What length are the stock piston rods in a 1990 LB9? Are they 5.7"? If they are, what would happen if I put in 6.0" rods? Keeping the 305, of course. Is that possible? Will I run into any problems?

[This message has been edited by 90Iroc (edited March 05, 2001).]

 

Most likely you'd need different pistons with a higher wrist pin location.

 

Would this increase compression? Do I need the different pistons to keep from banging the rod on the cylinder wall or something?

 

yes. your rod is now .300 longer. you will need a different set of pistons. this affects what is called your compression height. you block will have a deck hieght. then you piston and rod compination make up you compresiion hieght then the volume of you chamber figures in for your compresion ratio.

 

you can swap to 6" rods and keep the same compression if you want.

either way you will need a very expensive set of custom pistons.

and they are 5.7" length rods.

 

Keith black makes pistons for a 305 and 6" rod, your choice of 10:1 or 10.4:1 compression. If I had to guess, they wouldnt be more than 400 for the set.

 

You can get those pistons from Keith Black Performace Pistons, part#KB153. These pistons are 10/1 cr with the stock heads and are made for 6'rods.

Hope this helps.

Steve

 

For the cost of the rods and pistons you should be able to get a good 350 long-block that will pump out more torque, more HP and more GRINS!
In other words don't waste your money on rods and pistons when you won't see much in the way of performance increase. Get a 350 or 383 and be happy!

 

Flash, I've been trying to find a 350 for a while now, no one's got an 87 or newer block that will work with the TPI. I found an 85 today, will that work? It came out of a 1/4 ton pickup. Can my TPI bolt onto it?

 

I agree with FlashGTA.

I cant figure out why you would change to 6" rods in your 305. It does not increase your displacement and barely makes a difference on performance.

If you cant find a 350, then buy a stroker kit for your 305, that is, a 3.75" crank along with new pistons, usually new rods are also included in the stroker kit. Going 30 over with the stroker crank you will end up at about 343 cubes (dont remember exact number).

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Thanks Doc, I didn't realize that displacement would not be changed. I've got a guy who wants to sell me a 350 from an '85 1/4 ton pickup truck. Will this block be okay to use? Also, what exactly does it mean for an engine to be a stroker?

 

that engine should be fine.

a stroker is an engine that uses a crankshaft with a longer stroke. The crank moves the piston down the bore further and displaces more cubic inches.

 

An 85 wont be a roller block, but if it is coming out of a truck, it is most likely a 4 bolt main..(most are)

 

A couple of more questions: How come the 6.0 rods won't increase displacement, and can I make that 85 block a roller cam block? It does in fact have four bolt mains.

 

Well, this is not easy to explain in words.

ODB has the short answer for you. The crankshaft has a "throw". The journal on the crank where the rod connects is not a the crank centerline, in fact it is out one half the stroke of the crank. For us the throw is 3.48"/2 or 1.74". The stroke is 3.48" for a Chevy 305 and 350 (same crank in both engines). When the crank/rod journal is in the up position for a specific cylinder, the piston is said to be at TDC (top dead center). When the crank is in the down position, the piston is at BDC (bottom dead center). It makes no difference how long the rod is. The piston will go up and down in the cylinder bore a distance equal to the stroke of the crankshaft.

Some side notes:
You determine the upper most position of the piston with respect to the crankshaft centerline by adding the crank throw plus the rod length plus the piston compression height. For example, 3.48"/2 + 5.7" + 1.56" which equals 9.00". With a deck height of 9.020", the piston will be 0.020" down in the bore when the piston is at TDC.

The compression height of a piston is the distance from the wrist pin centerline to the flat top portion of the piston.

I think the typical deck height of a SBC is 9.020"

The standard rod length is 5.7" for both the 305 and the 350.

Therefore, the 305 and 350 share the same stroke and rod length. They differ in the cylinder bore. The 305 bore is about 3.75", while the 350 bore is 4.001".

A stroker crank is a crank with a longer stroke, such as, the standard is 3.48" versus a stroker of 3.75". Here the throw of the stroker is 3.75"/2 or 1.875". When you install a stroker shown above, you must install new pistons having a smaller compression heigth. If you dont, the piston will strike the valves or heads at TDC. In fact, you reduce the piston compression heigth by exactly the same amount that you increase the crank throw, which in this case is 0.135". Therefore, your new pistons have a compression height of 1.425".

Putting a 3.75" stroker crank in a 350, will net you about 377 cubes in displacement. If you overbore the cylinder by 0.030", we get the magical 383 cubic inch motor.

Putting a 3.75" stroker crank in a 305, will give you about 329 cubes.

Well, did I make things better or worse. I'm not very good at explaintions.

 

I get it now, thanks! I thought that I would get an additional .3 inches of movement up an down the bore. But since I would have to have different pistons for the 6.0's, the new pistons accomodate for that don't they? I thought I could just stick in the 6.0's and everything would be fine. Even if it would work, I guess I'd be hitting the valves wouldn't I? Thanks doc, that was very informative! Now it's time to talk to dad and see what he will and won't let me do. Oh yeah, can I make that 85 block into a roller block?

 

You cant make a '85 block into an '87 block which uses hydraulic roller lifters, not enough metal in the '85 block for the machining required.

However, you can purchase roller lifters for any non-roller block. Crane and Comp Cams have these retrofit kits.

I highly recommend that you be patient and find an 87 or newer block.