Experts on Carburetors and E85?

[pkjorlie]

I was looking at my Edelbrock carb manual and it says that if I want to get better economy I can change out to smaller primary jets. Also says if I want to switch to E85 I need to change out to larger jets. So what if I keep the same jets and try running on E85? What should I expect if I did this like overheating, dieseling, etc.?

[cgrey8]

Those are general recommendations with no quantity guidance at all. The amount smaller you'd want to go to get better fuel economy is MUCH less than the amount larger you'd need to go to run E85. My guess is if you plan to run E85, you'll need to go to a larger jet size regardless. Now how much larger will depend on what lambda (AFR) you want to run.

While there isn't much you can compare between EFI and carb other than they both control fuel flow, you can think of jet sizing as something like injector sizing. Smaller is OK for engines that don't need as much fuel. But with a built engine, smaller may get you better fuel economy, but may not be able to deliver the amount of fuel necessary at heavier RPM/Loads. If you have a progressive carb (secondaries larger than the primaries), then you can jet the primaries for fuel economy since they'll only be used for cruise...but with E85 will still need to be larger than they are for gasoline. Then setup the secondaries for whatever lambda you are targeting at WOT. However once you get jets selected, you still need to retune the carb to deliver the lambdas you are expecting at idle, cruise, moderate acceleration and WOT over the engine's RPM band.

Different carbs have different ways to adjust them. The carbs I dealt with were Edlebrock/Carter style which had the distinctive 2 screws right up front for adjusting idle. Then they had a tuning kit that had different diameter jets, different diameter metering rods, and various strength springs used to lift/lower the rods out of the jets to vary how much fuel flowed at various loads determined by vacuum. I'm sure some people could set them up purely by ear and smell. But I was never any good at carbs...never pretended to be. Even today, I'd need a Wideband O2 sensor (aka WB) to do any kind of justice beyond what the carb was setup for stock. In fact since I never had a WB nor did anybody I ever knew that ran carbs, the best recommended thing to do was not oversize the carb and run them as they were setup from the factory. And the only fiddling should be with the idle screw.

I do recall people used to love the Holley 650 for 350 SBCs and 351w SBFs, but I don't recall a single one running quite right. The Edelbrocks and Carters always seem to run better. My friend with the 327 '68 Camaro and my stepfather with an Olds 455 in a Caprice station wagon both ran the Edelbrock/Carter carbs and neither had a bit of problem with running them as-is out of the box. The 68 Camaro got 25 MPG at 75MPH which impressed us given it was only a 3 speed. But we were pretty sure the car originally came with a 396 and thus still had BB gears. I remember the 1st time he took it to the track, the car shifted into 3rd just as it was crossing the 1/4 line...and it still clocked in at 14.3 seconds. With better gears, it'd have done a bit better but wouldn't have gotten the fuel economy.