Timing gear considerations on 1978 Ford 460

[evranch]

Hi everyone,

Been working on a 1978 F350 conversion for a farm truck. Running a single Impco 425 over a governed mechanical 4-barrel Holley plate. The whole setup was pulled from a bus with a 390. Seems to have enough flow for what I want the truck to do. The mechanical secondaries are not delayed but come in right away. I have not adjusted the power valve from how it was set on the bus, but have adjusted the idle mixture to get the highest idle.

I'm a little confused with the timing on the 460 though. This is supposedly an emissions 460 by the year and as the factory timing sticker says 12 degrees BTDC. However, it has no EGR and is a non-catalyst setup (no smog pumps etc.) as it probably was exempt as a farm vehicle. EGR was not deleted as it turns out the manifold is not drilled for it. I'm a little concerned about not having EGR to cool combustion and preserve my exhaust valves as I was hoping to re-enable it.

So from what I read about these motors, the 12 degrees is actually more like 8 due to the cam on these motors being retarded for some emissions thing. Apparently earlier 460s ran at 8 degrees?
I'm wondering where I should actually look for my ~14 degrees of initial advance. 14 actual degrees, or 18 degrees to compensate for the timing gears (12 - 8 + 14)?

Running at the stock 12 degrees the truck starts easily, pulls hard and sounds great, but in initial testing is going through fuel at a high rate. By that of course I mean even more than a 460 usually goes through fuel

With vacuum advance in it idles around 35 degrees BTDC. Vacuum was not ported originally and no ported vacuum source is available on the carb/mixer setup. Everything but the carb is currently stock including the Duraspark distributor. Thanks for any help!

[Tom68]

You don't have anywhere near enough low RPM ignition advance for an LPG engine, it'll run hot and burn gas like it's going out of fashion. Have you got vac advance connected ?

[storm]

I know it's sometimes difficult, because I have done it myself, but don't confuse cam timing and ignition (crank) timing.

I, personally, would keep the standard timing specs for the engine (year and specification) and then adjust it over time for best vacuum at idle and best economy at cruise speed. If you can fit an adjustable vacuum advance you can adjust it for best economy.

[evranch]

Yes, vacuum advance is connected straight to manifold, no porting or VDV. I believe duraspark has an adjustable vacuum can but I haven't touched it yet. I'm also considering swapping distributor weights from the bus as it was likely recurved for propane.

I don't have a ton of Ford experience and have been more of a Chevy guy for years so that's why I'm a little confused here.

The performance guys say if I swap the timing gears for the "straight up" 1972 gears for more low end torque and economy, I should change my initial timing to 8 degrees for the same performance.

Since changing cam timing doesn't move the crank, the best explanation to me is the '0' mark on the flywheel is not actually TDC with this set of timing gears and the factory "12 degrees" is actually 8 BTDC. Which would explain the excessive heat and fuel consumption.

[storm]

There is no easy way to say this, again I know because I made the same mistake (it was even in a discussion on this forum), DON'T confuse cam degrees with timing (crank) degrees. If you swap the cam gear you are STILL timing by the crank and shouldn't change the way you set timing. Sure set it at a different degree but don't change it because the cam timing is now different.

Crank and cam spin at different speeds so you wanting to add or subtract 8 cam degrees from the crank timing doesn't take this simple (and this is where I got stuck) fact into account. Through scientific testing it has been found the best timing is when the highest pressure is applied at 16 ATDC. Remember pressure builds up as the flame front expands so if you set the base timing to 14 BTDC at idle and then adjust it for best vacuum and you end up at say 20 BTDC (it's just an example) you now know it is taking 36 degrees for the flame front to build maximum pressure.

Use your timing light and set the BASE timing (no vacuum advance), set the idle mixture for 15.5 AFR (or 1.0 Lambda), check your timing again (just incase your rpm changes). Reconnect the vacuum advance check the timing and write it down. Drive the vehicle with a vacuum gauge attached and see how it feels.

Adjust your BASE timing to where you are happy with it. It may take a few days for you to be happy with it and you may find when seasons change you want to change it lower due to slow cranking. When you are happy with it set it and leave it alone.

Now adjust the timing for best vacuum per degrees at idle (no vacuum advance), there is a point where you get diminishing returns and the vacuum doesn't increase much over multiple degrees so it is good practice to stop increasing when you get to this point. This is the difference between BASE timing and what you want when your vacuum advance is connected. Reset the timing to BASE timing and adjust the vacuum advance canister to get your idle timing with vacuum advance connected to this new timing. DON'T change your BASE timing anymore because manufacturers set the base timing to stop things like dieseling/run on when engine is turned off. Increasing BASE timing also makes an engine and starter system work harder to start the engine.

After you have done all that set your timing curve for best performance/economy (without detonation).

[evranch]

OK, I get it, instead of thinking in circles about how the cam timing affects the camshaft-driven distributor and what they did to it, I'll just use a piston stop to confirm TDC on #1 and make sure it lines up with the '0' timing mark.

If it turns out that Ford has done something strange and it doesn't line up, then I'll set my initial base timing to 14 degrees from that mark and not worry about the cam timing at all. It just seems odd to me to be chewing so much fuel (about 5 MPG imperial) and throwing so much heat if the stock timing is only 2 degrees off from the usual starting point of 14 BTDC, so that's why I'm wondering if the timing marks are off from the actual crank timing.

Seems strange that they would change the factory base timing from 8 to 12 degrees with no other changes in the engine unless it is a fudge factor (8 cam degrees of retard = 4 crank degrees of advance needed to get back to the same point from TDC if everything else remains the same)... OK, I'm wasting my time thinking about this again...

I like your method of determining the amount of vacuum advance to run by twisting the distributor and measuring the vacuum rather than adjusting the vacuum advance can every time. That will save a lot of time messing with the advance can adjustment since I only have to set it once after determining the maximum advance at idle. I'll give it a try once it cools off and my shop is bearable, there is a heat wave going on and that motor is way too hot to be leaning over today!

[evranch]

Alright! The piston stop tells me the '0' mark is accurate on the flywheel. That puts the mystery to rest, I guess the cam timing is retarded and nothing else. Someday I'll have to advance it to get more torque and economy.

I did find out that somehow the spark timing slipped to under 10 degrees when I installed the LPG setup, even though the distributor was so tight I had to soak and bang on it when I bought it. Explains a lot about the performance. Oh well, I reset it at the proper timing speed and now have a decent starting point.

So now I have:
- base timing 15 BTDC
- mech advance, all in before 3000, 14 degrees
- vac advance 15 degrees

Turns out the throttle plate vacuum source beside the PCV port is ported vacuum, where it usually is manifold on a gas carb. Maybe because it was part of an LPG setup? So that's convenient.

All in with high RPM and low throttle opening is ~42-45 degrees

Mileage has improved to 7.2 imperial MPG (almost 50% better!). This is... decent for an old 1-ton grain truck with a big block. But I'd still like to make 10MPG if I can. Power is decreased a little but the truck still has plenty to spare, and economy is more important than maximum power. No knock, and it looks like a lot of guys run 15 degrees with gasoline anyways without issues.

All my previous propane conversions have been governed engines so I'm learning more about tuning throughout the powerband now.
I'm more concerned about knock than I was on my previous conversions. You would recommend trying to increase base advance until I see the maximum vacuum at idle, and then road testing that way without adding any retard for safety?

[storm]

Ok let's clarify a couple of terms here so we are all on the same page with regards to what is happening.
Base timing = timing setting that allows the engine to start easily.
Idle timing = timing setting that enables the engine to idle efficiently.
Timing curve = timing settings at different loads and/or rpms.
Mechanical Advance (MA) = the timing advance curve set by the advance weights and springs in the distributor.
Vacuum Advance (VA) = the timing advance curve set by the effect of engine vacuum on the vacuum advance canister on the distributor.

Now when setting Idle timing it is possible, and if using manifold vacuum highly likely, for the VA to have an effect. It is also possible if light springs are used on the MA for MA to have an effect on Idle timing. Neither of these are a bad thing but you need to be aware if they are having an effect or not.

I set timing to the maximum vacuum achieved before diminishing returns occur. for example on the imaginary table below I would set idle timing to 14 degrees. Why? because adding more timing for very little return can load the starter motor on cranking and also push the engine to the point of detonation on tip in (which shouldn't really happen with LPG but everything is a compromise).
timing vacuum
10 10
11 12
12 14
13 16
14 18
15 18.1
16 18.2
17 18.3
18 18.4

For the rest of the advance curve I would set MA (without VA attached) to achieve maximum "trap speed" and acceleration (dataloggers are a brilliant tool for this). For cruising (VA) set the timing with VA attached for maximum cruising vacuum at each load point and check for detonation as you go.

[evranch]

Right, when I was talking about trying to get the most vacuum at idle I was referring to idle RPM ("Timing speed") with the vacuum advance disconnected.
So to match your list of terms, that would be trying to set the base timing to get the maximum vacuum.

For a test I brought my idle speed way down to confirm that at timing speed there is no contribution from the mechanical advance. There is no change as I slow the motor down. So at timing speed, 650RPM, with the vacuum advance disconnected, I have only the base timing active which is currently 15 BTDC.

So what you are saying is to advance my base timing, adjusting my idle stop screw as needed to maintain timing speed, until I get diminishing vacuum returns from adding more advance.

Assuming I make it as far as 20 BTDC, then once I add my 15 degrees of mechanical I should be at a max advance of 35 BTDC without any vacuum advance. That would be fairly similar to the max advance that this motor should run on gasoline, but it would be in sooner due to the higher base and lower mechanical contributions, which most documents say is the goal for LPG.

If I don't make it to 20 base, then I need to start adjusting the mechanical stops to get my total advance back into that range, and look at changing the curve with the springs after road testing. When I bought this truck it was running 25 base and 15 mechanical with vacuum advance disconnected. And the choke stuck on and a huge vacuum leak - but decent performance and 10MPG. So I'm not sure what the previous owner did in there and I should probably look at the weights to make sure they aren't as messed up as the rest of the truck was.

After all that, I should try to get the vacuum advance dialled in. At the target setting of 35 + ?15? that would be 50 degrees which wouldn't be out of line in a modern motor, but sounds way too high for this old dog. What would you recommend as the limit for an absolute max all-in cruise timing?


Unfortunately testing for max trap speeds or 1/4 mile times is a challenge, I live on the gravel 20km from the nearest lumpy potholed strip of pavement and 150km from the track. When I put my foot down for a WOT test I dig two big holes in the road, and my top speed is 80kph before I'm risking hitting the ditch for lack of traction!

[storm]

I know what you mean about road tuning. I used to live, actually I do at my new place as well, on bad roads where timing for trap speed is very difficult. In your case it is either a trip to the dyno or trial and error over a period of weeks everytime you go out in it.

As for VA timing I suggest you watch this video https://www.youtube.com/watch?v=jmvTz9iftl0&t=9s. It is part 2 but it shows the actual tuning process rather than part 1 just being a talk fest between David Vizard and the guy from Unity Motorsports.

LPG and petrol are very different but so is each individual engine. Time the engine for what it can take not for what you expect it to take based on when it was originally built.

[evranch]

Probably going to be successive tuning by trial and error as you mention. Especially since at this early stage of conversion testing I don't have proper fuel tanks mounted, thus no ability to make it to a dyno and back or refuel away from the farm! Now that I have it running decent, fuel tanks are the next immediate thing I need to add.

Manifold vacuum stabilized around 20 BTDC base so I set it there. I opened up the distributor and found a 16L cam with a bushing on the post that must significantly reduce travel, since I could never get more than 15 degrees or so out of it no matter how much I revved up the motor. With one very light and one very heavy spring the advance was coming in in two stages with the bulk of it above 2000 RPM. I pulled the old trick of slightly bending the heavy spring post to decrease the tension on it, getting all 15 degrees in at 2250 RPM and a more linear curve.

That gives me 35 BTDC all in at 2250 without vacuum advance, and vacuum advance pushes it up into the high 40s under full vacuum (estimated, no timing marks).

It was a nice evening, rain settled the gravel and there was no wind so I could actually hear the motor. Took the truck for a run and got no knocking and about 8 mpg. Driveability was excellent. Hot restarts are no problem. I could probably push it a bit further, but always have to consider in these tests that I'm driving on very hilly gravel roads in a truck with the aerodynamics of a brick and the weight of several pallets of them. Even my modern car gets 11L/100km out here while it will do 7 on the highway. The emissions era 460 had a reputation for eating fuel anyway, so I should probably accept the timing as functional rather than spend too much time on it. I should now take a look at some other mods that will bring up efficiency like replacing the timing gears, tuning the shift points and adding the dual exhaust system that is on a neighbour's wrecked truck.

I might leave it like this for awhile since it's useable for farm tasks and I need to get the truck out of the shop and work on equipment for haying time which is fast approaching.

Thanks for all your advice and for the vacuum advance video which looks like a great technique, I have everything I need to give it a try. In fact the vacuum advance can could be used to test varying degrees of base timing in the same way, if I wrote down the number of degrees it will advance at certain vacuum levels. I just need to drive the truck down to the nearest stretch of highway for sustained flat cruising, and make sure I bring enough fuel to get home!

[Tom68]

Probably going to be successive tuning by trial and error as you mention. Especially since at this early stage of conversion testing I don't have proper fuel tanks mounted, thus no ability to make it to a dyno and back or refuel away from the farm! Now that I have it running decent, fuel tanks are the next immediate thing I need to add.

Manifold vacuum stabilized around 20 BTDC base so I set it there. I opened up the distributor and found a 16L cam with a bushing on the post that must significantly reduce travel, since I could never get more than 15 degrees or so out of it no matter how much I revved up the motor. With one very light and one very heavy spring the advance was coming in in two stages with the bulk of it above 2000 RPM. I pulled the old trick of slightly bending the heavy spring post to decrease the tension on it, getting all 15 degrees in at 2250 RPM and a more linear curve.

That gives me 35 BTDC all in at 2250 without vacuum advance, and vacuum advance pushes it up into the high 40s under full vacuum (estimated, no timing marks).

It was a nice evening, rain settled the gravel and there was no wind so I could actually hear the motor. Took the truck for a run and got no knocking and about 8 mpg. Driveability was excellent. Hot restarts are no problem. I could probably push it a bit further, but always have to consider in these tests that I'm driving on very hilly gravel roads in a truck with the aerodynamics of a brick and the weight of several pallets of them. Even my modern car gets 11L/100km out here while it will do 7 on the highway. The emissions era 460 had a reputation for eating fuel anyway, so I should probably accept the timing as functional rather than spend too much time on it. I should now take a look at some other mods that will bring up efficiency like replacing the timing gears, tuning the shift points and adding the dual exhaust system that is on a neighbour's wrecked truck.

I might leave it like this for awhile since it's useable for farm tasks and I need to get the truck out of the shop and work on equipment for haying time which is fast approaching.

Thanks for all your advice and for the vacuum advance video which looks like a great technique, I have everything I need to give it a try. In fact the vacuum advance can could be used to test varying degrees of base timing in the same way, if I wrote down the number of degrees it will advance at certain vacuum levels. I just need to drive the truck down to the nearest stretch of highway for sustained flat cruising, and make sure I bring enough fuel to get home!

Other than 35 at 2250 less vac being up there, looks like a reasonable curve, Gas has a pretty wide range of ignition point that will work.

[Tom68]

Somehow double posted and don't know how to delete.

[evranch]

Other than 35 at 2250 less vac being up there, looks like a reasonable curve, Gas has a pretty wide range of ignition point that will work.

As far as being "up there" do you feel like 15 degrees is too much mechanical? These years of the 460 motors had very mild compression (8:1 range) so I'm not too worried about overdoing it at 35.
When I bought this truck it had been running on 87 octane for years with 40 degrees of advance at 3000 RPM (vacuum can plugged off), so I'm actually running less total advance now on LPG.

[Tom68]

Often stated that you should keep total less vac under 30 for even snall block chevs that need a lot of advance to be efficient. but no pinging means no problems of course.

I'm running a 327 with a fixed mechanical dizzy, I'd run over 30 but it kicks back on the starter when hot on hot days, so backed it to 30 plus 15 degrees vac straight from the manifold, only my primaries are on LPgas. Secondaries are running 98 octane petrol, that's why I'd like to run more timing.