Strange man! Tonight I go to move the '29 out of the garage to do an oil change on my brother's car and it works great! And the only thing I did last night was advance the timing a hair. We'll see how long this works this well- In the mean time...... Thanks so much for all the help guys!!!
Advancing the timing will increase the manifold vacuum. The modulator operates off of a required vac input. Actually timing with a vac gauge is probably the best way on engines that are modified. Advance until vac gauge reads maximum amount. Advancing further will cause a slight drop. Retard distributor to again reach the peak amount of vacuum then continue retarding until the gauge shows 1/2" to 1" of vacuum below peak to reduce ping. Rest idle speed and repeat.
...And we're back to fixing the tune-up which could have been done before throwing money at the torque converter. Anything you can do to pick up vacuum is going to help improve the operation of anything that depends on vacuum to operate. Vacuum modulator. Power valves. Distributor vacuum advance. Choke pull-off. Brake booster. Vacuum wiper motor. And then if it's still a bit low on vacuum due to the hopped up cam you change the power valves, vacuum advance or add an adjustable vacuum modulator. If your fighting a brake booster problem it's time to add a vacuum reservoir or a seperate vacuum pump. Install a vacuum gauge in the car and you'll soon see what's really going on.
Completely understand the tuning issues if any, and would easily agree. However, all I was trying to do is get comfy at stop signs. I have no brake booster, and really all that works off vacuum on my motor is advance and tranny mod. So even with working with advance, I simply HAD to get the tranny to stop pulling so damn hard at idle. 3000 stall remedied that. I am going to work on the "best vacuum/timing" procedures you guys are speaking of though cause that makes a lot of sense to me. Talk later-I'm bolting out to the garage to check vac settings! Thanks!
...And don't forget about power valves or power pistons in the carb that also need vacuum to work properly. If you know the operating vacuum range of all these things you can do a lot of tuning from watching a vacuum gauge as you drive. A lot of the "pull" you felt against the converter may have been due to having the idle set faster due to the new cam. A looser converter is just part of the solution. There's been some solid advice posted here and I suspect you'll get it all sorted if you stick with it.
http://books.google.com/books?id=bu...a=X&ei=a3fHUfHDOo_49gT0pIHQCQ&ved=0CCoQ6AEwAQ Decreasing manifold vacuum is an excellent indicator of torque demand on the engine. That modulator adjusts line pressure accordingly. (Decreased vacuum increases line pressure) I wasn't there, but if all you did was increase the timing... I know that increased your manifold vacuum (unless you went past the peak vacuum) That vac increase pulled the modulator back a little bit more. Pulling the modulator back decreased the line pressure and that let's the converter relax.
So, most of the talk has been about fire breathers that are street driven, my question is this: basically stock 305/350 in light street roadster. The conversation here indicates I'd need a loose converter. Would a 10" Vega torque converter hold up?
When I first built and started driving mine a year or two ago now,it was a 350/350 combo with lakes pipes, and it drove great-never had the fighting the brakes problem I got later on after mildly hopping up the motor!
I think it would hold up fine. The problems start when you put a lot of torque and hp to them and beat them up. But on a street driven car there should be no problems. Don
I know this is a pretty old thread, but I'm going to ask a question anyhow and see what happens. I have a Chevy II 4-banger I built for the model A jalopy I'm working on. I'm going to use a Powerglide behind it. I also have a Powerglide spline Vega converter I want to use. Now, heres my problem and question. I know for pretty sure, when we put a Vega in the 396/T400 combo of my brothers Chevelle years ago, we didn't do any re-drilling of the flywheel. The one I have and the two my buddy has, don't have a bolt pattern (it's too small) to line up with any of the dual pattern flywheels we have. Anyone know what gives?? Thanks, Gene.
Nope. it doesn't work like that, vacuum modulator does not influence converter charge pressure enough to make any changes like that. Converter charge pressure is a lot lower than pump line pressure. Increased vacuum just makes the engine idle cleaner, with a little more power, and that is what will make for easier idling against a tight converter stall speed.
What is the LSA on that cam? I ran a cross ram on a 377, back in the 1980's. I could not get it tuned correctly until I changed the cam to one with a 114º LSA. After that, it was a beast.
I’m still amazed with the “speed shop down the street idea”. I don’t know of a speed shop within 100 miles of me.
Ok so What happens mechanically in the trans when the vacuum drops ? because you stick you foot in it,? what about a small vacuum leak in modulator line? how about the modulator line falls off or breaks? What about when it’s not even hook up but ports are plugged and no vacuum leak? Over here, when the vacuum isn’t on the modulator vehicles lurch and it’s hard to hold them from moving. Say it’s a truck, granted without the bed, and questionable rear brakes it just slowly spins the tires but putting vacuum back to the modulator stops that behavior.
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