Seeing I got the idiot part out of the way I have some questions, seeing I am not car person I know I will come off like an idiot.

1. What is the difference between Drive and Overdrive??? I have asked many car people this and have yet to get a solid answer. Is there a speed difference, MPG Diff, I just want a solid answer.

2. What is the diff between TPI or TBI (keep in mind I am a car idiot)?

3. I have an 84 v6 with T-tops what are some of the best ways to raise my fuel economy..or HP cost effectively.

4. Do you all hate me for asking what seems to me to be such stupid questions?

If anyone needs any computer questions answered no matter how stupid I'll be happy to try seeing computers are my forte.

 

1. In "Drive", your highest gear is such that the engine and driveshaft turn at the same speed. In "Overdrive", another gear is added in which the engine turns slower than the driveshaft, that the trans shifts into when it reaches a relatively high road speed. This helps improve gas mileage at cruising speeds. Many cars with overdrive will go faster in drive however, because the engine RPM is too low in overdrive to produce enough power to overcome the additional air drag at higher speeds.

2. TBI is Throttle Body Injection. It uses "injectors" that sit above the throttle blades, exactly like a carburetor's discharge nozzles do. TPI is Tuned Port Injection. It is GM's tradename for a multi-port injection system that uses long, small-diameter runners to produce a "ram-air" effect at low to medium RPMs, to help small motors produce torque more effectively. It has individual injectors for each cylinder positioned immediately before the intake valve; there is no fuel in the upper portion of the intake tract, as opposed to a carb or TBI. TBI was used in lower-performance versions of Chevrolet V8s. Engines equipped with it produced considerably less power even than some of the older carbed versions from the early 80s. TPI was used on the higher-powered versions of the V8 engines installed in these cars and Corvettes from 85 to 92.

3. First and cheapest, is regular maintenance. Keeping good spark plugs, clean filters, etc. will all help the car work as efficiently as possible. Synthetic fluids, especially motor oil and rear end grease, can make a significant improvement in both ecenomy and power by reducing friction losses. Next would be improvements to the car's exhaust system, which allows more efficient combustion by getting waste gases out of the engine.

4. Everybody starts learning somewhere. We all knew absolutely nothing whatsoever about cars at one time ourselves; hopefully we are all humble enough to remember that, and treat beginner-type questions with respect.

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"So many Mustangs, so little time..."

[This message has been edited by RB83L69 (edited January 02, 2001).]

 

Welcome to the messageboards. Don't ever think any question you have is "stupid," the people here are willing to answer all of them. As for your questions...

1. Drive is third gear, and overdrive is fourth. There is a speed difference, since when you're in fourth gear at 2000 rpm's your tires will be rotating faster than they would be in third gear at 2000 rpm's.

There are some older cars without overdrive in them at all. Overdrive cars will get better average gas mileage, as they can go the same speed at lower RPM's, thus saving gas.

2. I'm going to answer this question as basically as I can, not to insult your intelligence, but so I can try to make sure you understand. Both TPI and TBI are ways of delivering the air/fuel mixture into the engine. (Other ways are carburators, MPFI, EFI, and so on.. but you didn't ask about them, so I won't go into them). TBI's, or Throttle Body Injection, have two injectors sitting on top of the intake manifold. TBI's resemble a stripped-down 2-barrel carburator with injector nozzles stuck on them.

TPI's, or Tuned Port Injection, stick the injectors (I believe) in the runners of the manifold.. something like that. There's more injectors in TPI's than there are with TBI's... and because of this, more precise fuel/air measurments/deliveries can be made, thus generally resulting in better overall performance.

How can you tell what you have in your car? Well, if there's a sticker on the back bumper that says "Tuned Port Injection" that's a dead giveaway. Or.. look under the hood. If you have a large, black metal circle with a tube sticking out of it running to the front drivers side of the car, you have either a carburated engine or an engine with a TBI system. If you remove the cover of the air filter (the black circle), in the middle of it is either the carb. or the TBI. If it's got two injectors sitting slightly out of it with wires running off them, it's a TBI.

TPI systems are easier to spot. It's a rectangular metal device with several tubes running out of it mounted where the black circle of the air filter would have been. I always thought it resembled a facehugger alien clinging onto the intake of the car, but I'm a bit loopy.

3. A really easy way to boost performance/economy is a good K&N open element air filter kit. There's a tech article on how to properly install this, so I won't mention it here. It's about 40-50 bucks, gives you power you can feel (granted, not TOO much power) and improves gas mileage (a bit.) Headers are also a great way to improve economy and horsepower, as is exhaust systems... but these get to be a tad more pricey.

4. Again, nobody minds any questions you have to ask.

Computers are my strongpoint too.. but I got a camaro a few years back and have been slowly learning more and more about my car. (Very slowly, as the only tools I have are a screwdriver.)



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89 iroc-z 305 tbi
everything stock, except for a mirror that fell off

 

thanks for all of your help.. Well I have found out I a carb. at this point in the game it doesn't mean much to me but in the future I have a feeling that it will. I did an estimate on my mpg and I seem to be getting 12-15 a gallon. Hopefully I can manage to raise it at least a little but.

tanks again

 

If you've got the RPO sticker sheet from your car it will tell you exactly what engine you've got. Its usually located inside the middle console attached to one of the inner walls or could possibly be in the rear locking compartment. The engine designation will start with an "L" and you can decode all these with the RPO coder on the main thirdgen.org page. If you don't have one of these sheets then you can do what I did and take your registration down to your local Pontiac dealer and have them do a look up for you and print out a sheet. Didn't cost me anything (and really shouldn't) but they were pretty nice guys. One thing I could add to the overdrive definition is that it basically locks up the engine to the rear wheels. Your torque convertor (in your tranny) disengages and all power is transferred directly from the engine through a constant shaft to the rear wheels. I do believe that this can happen in 3rd gear also (D). Hope that helps, good luck with your thirdgen!

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"The world would have no problems if everyone had a presence of mind."
'91 Firebird
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'87 Firebird
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