Why do the TBI motors redline so early?

[FunInTheMud]

Hey Guys,

 

I was just wondering why the TBI motors redline so early.  4.5K is really early.  Would it hurt to take it further?  When I do a break torque, I usually peg the tach all the way to the end.  I hope they at least have a rev limiter in there to save my but I know my truck has a speed limiter in it.  It kicks in a 90 m.p.h., and there is still plenty of pedal left.  Anyway, just wondering if it would hurt the motor to take it past the redline.

 

FunInTheMud

[Won_Hunglo]

GM is trying to keep the engine parts inside the oil pan. If you over rev the motor them parts will leave the oil pan. That is a truck production 350 built for low end power not a race built 350 designed foe 7000 RPMs.

 

The Great Won

[FunInTheMud]

Hey Guys,

 

I was out racin the truck tonight.  I raced my friends 93 Sierra SLE, and of course, I won the race.  I hit 5K before I slammed it back into second.  Boy, did those tires squeal   Engine still runs fine.  It seems the harder you beat them, the better they run.

[dk1500]

This guy only wins for now!!!!

 

He didn't say that i have a bad clutch, x-cab, and a cab, there will be things done to prevent this again....

 

 

 

sorry bud!!

[Kansas Kid]

Honestly, the TBI motors are running out of steam well before they get up to 5000rpm.  The intake, fuel injection, etc just don't make power that high in the bad.  It would be much better for the truck to shift right at 4000rpm at full throttle.  I'm fairly sure they have a rev-limiter in the truck, but I wouldn't want to test it out.  Unless you like the idea of floating your valves (which can destoy both pistons and valves) or chucking a rod through the oil pan.  These motors are just not intended for high rpm operation.

 

btw, no speed limiter on my truck.  

[76BBSub]

Actually, holding the engine in gear up to 5000 or 5500 RPM probably won't throw a rod or float the valves, but you will be out of the useful power curve of the engine.  The TBI engines in the trucks have small intake/exahust ports and small valves, and a short duration low-lift cam.  They are designed for low end torque and not alot of horsepower.  It is easier to float the valves on a low-lift engine, since the springs don't need to be as stiff as with a higher lift, longer duration camshaft, but the low-compression ratio used with these engines leads to a fairly large combustion chamber so I doubt you'll do any damage.

Having said all that, Kansas Kid is right that you are most likely not gaining any acceleration by holding it in gear, unless you have really low (numerically) rear end gears...

Only way to know for sure is to go out and time it...wanna do some experimentation??

Jeff

[Kansas Kid]

Quote from 76BBSub, posted on Nov. 12 2001,07:32

Actually, holding the engine in gear up to 5000 or 5500 RPM probably won't throw a rod or float the valves, but you will be out of the useful power curve of the engine.

 

Yeah, I wouldn't think it would cause stuff that serious just holding it in gear that long.  Just thought I would throw it out there though.  I'd think he would cause more problems when he is doing his brake torques with the motor running at those kind of rpm for a long period of time.  But even then the odds of it happening aren't very large.

[FunInTheMud]

Hey Guys,

 

Thanks for the informative replies.  I didn't think it would hurt the truck to take it that far, but I thought I would ask.  According to my 1992 Chevrolet Light Truck Facts Book, my L03 305 TBI pust out 175 hp and 4000 RPM.  The torque is 270 ftlbs and 2400 RPM.  Now after compensating for 100% free flowing exhaust, and an air intake, I figure my truck puts out close to 200 hp and 300 ftlbs or torque.  At 2400 RPM, my exhaust is the loudest, loud enough to set car alarms off, so I figure I make the most power there.  However, if I try to do a break torque at 2400 RPM, the engine will most likely die.  Now, if I take it up to 4500 RPM, I can do a hellish break torque.  

 76BBSub, in the spring, I am going to take my truck down to Maple Grove Raceway, the closest Raceway to Lebanon, PA, and run my truck in the quarter mile.  Before I do that, I think I will try to shift at different RPM to see which one yields the best results.  I am very interested to see if shifting at 2400 RPM is better than 5K.  BTW the rear in my truck is a 3.73, but unfortunately it's open.  Kansis Kid, it's a Chevy motor, it can take loads of abuse, including my 30 second break torques at 5K.  And if it blows, it gives me an excuse to put a 350 in.  dk1500, I did forget to mention that you have a have a bad clutch, x-cab, and a cap, but I also forgot to mention that you have a 350, Posi, and a 14 bolt rear

 

FunInTheMud