How much RPM drop from park to drive with stock type 340 convertor

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340doc

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We put a 340 with a 904 in my sons Dart, stock engine with 4 spd type cam. The trans and convertor were bought used. The convertor was supposed to have a 2500 stall. I tried to find a number or something on the convertor to verify but no luck, and I didn't think to measure it at the time. It took 7/16 convertor bolts, so I know its not a factory one. I figured stick it in and see. Well the trans seems to work fine but the convertor seems real tight. I get a 300-325 rpm drop when it goes in gear and its definitely not a 2500 stall. What kind of rpm drop would I see with a stock 70 340 convertor? Thanks.
 
Not an expert at this.... 100-200 RPM max.
But that is what I get on stock vehicles at worst and my wife’s [email protected] cam in here 360. Performance cams, even small ones may drop more but I don’t think it should if the right converter is in there.
 
Hold the brake as tight as you can to prevent the wheels from spining. Put the throttle to the floor and see where the tach stops. That would be your brake stall. Any stock performance motor should have at least a 3000 stall converter. Different motor combos stock or not will stall different. The highest I saw a factory 340 converter stall with my own eyes was 3100. the lowest was 2700. Normal is around 2800. I have a 80's 318 4bbl police 904 in a duster right now and it stalls at 2500 but it spins the tire. Once the car gets a suregrip it will probably go a little higher.
 
Not an expert at this.... 100-200 RPM max.
But that is what I get on stock vehicles at worst and my wife’s [email protected] cam in here 360. Performance cams, even small ones may drop more but I don’t think it should if the right converter is in there.
Yeah, my buddy thought no more than 200 also. I don't have access to something to compare it to. The higher idle in park required for a good idle in drive is uncomfortable. I guess its coming out.
 
Hold the brake as tight as you can to prevent the wheels from spining. Put the throttle to the floor and see where the tach stops. That would be your brake stall. Any stock performance motor should have at least a 3000 stall converter. Different motor combos stock or not will stall different. The highest I saw a factory 340 converter stall with my own eyes was 3100. the lowest was 2700. Normal is around 2800. I have a 80's 318 4bbl police 904 in a duster right now and it stalls at 2500 but it spins the tire. Once the car gets a suregrip it will probably go a little higher.
Man that's some good stall from a stock converter. A friend offered me a 3500 to try so I'll probably do it. The car will never see the track but I want it to do what its supposed to when you wack it from a light. I will ck the brake stall but not expecting much. Thanks OMM
 
Put it in drive, putt along about 10 mph and stab the gas, watch where the tach needle jumps up to.
That is the flash on the convertor. At the end of the day, that is all that matters.
Where the car wants to creep past where the brakes will hold it is entirely case by case. How good are brakes, loose is convertor, size of tire,etc... Hard to draw much conclusion on what the convertor is based on that.
I have had 5k convertors i couldn't hold without creep at 3k....sounds like what you have wont be much different than what a stock one will do.
 
I believe the "Hold the brake, florr the gas watch the tach" is called a "stall speed " test. it is used by tranny guys to check the converter.

here is a description from the web:

Put the pedal to the metal While pressing on the brake pedal, press the accelerator to the floor for two to three seconds. Don't exceed five seconds, or you risk blowing out the transmission. The RPM the engine maxes out at is the stall speed.
 
I believe the "Hold the brake, florr the gas watch the tach" is called a "stall speed " test. it is used by tranny guys to check the converter.

here is a description from the web:

Put the pedal to the metal While pressing on the brake pedal, press the accelerator to the floor for two to three seconds. Don't exceed five seconds, or you risk blowing out the transmission. The RPM the engine maxes out at is the stall speed.

i will stick with how to test in my earlier post. Everybody i know checks a convertor that way.
What you are talking about has a ton of variables, that i listed. Just changing tires or brakes could change that number, irregardless if zero changes to the drivetrain.
 
i will stick with how to test in my earlier post. Everybody i know checks a convertor that way.
What you are talking about has a ton of variables, that i listed. Just changing tires or brakes could change that number, irregardless if zero changes to the drivetrain.
Wasnt arguing just saying that is how its done in the transmission repair business. That test is as old as dirt. :)
 
2500 is a stock converter for a Mopar.
 
When I say hold the brake tires don't move or the car. That is brake stall. If the tires move the test won't be accurate. Flash stall is at idle just tromp the throttle and watch the tach . That is flash stall. Weather you are rolling or not at idle when floored the tach reading is flash stall. Here I am flash stalling and trying to brake stall but could not hole the car back so brake stall is not accurate. It just started pushing the front tires.
 
Put it in drive, putt along about 10 mph and stab the gas, watch where the tach needle jumps up to.
That is the flash on the convertor. At the end of the day, that is all that matters.
Where the car wants to creep past where the brakes will hold it is entirely case by case. How good are brakes, loose is convertor, size of tire,etc... Hard to draw much conclusion on what the convertor is based on that.
I have had 5k convertors i couldn't hold without creep at 3k....sounds like what you have wont be much different than what a stock one will do.


^^^^^^^THIS^^^^ is the correct way to do it. As he mentioned, you take the variables out of it.
 
We put a 340 with a 904 in my sons Dart, stock engine with 4 spd type cam. The trans and convertor were bought used. The convertor was supposed to have a 2500 stall. I tried to find a number or something on the convertor to verify but no luck, and I didn't think to measure it at the time. It took 7/16 convertor bolts, so I know its not a factory one. I figured stick it in and see. Well the trans seems to work fine but the convertor seems real tight. I get a 300-325 rpm drop when it goes in gear and its definitely not a 2500 stall. What kind of rpm drop would I see with a stock 70 340 convertor? Thanks.
The drop in rpm at idle has only a small relationship to the change in converter.
I'll bet the drop at idle is mostly due to the engine and the idle tune.
A cam similar to the factory cam for a 68 340 with 4 spd. it was slightly more aggressive than the auto cam.
Retune for minimal idle rpm drop.
If that works, then a slight recurve of the advance may be neccesary. That will depend on what the distributor has in it to begin with.

In general people set the engines up with idle mixtures that are relatively lean and therefore lose power. Since there is not much power at low rpm to begin with, a few hp difference can be the difference between the engine easily turning the transmission pump or not.

Try making the idle mixture richer, and find an initial timing that matches the somewhat richer mix.
The real test is when the transmission is in gear. Without that load the engine can be run leaner and with more timing than when loaded.
To give you some idea for a starting point, a non smogged 4 bbl small block had an initial timing of 10* BTC around 600 rpm. Plus/minus 2* always allowed for specific engine and local fuel, etc.
It's safe to assume your 340 with its cam has more reversion at idle, and probably no more compresion than the 273. So, most likely the best initial timing for your engine will be in the 12 to 16* range.

Make small changes and see if you can reduce that idle drop.
You'll burn a bunch of fuel and there will be some steps back as well as some forward.
Make sure the engine is warm and if the rpm changes recheck the timing. Sometimes we set 'initial' but the distributor has already advanced a little. That throws the whole testing off and can be frustrating.
For example timing is set 15* at 900 rpm, but the weights in the distributor move out at 650 rpm.
 
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At what rpm do you idle at?
This maybe an issue worth chasing!
 
I'd set timing at 15* and 750 rpm and see what you get. It should run fine right there with minimal drop if everything else is in order. As Mattax mentioned, check timing both in gear and in park, they should not vary.

Need to make sure your total number doesn't go too high if adding initial timing. If it does, you need to limit mechanical advance.
 
The drop in rpm at idle has only a small relationship to the change in converter.
I'll bet the drop at idle is mostly due to the engine and the idle tune.
Mattax nails it!

The idle drop is caused by one major thing; and maybe two other minor things.
1) transfer slot sync, and
2) transfer slot sync, and
3) probably you guessed it, the transfer slot sync.

What I see most of, is guys throwing too much idle timing at the engine, which then causes the slots to be too short, which leans out the idle, requiring major compensation from the mixture screws. And BadaBoom she idles.
But then you put her into gear, and just like that, the airspeed slows and the fuel stops flowing, and she stalls, or near-stalls.
Your engine does not need 20* of idle timing.
It doesn't even need 18 or even 16.
It needs whatever it takes to get the transfer lot working right.

Lest you doubt this;
my manual trans 367 will pull itself around, on level flat hard pavement, at 550 rpm, @5* idle-timing, with a 230*@.050 cam in first gear with a 10.97 starter gear. It also did that with a 292/292/108 cam, but at 600rpm with a 9.44 starter gear.
The point is, let the engine tell you what she wants for timing by first establishing the transfer slot exposure under the primary throttle blades. Then don't be in a hurry to crank up the advance.
For your combo, if no one has mentioned it, I would set the transfer-slot starting point, to square; that is to say, just as high as wide, and put the mixture screws to the center of their adjustment range, about 2.0 turns on an AVS-type carb. After this; DO NOT Touch the speed screw! Set the idle-speed with timing. Do not look at the number; just push/pull the Vcan until you get 600 in gear.

That will be your starting point.
Now put it into Park/Neutral and see what happens.

Then go back to post #15
 
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When I say hold the brake tires don't move or the car. That is brake stall. If the tires move the test won't be accurate. Flash stall is at idle just tromp the throttle and watch the tach . That is flash stall. Weather you are rolling or not at idle when floored the tach reading is flash stall. Here I am flash stalling and trying to brake stall but could not hole the car back so brake stall is not accurate. It just started pushing the front tires.


Seen that. Don't mind watching it again!!!! Too bad it's low rez. Is that mill goin' back in the new car?
 
The idle drop is caused by one major thing; and maybe two other minor things.
1) transfer slot sync, and
2) transfer slot sync, and
3) probably you guessed it, the transfer slot sync.

What is 'transfer slot sync' ?

Is it the 'off idle' circuit - idle air bleed ?
How does drilling holes in the primary throttle plates work ?
 
I tune the carburetor on auto transmission cars in gear.
 
What is 'transfer slot sync' ?

Is it the 'off idle' circuit - idle air bleed ?
How does drilling holes in the primary throttle plates work ?
Back in the day drilling bleed holes in throttle blades was "common" to help idle with a wild cam. If you upend the carb (or just google it) there are slots that the throttle blade passes near the idle point. These are the transfer slots and help with just off idle and idle to a bit open transfer into the higher speed carb circuits. If you have to crank the idle screw too much open to get a wild cam to run, the transfer slots will be "out of" the range of the throttle blades. But nowadays many of us have wised up. Run a short VERY short distributor curve so you can run LOTS of initial timing. This helps fight that problem and allows you to close down the throttle. There are exceptions. Way too much compression ratio with certain cam profiles will generate lots of low RPM compression and you will have starter "kick" on starting........which can literally break a starter/ gear/ flywheel, and might have "ping" problems at low speed.

NOTHING is worse for park-to drive drop than the 70's--80's retarded timing and really lean idle mixtures.

But you cannot just bump the timing way up. It will over--advance. You need to "recurve" the distributor
 
1) What is 'transfer slot sync' ?
2) Is it the 'off idle' circuit - idle air bleed ?
3) How does drilling holes in the primary throttle plates work ?
As to #3; Yur getting ahead of yourself, that cam may not need any additional bypass air to what the PCV will supply.
As for #2; no, none of those
As for #1; if you have a Holley, remove the carb, drain it and flip it over. Look into the primary throttle bores up where the mixture screws are. Close by, you will see a couple of vertical slits. These are your transfer slots. These are your main fuel supply for low-speed operation. The mixture screws are basically just idle enricheners. But if your throttle blades are not in the right place on the transfers, at idle, you can make it idle with the trimmers, but as soon as you open the throttle, the mixture is wrong, and so the engine complains about it right away.

At idle, your engine will like lots and lots of advance, maybe mid 20s to even 30s. To see this, just tug on your Vcan until the rpm peaks, then read the timing.
But you can't set it there with a non-computerized timing, vehicle, because the Power-timing will be all wrong, and the thing will likely detonate itself to death.

So, I usually recommend the transfer slot exposure, underneath the blades, with the throttle on the curb-idle screw; be adjusted to a lil taller than wide. In your case with a more or less stock engine, Ima thinking closer to square will do it; but it's just a best guess. We can figure that out later; the engine will tell us as we go along.
Now, when you put this carb back on, the idle-speed is apt to be way too fast, and to slow it down, you will have to reduce the timing.

Once the T-slot exposure is set, Do NOT Mess with it. Use the timing to change the idle rpm.

Don't even look at the timing, just push the Vcan back until yur speed is in the ballpark. Mattax likes 750 in N/P, which is a good number, but I like 600/650 in gear. We'll let the engine tell us what it wants as the tune progresses.

OK here is the theory;
By retarding the idle-timing, we are reducing idle power, which with a fixed throttle blade, stabilizes the airspeed past the transfer slots. Now you an use the trimmers to compensate for the airspeed loss when you put it into gear. If your trimmers get out of range, either too far open or too far closed,
THEN, we will change the transfer slot flow, to get the trimmers back into a working range.
If it should come about that we cannot get into range
Then, we will have to find air elsewhere, which we will call idle-air bypass.

I had a 4-speed type cam in my 360; a Hughes HE2430AL. the specs were 270/276/110, and 223/[email protected]. I put a Holley 750DP on it and it required NO idle air bypass.
Your engine may vary,lol.
 
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What Carburetor are you trying to tune. There is a difference between a Holley and carter.
 
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