Highest Reliable Compression Ratio
 

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?

Re: Highest Reliable Compression Ratio
 

I think two major points would be: How good is the bottom end now and how good are you at managing the spark advance lever. I prefer the original manual advance set up because you have total control of the spark setting in any situation. There is nothing wrong with quality babbitt if it is correctly poured, peened, and machined. That being said my present motor has inserts because I didn't have access to a suitable babbitt shop but I do know an excellent machinist. Not an uncommon situation today. :)

Re: Highest Reliable Compression Ratio
 

Flathead, how does that compare in price? Did you have to upgrade the crank so it could be crossdrilled, and a higher volume oil pump?

Re: Highest Reliable Compression Ratio
 

I have Snyders 6.1 with new studs, babbitts, stock oiling. No issues other than less advance is needed.

I have done about 5k per year for the last 2 years and would never go back to a stock head.

Re: Highest Reliable Compression Ratio
 

Stock Burlington crank and original oiling system. Don't know the price for a good babbitt job, so can't give comparison.

Re: Highest Reliable Compression Ratio
 

B Engine with Babbitt bearings running a 6.0 head in a car I have done a lot of touring in. No problems at all except watch the spark advance.

Chris W.

Re: Highest Reliable Compression Ratio
 

For what it is worth, I ran a little over 10 to 1 on a babbit engine for about 7000 miles. The bearings were still like new when I sold it. There were 2 hill climb meets on it besides the usual street driving. It had a Dan Price head and a 404A radius lifter cam.

Re: Highest Reliable Compression Ratio
 

I will be following this one as well as I am thinking about a 6:1 head. In my opinion 6:1 should not cause any issues at all but let's see the comments.

Ron

Re: Highest Reliable Compression Ratio
 

I have run 8,000 miles on an engine with babbitt bearings, stock oil system, with a 7:1 Thomas fin head. No issues, no knocks, no oil drips.

Re: Highest Reliable Compression Ratio
 

What about those of us who are running 8:1 compression? :eek: Don't we get to comment?? :D


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. :eek: 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.

Re: Highest Reliable Compression Ratio
 

Original Winfield red head,7 to 1 compression on 'hard' babbit. Babbit does not fail from compression it fails from torsional vibration of a stock crankshaft and improper timing. Any increase in compression reduces the engine tolerance for improper timing, I prefer running a mechanical advance distributor for consistent proper timing at all speeds. To combat torsional vibration I use a murray horn harmonic balancer and have a mitchell OD to keep the rpm down.

Herm Kohnke felt the torsional vibration would push the center main journal .001 off center at 2400 rpm. Counterbalancing helps. Folks hear about racing upgrades like cross drilling the crankshaft and fully pressurizing the engine and invest big money into doing it,not necessary for a street car..Fords stock system works fine,the hydrodynamic principle (crankshaft floating on a film of oil) maintained by gravity in the A engine is capable of sustained high rpm (2400)..

Re: Highest Reliable Compression Ratio
 

Thanks to all of you who have shared your experiences. Brent & Pete, I admire your hi-compression experiences.
Is there a stock-looking 7:1 head available?

Re: Highest Reliable Compression Ratio
 

Bob,

There is another aspect to the equation, the shape of the combustion chamber will have a large effect in the resistance to detonation, not just the CR.

Brett and Pete are spot on with their explanations. The Babbitt material and process of installation is key. FWIW, I do insert bearing installations in "B" motors and 4 out of 5 blocks will have inadequate lead based babbitt, just beat out.

John

Re: Highest Reliable Compression Ratio
 

Tod Buttermore makes one.

Re: Highest Reliable Compression Ratio
 

My personal opinion is that crypto-HC heads (heads that are HC but look stock) are OK but one needs to be extra-careful to document them. You won't own the car forever, and whomever you pass it on to will need to remember that their car will behave differently than one with the stock head. A 5.5 head can probably pass without documentation, but certainly once you get to 7 you should either mark the head visibly or keep good records.

That's one reason I like the Winfield head – it keeps the stock shape, but it says Winfield on it and the spark plug locations are different, so anyone buying the car will be able to identify that it's a 6 or 7 head without taking the head off.

Also I painted mine fire engine red, but that's just me.

Re: Highest Reliable Compression Ratio
 

There are some very good answers above! I ran a 7.5:1 Cyclone flathead for years with no problems. Included were several trips from SoCal to Las Vegas to visit Ed Winfield. The engine was a B/C, stock oiling, babbitt by an unknown guy but done at Trade Tech school in L.A., so of unknown quality. It is not compression, but detonation that will ruin babbitt. Something to think about, ... when 2 Model A's are traveling down the street together at the same speed, one totally stock, the other with high compression, dual carbs, etc., they are both using the same amount of power, so both have the same amount of pressure in the cylinders. At that point they are totally equal. Now if they both put the pedal to the metal, the non-stock A will put more pressure on the bearings and will pull away from the stocker. With oil as a cushion, the bearings will not be harmed.

Re: Highest Reliable Compression Ratio
 

Yes, ...before Tod, Larry Brumfield offered the Super Brumfield which was 7:1. I have two of them, and can send you photos of you like.

Re: Highest Reliable Compression Ratio
 

Re: Highest Reliable Compression Ratio
 

I am amazed with all the interest in high compression heads. How fast do you really have to go in a Model A? My Victoria has a touring cam and it idles lousy. With the fast steering and bias ply tires, my car is made for secondary roads doing less than 50 mph. One of the pleasures of the car is getting there eventually and enjoying a 1930s motoring experience safely. Ed

Re: Highest Reliable Compression Ratio
 

Hi Ed. In the typical Model-A, a high compression head really does not make you go any faster by itself, -it helps you maintain your speed when additional power is needed. Where it helps you go faster is up a hill, -or it allows you to make additional power to pull a faster rear end gear ratio.

A typical 'touring camshaft' should never make an engine idle lousy. The Stipe camshafts typically have more lift (generally a minimum of 0.030" more) than a typical reground 'Touring Cam'. The new Stipe cams including the IB340 and under all idle very nicely, -so I am believing you have other issues that are causing your poor quality idling.