How much better highway gas mileage can I get by changing my fixed pitch turbo 400 over to a switched pitch 400 in my 67 LeSabre 400 w/a stock high compression 340 under the hood and stock gears out back? Currently I average around 13 on the highway. Patrick
What fixed pitch converter do you have?? If it's a stock converter, they stall around 14-1600 RPM, and you would gain nothing with a switch pitch, as far as gas mileage goes. Stock gearing on your car is what? 13 MPG seems a little low unless your gearing is 3.42-3.73, which I doubt. When was the last time you checked your vacuum advance/ Are you sure it is working?
Wait isn't a fixed pitch converter just a standard turbo 400? I thought the difference was the pitch. The engine is totally stock but the carb is an edelbrock installed before i owned it and might need to be rebuilt. I've asked about the rear gear ratio for the LeSabre 400 and didn't get a response so I'm really not sure what gears are back there but I'm assuming its between 2:93 and 3:08 because it seems to rev around the same on the highway as my Apollo did (I'm guessing by ear), and the Apollo had 2:73 gears outback and a turbo 350. I read in another article that someone with an SP 400 was able to get 26 miles on the highway. It was in the article on www.buickperformance.com. Patrick
No, both the fixed pitch, and switch pitch are THM 400's. The switch pitch uses a different front pump, input shaft, and converter. The switch pitch converter has stator blades that can change angle. The switch pitch converter has a piston concentric with the converter hub. The piston operates the stator blades, and moves them to 2 different angles, giving 2 different stall speeds. Gives you good launch, and maximum efficiency with low stall, once the car is moving. Any THM400, can be converted with the right parts, and a switch pitch converter. There are several different switch pitch converters with different stall speeds. I had a 13" converter with 2300/1100. I now have a 12" with 3200/1800. Your stock converter isn't hurting your gas mileage, unless it is slipping, and you would gain only performance by going to a switch pitch 400. I would look at your ignition, spark curve, and vacuum advance, and make sure the jetting on that Edelbrock is optimal. My friend Ron (Tufbuick on V8) has a 70 Electra with a 455. His vacuum advance was defective. When we replaced it, his highway mileage went from 12 to 16 MPG
Wow thats a lot of info thanks :Smarty: . I have a tax return coming to me so I think I will check into both of those. Thanks A LOT for your help, :beer :3gears: :TU: Patrick P.S. can a Switch Pitch be installed by a transmission shop or is that something that should be done by someone who knows the Switch Pitch system?
It involves changing the front pump and input shaft, and the converter. It involves transmission disassembly so it is best done at rebuild time. There is also a passage in the case (behind the front pump), that has to be blocked. Alot of trans shops don't have the experience, so you have to ask, if they are familiar with it. But even if they aren't, if they are competent, they should be able to handle it. There is a thread in this forum on switch pitch conversion, take a look, it's recent.
