Why can I put a 3.73 with my tbi but not with a tpi?

 

huh?

Can you be a little more specific?

 

i dont see why not it wouldnt matter if you had a steam powered motor.

 

hmm, why couldnt you?

 

TPI, otherwise known as "TUNED Port Injection", will enforce a certain rather specific torque curve; as long as those 22" runners are on there. In particular, the design of TPI causes the torque to fall off rapidly above the tuned RPM. Which means, you step on the gas and you get this great high-torque thing going on, and then at some given RPM (about 4500) the motor just runs into a brick wall and won't go any faster. The car won't accelerate again until you shift.

The torque curve it imparts to the motor is such that 3.73 gears take the motor up beyond 4500 RPM, out of the "optimum" RPM range, too quickly and easily.

It's easier to mod a TBI motor to not have that drastic torque fall-off. The only way to get around it with TPI is by de-tuning it; siamesing runners, changing the length, etc.

It's always better to work WITH the strong points of your induction system, rather than trying to force it to do something contrary to its design. A somewhat lower gear keeps TPI in its "ideal" RPM range, where it makes the most power.

 

Golden,

Is your car a daily driver?

 

sofa you have a good theory but ill make a few points. yes tpi lays down like a old dog. but with a 3.73 gear you turn you rpms up faster. you get a better launch quicker 60 foot. in other words more mph faster face it in a auto your going to be in 3rd before the end of a 1/4 even with a 2.77 gear. why not get what you can faster. i started out with a 2.77 gear in my car with a 350tpi a T/A stock as well with i think a 3.27 gear 350tpi ran a tenth or 2 faster consistantly. i have a 3.27 gear now but havent got to run with it but im not stock either. besides you get a 6speed and hsr you can run a 4.10 gear and have power at all rpms with just hsr and headers.

 

i never thought about this. i bought my car from someone who built it up a little. a zz1 motor (put tpi back on it) and a new 10 bolt rear with 3.73's in it. i guess this is a bad idea. and my next mod after i get the new tranny is in (he forgot that and it broke pretty quickly) would be to do the intake. would shorter runners, ported plenum, etc do it for me. or would you suggest something like hsr to match the gears? thanks a lot and sorry to hijack, but i think your question was answered.

 

hsr would go great with that zz4 it was made to work with up to 8,000 rpms. if never stops making hp the more rpms you turn the more hp you make. you need headers definitely with it tho. I didnt have money for headers when i first put it on i actually lost 1 tenth at the track with it without headers it was flowing more air in than my manifolds could flow out plus i had crappy exhuast. i put slp headers on and a new exhuast and cut 9 tenths off of my time. I have a 350 4 bolt main block im going to build when i get some money. its already bored and has new freeze plugs ect.. i plan on making a 383 stroker. my 350 will go in my T/A minus the hsr it will go on the 383. I kept all my stock parts becuase i planned to do it but i wanted to see what the hsr would do with headers and a stock motor works great. stock my car was 230hp at the fly of course now workin with 1/4 times speed and car weight all the calc's say i have around 330hp at the fly. never turned a head bolt and i still havent tuned it. i have a 58mm tb on it now and working on ramair i broke my trans so i got time. when taxes come in it will get a T-56.

 

sorry i miss read thought you said zz4 well it doesnt matter hsr and headers will make it flow and fast. if your worried about hood clearance it will fit it will touch but it wont hurt anything i ran it like that for over a year. you can get the unpolished hsr and fuel lines and rails for 900. its not nearly as much as a mini ram or anything like that and it makes more hp. you can mod a tpi and run fast but its limited. the hsr you could put on anything you built and it would work great. if i make no sense sorry i work a 12 hour shift and i have 2 kids that keep me up and im tired. and if you wasnt asking me sorry aswell. good night

 

TPI and 3.73 gears, IMO, just don't work well together. The engine in my Z28 right now has about the same powerband as a 350 TPI (as in... its done my 4500 rpm). I have 3.73 gears as well. The problem is that TPI can't spin very high, and to go fast, 3.73 gears need an engine that can rev.

As I said, my car has the same powerband as a 350 TPI, and I have 3.73 gears, and at the track (1/4mi) it litterally runs out of gear 200ft before the end of the track. It just stops accelerating. With 3.73 gears, the engine just tops out too quickly.

Thats why a gear set like 3.23 or 3.42 is a much better match for the TPI powerband. A taller gear set like 3.23 or 3.42 will take advantage of the low end grunt that a TPI engine makes, and use that torque to accelerate the car.