In my opinion, I don’t think there is a mystery about the differences between OEM and aftermarket performance.
Here’s why.
It boils down to a balanced system design and vehicle integration – the OEMs have design control, the aftermarket does not. The OEMs can determine overall system strength/ durability and noise performance and roll down the specs for each of the driveline components accordingly. They’re responsible … or delegate responsibility to assemble each new component into various new sub assemblies with optimized processes for a given axle and driveshaft. The suspension design needs to be figured out and sourced. The transmission specs determined and sourced. Then it all has to play nice with each other under variable operating, manufacturing, and environmental conditions over a 10 to 15 year service life with no loss in performance and minimal service requirements within a given price point for each sub assembly.
Piece of cake. What do we do after lunch today?
On the oem side - regardless of which vehicle manufacturer it takes hundreds if not thousands of man hours to integrate a common gearset family into multiple vehicle platforms. Example, GM’s 8.5 10 bolt goes into pickups, SUVs and vans. It used to go into the B body, G body, 2nd gen F-body etc. Each has its unique needs to meet the performance requirements the vehicle designers want out of their vehicle. For economy of scale purposes they limit the number of ratios to make the number of combinations manageable to validate the total vehicle design for strength / durability / noise performance and fuel economy within the market window.
The aftermarket has to supply a general application gearset that has to go into off road trucks with big knobby tires, replacement gearsets for stock cars and trucks and high performance muscle cars. They have to make it easy for an unknown end user to assemble into components with unknown age and condition. They have to reverse engineer the specs that each OEM has created.
The GM 8.5 10 bolt has been in production from the early 1970’s and continues to this day with improvements as the 8.6 currently installed in half ton trucks, utilities and vans. 40 + years of applications on one axle size for one vehicle manufacturer. Now, GM had gear ratios available from a 2.41 to 4.56 using different carrier breaks. The aftermarket manufacturer needs to decide which of those OEM ratios will sell and which ones won’t and at what price point. They also need to decide what else to offer in the higher numeric ratios as the end customer re gears their truck / car for performance modifications and at what price point.
GM also has the 7.5, the 14bolt semi float (9.5), the 14 bolt full float plus the new stuff on the C5 / C6 / Holden cars / New Camaro, Etc to cover. GM isn’t the only vehicle manufacturer. There’s Ford, Chrysler, Toyota, Nissan, Suzuki cars, trucks and vans to consider.
Once the “what” to make is determined, now the “how” to make it begins.
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