Gromm Racing

Cool technical write up by Nick Norris on the custom crank and rods Callie’s designed for us that we used to build Matt Fields C8 corvette engine and several of our Ultra 4 engine combos.

Super grateful to have such amazing friends and awesome partnership with Heath , Nick and all the amazing people at Callies Performance Performance Products.

“ Gromm Racing LS Ultra Dirt Package
Crankshaft

The crankshaft we designed for Semir at Gromm Racing is based on our Ultra Dirt line of cranks.

These cranks utilize greatly reduced counterweight mass at the front and rear in an effort to reduce end-
to-end torsional flex. At the same time, we add strategic mass to either side of bearings 2 & 4 to improve loads in those critical areas.

The center counterweights are reduced to the minimum required to counteract the rod journal mass. All of these characteristics combine to reduce overall weight and moment of inertia, which means quicker acceleration.

Specific to Semir’s application, we wanted to take advantage of the lower weight and reduced bearing
speed of a 2.000” rod journal diameter, but with a longer 4.250” stroke. To accomplish this with
confidence, we chose to use a 1.825” NASCAR width rod journal. By reducing the rod width, we shorten
the bending area, and increase the rod pin arm cross section. Additionally, narrowing the journal also
adds thickness to the counterweights, which aids in balancing.

• Reduced overall weight
• Reduced counterweight mass on front and rear counterweights
• Reduced end-to-end torsional flex
• Lower inertia
• Improved bearing loads
• Increased rigidity of the rod journal structure
• Easy to balance

Connecting Rods

As part of a rotating system of components, the connecting rods we designed for Semir are also based
on our Ultra Dirt product line.

These rods are a tapered H-Beam design, which allows for lower wristpin end weight, which reduces
tensile loads at high RPMs. They use 7/16 ARP bolts made from L-19 material. They provide a 2X
clamping force margin of safety, and are reasonably priced when considering replacement at rebuild
intervals.

The Ultra dirt design features a tapered beam face that starts out thicker in the bearing housing area,
and gets thinner as you transition towards the wristpin end of the rod. This provides a more stable
housing and better section thickness in the thread and bolt spotface area. To offset the added weight of
the thicker big end, we have cut out portions of the sidewall thrust face.

Specific to Semir’s application, we’ve reduced the big end thickness to match the narrower NASCAR rod
journal width of the crankshaft. We also relocated the bearing tab slots to move the bearings away from
the crankshaft fillets to eliminate having to modify the bearings to clear the sidewall radius.

The rods are made to a unique center to center length to take advantage of readily available shelf pistons.

• Great strength to weight ratio
• Tapered H-Beam design
• Lower tensile loads at high RPMs
• Tapered thickness transition at bearing housing bore
• 7/16 L-19 ARP rod bolts
• Improved bearing housing roundness and clamping force “