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