Vacuum advance or not ?
- Thread starter elwood
- Start date
rotorwrench
Well-known member
One thing that makes the vacuum can use more complicated on a flathead is that the flathead has different manifold pressures than an OHV engine. Some cans are adjustable in the vacuum port entry to the can so that the amount of advance movement can be adjusted. It take trial and error to ajust one for manifold vacuum source. Most source from the venturi on the carb but multiple carbs make this almost impossible. I'm sure a few folks on here can lead a person in the right direction about where to connect your source and how to adjust the can. Every different combination of carbs, manifold designs, etc., will require different adjustments.
Ol' Ron
Well-known member
I would think alittle common sense would set in about now. For max power you need approximately 20 degs of advance at 2000 rpm. JWL ran dozens of WOT dyno tests and found tjis to be the optimum. Agree, not all engines are the same so this can vary some what. For thse that have taken advantage of higher compression and a tighter chamber, this number becomes critical. Most street engines have increased displacement, higher CR and a longer duration camshaft create a different requirement for the ignition system. Because of this modern engines have computes that regulate the fuel flow and ignition advance depending on engine load.
We don't have that luxury, we're stuck with a distributor and a carburetor.
Richard has mad an engine monitor that displays the RPM, timing, vac,and AF. This is recorded for future observation. Over the past few years we've given thin information to everyone interested and shared here on the Barn. Now most of you foks don;t have this kind of instrumentation, so we make it easy for you to do this with just a timing light and a tachometer . The following is a simple way to set up your engine.
Now some of you may be running a ot cam like the L-100 and the vac at idle will be jumping around abit. Install a plug in the vac line with a small hole in ti like .060" this will tame down the effect of the cam.
I probably missed something but some one will probly jump on me, like Richard.
1. Install the modified SBC dist with adj Vac adv. do not connect vac
2. run engine up in rpm and using timing light, determine the amt of advance.
3. If more than 22 degrees send back for adjustment to 20 degs.
4 set dist at idle to 2 degs
Run up in RPN and check for max adv and rpm. If over 2200 rpm send back to mfg.
5. while the eng is idling, add vac to canister. timing will adv the amt set in the canister. You can adj this with the Allen wrench supplied, start with 6 degs.
In my case the finished timing was machanical advance 16@2000
Vacuum advance 8 degrees,
Have fun
We don't have that luxury, we're stuck with a distributor and a carburetor.
Richard has mad an engine monitor that displays the RPM, timing, vac,and AF. This is recorded for future observation. Over the past few years we've given thin information to everyone interested and shared here on the Barn. Now most of you foks don;t have this kind of instrumentation, so we make it easy for you to do this with just a timing light and a tachometer . The following is a simple way to set up your engine.
Now some of you may be running a ot cam like the L-100 and the vac at idle will be jumping around abit. Install a plug in the vac line with a small hole in ti like .060" this will tame down the effect of the cam.
I probably missed something but some one will probly jump on me, like Richard.
1. Install the modified SBC dist with adj Vac adv. do not connect vac
2. run engine up in rpm and using timing light, determine the amt of advance.
3. If more than 22 degrees send back for adjustment to 20 degs.
4 set dist at idle to 2 degs
Run up in RPN and check for max adv and rpm. If over 2200 rpm send back to mfg.
5. while the eng is idling, add vac to canister. timing will adv the amt set in the canister. You can adj this with the Allen wrench supplied, start with 6 degs.
In my case the finished timing was machanical advance 16@2000
Vacuum advance 8 degrees,
Have fun
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BUBBAS IGNITION
Well-known member
Theres no problem with using the vacuum on our distributors, i am just saying it will be hard to tune and get correct. Most just dont have the patience ..
Richard pulled some pretty good mpg by messing with the distributor and the vac can. (hes been on the project for a couple years)
Richard in Florida
Well-known member
He can always count on that!
If you're using a converted GM distributor and you don't have a distributor machine (who does?) or even a simple static setup fixture (I'll gladly share my design with you), then you have to use the engine to arrive at the desired adjustment. Fortunately, you can make all the changes you might need without pulling out the distributor.
I go about doing with the engine a bit differently than does Ol' Ron, but he taught me the basics. It's important to say here that THIS IS JUST AN EXAMPLE AND IS NOT A CUT AND PASTE FORMULA FOR ALL FLATHEADS. However, this works really well for us.
For this example the goal will be a distributor set up to produce 20 crankshaft degrees of mechanical advance which will all be in by 2000 crank rpm. The vacuum advance is like icing and comes later.
1. If you can, warm up the engine before pulling out the old distributor, assuming you're making a change.
2. Install the GM conversion with the rotor aimed the same as the old distributor's. Leave the vacuum hose off the distributor, but plug it.
3. Before you install the rotor and cap, remove the springs. Without the springs pulling on them, the weights will be allowed to swing out to full advance at idle or low rpm.
4. Get the engine started and while at fast idle use your timing light to set the engine advance to 20 degrees BTDC. Stop the engine.
5. Put the springs back in and start the engine again. Set the idle rpm where you want it and see what the advance is. It'll probably be between maybe 2 and 6 degrees BTDC. The light springs allow some mechanical advance even at idle.
6. With your timing light, watch the timing as you or a helper slowly raises the rpm. You're watching to see at what rpm those 20 degrees of mechanical advance are all in.
If it's all in between 1800 and 2000 crank rpm, you're done. If it's not, then you'll have to play with different springs and spring combinations to get what you want.
As for vacuum advance, the icing, we use a B33 adjustable canister and set it for 8 degrees of advance. This advance gets added to the mechanical advance ONLY during periods of high vacuum as someone stated above. That means when the engine's not working hard as when at idle or easy cruise.
So at idle you may see 10 to 14 degrees BTDC, and this is fine. Nice smooth cool idle.
When you connect the vacuum hose, you may have to reset your idle speed.
At easy cruise it'll be running at about 28 degrees BTDC. Good all around.
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Smart distributor people: If you are a person without a machine, is it prudent to start out with the medium advance springs as you begin your on engine analysis? Or, do you start with the lightest and go from there? Heck, just cut to the chase, which set of springs are you guys using?
Also, I drilled the plate as Richard showed us long ago in a thread to put in the set screw stop. That is when I discovered on one of the distributor cores the little rubber (?) collar that goes on the pin as a limiter. So, my question is, do you guys start with that set screw set to approximate the thickness of that collar initially?
For my un-scientific testimony, my Bubba chevy distributor with a Petronix has worked flawlessly over the last couple of years but my mileage sucked (about 11) and the plugs were sooty. I leaned up the 48 on the 8BA to Uncle Max's recommendations and I don't have the actual numbers yet, but I can tell that the mpg has improved dramatically. The plugs are tan now. I guess I had plenty of spark so the truck ran well even as rich as it was. The only notable part was pinging when it was under load like pulling up long hills at speed. I want to add the adjustable can as the next improvement experiment.
Also, I drilled the plate as Richard showed us long ago in a thread to put in the set screw stop. That is when I discovered on one of the distributor cores the little rubber (?) collar that goes on the pin as a limiter. So, my question is, do you guys start with that set screw set to approximate the thickness of that collar initially?
For my un-scientific testimony, my Bubba chevy distributor with a Petronix has worked flawlessly over the last couple of years but my mileage sucked (about 11) and the plugs were sooty. I leaned up the 48 on the 8BA to Uncle Max's recommendations and I don't have the actual numbers yet, but I can tell that the mpg has improved dramatically. The plugs are tan now. I guess I had plenty of spark so the truck ran well even as rich as it was. The only notable part was pinging when it was under load like pulling up long hills at speed. I want to add the adjustable can as the next improvement experiment.
Richard in Florida
Well-known member
We've found that you want ALL of the mechanical advance to be in by 2000 rpm tops, so we start with light springs... and usually stay with them.
Here's where a simple degree setting fixture can really save you time and aggravation.
I use the fixture to set the range of mechanical advance to 20 degrees. Without a fixture, you'll have to use the engine as I described.
Charlie ny
BANNED
Richard,
Great job.
As a double check I mark the crank pulley by measuring it's circumference ,
say 17 1/2 inches, and divide that number by 16 which gives the length of 22 1/2
degrees on that particular crank pulley. 17 1/2 " circ / 16 = 1.1 ". So I measure 1.1"
from the zero TDC existing mark on the pulley and scribe a line. Note here 360
degrees dived by 16 is 22 1/2 degrees which is close enough to the theroetical
20 degrees.
Charlie ny
Great job.
As a double check I mark the crank pulley by measuring it's circumference ,
say 17 1/2 inches, and divide that number by 16 which gives the length of 22 1/2
degrees on that particular crank pulley. 17 1/2 " circ / 16 = 1.1 ". So I measure 1.1"
from the zero TDC existing mark on the pulley and scribe a line. Note here 360
degrees dived by 16 is 22 1/2 degrees which is close enough to the theroetical
20 degrees.
Charlie ny
Richard in Florida
Well-known member
Here's another approach. If the circumference is 17.5", then dividing by 360 gives you 0.0486111 (I did it in my head.) That's one degree. So 20 degrees is 20 times that or 0.972222".
Richard in Florida
Well-known member
Charlie alludes to a very important idea. Unless you have an ACCURATE TDC mark on your pulley, all the calculations and distributor setting in the world doesn't mean squat.
Richard, very cool jig. Darn, something else I will probably try to copy. A bit OT but something Richard and Bubba probably have is some jig they use for center drilling the shaft for new pins in locating the flathead drive gear on the Chevy distributor. Been looking here on line for about an hour and all I can find is a kit offered by Hemingway in England to build a cross drill jig. Really?! Can anyone point me to something off the shelf offered domestically? Of course, I don't want to spend an arm and a leg.
Ol' Ron
Well-known member
Ain't these fFathead great, they'll run no mater what you do to them. We replace the rubber bushing on the distributor with a metal one. Richard has the dimension. You can get then in a tune up kit for the dist from Summit. I think along with some springs Now the springs I use are the light ones, this brings the advance very quick. this might make it ping while driving around town at hi vac. I can't tell you what to do, but it just means you have too much advance and you'll just learn as you go. The timing light is your best friend here.
Richard in Florida
Well-known member
Photo 1: I put the gear by itself on the bit in the drill press and capture it in the vise. I secure the vise to the drill press with clamps
Photo 2: Next I slip the end of the shaft installed in the housing into the captured gear, supporting the big end to keep everything level.
Photo 3: Then I drill the hole for the roll pin. Works like a charm.
Attachments
Wow, what a concept! You are using the gear as your jig, if I read this right. The shavings on the bit are there to fool me, right? You don't re-drill the holes in the gear or do you? 1/8" spring pin?
So, from Ol'Ron I learn you used to work on airplanes and I used to fly 'em. Definitely will have to plan a trip with the flathead to visit now. Thanks for the pics.
So, from Ol'Ron I learn you used to work on airplanes and I used to fly 'em. Definitely will have to plan a trip with the flathead to visit now. Thanks for the pics.
51 MERC-CT
Well-known member
I wonder what happened to the plane??
Richard in Florida
Well-known member
Yes, that was my last flying toy. It was based on the venerable Rand-Robinson KR-1, an everyman's airplane of the '70s that you built from plans and raw materials. Ken was one of the pioneers of spruce and foam and epoxy construction.
My airplane, however, was HIGHLY modified and didn't much resemble the KR-1. It was a sort of cross between a Spitfire and a Mustang. The markings on it were those of Douglas Bader, the famous legless RAF fighter pilot.
It had a sliding canopy, trim on elevators, rudder, and ailerons, a sidearm control stick... the list was long.
The engine was BASED on the VW, but there was little about it that was stock. It had a wild Erson cam, 2180 cc, electronic ignition, and so forth.
It would do 165 mph straight and level, and I had it in dives of 240 and more without a hint of flutter or vibration.
Government regulations, outrageous hangar rents, and waning interest prompted me to sell it.
The guy who bought it ran it out of fuel the next day and totally it in some farmer's field. He walked away unhurt, but the little airplane was completely destroyed. Six and half years of work down the tubes.