Go Back   The Ford Barn > General Discussion > Model A (1928-31)

Sponsored Links (Register now to hide all advertisements)

Reply
 
Thread Tools Display Modes
Old 10-10-2019, 05:59 PM   #1
Mulletwagon
Senior Member
 
Mulletwagon's Avatar
 
Join Date: Jan 2019
Location: Florida Panhandle
Posts: 205
Default Max Compression Ratio

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 ?
Mulletwagon is offline   Reply With Quote
Old 10-10-2019, 06:17 PM   #2
Patrick L.
Senior Member
 
Patrick L.'s Avatar
 
Join Date: May 2010
Location: Upstate NY and western Florida
Posts: 5,188
Default Re: Max Compression Ratio

It seems I remember the limit for a flathead is about 8.
Patrick L. is offline   Reply With Quote
Sponsored Links (Register now to hide all advertisements)
Old 10-10-2019, 06:59 PM   #3
Dick Steinkamp
Senior Member
 
Dick Steinkamp's Avatar
 
Join Date: Apr 2016
Location: Bellingham, WA
Posts: 1,158
Default Re: Max Compression Ratio

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.
__________________
All steel from pedal to wheel
Dick Steinkamp is offline   Reply With Quote
Old 10-10-2019, 10:09 PM   #4
Railcarmover
BANNED
 
Join Date: Apr 2017
Location: Sacramento
Posts: 1,262
Default Re: Max Compression Ratio

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.
Railcarmover is offline   Reply With Quote
Old 10-10-2019, 10:49 PM   #5
Pete
Senior Member
 
Pete's Avatar
 
Join Date: May 2010
Location: Wa.
Posts: 3,505
Default Re: Max Compression Ratio

Quote:
Originally Posted by Mulletwagon View Post
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.
Pete is offline   Reply With Quote
Old 10-11-2019, 05:34 AM   #6
Patrick L.
Senior Member
 
Patrick L.'s Avatar
 
Join Date: May 2010
Location: Upstate NY and western Florida
Posts: 5,188
Default Re: Max Compression Ratio

Quote:
Originally Posted by Pete View Post
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.
Patrick L. is offline   Reply With Quote
Old 10-11-2019, 07:41 AM   #7
Bob Bidonde
Senior Member
 
Bob Bidonde's Avatar
 
Join Date: May 2010
Location: Long Island, NY
Posts: 1,531
Default Re: Max Compression Ratio

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?
Attached Images
File Type: jpg 190A Model B Engine Rebuild by Schwalm 555KB.jpg (64.7 KB, 132 views)
__________________
Bob Bidonde
Bob Bidonde is offline   Reply With Quote
Old 10-11-2019, 11:16 AM   #8
PC/SR
Senior Member
 
Join Date: May 2010
Location: Santa Rosa, CA
Posts: 1,149
Default Re: Max Compression Ratio

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.
PC/SR is offline   Reply With Quote
Old 10-11-2019, 11:43 AM   #9
Dodge
Senior Member
 
Join Date: Nov 2011
Posts: 1,173
Default Re: Max Compression Ratio

Mines been running fine, I think it’s close to 7-1, the compression gauge says 120 lbs.
Dodge is offline   Reply With Quote
Old 10-11-2019, 12:20 PM   #10
Jim Brierley
Senior Member
 
Join Date: Jun 2010
Location: Temecula, CA
Posts: 2,938
Default Re: Max Compression Ratio

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.
Jim Brierley is offline   Reply With Quote
Old 10-11-2019, 01:11 PM   #11
Ak Sourdough
Junior Member
 
Ak Sourdough's Avatar
 
Join Date: Jul 2014
Location: Fairbanks, Alaska
Posts: 11
Default Re: Max Compression Ratio

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.
Ak Sourdough is offline   Reply With Quote
Old 10-11-2019, 03:26 PM   #12
BRENT in 10-uh-C
Senior Member
 
BRENT in 10-uh-C's Avatar
 
Join Date: May 2010
Location: Eastern Tennessee
Posts: 9,017
Default Re: Max Compression Ratio

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 in 10-uh-C
.
www.model-a-ford.com
...(...Finally Updated!! )

.
BRENT in 10-uh-C is offline   Reply With Quote
Old 10-11-2019, 03:34 PM   #13
Patrick L.
Senior Member
 
Patrick L.'s Avatar
 
Join Date: May 2010
Location: Upstate NY and western Florida
Posts: 5,188
Default Re: Max Compression Ratio

Quote:
Originally Posted by Ak Sourdough View Post
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.
Patrick L. is offline   Reply With Quote
Old 10-11-2019, 04:19 PM   #14
Benson
Senior Member
 
Join Date: May 2010
Location: Central Highlands, Cen~Col.
Posts: 1,887
Default Re: Max Compression Ratio

...
Benson is offline   Reply With Quote
Old 10-11-2019, 05:57 PM   #15
Railcarmover
BANNED
 
Join Date: Apr 2017
Location: Sacramento
Posts: 1,262
Default Re: Max Compression Ratio

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
Railcarmover is offline   Reply With Quote
Old 10-11-2019, 06:00 PM   #16
40 Deluxe
Senior Member
 
Join Date: May 2010
Location: now Kuna, Idaho
Posts: 3,033
Default Re: Max Compression Ratio

Quote:
Originally Posted by BRENT in 10-uh-C View Post
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!
40 Deluxe is offline   Reply With Quote
Old 10-12-2019, 03:41 PM   #17
Railcarmover
BANNED
 
Join Date: Apr 2017
Location: Sacramento
Posts: 1,262
Default Re: Max Compression Ratio

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.
Railcarmover is offline   Reply With Quote
Old 10-12-2019, 06:07 PM   #18
old31
Senior Member
 
Join Date: Jan 2016
Posts: 984
Default Re: Max Compression Ratio

Rail, what would you recommend for a harmonic balancer?
old31 is offline   Reply With Quote
Old 10-12-2019, 08:13 PM   #19
Railcarmover
BANNED
 
Join Date: Apr 2017
Location: Sacramento
Posts: 1,262
Default Re: Max Compression Ratio

Im searching for one right now..
Railcarmover is offline   Reply With Quote
Old 10-12-2019, 09:09 PM   #20
Pete
Senior Member
 
Pete's Avatar
 
Join Date: May 2010
Location: Wa.
Posts: 3,505
Default Re: Max Compression Ratio

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.
Pete is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Sponsored Links (Register now to hide all advertisements)


All times are GMT -5. The time now is 12:57 PM.