How much compression will babbit take?

dynaflash_8

dynaflash_8

Member
So im thinking of going the route of a high comp head on my Bone stock A banger. Motor is completly stock internally, but seems to be in decent shape sans a noisy lifter thats gonna get adjusted. My question is, how much compression will the babbit bearings take? how much was stock on this motor?

Thanks,

Marty
 
Marty, this is a loaded question. Stock was about 4¼ but some babbitt that has been poured in modern times has difficulty even withstanding that amount of compression. On the other hand, many diesel engines back in the 20's & 30's had better that 15:1 compression and they were strictly babbitt, ...and survived. I have run (raced) as much as 10:1 on a 'Banger engine' with success too. Therefore the real question back to you is how good is your babbitt? If it was a good quality babbitt and properly poured, it will probably withstand all you can throw at it. If not, you'll soon get the chance to have it properly installed!!
 
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brent , do you have experience with inserts ? how much compression can i run on a daily driver ? .. thank you .. steve
 
It's not compression that will hurt babbitt, or inserts for that matter, it is detonation. My current Bonneville engine has the same block, crank and babbitted mains that I ran up there in the 70's, and ran it a lot at 1/4 mile drag races. It has gone 157 MPH on the salt and 116 was normal for it in the dragster, and turned it 6,000 RPM on a regular basis. This is a B block, C crank.
 
steve norcal said:
brent , do you have experience with inserts ? how much compression can i run on a daily driver ? .. thank you .. steve
Click to expand...

Ummmm, ....some. ;)

Jim said it all above as it is all about detonation. The other thing on a flathead is at some point you lose volumetric efficiency as the roof of the chamber is lowered to gain higher compression. It is a trade-off based on RPM, chamber design, and cam design. For example, which do you suppose you would make more power with? Two identically built short blocks yet one has 7½:1 compression that that has a volumetric efficiency of 81%, or a head that is 10:1 compression ratio yet has a volumtric efficiency of 65%? My point is what good does it do to have 10:1 compression if you can only load 65% of the cylinder with air & fuel before the piston squeazes it? The higher compression does nothing if there isn't anything there to "compress".
 
brent ,
i have noticed a big difference in chamber design @ the same compression ratio .
example ... 6-1 snyder .. 85-90# compression ... 5.9-1 brumfield ... 95-100# compression . im guessing better flow on the brumfield .............. steve
 
Jeff,
My wifes Vue has a Honda J35 series V6 running 10.5:1 compression and its rated to run on 87 octane. Its actually now one of my favorite engines, 250HP from a 3.5L v6 with good low rpm power.

I believe many engines out there are running just under 12:1 compression and some such as the direct injected VW TFSI 2.0 are running 10:1 and even 10.5:1 with boost.

Sorry... that was kind of off topic but there are some impressive engines out there once again. I'm sure the gas prices will kill them off again :(
 
steve norcal said:
brent ,
i have noticed a big difference in chamber design @ the same compression ratio . example ... 6-1 snyder .. 85-90# compression ... 5.9-1 brumfield ... 95-100# compression . im guessing better flow on the brumfield .............. steve
Click to expand...

1. The comparison must be done on exactly the same engine and configuration with that engine broken in for both tests.

2. Efficiency differences as Brent described will not be evident at cranking speeds such as doing a compression test so the difference is due to something else.

3. You cannot assume the compression ratio stated by the vendor or manufacturer are as stated. You will only know or be able to compare if you cc (measure) the combustion chambers.
 
Good babbitt was used in these vehicles originally which is basically what is used today.. Poor babbitt I don't think would last long regardless of compression ratio or combustion pressures.. These engines were still in the 'infancy' of engine design.. Now to answer your question; 4.2 to 1..
 
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