OD power loss

Pandersen33

Pandersen33

Member
I seem to be having an issue and cant quite pinpoint where to go with it. My car has plenty of power going through the gears including 2nd high but *** soon as i go to 3 high it just dogs. Loses power and i have to switch back to 3 low. Motor was rebuilt with snyder head, oiled mains, inserts, weber carb and fsi distributor. Ive messed with everything and cant figure it out. Anyone have the same problem? Thanks
 
Do you know what gearing your overdrive is, and what rear axle ratio you have? Most Model A’s have the stock 3.78 gearing, and this works well with OD’s that are around 26%. If you happen to have an OD that is 36%, it will be more difficult pull well, unless you are on flat ground. If your car has higher gearing, such as a 3.54 that combined with a 36% OD would pull even worse. The Snyder HC head help quite a bit. Be sure you are not shifting into OD until you are traveling at least 40 mph.
 
Its a 3.75 i believe with 26 OD. I tried shifting into od at 45 and 50 all the way up to 55. 3 low cruises all the way up to 55 no problem then i shift into OD and it drops to 50 ish and wont accelerate unless im on a decline. 2 high has no problem accelerating. I tried timing at o advance at idle as well as timing from full advance with 25 and 28 degrees. Im stumped.
 
Try putting the original carb and manifold back on and see how it runs. If that doesn't help, put the original distributor back on and try it. Is the compression within specs? What is the RPM range of the cam? If it's a high RPM cam, it will be a dog at lower RPM. Suppose the cam reaches peak power at 3,000 RPM. In overdrive you may be only turning 2,000 RPM. Thus the engine is way under it's power band.
 
Pandersen33 said:
Its a 3.75 i believe with 26 OD. I tried shifting into od at 45 and 50 all the way up to 55. 3 low cruises all the way up to 55 no problem then i shift into OD and it drops to 50 ish and wont accelerate unless im on a decline. 2 high has no problem accelerating. I tried timing at o advance at idle as well as timing from full advance with 25 and 28 degrees. Im stumped.
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I see where you did not mention camshaft.


While many people struggle with believing this, it is my opinion that a camshaft is a better performance gain than a high compression cylinder head. As mentioned above, this Model-A engine was designed around the premise of strong low-end torque. When you compare dyno performance numbers with a NOS stock camshaft vs. a reground 'Touring Cam' that the majority of the engine rebuilders use, you will see the max torque is above the useful range of an overdrive-equipped Model-A. Again, I do not know which camshaft you have inside your engine, but I am wondering if that is your issue. Also, aren't you the one that was wanting to use a MG carburetor on your car?
 
The cam is important but it is the overall combination that determines performance. Terry H nailed the gearing. A good, stock B cam is almost ideal for normal driving and especially with OD. The latest B cam had more lift. .339" and slightly more duration than earlier B's and are the best, but are harder to find.
 
Aside from the fact of making sure your timing is correct and checking he health of the engine by checking compression, taking s vacuum reading for reference, plugs and points properly gapped, I think you are out of the power curve for the axle/iverdrivecratio forcthr power of your engine. A high compression head will help a little but it may be your rear axle ratio as terry indicated as well as a 26 percent Is probably better suited to a sedan .


Hopefully you can figure it out and make appropriate changes.
Larry shepard
 
Your OP is not clear, has the setup been running properly and then this just happened? Is this a new setup? Did the performance decline gradually? Suddenly? Answering these questions will, I believe, give you more accurate answers from those trying to help.
 
Perhaps I missed it, but what bodied A are you driving?

A large sedan, or smaller like a coupe?...gearing could make a big difference here.
 
Pandersen33 said:
I seem to be having an issue and cant quite pinpoint where to go with it. My car has plenty of power going through the gears including 2nd high but *** soon as i go to 3 high it just dogs. Loses power and i have to switch back to 3 low. Motor was rebuilt with snyder head, oiled mains, inserts, weber carb and fsi distributor. Ive messed with everything and cant figure it out. Anyone have the same problem? Thanks
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If your FSI distributor uses a centrifugal spark advance, and you no longer have the spark lever advance, that may be your problem. When you engage the overdrive, your engine RPM drops and the spark advance may fall back. If it does, you will lose power.

Talk to the FSI people about this. They may have a slightly different centrifugal advance setup to use with an O/D.

Also, what compression ratio Snyder head are you using?

And what muffler?
 
I'll be glad to know what you find out from FSI. Just got one of their distributors. Not sure if I'm gonna use OD yet, but will be good info to have if I decide to.
 
I believe it is a 6:1 head. Lightened flywheel. Tudor sedan. 26% OD. I have not talked to FSI yet as i have been too busy. I cant imagine being out of the power curve since it runs so nice all the way up to 55 u til I shift. Switching back to the original distributor is not really an option since the reason i upgraded was the old one was pretty trashed
 
I recommend you connect up a tach on the engine, even if just temporary, and measure/record some actual RPMs in 3rd gear at 55 MPH and then again after you engage the OD.

I also recommend you jack up one rear wheel and hand crank the engine, with the transmission in 3rd gear (1:1 inside the transmission) to determine what the actual gear ratio your rear axle is. At this point you can't be guessing or assuming. You really need to know, or you'll not figure things out.

And talk to FSI, after you have actual RPM numbers.

.
 
Might be worth checking the overdrive for stiffness first, and oil.

Take the plugs out of the engine, put one back wheel up on an axle stand, 3rd gear, no overdrive and see how easy it turns over on the crank handle. Then do the same in overdrive.
 
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