Max Compression Ratio

[Mulletwagon]

Is there an accepted or practical maximum compression ratio for Model As beyond which will result in exceeding design limits of internal engine components and cooling/lubrication systems ?

[Patrick L.]

It seems I remember the limit for a flathead is about 8.

[Dick Steinkamp]

I don't know the answer, but I would think it would depend on babbitt vs inserts, counter weighted crank vs stock, lightened flywheel vs stock, cam grind (especially overlap), valve size, port work, intake and exhaust systems...probably other factors.

[Railcarmover]

Reducing the combustion chamber in size above about 8 to 1 begins to limit how well it can breathe,the chamber is too small. Nothing is really new in model a speed,just about everything available today existed 80 years ago..High compression heads,OHV heads,cams,you name it..all of it ran on dynamically balanced crankshafts with babbit bearings.Like anything else you have to do it correctly,stacking compression on top of a unknown lower end will buy you trouble,be it babbit or inserts,build from the bottom up,it will last.

[Pete]

Originally Posted by Mulletwagon

Is there an accepted or practical maximum compression ratio for Model As beyond which will result in exceeding design limits of internal engine components and cooling/lubrication systems ?

The PRACTICAL limit is usually determined by the octane of the gas you run.
The design limit of the parts and the cooling system are way beyond what they can be stressed by raising the compression alone. I have run 10 to 1 in street model B flathead engines. 14 to 1 flathead model B on alcohol.

It was common for overhead valve conversion model B race engines in the old days to run 15 to 1 on alcohol. That was with babbit bearings.

[Patrick L.]

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Originally Posted by Pete

The PRACTICAL limit is usually determined by the octane of the gas you run.
The design limit of the parts and the cooling system are way beyond what they can be stressed by raising the compression alone. I have run 10 to 1 in street model B flathead engines. 14 to 1 flathead model B on alcohol.

It was common for overhead valve conversion model B race engines in the old days to run 15 to 1 on alcohol. That was with babbit bearings.

WOW, learn something every day.

[Bob Bidonde]

Is there anyone out there running a 7:1 or higher compression flathead for general touring? If so, what is your experience re babbitt life?

[PC/SR]

I run 7-1 flathead for general driving and hill climbs on babbitt. No problems. The rod and main bearing journals are very wide for the engine size and have a lot of surface area. Lack of pressure oil is the limiting factor of the Model A, not babbitt. Conventional wisdom has it to not exceed 4000 rpm on the stock splash system (though some have). But for regular driving you do not need anywhere near 4K anyway.

[Dodge]

Mines been running fine, I think it’s close to 7-1, the compression gauge says 120 lbs.

[Jim Brierley]

Good answers by Railcar, Pete and PC/SR! My new head is 7:1 but has a raised area between the intake valve and the cylinder, to make higher ratios more viable. I ran a 7.5:1 Cyclone on the street for many years, on dipper-babbitt, no problems. My current Bonneville engine is still on full pressure babbitt mains, turbo-charged, 7.5:1 Cook OHV, on racing gas or alcohol, 6,000 RPM. Last run in '13, at 167 MPH. I would recommend 7:1 for most street use.

[Ak Sourdough]

Having raced Sprint Cars for 10 years in the 80's, my understanding of alcohol fuel is that it is very easy on the bottom end of an engine. Here's how it was explained to me and seems to be true.



Gasoline burns quickly and makes a big explosion at the top of the cylinder causing big shock loads to the pistons, rods, bearings and crankshaft. All the work and the heat is produced at the top of the cylinder and the combustion gas expands very rapidly, pushes down on the piston, then the explosion is over.



Alcohol burns very slowly in comparison and has a much less powerful explosion at the top of the cylinder when it starts to burn. The advantage comes from the slow burn. Rather than the big bang at the top of the cylinder it burns and expands the gasses all the way down the cylinder making a long powerful push on the piston instead of a jolt and thereby dramatically increases the total force applied to the piston and reduces the shock loading on all the related parts. By burning more slowly, it allows much more compression and more total timing advance than gasoline without causing pre-ignition or detonation. 14 or 15 to 1 compression and 38 to 40 degrees of timing was common in Sprint Car engines at the time.



There are a number of disadvantages too for street use.

[BRENT in 10-uh-C]

To muddy up the waters a bit, does anyone remember the diesel fuel conversion for a Model-A engine? As I recall, it was like 16:1 CR on cast bearings. It sure wasn't the compression ratio that was the downfall.

And, FWIW, several of the mid-50s build OHV conversions were 10:1 weren't they Jim? I had a single stock Gemsa that was at least that. As stated above, it is Volumetric Efficiency that is/was the issue with high compression flat heads. F-heads and OHV have no issue with higher compression.

[Patrick L.]

Originally Posted by Ak Sourdough

Having raced Sprint Cars for 10 years in the 80's, my understanding of alcohol fuel is that it is very easy on the bottom end of an engine. Here's how it was explained to me and seems to be true.



Gasoline burns quickly and makes a big explosion at the top of the cylinder causing big shock loads to the pistons, rods, bearings and crankshaft. All the work and the heat is produced at the top of the cylinder and the combustion gas expands very rapidly, pushes down on the piston, then the explosion is over.



Alcohol burns very slowly in comparison and has a much less powerful explosion at the top of the cylinder when it starts to burn. The advantage comes from the slow burn. Rather than the big bang at the top of the cylinder it burns and expands the gasses all the way down the cylinder making a long powerful push on the piston instead of a jolt and thereby dramatically increases the total force applied to the piston and reduces the shock loading on all the related parts. By burning more slowly, it allows much more compression and more total timing advance than gasoline without causing pre-ignition or detonation. 14 or 15 to 1 compression and 38 to 40 degrees of timing was common in Sprint Car engines at the time.



There are a number of disadvantages too for street use.

Yep, there are disadvantages in alcohol for any use. There are also some advantages.

[Benson]

...

[Railcarmover]

and to answer part of your question,raising compression raises efficiency.the engine will run cooler than stock

yep patrick l,alcohol has been helping ugly women for years

[40 Deluxe]

Originally Posted by BRENT in 10-uh-C

To muddy up the waters a bit, does anyone remember the diesel fuel conversion for a Model-A engine? As I recall, it was like 16:1 CR on cast bearings. It sure wasn't the compression ratio that was the downfall.

And, FWIW, several of the mid-50s build OHV conversions were 10:1 weren't they Jim? I had a single stock Gemsa that was at least that. As stated above, it is Volumetric Efficiency that is/was the issue with high compression flat heads. F-heads and OHV have no issue with higher compression.

Brent, can you tell us more about this Diesel conversion? When was it made? How many made? Did it work at all? Thanks!

[Railcarmover]

I found an article about it in old SOSS magazines..college kids set up an A with 18 to 1 compression and a scroll type fuel injection pump and nozzles as part of an engineering program. As I recall from reading the article the engine performed well. The square inch contact area of A main bearings exceeds a chevrolet 6 cylinder main bearing contact area.I understand why builders push inserts,they are fool proof,line bore and install,no risky tinning and peening,no temperature issues,perfect for business,reduce the comebacks.But the stories that babbit doesnt hold up or isn't as robust as inserts is just plain wrong.On a low rpm engine there is even an argument between counter balancing and dynamically balanced cranks as well.A decent harmonic balancer with a dynamic crank and a balanced rotating group should yield effective results at a fraction of the counterbalance costs.

[old31]

Rail, what would you recommend for a harmonic balancer?

[Railcarmover]

Im searching for one right now..

[Pete]

The one that will do the most good is the heaviest you can find and adapt.
This may work best but it is not always the best solution for people that do not want to modify the frame.
Most ones that are big enough to do some good require trimming the front crossmember.
The amount trimmed does not weaken the crossmember and done properly will hardly be noticable.

I have been using the stock 8 inch diameter one from a big block Chev. Easy to modify.

Once you have one installed and drive around for a half hour or so and then feel it, if it is warm it is working. If it is cold, you probably don't need one.

[Railcarmover]

What is the modification? Bore it out some?

[Pete]

Originally Posted by Railcarmover

What is the modification? Bore it out some?

There are a couple ways to do it.
Bore it and make a split sleeve.
Bore it bigger. Sleeve it. Broach a new keyway.

[Railcarmover]

Thanks Pete,going to give one a shot.A fella on here claims it helps with a cam thrust issue too.It makes sense,it should.

[Jim Brierley]

Brent, Some of the OHV's had very high compression, most did not. The last of the aluminum Riley 4-ports were 10:1, Cooks and other racing OHV's were only 6.5 or 7:1 high for the day but considered low today. The diesel conversion that I am aware of was built by a trade-school teacher in L.A. It was an F-head design, OHV on the intakes, exh still in the block. Yes it was very high compression, necessary to ignite diesel fuel. Story goes something happened to the B engine he was running, so he hurriedly built an A block for it for his honeymoon. It worked well for that trip. I once had a set of B rods that he had supposedly built for the diesel, they were double I-beam rods, 2 stock rods welded together, 4 bolts, extremely heavy. I thought too heavy for racing, so sold them to a friend. That friend raced them at Bonneville, and one broke! There was a reinforcing rod in the middle of them! Yes, we are still friends!

[Pete]

Originally Posted by Railcarmover

Thanks Pete,going to give one a shot.A fella on here claims it helps with a cam thrust issue too.It makes sense,it should.

Here is a pic of one. The cam thrust bolt is visible also.

[Bob Bidonde]

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Hi Jim,
Do you sell 7:1 heads?

[BRENT in 10-uh-C]

Originally Posted by Jim Brierley

Brent, Some of the OHV's had very high compression, most did not. The last of the aluminum Riley 4-ports were 10:1, Cooks and other racing OHV's were only 6.5 or 7:1 high for the day but considered low today. The diesel conversion that I am aware of was built by a trade-school teacher in L.A. It was an F-head design, OHV on the intakes, exh still in the block. Yes it was very high compression, necessary to ignite diesel fuel. Story goes something happened to the B engine he was running, so he hurriedly built an A block for it for his honeymoon. It worked well for that trip. I once had a set of B rods that he had supposedly built for the diesel, they were double I-beam rods, 2 stock rods welded together, 4 bolts, extremely heavy. I thought too heavy for racing, so sold them to a friend. That friend raced them at Bonneville, and one broke! There was a reinforcing rod in the middle of them! Yes, we are still friends!

I definitely agree with you Jim. Most I think were 'power adders' for AA or BB 4-cylinder trucks to help them keep up with 6 cylinder and V-8 powered trucks. These would not have needed much compression, -just better airflow.

[BRENT in 10-uh-C]

Originally Posted by Pete

There are a couple ways to do it.
.....Bore it and make a split sleeve.
.....Bore it bigger. Sleeve it.
.....Broach a new keyway.

...or just mount it inside the flywheel!

[Jim Brierley]

Bob, I do, that's the only ratio mine are. e-mail me at [email protected] and I'll send you details.

[Terry Burtz, Calif]

...or just mount it inside the flywheel!



Adding it to the flywheel does nothing to mitigate crankshaft torsional deflections!

The following is a simple explanation of what a harmonic balancer does and is from https://auto.howstuffworks.com/harmonic-balancer.htm

As the cylinders in your engine fire, they move up and down, generating torque that's transferred into the crankshaft. As you may already know, the crankshaft is what converts the engine's power into rotational movement, eventually turning the wheels of the car.

But consider for a second the forces that are acting on the crankshaft -- they're tremendous. Each time a cylinder fires, a force acts upon the crankshaft, causing it to twist. But this force also causes vibrations in the crankshaft, and at certain frequencies, the shaft can resonate, which makes the vibrations even worse [source: Mach V].

These vibrations from the engine can become too much for the crankshaft to bear, causing it to fail. And when that happens, your car won't run and you'll be facing some expensive repairs.

This is where the harmonic balancer comes in. The circular device, made of rubber and metal, is bolted at the front end of the crankshaft to help absorb vibrations. It's usually connected to the crank pulley, which drives accessories like the air conditioner. The rubber inside the pulley is what actually absorbs the vibrations and keeps them at a safe level. In essence, the device is designed to help prevent crankshaft failure. It's also sometimes called a "dampener."

A more technical explanation can be found at https://en.wikipedia.org/wiki/Harmonic_damper

[BRENT in 10-uh-C]

Originally Posted by Terry Burtz, Calif

...or just mount it inside the flywheel!



Adding it to the flywheel does nothing to mitigate crankshaft torsional deflections!

Terry, not according to an engine rebuilder from Texas who mounts them there and has argued to many that we don't know what we are talking about when we stated that wouldn't work!

Trust me when I say we have had that discussion here many times and Pete has been involved. (Hence the reason why I put the rolling on the floor emoji.)

[Terry Burtz, Calif]

Brent, mounting a torsional damper on the flywheel end of the crankshaft will make for a smoother running engine, however, it does nothing to minimize crankshaft torsional deflections, and it does nothing to minimize harmonics.

Look at any modern engine designed by engineers and please find one that has a torsional damper mounted on the flywheel end of the crankshaft instead of at the front of the crankshaft, and post your findings here for all to read.

[40 Deluxe]

Originally Posted by Terry Burtz, Calif

Brent, mounting a torsional damper on the flywheel end of the crankshaft will make for a smoother running engine, however, it does nothing to minimize crankshaft torsional deflections, and it does nothing to minimize harmonics.

Look at any modern engine designed by engineers and please find one that has a torsional damper mounted on the flywheel end of the crankshaft instead of at the front of the crankshaft, and post your findings here for all to read.

Just a side note on the "crankshaft torsional deflections" that Terry mentions here: These deflections are large enough to be easily measured. In fact, every modern vehicle has such a device. It's called a "crankshaft position sensor", and one of its functions is to detect any misfire. It measures this crankshaft deflection every power stroke. If the cylinder doesn't fire, there is no deflection and the ECM records this. After a certain number of misfires in a given time frame, the ECM sets a code and begins flashing the "check Engine" light. The code will indicate which cylinder is misfiring (P0301, P0305), etc.) unless it's a random, multiple cylinder misfire (P0300). Interestingly, in the early days of electronic engine control, driving on a washboard road would rattle the crankshaft enough to upset the ECM and it would set a misfire code!

[Pete]

There is an old saying:

Just because you personally don't believe, doesn't mean the laws of physics don't apply.

[BRENT in 10-uh-C]

Originally Posted by Terry Burtz, Calif

Brent, mounting a torsional damper on the flywheel end of the crankshaft will make for a smoother running engine, however, it does nothing to minimize crankshaft torsional deflections, and it does nothing to minimize harmonics.

Look at any modern engine designed by engineers and please find one that has a torsional damper mounted on the flywheel end of the crankshaft instead of at the front of the crankshaft, and post your findings here for all to read.

Terry, please go read what I am saying. My comment to Pete was said 'tongue in cheek'. You don't need to try to convince me that it didn't work as we hashed that out here many years ago.

[Railcarmover]

Originally Posted by 40 Deluxe

Just a side note on the "crankshaft torsional deflections" that Terry mentions here: These deflections are large enough to be easily measured. In fact, every modern vehicle has such a device. It's called a "crankshaft position sensor", and one of its functions is to detect any misfire. It measures this crankshaft deflection every power stroke. If the cylinder doesn't fire, there is no deflection and the ECM records this. After a certain number of misfires in a given time frame, the ECM sets a code and begins flashing the "check Engine" light. The code will indicate which cylinder is misfiring (P0301, P0305), etc.) unless it's a random, multiple cylinder misfire (P0300). Interestingly, in the early days of electronic engine control, driving on a washboard road would rattle the crankshaft enough to upset the ECM and it would set a misfire code!

No it doesnt,the crankshaft position sensor works on hall effect,the passing of a steel reluctor wheel or tone ring (it has teeth,not unlike a gear) above a magnetic pickup.As the crankshaft spins the teeth/space coupled with the magnet makes an impulse,the impulse is read by the ECM.

https://www.samarins.com/glossary/crank_sensor.html

[Railcarmover]

The closest thing to a 'harmonic balancer' flywheel is a flex plate/torque converter on an automatic transmission,it provides centrifugal and well as some fore and aft dampening.

[Pete]

Originally Posted by Railcarmover

The closest thing to a 'harmonic balancer' flywheel is a flex plate/torque converter on an automatic transmission,it provides centrifugal and well as some fore and aft dampening.

It is fluid inertia.

[40 Deluxe]

Originally Posted by Railcarmover

No it doesnt,the crankshaft position sensor works on hall effect,the passing of a steel reluctor wheel or tone ring (it has teeth,not unlike a gear) above a magnetic pickup.As the crankshaft spins the teeth/space coupled with the magnet makes an impulse,the impulse is read by the ECM.

https://www.samarins.com/glossary/crank_sensor.html

The cited article states: "The signal from the crankshaft position is also used to monitor if any of the cylinders misfires."
Other references state that what is monitored is the slight accelleration of the crankshaft at each firing. If this slight "kick" is not detected, it is recorded as a misfire. Another article mentioned that the system "has to be invulnerable to torsional oscillations" and other 'noise'. So I had things somewhat mixed up! A good reminder for me to check things out first.

[johnneilson]

ummmm, some sensors are hall effect, most are not, just magnetic.

crankshaft position sensor just reports pulses to ECU, and gap, usually at TDC.
if engine has only crank sensor, it fires plugs every revolution
if engine has cam position in addition to crank plugs fire in sequence

[Terry Burtz, Calif]

Brent, I'm happy that we agree.
My problem is that I don't understand 10-uh-C humor.

[BRENT in 10-uh-C]

Originally Posted by Terry Burtz, Calif

Brent, I'm happy that we agree.
My problem is that I don't understand 10-uh-C humor.

Ohh OK. Well typically when someone puts some type of funny symbol ( etc. ) beside their comment, it typically means they intend for their comment to be taken as humorous.

[Railcarmover]

Originally Posted by johnneilson

ummmm, some sensors are hall effect, most are not, just magnetic.

crankshaft position sensor just reports pulses to ECU, and gap, usually at TDC.
if engine has only crank sensor, it fires plugs every revolution
if engine has cam position in addition to crank plugs fire in sequence

The 'hall effect" is the name for a magnetic pulses impact on current flow.All magnetic switches work on this principal.The beauty of it is precision,you can accurately measure shaft position at any speed,its why electronic based ignition systems work so well.As an old fossil I dont care for 0 to 5v controlling my destiny,I prefer gears and breaker switches..as an old mechanic I love plugging in a box,pulling a code and changing a part,it gets you home faster

[johnneilson]

Railcar,

close, but no. Hall effect sensors are amplified to increase the magnetic inductive signal up to 5v. These require 3 wires, one for powering the amp.
Most position sensors are 2 wire, magnetic inductive type only.

BTW, there are several cam position sensors that are optical LED.

John

[Railcarmover]

Either can bus or magnetic pulse both use the hall effect. I have alot of respect for your knowledge base.Hell the reason we are here is to avoid the temple of the screaming electron,not argue about it.The merits of the computer on the design and function of the internal combustion engine is undisputable,that doesnt mean I have to accept it..