total timing reading accurate with no load?

[Mattax]

The reason for advancing timing with rpm is there is less time in miliseconds for the burn to build pressure to get the most leverage.

But with increasing rpms, combustion conditions often improve, so advance can be slowed down as burn speeds up.
see The New Distributor (Session 136) from the Master Technician's Service Conference

The purpose of vacuum advance is provide more lead time for lower density mixtures.

 

[adam83]

Thank you I did read into that, and I figured since idle was a light load as well it may as well get the vacuum advance by hooking to manifold vacuum.

What I can't seem to figure out is if the initial timing is bumped almost 10 degrees over stock, should I bump the stock idle speed as well? For my truck the recommended idle was 750rpm. After I recurved the distributer and adjusted the timing to 16 intitial, it bumped my idle up to 850. I'm wondering if I should just leave it at 850 or do some more adjusting and set it back at 750

 

[Mattax]

Thank you I did read into that, and I figured since idle was a light load as well it may as well get the vacuum advance by hooking to manifold vacuum.

Ah. So many of us did 'cause it seems that way. The explanation seems to be that at very low rpm the engine's power is very low too. So as a percentage of what the engine can make at idle speed, its a moderate load. maybe the engine make 30 hp and loses 5 - 10 to internal losses. Slipping it into gear, especially with an automatic might take another 10hp.

What I can't seem to figure out is if the initial timing is bumped almost 10 degrees over stock, should I bump the stock idle speed as well? For my truck the recommended idle was 750rpm. After I recurved the distributer and adjusted the timing to 16 intitial, it bumped my idle up to 850. I'm wondering if I should just leave it at 850 or do some more adjusting and set it back at 750

Can't think of any reason why it would be good to idle higher.
If it idles nice at 750 then I would go back to that. *

Without any other info. the practical test with an automatic to tune in will be to set it for most power at idle in gear.
When shift into drive, the rpm shouldn't drop, or drop very little.
Another check is the manifold vacuum in drive, at that rpm is an indicator of how strong it is with the load.

So an example of this first round of testing.
850 rpm, 16* in N
reduce idle speed via throttle stop screw.
750 rpm, 16* in N
shift into D
650 rpm, 16* in D

Compare with previous settings. Lets pretend it was
750 rpm, 14* in N
shift into D
700 rpm, 14* in D

Therefore in the above example, 14* initial is the stronger one. Go with that.

*There is another effect when changing the throttle position at idle.
That's why I put an asterisk up. The throttle can only be changed within a small range before the effecting both the idle mixture and the off idle mixture.
This is particulaly true with Holley 4150/60 type carbs.

So...the practical next test is to try some very gently accelerations from stop, along with some more moderate accelerations from stop. If there is a lag or delay in response, then the initial throttle position is out of range.
Adjusting the idle mix screws can help make up for loss of fuel at dead idle (probably need to open them more when reducing throttle position)
They will only have limited impact in helping the off-idle.
Keep track of how much you turn each of these three screws (idle stop, and idle mixture screw).

 

[Mattax]

Some photos showing throttle plate's relationship to the transfer slot in this post
Carburetor Question - Is this a good call?
The portion of the transfer slot above the throttle is at higher pressure - and the portion below is at same pressure as the idle port.
At idle - fuel comes from both! The mix screws only control the portion through the idle port.
As the throttle opens, a larger portion of the fuel comes from the transfer slots. Off idle performance depends on the throttle and transfer slot relationship to be correct - its like a variable jet and variable airbleed combined.

 

[IQ52]

I'm going to learn how to spell destibbertur so I don't look like a dizzy blonde.

 

[MopaR&D]

Interesting the manifold vacuum advance is working for you. I've thought about trying it on my 360 but the way it is now it's very happy with 20 initial and 39-40 total (9:1 comp, aluminum heads, high altitude), if I added the 14 or so degrees of vacuum advance at idle I'd probably need to back down the initial and then readjust the mechanical advance limit so total timing stays the same. Kinda doesn't sound worth it, anyone think it's worth a shot...?? It runs perfectly the way it is now.

 

[yellow rose]

Interesting the manifold vacuum advance is working for you. I've thought about trying it on my 360 but the way it is now it's very happy with 20 initial and 39-40 total (9:1 comp, aluminum heads, high altitude), if I added the 14 or so degrees of vacuum advance at idle I'd probably need to back down the initial and then readjust the mechanical advance limit so total timing stays the same. Kinda doesn't sound worth it, anyone think it's worth a shot...?? It runs perfectly the way it is now.

You can also do what you are thinking and limit the amount of timing added by the Vacuum advance and delaying its start vacuum.

I assume the 40 total is without the VA.

 

[Mattax]

Interesting the manifold vacuum advance is working for you. I've thought about trying it on my 360 but the way it is now it's very happy with 20 initial and 39-40 total (9:1 comp, aluminum heads, high altitude), if I added the 14 or so degrees of vacuum advance at idle I'd probably need to back down the initial and then readjust the mechanical advance limit so total timing stays the same. Kinda doesn't sound worth it, anyone think it's worth a shot...?? It runs perfectly the way it is now.

Khalid,
For your situation, and I think Adam83's, the answer is no.
Adam83 posted he was going to take it for a test drive - but hadn't yet. So be interesting to hear his experience after the test drives.

In my opinion if the distributor can do it, its better to use mechanical advance to match the rpm related timing requirements and use vacuum advance for load related timing adjustment. The only situation where load is somewhat high with closed throttle is right at idle when slipping into gear. Hence the widespread use of ported vacuum.

But there are distributors that can't be made to get enough advance at idle, then manifold vacuum can be used.
There are also combinations where more advance is needed or just works out. Over in the compression testing thread yellow_rose writies about running high compression engines pretty cold (160* thermostat etc). I don't have experience working with anything that high of compression but running cold combustion chamber it makes sense that more advance could help insure (a) the burn gets done (b) little heat gets put into the cylinder walls.

Lets go back to the first situation, distributor that can't get enough advance at idle.
For Joe jr hotrodder that's not comfortable taking apart and modifying a Chrysler distributor, manifold vacuum could be an answer.
A typical Chysler replacement distributor was made in the smog control era and all of the early emissions efforts involved putting more heat into the cylinder walls at idle. The goal was reducing CO and left over hydrocarbons. Trade off was more complete burning in exchange for efficiency (power). Slightly higher idle speeds made up for the loss.

A good comparison of timing for Clean Air Package vs. regular timing can be seen specs for 1968 318 2bbl posted on the bottom of page 1.
The initial timing for a manual transmission with CAP was 5* ATC and the distributor has a 10* longer advance curve than a non-CAP distributor.

If Joe jr hottrodder has a distributor with a long advance in it like that, but doesn't want to take it apart and weld the slots, an easy fix (or crutch) might be to use vacuum advance at idle. Depending on the vacuum Jr hotrodder's engine pulls at idle, and the vacuum canister, it may pull timing at idle up to 10 or 15*. Suddenly his engine seems much happier. In fact it probably is. Especially if has a little hotter than stock cam and a carb that doesn't have idle mixture limiters.

 

[Mattax]

For road course events, out there 20 minutes at a time, you'll be heat soaking the block in way that never happens at the dragstrip. In fact it doesn't happen on the street most of the time. Exceptions are long trips at highway speeds - and more so with mountains. Considering where you live I'll assume hills are routine..

 

[MopaR&D]

You can also do what you are thinking and limit the amount of timing added by the Vacuum advance and delaying its start vacuum.

I assume the 40 total is without the VA.

Yup that's correct. I don't have much incentive to mess with the timing, I actually just put in a hotter thermostat (it was running only around 170 now it's around 195-200) and reduced the advance from its previous 24/44, actually has noticeably more power now everywhere and still feels just as strong at WOT in the higher revs (where I expected it to lose power). I think my hunch of the low compression/cylinder pressures with aluminum heads liking more heat in the chamber was correct.

@Mattax I have experienced the "full heat" soak you mention driving up through the mountains where you're fairly hard on the throttle for minutes at a time (maintaining 65 mph up a 6% grade takes quite a bit of HP). I have a high-flow Milodon water pump, high-flow 195* EMP-Stewart thermostat and ECP aluminum radiator which I hope will be able to reject all that extra heat quickly enough doing road course shenanigans.

 

[Mattax]

Yup that's correct. I don't have much incentive to mess with the timing, I actually just put in a hotter thermostat (it was running only around 170 now it's around 195-200) and reduced the advance from its previous 24/44, actually has noticeably more power now everywhere and still feels just as strong at WOT in the higher revs (where I expected it to lose power). I think my hunch of the low compression/cylinder pressures with aluminum heads liking more heat in the chamber was correct.

@Mattax I have experienced the "full heat" soak you mention driving up through the mountains where you're fairly hard on the throttle for minutes at a time (maintaining 65 mph up a 6% grade takes quite a bit of HP). I have a high-flow Milodon water pump, high-flow 195* EMP-Stewart thermostat and ECP aluminum radiator which I hope will be able to reject all that extra heat quickly enough doing road course shenanigans.

A little secret - I want to say Tuner had Dick Ott's engines running around 28* for road racing at Laguna and Portland. I was quite surprise when Dick mentioned that, and pretty sure Mike Ritz (Team Starfish) said something similar - engine also built by Ott. You're at different elevation and obviously different engine - I don't know what secret sauce went into theirs but apparently they did not need or want a lot of spark lead in the conditions they were running. Probably built to be very efficient in the rpm range needed for vintage racing.

My understanding is the coolant temps should not climb too much on the road course. The speeds help get rid of the heat. The oil temperature is more often the issue. As long as it stabilizes at some point then probably can get away without a cooler by choosing an appropriate grade oil to account for the higher temperature. That's been my plan since track events are secondary. My one road course event with the '67 didn't really get the oil too hot. But that was dip your toes type event. Even there I was ready. I brought heavier grade oils andran with the accusump empty. That would let me put in close two additrional quarts of heavier weight oil if needed.

 

[yellow rose]

A little secret - I want to say Tuner had Dick Ott's engines running around 28* for road racing at Laguna and Portland. I was quite surprise when Dick mentioned that, and pretty sure Mike Ritz (Team Starfish) said something similar - engine also built by Ott. You're at different elevation and obviously different engine - I don't know what secret sauce went into theirs but apparently they did not need or want a lot of spark lead in the conditions they were running. Probably built to be very efficient in the rpm range needed for vintage racing.

My understanding is the coolant temps should not climb too much on the road course. The speeds help get rid of the heat. The oil temperature is more often the issue. As long as it stabilizes at some point then probably can get away without a cooler by choosing an appropriate grade oil to account for the higher temperature. That's been my plan since track events are secondary. My one road course event with the '67 didn't really get the oil too hot. But that was dip your toes type event. Even there I was ready. I brought heavier grade oils andran with the accusump empty. That would let me put in close two additrional quarts of heavier weight oil if needed.

LOL. Tuner and I talked last night (trying to get him to let me buy him lunch next week but he's so damn busy it's hard to nail him down) and we discussed the quality of the air at PIR and even SIR (yes...I'm old...it's now Pacific Raceways again but it's still SIR to me) and how the air is so different between PIR and Ohio, or North Carolina and how it affects the tune up.