i know everyone says to go with a 52mm t/b if ur motor is under 383 ci. i don't see what is wrong with running a 58mm t/b on a 305 or 350 tpi motor, especially if ur running nitrous. can someone fill me in on this. and the reason i'm asking this is because a friend and i got into a big argument bout this subject.

 

Well...At low rpm, you go from low kPa to 100kPa with very little throttle movement, making driveability 'worse'. For example, with a very large throttle body you may get 100 kPa at 20% throttle at 2000 rpm. This means if you want to hold it at 40 kPa for cruise, you have to be very steady on the throttle, as small movements may produce large changes in engine output (so it's harder to be smooth), and
A small throttle movement (and a small V/sec TPS signal change) can result in a very large change in MAP (as mentioned above) at low rpms. The result is little (or no) accel enrich when the engine needs it most. However, you can usually tune around circumstances like this by richening the VE table at low rpms and higher kPa (say < 2500 rpm and> 70 kPa) by about 5-7%. This has a negligible affect on fuel economy, since you likely never see 70 kPa while cruising.

 

ok i see where that would make a differnce with speed density but what about mass air.

 

I had a 48 mm on my 305, then, I bought a 52 mm for my 350, then I had to buy a 58 mm for my new engine. My advice is buy the big one and be done with it. Drivability is not bad. Save some bucks. Wish I did!

 

i thought the 48 was a stock t/b

 

Yes it is, on a 305, and 350. I meant that the 48 is what it had originally. I bought two TBs when I could have just bought the big one in the first place. That was my stupidity though, knowing I would not leave well enough alone with the 52. If you dont plan on ever building a big cube engine, then you would be doing the right thing with the 52.

 

the only big inch motor i build will be going into my 68, but my wife is wanting athird gen t/a and maybe run nitrous. i know nos makes a 58mm t/b for the lt1's running nitrous, so i wasn't sure if i would be better off running a t/b that big .

 

Very, very few people on this board need a 58mm tb. But there are alot of people running one. In doesn't help make you any faster to run a big throttle body if you don't need it, but you do lose some driveability as mentioned before. Think about what exactly your engine goals are, and do some searching on this site for flow numbers. See if you really need one. The stock 48mm throttle body isn't a restriction until you are running 12s.