The Ford Barn

Mongo · 07-16-2018, 04:46 PM

What vacuum should a good Model A engine pull at say 40 miles per hour.
At Idle my engine pulls a steady 18. At 35 - 40 it drops to above 10.

michael a · 07-16-2018, 04:56 PM

Man pulls 20 in at Idle and the same at 40 miles per hour steady speed

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Mongo · 07-16-2018, 05:13 PM

Could this indicate a muffler restriction?

Kurt in NJ · 07-16-2018, 06:38 PM

were is the spark advance ---too retarded and vacuum will be low

Mongo · 07-16-2018, 10:48 PM

spark is fully advanced.

100IH · 07-17-2018, 09:13 AM

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Ignition advanced fully usually not good either. All things being correct, the lever can provide a timing that is more than what is safe. Try retarding the timing slowly at a constant speed and see if the vacuum increases, if it does, that's good. Find the best and use that or 1"Hg below the best when backing off (retarding) lever. this is for any speed. Loosen the clamp that holds the muffler down pipe so it has an intentional exhaust leak. This will relieve back pressure if the flow is restricted as a test. The vacuum test is good and might indicate restriction. Dropping the pipe is proof.

Bill Pursel · 07-17-2018, 11:25 AM

Look at these

Corley · 07-17-2018, 02:59 PM

Depends on several items, but mostly it is power output vs wind resistance. At around 40 mph, out are getting close to using most of your power just to push the car through the air. If you have some modifications that allow more power to be generated than the stock 40hp, then you would expect less vacuum loss at that speed. However, on Elvira, my mostly stock, but high compression head, model A, the vacuum drops off pretty quickly between 40mph and 50mph (and above). I made several measurements when installing the Honda vacuum advance distributor, and I was at 10 inches of vacuum at about 43-45mph. I was down to 6 inches, where the vacuum advance drops out on the Honda Distributor at around 50mph.

It all depends on how far your foot is into the carburator, which is based upon how much power your engine puts out, vs what the resistance to vehicle motion is. For a totally stock engine in no wind conditions, 10 inches at 40mph is not too far off based on my measurements.

07-16-2018, 04:46 PM	#1
Mongo Member Join Date: May 2010 Posts: 62	Engine Vaccum readings What vacuum should a good Model A engine pull at say 40 miles per hour. At Idle my engine pulls a steady 18. At 35 - 40 it drops to above 10.

07-16-2018, 04:56 PM	#2
michael a Senior Member Join Date: Feb 2015 Location: Lone Jack Missouri Posts: 381	Re: Engine Vaccum readings Man pulls 20 in at Idle and the same at 40 miles per hour steady speed Sent from my SM-J700T using Tapatalk

07-16-2018, 05:13 PM	#3
Mongo Member Join Date: May 2010 Posts: 62	Re: Engine Vaccum readings Could this indicate a muffler restriction?

07-16-2018, 06:38 PM	#4
Kurt in NJ Senior Member Join Date: May 2010 Location: on the Littlefield Posts: 6,159	Re: Engine Vaccum readings were is the spark advance ---too retarded and vacuum will be low

07-16-2018, 10:48 PM	#5
Mongo Member Join Date: May 2010 Posts: 62	Re: Engine Vaccum readings spark is fully advanced.

07-17-2018, 09:13 AM	#6
100IH Senior Member Join Date: Dec 2011 Location: SW Idaho Posts: 970	Re: Engine Vaccum readings Sponsored Links (Register now to hide all advertisements) Ignition advanced fully usually not good either. All things being correct, the lever can provide a timing that is more than what is safe. Try retarding the timing slowly at a constant speed and see if the vacuum increases, if it does, that's good. Find the best and use that or 1"Hg below the best when backing off (retarding) lever. this is for any speed. Loosen the clamp that holds the muffler down pipe so it has an intentional exhaust leak. This will relieve back pressure if the flow is restricted as a test. The vacuum test is good and might indicate restriction. Dropping the pipe is proof.

07-17-2018, 02:59 PM	#8
Corley Senior Member Join Date: Aug 2011 Location: Battle Ground WA Posts: 293	Re: Engine Vaccum readings Depends on several items, but mostly it is power output vs wind resistance. At around 40 mph, out are getting close to using most of your power just to push the car through the air. If you have some modifications that allow more power to be generated than the stock 40hp, then you would expect less vacuum loss at that speed. However, on Elvira, my mostly stock, but high compression head, model A, the vacuum drops off pretty quickly between 40mph and 50mph (and above). I made several measurements when installing the Honda vacuum advance distributor, and I was at 10 inches of vacuum at about 43-45mph. I was down to 6 inches, where the vacuum advance drops out on the Honda Distributor at around 50mph. It all depends on how far your foot is into the carburator, which is based upon how much power your engine puts out, vs what the resistance to vehicle motion is. For a totally stock engine in no wind conditions, 10 inches at 40mph is not too far off based on my measurements. __________________ Corley ----------------- Subscribed to the KISS principle!