Vacuum/Mechanical Advance Dilemma SB340

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340inabbody

340inabbody

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Hey guys,
I am looking to get some inputs on a strategy/approach for setting up/tuning my less than optimal (mechanical advance) street performance motor. Need to try and improve for drive-ability with possibly a less than optimal ignition set up and over cam for an automatic.

Ideally, I would just de-cam the motor and add a vacuum to the billet MSD dizzy (but that is not an option according to MSD) therefore change out the entire new distributor (not sure which one but one with a vacuum advance that can work with the 6AL). However funding is to limited to replace these new components and or rip at the motor and replace the cam at this time.

So I am looking to get advice on the things I can tune, timing advance, AF, advance curve, jetting etc. At this point I have not done anything to the carb and just played with he advance and curve. I need a a more clear approach as I feel I am just shooting in the dark by myself. I have read articles on this and understand that vacuum secondaries are better for a street automatic not just for better burn efficiency but better drive-ability since you get the boost of advance at idle and cruise which is needed since the AF is lean. This lean condition takes longer to burn and more advance is needed. I get this. I do hear that people have solved or greatly improved this situation I am in but I have not located any threads as to how.

I have access to an AF meter/sensor that I plan to use to see where I am at.

The Problems:
  • Idle, needs to be >1100RPM out of gear which equates to roughly 500RPM in gear. Typical issue for high cammed motors right?
  • Not great vacuum 12in
  • Cold idle is very poor and takes a while to warm up even here in AZ.
Needless to say I regret not running a vacuum advance set up originally as that would have been the right choice for a more street-able set up.
Current set up:
  • 1974 block, 340 with unknown but very aggressive cam at least 10:1 compression
  • Factory heads (1973)
  • 650 CFM Holley Double Pumper 4150 with mechanical advance manual choke
  • M1 high rise
  • MSD6AL
  • Billet MSD Dizzy
  • Its a TF904 auto 2400 stall
  • 355 rear end (not yet installed)
  • Full length headers
Currently Tune:
  • 14 degree advance stop distributor bushing to limit all in timing help with detonation
  • Slowed the advance down to help with detonation using the largest springs
  • Base timing is currently set to 17 degrees and 31 all in at around 3800-4000. No detonation with this setting.
Thanks for taking the time to red this and for your comments.
 
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Without knowing what the cam really is, and where it’s installed it’s just a crapshoot.


Off the top of my head I’d say...... put a 10” converter in it which should really help with how the motor behaves at low speeds.

What the current combo wants is more timing and a quicker advance curve, but then it pings.

It’s hard to cover up a poor combo by just messing with the ignition curve.

The carb could probably benefit from being modified for 4 corner idle if it doesn’t already have it.
The carbs idle circuit needs to be on the same page as the cams idle requirements.

That 12” of vacuum...... is that in neutral at 1000, or in gear at 500?
 
PRH said:
Without knowing what the cam really is, and where it’s installed it’s just a crapshoot.


Off the top of my head I’d say...... put a 10” converter in it which should really help with how the motor behaves at low speeds.

What the current combo wants is more timing and a quicker advance curve, but then it pings.

It’s hard to cover up a poor combo by just messing with the ignition curve.

The carb could probably benefit from being modified for 4 corner idle if it doesn’t already have it.
The carbs idle circuit needs to be on the same page as the cams idle requirements.

That 12” of vacuum...... is that in neutral at 1000, or in gear at 500?
Click to expand...

Hi PRH thanks for replying!! Yeah I bought the engine and the guy did not have the info on the cam so I took the risk (maybe not the smartest thing I have ever done). But not much I can do about that now. If I recall correctly that vacuum was in park at 1100RPM. But I should characterize it in gear at the lower idle as well huh? Can you please explain the 10" converter? I don't know what factory diameters were used etc. I assume the 10" is a larger diameter than what I am running that the larger diameter has more flywheel affect???
 
17 and 31 are both "somewhat" low. I would try more initial timing and that of course will raise total. Small blocks generally like total timing figures in the 34-36 degree neighborhood. If you can run a compression test, that would give us more information to work with. It's probably a low compression build from your description so far and there's nothing wrong with that. They can be made to run well. They just need a lot of initial timing. If you can also tell us "what cam" and "WHERE" it was installed. In other words was it degreed or just slapped in dot to dot? Remember too, that vacuum advance is never included when timing an engine. It's initial and mechanical only and vacuum advance "just is what it is".
 
10 inch high stall converter, factory is 11 inch. Sounds like it needs gears, and a lot lower than 3.55. But run a compression test. With detonation coming in that quickly I would be highly suspicious.
 
RustyRatRod said:
17 and 31 are both "somewhat" low. I would try more initial timing and that of course will raise total. Small blocks generally like total timing figures in the 34-36 degree neighborhood. If you can run a compression test, that would give us more information to work with. It's probably a low compression build from your description so far and there's nothing wrong with that. They can be made to run well. They just need a lot of initial timing. If you can also tell us "what cam" and "WHERE" it was installed. In other words was it degreed or just slapped in dot to dot? Remember too, that vacuum advance is never included when timing an engine. It's initial and mechanical only and vacuum advance "just is what it is".
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I can add more timing. I assume this will help idle. There will be a limit as I believe the compression is not low. The guy who built it put a big cam in it so he probably went min with 10:1 my guess. I sure wish I could upload a video so you guys could hear the idle.
I will do a dry compression test on all cylinders its going to be a while as I am still out of commission medically. So I I weave that into the plan. What information will the compression test give us with respect to tuning etc?

Ok now regarding the cam. I had an engine builder shop look at the engine as it was taking more than 52 degree of total timing. The guy said something was not right. He pulled the water cover off and as suspected there was an offset on the chain As can be seen here. Was this a problem? Is this how you advance an engine internally? By 10 degrees?

IMG_3667.PNG
 
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340inabbody said:
I can add more timing. I assume this will help idle. There will be a limit as I believe the compression is not low. The guy who built it put a big cam in it so he probably went min with 10:1 my guess. I sure wish I could upload a video so you guys could hear the idle.
I will do a dry compression test on all cylinders its going to be a while as I am still out of commission medically. So I I weave that into the plan. What information will the compression test give us with respect to tuning etc?

Ok now regarding the cam. I had an engine builder shop look at the engine as it was taking more than 52 degree of total timing. The guy said something was not right. He pulled the water cover off and as suspected there was an offset on the chain As can be seen here. Was this a problem? Is this how you advance an engine internally? By 10 degrees?

View attachment 1715360797
Click to expand...

Run the compression test and report back. That will give us a lot to go on. Keep in mind compression ratio has zip to do with cylinder pressure, which is what you will be measuring with the compression test. The two are related, but that's it.
 
Garrett Ellison said:
10 inch high stall converter, factory is 11 inch. Sounds like it needs gears, and a lot lower than 3.55. But run a compression test. With detonation coming in that quickly I would be highly suspicious.
Click to expand...

YES! I was running a peg leg 271 when I did. I just got my Suregrip 3:55 and need to get that in my 8-.25 ASAP! Next on the list actually.
 
340inabbody said:
I can add more timing. I assume this will help idle. There will be a limit as I believe the compression is not low. The guy who built it put a big cam in it so he probably went min with 10:1 my guess. I sure wish I could upload a video so you guys could hear the idle.
I will do a dry compression test on all cylinders its going to be a while as I am still out of commission medically. So I I weave that into the plan. What information will the compression test give us with respect to tuning etc?

Ok now regarding the cam. I had an engine builder shop look at the engine as it was taking more than 52 degree of total timing. The guy said something was not right. He pulled the water cover off and as suspected there was an offset on the chain As can be seen here. Was this a problem? Is this how you advance an engine internally? By 10 degrees?

View attachment 1715360797
Click to expand...

Put a degree wheel on it. That's the only way to know where the cam is installed.
 
RustyRatRod said:
Run the compression test and report back. That will give us a lot to go on. Keep in mind compression ratio has zip to do with cylinder pressure, which is what you will be measuring with the compression test. The two are related, but that's it.
Click to expand...
Thanks Rusty so what do we learn from the cylinder pressure? Can we back into the CR or estimate the CR from this?
 
With it being so ping happy, check your plugs. No iridium, platinum, or unobtanium. Champion copper plus truck plugs one step colder, maybe two steps colder. You got race gas available? If the engine is cammed that much, plan on trading the 3.55s in on 3.91-4.10 gears. I stand with RRR on 3.55s usually being more of a halfway compromise instead of a viable gearset in a real performance build. Great for a lot of stock stuff, but when you have something that doesn’t turn on until 3000 rpm not so much.
 
I believe you need a looser converter as stated. That converter you are using is for a truck or a passenger car motor. All 340's from the factory came with a converter that would brake stall at 2800-3000. A advertised 3500 will stall at that with a mild 340 motor as yours. I have a 5500 and I drove the car on the street and raced it. 10 second car through the exhaust. full interior spare in the trunk 3650 lbs. Distributor is locked timing at 34. MSD billet with a "digital 6" , I ran pump gas on the street for cruise night.

 
Oldmanmopar said:
I believe you need a looser converter as stated. That converter you are using is for a truck or a passenger car motor. All 340's from the factory came with a converter that would brake stall at 2800-3000. A advertised 3500 will stall at that with a mild 340 motor as yours. I have a 5500 and I drove the car on the street and raced it. 10 second car through the exhaust. full interior spare in the trunk 3650 lbs. Distributor is locked timing at 34. MSD billet with a "digital 6" , I ran pump gas on the street for cruise night.

Click to expand...


GAWD that thing was awesome. You gotta get it back goin again.
 
Its getting there . I need to sell the brown one. That will give me the Funds. I bought the brown Duster with intentions of using that car . I spent the rest of the money for the build buying another body I should have bought first.

Wait until you all see the brown Duster. Its going to be a nice car and it will be cheap. So far I have about $3000 invested total out of pocket. That is with buying the car. I would have just sold it as a drag car but I traded the motor to buy the red 70 back. One thing led to another and I just could help myself. The body was to nice to keep my fingers out of it. I put a 318 in .It is not an original numbers matching car and I added some options it didn't have.

Finally paint
 
I’m not sure what I’m looking at with that pic of the lower timing gear.......but unless the dots on both gears weren’t aligned, it should have been fine like that.
The lower gear is on the “0” keyway, and the 0 tooth is basically up.

But...... nonetheless, if the timing cover was off...... shoulda put the degree wheel on it.
 
PRH said:
I’m not sure what I’m looking at with that pic of the lower timing gear.......but unless the dots on both gears weren’t aligned, it should have been fine like that.
The lower gear is on the “0” keyway, and the 0 tooth is basically up.

But...... nonetheless, if the timing cover was off...... shoulda put the degree wheel on it.
Click to expand...
Makes two of us. But they said it was one tooth off....how many teeth are on these gears? They equated it to 10 degrees which implies 36 teeth. If is was running well at 52 degrees it now runs well albeit slight detonation with the 271 poor gears minus 10 degrees puts it at 42 degrees or there abouts which is on the high side of total...it made some sense to me but they did not degree it when I asked afterwards. My thoughts were the same as you guys. No reference point right?
 
Oldmanmopar said:
I believe you need a looser converter as stated. That converter you are using is for a truck or a passenger car motor. All 340's from the factory came with a converter that would brake stall at 2800-3000. A advertised 3500 will stall at that with a mild 340 motor as yours. I have a 5500 and I drove the car on the street and raced it. 10 second car through the exhaust. full interior spare in the trunk 3650 lbs. Distributor is locked timing at 34. MSD billet with a "digital 6" , I ran pump gas on the street for cruise night.

Click to expand...

What rear gear were you using on that run?
 
Oldmanmopar said:
Its getting there . I need to sell the brown one. That will give me the Funds. I bought the brown Duster with intentions of using that car . I spent the rest of the money for the build buying another body I should have bought first.

Wait until you all see the brown Duster. Its going to be a nice car and it will be cheap. So far I have about $3000 invested total out of pocket. That is with buying the car. I would have just sold it as a drag car but I traded the motor to buy the red 70 back. One thing led to another and I just could help myself. The body was to nice to keep my fingers out of it. I put a 318 in .It is not an original numbers matching car and I added some options it didn't have.

Finally paint
Click to expand...

Spoken like a true addict. lol
 
PRH said:
Without knowing what the cam really is, and where it’s installed it’s just a crapshoot.


Off the top of my head I’d say...... put a 10” converter in it which should really help with how the motor behaves at low speeds.

What the current combo wants is more timing and a quicker advance curve, but then it pings.

It’s hard to cover up a poor combo by just messing with the ignition curve.

The carb could probably benefit from being modified for 4 corner idle if it doesn’t already have it.
The carbs idle circuit needs to be on the same page as the cams idle requirements.

That 12” of vacuum...... is that in neutral at 1000, or in gear at 500?
Click to expand...
So what exactly is a 10” converter do? If I am running a stock 11” which I am can only assume does the 10” work differently than the 11 or does it come in different stall ratings etc? How does a 10 improve low speed drivability? Who makes them and how are they specified if different than 11’s?
Thanks
 
340inabbody said:
So what exactly is a 10” converter do? If I am running a stock 11” which I am can only assume does the 10” work differently than the 11 or does it come in different stall ratings etc? How does a 10 improve low speed drivability? Who makes them and how are they specified if different than 11’s?
Thanks
Click to expand...

My understanding, smaller diameter converter has a higher stall speed but also more efficiently transfers the torque to the trans. Generally converters get smaller as the stall speed goes up. When you get a converter the proper way is to call a known good converter company (PTC and Dynamic are two off the top of my head) and give them the full setup of your car including engine specs, trans type, vehicle weight, rear gears... and they will custom-build a converter to match your combination which will perform the best.

What you described about checking the cam timing doesn't really make sense to me, they basically told you just to change the spark timing but didn't bother to reset the sprockets so the dots lined up? WTH? If the timing chain is off a tooth usually it will cause all kinds of issues, least of which is inaccurate ignition timing. If you put a degree wheel on the crank with a dial indicator measuring lift on one of the lifters you can get a pretty close idea of what the duration numbers and lift of your current cam are.

I'm betting those 2.71 gears are putting way too much load on the engine and with decent compression/cam it really doesn't like it, hence your pinging issues. 3.55 gears will take some load off the engine and allow it to rev up quicker which is what the engine really wants to do. Tall gears feel the same to an engine as a heavy vehicle, i.e. with 2.71 gears in a 3200-3600 lb. car your engine is feeling the same load as if it was in a big 5000-lb. truck with 3.55s.

I wouldn't worry about vacuum advance for now seems like you have plenty of other things to fix and mess with...
 
MopaR&D said:
My understanding, smaller diameter converter has a higher stall speed but also more efficiently transfers the torque to the trans. Generally converters get smaller as the stall speed goes up. When you get a converter the proper way is to call a known good converter company (PTC and Dynamic are two off the top of my head) and give them the full setup of your car including engine specs, trans type, vehicle weight, rear gears... and they will custom-build a converter to match your combination which will perform the best.

What you described about checking the cam timing doesn't really make sense to me, they basically told you just to change the spark timing but didn't bother to reset the sprockets so the dots lined up? WTH? If the timing chain is off a tooth usually it will cause all kinds of issues, least of which is inaccurate ignition timing. If you put a degree wheel on the crank with a dial indicator measuring lift on one of the lifters you can get a pretty close idea of what the duration numbers and lift of your current cam are.

I'm betting those 2.71 gears are putting way too much load on the engine and with decent compression/cam it really doesn't like it, hence your pinging issues. 3.55 gears will take some load off the engine and allow it to rev up quicker which is what the engine really wants to do. Tall gears feel the same to an engine as a heavy vehicle, i.e. with 2.71 gears in a 3200-3600 lb. car your engine is feeling the same load as if it was in a big 5000-lb. truck with 3.55s.

I wouldn't worry about vacuum advance for now seems like you have plenty of other things to fix and mess with...
Click to expand...
Thanks for the info. Yup the gears were first down n my list and I got the 3:55 but now seriously thinking of going with the 4:10 since I want lower end torque and am willing to trade off MPG and lower RPM on the highway. This car is not meant for touring but hacking around and driving to local meets etc. So a 4:10 should be better for this than the 3:55 I presume. Heck it might also mean the TQ Converter as is might not need to be changed....
 
340inabbody said:
Thanks for the info. Yup the gears were first down n my list and I got the 3:55 but now seriously thinking of going with the 4:10 since I want lower end torque and am willing to trade off MPG and lower RPM on the highway. This car is not meant for touring but hacking around and driving to local meets etc. So a 4:10 should be better for this than the 3:55 I presume. Heck it might also mean the TQ Converter as is might not need to be changed....
Click to expand...

Ah gotcha I misread that part. In that case run with the 3.55s now and get as much of the tuning (cam timing, ignition, carb) and diagnostics out of the way FIRST then see how it runs. No point in throwing more upgrades at the car if the current combo isn't working to its full potential. Get your engine tuned up proper and go from there.

Honestly if it was mine I'd be first doing a compression check, then pulling the timing cover (again), doing a full check with degree wheel of your cam timing and lift after lining up the sprockets dot-to-dot, then put it back together and move on to the ignition. If the cam checks out to be installed correctly my first move would be to crank up that initial timing and see if it idles better without so much of a drop shifting into gear.
 
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