This forum has a lot of guys who might know about this type of engine so hopefully some of you can chime in here.

My parter and I run a GT-1 car. If you don't know that's a road race car that will have a post race weight of around 2980lbs. We currently have a high-effort SB2.2 engine in this car. The crank has a "283" main and 2.00 inch rods. With a Bryant crank, how small can we go before the lack of material in the rod throws becomes a problem? Most of what's out there these days is 1.880 or smaller. The 1.770 rod size is the most common.

We generally run about 65 minutes a weekend. This motor makes 800+hp and runs to 9500rpm

 

What is the stroke ?

 

Stroke is 3.268"

 

1.850" is cup size and 1.770" would still be ok, especially with a bryant crank.

Bret

 

That will surely be ok with the 1.880 journal.

We do plenty of 3.500" stroke cranks with 283 mains and 1.880" pins. One thing to keep in mind with the 1.770" pin is that by reducing journal size that small, it's possible you may be getting very close to the oil transfer passage. With your short stroke, it may not be an issue, but it can be with the 3.500" stroke. I'll have a chat with one of our engineers in the morning and let you know.

 

Thanks for the info, guys. How much is bearing life lessened with those tiny rod journals? It's just hard for me to imagine how a journal that small could stand the abuse of a road race car. Unlike cup cars that stay in a small rpm range, we operate from about 5,500 to about 9000 and there is a lot of engine braking under downshifts.

What bearing sizes are the cup guys using on the road courses? Maybe I should call our contact and ask that.

 

I wouldn't expect bearing life to be lessened. The smaller journal diameter reduces bearing speed, meaning that there is less bearing surface per revolution. Because of this, oiling requirements and temperature are somewhat reduced.

Besides the oil passage issue I mentioned in my previous post, you can also have issues with lightening holes. These are possible problems you might encounter if grinding your existing crank to the smaller 1.770" journal. They wouldn't be a problem with a new crank that was designed for the small pin diameter.

We're in the early stages of designing LS6 cranks with the 1.770" pin diameter for some DP engines.

 

Just for reference, if you had a 1.888 rod journal crank ground down to 1.770, would that also necessitate running a different set of rods with their journal sizes matched accordingly or could you get away with running thicker bearings to make up for the difference?

 

When you change the journal size that much you need to change the rods as well.

Bret