Quote:
Originally Posted by Bob Bidonde
I want to hear from those who are running 6:1 and 7:1 compression heads on Model "A" and Model "B" engines with babbitt bearings, and stock oiling systems. Can 6:1 and 7:1 compression ratio heads be run reliably for cars that do a lot of touring? If any, what issues have you experienced?
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What about those of us who are running 8:1 compression?
Don't we get to comment??
When we stop to analyze why there would be a failure, lets begin by determining what would
not cause the failure;
- The stock Model-A block has a much larger surface area of the main & rod bearings when compared to other higher compression engines. Therefore it is not the size of the bearing that causes failures.
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.- We also know that era diesel engines used in truck and marine applications used cast (babbitt) bearings with great success. These were engines that had much higher compression ratios too. So we know it is not the higher compression ratios that cause bearing failures.
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.- The stock Model-A oiling system is very adequate at keeping a supply of oil to the Main and Rod bearings. The Main bearings are distributing each cylinder explosion over two pins, and the ability to gravity feed the Main bearings with sufficient oil exceeds what the bearing can use. The Rod bearing in a stock Model-A is pressure fed with oil that exceeds the amount it needs or can use. Therefore the oiling system is not the issue to reliability and longevity.
So why do some people experience failures whereas others do not? IMO there are several key factors that when they are followed, the longevity will rival or exceed that of using an insert bearing.
To begin with, the proper composition of bearing material must be used. Ford originally used a mixture of Tin, Copper, and Antimony, -with over 90% of that being Tin-based. With certainty, failures come when a Lead-based material is used instead. Lead was used by many rebuilders in the 50s - 70s because it was easier to obtain and much cheaper -however, its use also gave a bad rep. for Model-A cast bearings.
Next, the casting process is one that must be done correctly, -and thoroughly. A huge key to success for longevity that is often omitted is the peining and burnishing process. Without these two steps, the material is not as dense which leads to faster wear. Additionally, the burnishing process makes the bearing harder and during the burnishing process it momentarily becomes fluid where it conforms to the shape of the journal pin. How this burnishing is beneficial is because the load-bearing surface of the bearing is now greater which leads to slower wear.
The biggest reason why cast bearings fail is because the cast material gets hammered-out. When the clearances become excessive, it fails fast. For example, when you use a 5# hammer and hit a piece of solid copper wire, which will flatten it quicker? Lifting the hammer ¼" and striking the copper, -or lifting the hammer 1" and striking the copper. The same can be applied to loose bearings. The reason insert bearings seem to last longer is because the babbitt-like material is much thinner and has been placed onto the shell properly. When a block uses cast (babbitt) bearings installed correctly, they are equal in strength to the insert bearing, -and will actually give longer service life since there is a greater (thicker) amount of material.
Ohh, ...and regarding the controlling the timing, I have found this should not to be a huge deterrent for having higher compression ratios. If someone is concerned, then buy a higher octane fuel at the gas pump when refueling.
Then, there are many Model-A hobbyists that subscribe to the theory that if a little advance of the timing lever is good, then more advance must even be better.
Most Model-A engines are operated with more advance in the ignition timing than what is optimum. Operate it at an advanced timing level just under the point where detonation is found, and the cast bearings will be happy.