lets revisit timing

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ir3333

ir3333

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A good curve for a 10:1 340 usually falls around 10 initial with another 24 mechanical for a total of about 34 at around 3000 rpm without vacuum advance which may add another 16 or so for a cruising total around 50.
how might the engine performance differ with totals of 30 - 36 at wot ( no vac adv.)
..would it be noticeable?
 
you thinking gas mileage,or power.me i just time it by ear till its a little hard to start.lots of pull, no ping either.now i know mine is 9'' 1
 
Fuel, compression ratio, chamber design, cam design and cam timing, AND WEIGHT AND USE of the vehicle all play a part

The "correct" answer for fuel mileage has always been "vacuum advance."

"Back in the day" with "real" fuel and with a factory "high compression" 340 or 440 I used to run as much as 38-40. But approach that carefully, and even though the plugs did not show it that " I could see" one 440 ate a couple of top rings/ ring lands
 
Ok...38 or 40 degree totals are definitely risky!
...will a noticeable power loss occur at 30 or 32 degree total?
 
My opinion, my way, I let the engine decide what the total timing should be. My 360 in one Duster is about 35*, and the 318 in my other Duster is about 40*. So many variables, that honestly, for best performance I believe the engine must decide.
 
do you arrive at those timing figures based on actual performance time or feel?
I'm not a racer.
 
What is you vacuum at WOT ? You probably don't have enough to pull the vacuum advance unit. If you don't know you need to check that first.
 
I am terrible at explaining things..sorry.
All i am wondering is what effect running a total of 30 degrees would have compared to a total of 34?
 
You should notice a difference between 30 and 34, add some race fuel and run it up to 36 or 38.
I raced my 340 at 40 total with no problem but that was with good fuel I think was 114 or 116 octane.
 
"Advance Curve" Kind of a misnomer here. You are talking about initial and total advance only.
The "curve" would be a graph showing how many degrees at different RPM's until you hit the total.
How many degrees at 2500 rpm, 3000 rpm, etc...There are slow curves and quick curves. These were done on a Distributor machine off the car using different weights, springs, slot lengths, different Vacuum advance units.
More to the story...
Quick Tech: Why You Still Need Vacuum Advance
 
no...i said 10..34...50.I welded the slots to .4 and curved my distributor to 14 ..34 ..and about 46.( I have done many)
The question i am asking is what effect will changing the total from 34 to 30 have on engine performance?
 
Did you read the link I posted? It is talking about GM's but the info is the same for any internal combustion engine.
 
A couple of basics that many of you know, but some may not:
  • Fuel does not "explode" in the combustion chamber, it burns.
  • The burnt fuel creates gasses that take up more space and creates force that pushes down the piston.
  • It takes some time for fuel to burn completely it is not an "instant burn".
  • If you ignite the fuel too late, like at top dead center (TDC), the piston is already moving down and not much force is created.
  • If you ignite the fuel too soon, the force of the expanding gasses pushes against the piston moving up towards TDC, and you get pinging. This also takes away power from the engine and can damage the engine.
  • So for an engine to idle well, it needs some static or initial advanced timing to optimize the way it runs due to its idle speed. This is set by how you turn your distributor.
  • As the engine speeds up, the fuel has to be ignited sooner so the fuel has time to burn. So centrifugal (mechanical) advance is added as the engine speeds up. This is controlled by small weights and springs in the distributor.
  • A rich mixture burns more quickly and a lean mixture burns slowly. When an engine has a high vacuum, low load situation the fuel mixture becomes lean. So Vacuum advance ignites the fuel sooner so it has time to burn effectively. By the way, since a drag race car never does this, vacuum advance is not needed in these cases, but it is very beneficial on the street.
(The above assumes an "old fashioned" car with a distributor like our old Mopars.)
How much initial, mechanical and vacuum advance any car needs is based on complex factors such as compression, design of the combustion chambers, cam shaft profile, octane in the fuel, etc. What works for one car may not work well for another.

Hope some of this is helpful.
 
I understand what you are asking.
At a given rpm and load, the engine will make the most power when the spark is lead by enough time to develop maximum pressure when the piston can apply the maximum leverage on the crank.

An engine that makes the most power at 3000 rpm with 34*BTDC will seem a little sluggish at 30*. You will see on the drag strip even if you can't feel it in your butt. I can tell you from experience that there is a range of timing that once it is outside of you absolutely can feel it in your butt.

Can a few degrees make a difference? Absolutely.
I worked with mechanical tech that had been involved in Stock Eliminator for while. While searching for 'a little more' they decided to set up the new Direct Connection race distributor like they had the points distributor. Unforunately they started running slower. They lost several weekends until they figured out that by eliminating the heavy spring to bring the electronic distributor "all in" earlier, the spark was retarding at high rpm. That was it. A few degrees retarded was enough to lose races.

You can see it here.

The main culprit is the time it takes for electronic switching. There's ways around it - one of the simplest being to compensate in the distributor. That's what the Chrysler performance distributors did. Because few people race in stock eliminator or similar classes, they don't test for and/or see the loss.

Another example here from Bill Baldwin in a FB video with a locked out distributor run up from 1000 to 6000 rpm. Almost 5 degrees 'lost'

____________________________________________
Getting a little off subject, but if you're interested in slew rate:
drag race engine ignition systems - Speed Talk

Mallory Unilite timing issue___________________________

When Hot Rod Magazine ran tested a 340 at Edelbrock in '69, the timing was set to 39* BTDC.
Two points about this:
1. Drag race and sometimes with dyno pulls, the engines are not heat soaked and the combustion chambers are relatively cold. Because the chambers are colder than a heat soaked engine, lighting off the mixture a little earlier can often make up for the slightly slower burn.
2. Burn rate does not neccessarily continue to speed up enough to keep up at high rpm. A slight advance in timing in the upper rpms often will make more power. Again few are involved in the types of uses where this makes enough difference to chase. But in those motorsports where it is, its worth pursuing.

here's the link pdferized Hot Rod articles
Mopar Manipulatin’ , HRM August ‘69
 
Last edited:
ir3333 said:
A good curve for a 10:1 340 usually falls around 10 initial with another 24 mechanical for a total of about 34 at around 3000 rpm without vacuum advance which may add another 16 or so for a cruising total around 50.
how might the engine performance differ with totals of 30 - 36 at wot ( no vac adv.)
..would it be noticeable?
Click to expand...

I don't think it will make a difference at WOT if you have the vacuum hooked up or not, there wont be enough vacuum to pull the advance unit till you let off the gas. That is what I'm saying.
At wot there will be a difference between 30 and 36, it will have more power at 36.
 
ir3333 said:
The question i am asking is what effect will changing the total from 34 to 30 have on engine performance?
Click to expand...

Probably 15-40 hp at peak depending on actual details (exhaust, intake, etc). Mid-range and part-throttle will depend wildly on your actual curve (how 'fat' it is in the middle, how low/high the all-in rpm is, etc) and is much harder to predict.

It would be most notable from 3500rpm and up, and can definitely be felt. Likely only matter on on-ramps, etc. 32-36 is a pretty safe range for most builds, but details matter once you're at 34+ (cam timing, actual measured compression, altitude, etc). If you want a 'set and forget' figure where you don't need to worry about gas, temperature, altitude, etc - 30-33 is a probably OK for a mostly street driven vehicle. I myself see no reason at all to run 32 or less. So I'd be at 33-35 deg.

There's an engine masters episode where they went from something like 10 degrees less than optimal, and then kept pushing the timing up. Even 2 degrees has measureable results until they were within 2 degrees of 'optimal'. I think they got about 6 degrees past that with ever-increasing power, but were pushing the limits. They discussed exactly what @Mattax pointed out: dyno pulls are done cold, not in traffic, and not in a heat-soaked engine with cheap swill from the corner store..
 
TX Mat and PH, at 30 power loss may be noticeable...exactly what i was asking
... and some great info for newbies HS!
 
ir3333 said:
A good curve for a 10:1 340 usually falls around 10 initial with another 24 mechanical for a total of about 34 at around 3000 rpm without vacuum advance which may add another 16 or so for a cruising total around 50.
how might the engine performance differ with totals of 30 - 36 at wot ( no vac adv.)
..would it be noticeable?
Click to expand...
That depends on many factors, including cylinder pressure and camshaft. 10 initial is pretty conservative.
 
RustyRatRod said:
That depends on many factors, including cylinder pressure and camshaft. 10 initial is pretty conservative.
Click to expand...
To add to this....I'll give you a surprising example. My slant 6. Naturally, if something is gonna go against the grain, it'll be something of mine. lol

It is a measured 10.2:1, cranking compression at 175PSI WITH a camshaft that's 250 @.050 with an IVC event of 69*. lol You'd FIGURE it'd take a pretty conservative initial timing setting, right? Wrong. I HAD it at 20* initial and "about" 34* total with the points distributor. I just recently switched to the Mopar electronic ignition that I rebuilt and recurved the distributor for. I welded the governor slots to get 10* mechanical. Turned out perfect. So I put it back on 20* initial and it had right at 10* mechanical for a total of 30*. But I got to playing around with it and bumped it a little more until it stopped idling up and then back down "a tick". Checked it with the light and it's at 23.5* initial. It runs fantastic there. You can tell beyond a shadow of a doubt that's where it wants to be. ZERO spark knock. ZERO kickback on the starter when hot. I would not have fugured it would have taken that hot of an ignition curve, but it did.

I guess what I'm trying to get at is.....like our buddy @MOPAROFFICIAL has preached and preached and preached, don't just "assume" but let the engine tell you where it wants the timing to be. I did, and that's where it runs best.
 
10* BTDC was the initial at 600 or 650 rpm for the 273 Commando engines.
That's a good example because the idle were altered for cleaner emmissions.
Techs could increase the initial or reduce it depending on altitude and fuel available if the owner wanted.
upload_2019-7-30_21-36-48-png-png.png


Notice the timing advances at least up to 3600 rpm.

Ignoring the retarded initial for emissions, we see the same thing on the timing specs for a '68 340.
upload_2020-8-17_17-48-58-png.png


Too much advance advance between 1500 and 3000 rpm might not be very noticible drag racing, but for other uses, it might be.
Uses such as lugging up a long steep hill, pulling a trailer, or pulling a boat out of the water. In those situations the loads can be high even in the lower rpms. So the factory made sure those possibilities were all covered. Then they figured how much additional timing would be needed for light to moderate load situations were the afr was leaner and the burns take more time to develop.
 
so again give it timing till kick back, just back it a degree till its happy no ping, i get all the tricks to do with our dizzy to get timing just right but one has to draw a fine what the engine want.i just dont care about all the tech about some thingssssss. let just have some FUN.
 
Snake said:
so again give it timing till kick back, just a degree till its happy no ping i get all the tricks to do with our dizzy to get timing just right but one has to draw a fine what the engine want.i just dont care about all the tech about so thingssssss. let just have some FUN.
Click to expand...
You can do that!
But for guys like ir3333 who are looking for power, that's why they're asking about the effect.
 
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