In those days the Ford designers were trying every trick in the book to get their engine to pass the fed emissions specs. So besides lowering the compression, a lot of weird control configurations got put in place to squeak by. But I’m reading some folks owning those old vehicles now, they are reverting to the pre-emissions mid-1960’s configuration. For better idle quality & overall drivability. For a truck that isn’t used all that much emissions, maybe 4,000 miles a year, just curious what the experts here think?
For example the ignition timing. The oem spec is 6 deg btdc at idle, and advanced by centrifugal and vacuum advance to around 25- 30 deg btdc at higher rpms. The stuff I’m reading say this isn’t optimal for a Ford 302. Instead it should be around 18 deg btdc at idle, and go up to around 35 deg at higher rpms. Also they say the vacuum advance should be connected directly to the intake manifold, rather than to a port above the throttle plates as it is done oem-style.
So what do you think? Should I advance the idle from 6 to 18 degrees btdc and change the source of the vacuum advance to the intake manifold rather than above the throttle plates like it is now? Or would this just open up a can of worms?
Ford light duty trucks from 1970 until 1976 used 302s ranging from 8.0 to 9.0:1 compression ratio and all had a combined total spark advance of 22* +/- 1* with 6* BTDC base timing except for 1976 when the compression ratio was 8.0:1 and base timing ranged from 6* to 14* BTDC. I would strongly suggest checking the timing chain for slack and replacing the chain if needed and it likely is. Then setting the ignition base timing with a vacuum meter with all vacuum connections made as specified if all the external vacuum control mechanisms are working. If they aren’t it might be worthwhile to replace the vacuum diaphram with an older model.
My personal opininion of the 302 in a truck is poor. It is a gutless wonder in a hard stall. I don’t think that 4x3 engine can be modified to ever come close to the 300 I-6 in a truck… Just my opinion FWIW.
You should go no more than 10 degrees BTDC at idle. If there is any pinging at all during hard acceleration you need to go back and retard the timing a couple of degrees. Make sure the EGR system if so equipped is operating propertly.
Running that much advance at idle (18…) could possible lead to a wiped out engine. This would more than likely occur at extended highway speeds. Trust me on this; I’ve seen it happen a numbers of times and the holed or disentegrated pistons are not pretty.
I’d do a bit of experimentation as the other suggest. 6 degree BTDC is more than and bit weak but 18 is a bit too much IMHO. Push it 'til it pings and back off 2-4 degrees.
As for the vacuum. Ported seems like a better ploy. You certainly can try both and see what the driveability is like. If it idles well with the ignition timing you select, and has good off-idle throttle response with ported, there is no need to add any more with the distributor hooked to the manifold. Manifold porting adds more advance at idle. Ported doesn’t add advance until the throttle plate uncovers the port. So if the idle is good, as soon as you crack the throttle the you get more advance unless you are hard on it.
Agree with @Rod_Knox The 300 I-6 was a better truck motor. Had one in a van. A beast at low RPM!
Bypassing the ported vacuum is something that a fool would do. I can’t tell you how many vehicles that I have purchased in years past that have had this procedure done to it. A mechanic would know better than to do this but a fool only understands so much.
Thanks for the interesting comments, here’s some of my thoughts on this …
My truck runs and drives nearly like new (engine-wise), no complaints. Even on a cold start all I hear is “rr-pop-it’s running”… Doesn’t even get to the second rr. On cold starts it takes a little jigging of the accel pedal the first 15-20 seconds or so to get it to idle well, but after that it idles perfectly. Warm starts, it’s “r-pop-run”. Not even the first full “rr”. It’s main drivability problem is a little hesitation during accelerations. It’s always done that, not a recent thing. Modifying the accel pump setting doesn’t help. HP wise overall, it has plenty for me. But you don’t need a whole lot of HP in a truck. Too much HP in a truck seems dangerous b/c truck’s being top heavy. But a little more oomph during accelerations might be ok.
A chart of HP vs model year for the same engine would demonstrate the HP issue. The 302 engine HP spec fell like a rock during the early 70’s. I don’t have that chart in front of me, but as I recall the first year the 302 was produced was 1968, and that year had 225-250 HP. 250 required a 4B carb I think.
My truck’s engine is rated a mere 140 HP. Most of that fall was likely due to the reduction in compression from 9.5 (1968) to 8.2, but some is probably due to the set up. Like I say, HP isn’t much of a motivation for me, but a reduction in hesitation is. And improved HP might do that possibly. Another motivation is the 70’s 302’s tendency for overheating at idle. The lean-ness of the idle mixture combined with the low advance spec at idle I guess is part of the reason. It’s possible to buy higher volume water pumps to address this, a common thing done apparently for rock crawling with a 302 4x4 truck. To address this overheating at idle problem at the factory, Ford added another vacuum port on the distributor which advanced the ignition a little when the coolant got too hot. But why not just start out with it advanced to begin with? Why make it so complicated? I mean for a seldom driven truck.
I don’t really understand the motivation for a direct intake manifold source for the vacuum advance myself. Like you say, it seems like you want the advance to happen as you open the throttle, which a ported source does. If the intake manifold is the source, when you open the throttle you’d get less advance at throttle openings. But maybe there’s more to it. From what I can tell the early 302’s (like in 1968) used the manifold as the vacuum advance source, right from the factory. Ford then switched to a ported source in the 1970’s for emissions purposes.
The articles I’ve read about this topic (mostly in hot rod type magazines) seem to be pretty adamant that a ported vacuum advance doesn’t produce as good of results in a 302 as a direct manifold advance. My guess is that is may eliminate some of the hesitation during rapid accelerations. Why that would work, I don’t know. @missileman 's post above seems to indicate using the manifold as the source is a common thing '70’s 302 owner’s do though. So they must have a reason. But maybe the reason only shows up on drag strip use? Don’t know. I’m not doing any drag strip racing anytime soon… :
I found a link to the article that got me thinking about this topic in the first place …
"Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn. Ensuring that the mixture has a complete burn before leaving through the exhaust port also helps the engine to run cooler at idle. All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.
Ported vacuum sources are a result of emissions laws and manufacturers doing whatever they could to get big V8 engines to pass smog before the incorporation of the catalytic converter. The idea was that by using little to no spark advance at idle, the exhaust gas would leave the cylinder still-on-fire and help maximize the efficiency of antiquated air injection systems. Engines from this era often ran very, very hot, were prone to warped exhaust valves, cracked cylinder heads and all other manner of issues. Using a ported spark advance will still allow the vacuum advance to do its job at steady cruising, but all of the benefits of idle cooling will be lost."
I’ll agree with that up until the late 80’s/early 90’s The EFI 302’s out-torqued the EFI 300 of the same era. My brother had a 96 F-150 4WD/Automatic 3.55 gears. His best friend had a 90 or 91 F-150 4WD/manual with I want to say 3.08 gears. My brother’s truck was noticeably faster with or without a load.
I have never seen an engine that used manifold vacuum to operate spark advance. The engines with dual port advance pots used temperature control on manifold vacuum to modulate and cancel the advance made by ported vacuum. If manifold vacuum is connected to the advance pot on a distributor the engine will fall on its face when accelerating. And I am speaking from many years of unscrambling the spaghetti of vacuum hoses on engines. Try hooking the vacuum advance to manifold vacuum and driving the car around the block.
Part of the hp loss George notice is because they went from gross to net hp over that period, about a 20% drop just because of how it was reported, I think.
I found a power curve chart comparing the 300 I-6 to the 302 and hopefully I picked up the link
my search was for a '72 model.
The graph indicates that the long stroke six was ahead of the V-8 in power and torque from the start but when the V-8 picked up steam the 6 was running out of it.
I couldn’t find a charts on later EFI models @FoDaddy, but I don’t doubt that higher compression ratios on the 302 would push its numbers higher but I question that they could move the slope of the torque and power significantly to the left. UPS and FedEx were almost exclusively powered by that engine for 2 decades so they saw something in the numbers that benefited them. .
Having the optimum timing at idle isn’t very important and wont help your drivability or mileage at all. You want optimum timing in your normal driving modes.
Some recommendations you will see for timing were made when premium leaded (106 octane) fuel was readily available.
Here’s what I would do. First make sure the weights and springs for the mechanical timing advance are free and lubricated. In a Ford, I believe they are located under the points. Then make sure the vacuum advance is working properly, as designed. You don’t need vacuum advance at idle.
If you use a vacuum meter, play with the timing but after selecting a setting, check the vacuum under your most common driving conditions. Reset the timing and check again under the same driving conditions. Repeat until you have optimized the timing for your normal driving conditions and do not worry about the timing at idle.
If you really want to optimize your timing, take it to a shop with a dyno. They can find the optimum timing for all driving conditions and replace the weights and springs to optimize the advance curve.
