces

If it were my setup, I'd probably set the rev limiter to 6300 and talk to the cam guys about building me a cam that can get me the power desired within the safe rpm range.

You calculate piston feet per minute by multiplying the stroke times 2 then times the maximum rpm and dividing everything by 12.

Good luck!

 

A given bob weight is neither good nor bad. It is simply a function of the weight of the rods, pistons, wrist pin, bearings, and rings for a single cylinder. Obviously, lighter parts are better if they are sufficiently strong so I guess lighter bob weight is "better". But usually people talk in terms of the weight of the components and bob weight per se is only of interest to the peson balancng the rotating assy.

The formula for a V-8 is: 100% of rotating weight (big rod ends and rod bearings)+ 50% of recipricating weight (pistons, rings, pins, locks and rod small end)

There is no reason from the poiint of view of the short block that you coudn't use a 7,000rpm redline.

Rich

 

87Aerocoupe-

I'd recommend you thoroughly research this subject before deciding to spin the engine to 7000rpm. You have two posts on here that I sincerely disagree with, both saying it's fine to spin it that high.

There are two things to keep in mind. One is that a longer stroke crank isn't as stiff and the other is that the piston assembly has to stand up to more than 8000G as it gets yanked to a stop and then pulled in the opposite direction.....7000 times per minute. That means a 400 gram piston has forces of more than 7000 pounds applied to it! Add that concern to a crank that could be putting serious harmonic vibrations into the system. Make sure the rod, piston pin and piston can take that...by doing the research.

I'm not going to say it won't work, but, in the absense of hard data on the components I'll recommend against it. Perhaps some of the other comments would apply to the stock stroke where 7,000 rpm is possible without exceeding 4100 pfpm.

Good luck!

 

There are a LOT of guys spinning 383s to 7000rpms and beyond.

I can only guess that you heard some info based on 1970 metalurgy because you are completely wrong.

 

The issue with 7,000rpm is that unless you have some monster heads, intake, etc. it won't flow enough air to make power up there. I don't know what you are buiiding, but most 383 LT1's are done by 6,500 anyway.

Rich

 

I'm not completely wrong, sir. If you read all the words, I said to make sure the components can handle it. If they can, there is no reason you can't spin to 7000rpm. I said do the research and be sure it can handle it. That doesn't sound completely wrong to me....

 

My comment still stands, metallurgy these days is such that even cheap chinese cranks are fine at 7000 unless there is a defect or machining issue.