Compression Trivia

[Bob Bidonde]

Compression loading on rod and crankshaft bearings due to compression ratio of the cylinder head is not the design limit loading for babbitt bearings.

Raising the compression ratio significantly increases the combustion pressure loading on the bearings, so combustion pressure loading is the design loading condition of the engine.

In the case of the Model A & Model B engines, I calculate pressure loading during combustion to be in the neighborhood of 9000 to 10,000 lbs per square inch.

In addition to the pressure loading, the babbitts must endure inertial loads due to the motion of the reciprocating and rotating parts.

Connecting rod babbitts are the weak link.

[Jim Brierley]

What Bob says is true, however Babbitt will withstand a lot of loading, especially the mains. I have run a Bonneville lakester at 153 mph on a full-Babbitt B engine, B rods and all. With the same block, crank and Babbitt on the mains I have run 167, all still looks good. Replaced the rods a few years ago, before I started running the turbo.

[Juggler]

Originally Posted by Jim Brierley

What Bob says is true, however Babbitt will withstand a lot of loading, especially the mains. I have run a Bonneville lakester at 153 mph on a full-Babbitt B engine, B rods and all. With the same block, crank and Babbitt on the mains I have run 167, all still looks good. Replaced the rods a few years ago, before I started running the turbo.

What rpm do you need to be turning to hit 150+ Mr B?

[V4F]

10,000,000,000,000

[Synchro909]

Bob and Jim, While I respect your opinions and advice and I don't want to step on any toes, my experience is that the big ends rarely give trouble (so long as the motor is not over revved.) but it is the main bearings that give up first, especially the middle one.
When I do a long distance trip, I've given up carrying a set of rod inserts in my spares but still carry a set of mains. I've never needed them and I dread having to fit them "on the side of the road" but if I don't have them, I'll need them.
The figures given for bearing loads are fine but I think it should be noted for those who are not mechanically minded that the pressures go WAAAY UP if the ignition timing is too far advanced, thus leading to the their destruction.

[PC/SR]

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Some time back, with nothing else to do, I calculated that the A had more bearing surface per advertised horsepower than the 53 Chev stick shift, the last production auto with babbitt bearings. Do not recall the numbers, but the A had substantially more bearing surface per horse than the Chev.

[Art Bjornestad]

Crank whip is critical and Mr. B has paid attention to this.

[Bob Bidonde]

Synchro909,
No need to worry about my toes. I agree that the main bearings in the Model A can be problematic, but the higher bearing stresses are in the connecting rods. All of the main bearings in the engine are subjected eccentric loading from the crankshaft when it deflects, but the center main gets the worst of it because it reacts two segments of the crankshaft.

The Model B engine is more durable than the Model A engine because of its larger diameter main and rod bearings, its pressurized oiling system and I believe the Model B has a stiffer crankshaft.

Jim,
Your accomplishments are fantastic! The maximum speed I have ever done in my Coupe when it was running a souped-up Model B engine was 80 plus with the stock differential. It had more throttle left, but the Coupe's front suspension was scary.

[Jim Brierley]

I run 60 lbs. of oil pressure (less on my street engines), front and center main caps are heavy-duty steel, C crank and am turning right 6,000 RPM at the end of the 3 mile run. Some years back when drag racing the Cook, non-turbo, I turned it 6200 through the lights.

[Jim Brierley]

Another thing to think about ... compression ratio makes no difference when just cruising down the street. It takes the same amount of power to cruise a Model A at 45 (for instance) if it has 4.2:1 or 7:1 compression ratio, thus the bearing loading is the same. Body style has more effect than anything else. Just saying this as it seems some of us, like me, seem to have time on our hands to think up weird things!

[MikeK]

Originally Posted by Bob Bidonde

. . . In the case of the Model A & Model B engines, I calculate pressure loading during combustion to be in the neighborhood of 9000 to 10,000 lbs per square inch. . .

Bob, could you please post the algorithm you used to calculate this? My sorry engineering brain needs to know.

[johnneilson]

Originally Posted by MikeK

Bob, could you please post the algorithm you used to calculate this? My sorry engineering brain needs to know.

piston area 11.8" sq

if pressure is 10,000 psi at 90° crank angle ~=19,666 lb/ft torque.
(11.8*10,000/5.64)

Please share the calculations, John

BTW, I witnessed Mr. Brierley's 167mph run @ Bonneville.
From what I understand the Babbit in that motor is magic.

[Tom Wesenberg]

[QUOTE=johnneilson;1258987]piston area 11.8" sq

if pressure is 10,000 psi at 90° crank angle ~=19,666 lb/ft torque.
(11.8*10,000/5.64)

Please share the calculations, John

BTW, I witnessed Mr. Brierley's 167mph run @ Bonneville.
From what I understand the Babbit in that motor is magic.[/QUOTE]

Not magic, just a mystery because he must have used Marvel Mystery Oil.

[Jim Brierley]

No mystery oil, just good oil of current specs, usually 30 wt. when drag racing, switched to synthetic with the turbo as it can stand more heat.