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| Joined: | 03-14-2005 |
| Location: | Atlanta, Georgia USA |
| Posts: | 406 |
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As we all know, the 907 was really a prototype and there were a number of running changes that culminated in the extremely powerful and reliable 920 variant circa 1995.
The most critical change (IMHO) to the 9xx series oiling system was made circa 1987 when the HCI was introduced. Along with a number of changes (HTD timing belt, Nikasil liners, etc.) this marked a change from 360^ grooved bearings to 180^ grooved bearings and a cross-drilled forged steel crank.
At it's very core the 907 oiling problem begins with the main-bearing design which allows WAY too much side-leakage under pressure (groove is too wide and shallow). At temperature, it is impossible to maintain more than 50-60psi with stock bearings. Anyone understanding why the JH was retrofitted with an oil cooler???
When the mains can't hold pressure, the rods can't be fed enough oil at higher revs = boom! Utilizing a higher-volume pump and/or a dry-sump only throws more oil at the mains and makes them leak more.
Retrofitting to the HCI bearings are an obvious choice, but they are more expensive than JH bearings. Elsewhere in the forum I've posted my solution using Chrysler 318 bearings. I've used both the HCI bearings and the Chrysler bearings in a few cars including Ron's ITS car with great success.
Given the success I've had with changing over to 180^ bearings, I've since abandoned my dry-sump aspirations except for E-prod racing engines.