Is single turbo on a 305 TBI possible and if so what are the complications?

 

Its very possible if you have the money. However, people on the board have tried blow-through boost on TBI and everyone seems to think its a waste of money. One guy on here had a supercharged 305 TBI and it still ran 15s I think. For TBI nitrous seems to be the best power adder.

 

here is the boosted TBI thread:

https://www.thirdgen.org/techbb2/sho...hreadid=113322

I think its doable

since a TBI is so lackluster even a mismatched single turbo would likely add happy results on a small 305 if it was done the same way as was done for this supercharger. without a doubt nitrous would be the best deal. One of the carmagazines just swapped cam and vortec heads and intake onto a 305 tbi the other day with great results so dont forget to look into that.

 

I wouldn't do it. I would get a better EFI first befor putting a turbo on.

 

Do it… The TBI setup is much more flexible then people give it credit for and the farther upstream you inject the fuel the more of an intercooling effect it gives under boost.

 

Yes, it is totally doable.

You have to modify the housing at each end of the trottle shaft. You can see the Vortech instructions on how to do this on a TBI. ( drill two holes )

Install a better fuel pump.

At low boost levels you can use an FMU for extra fuel. You could also install larger injectors and replace the MAP-sensor with a 2-bar unit and tune for boost.


TBI respond just as well as any other system to boost.

At 7psi boost you get about 50% more power.

If you start with less you get less but it still works.

 

the procharger kit for TBI trucks doesnt even suggest anything like that because the throttle shaft leaking could never out leak the boost pumping in. It would be as miniscule as the vacuum that it leaks already.

 

It is not because of boost leaking out. It is because you have the injector before the throttle.

When you close the throttle you have fuel and boost above the throttle. This will force the fuel out along the throttle shaft.

This is not a problem on an NA engine because engine vacuum will suck air in along the trottle shaft.

 

sounds like a very good reason, but I am very curious to know why this was never a problem on the procharger kit. it blows down throught the TBI with a cap just like the vortech. I wonder how critical it is? I would try it without the mod first just to see how it happens.

 

It probably depends on how much play the throttle shaft has.
It might be a problem on a worn TBI, but not on a new one.

 

If you put forced air on a 305 tbi, and put it on a 305 tpi with the same boost, there is no way the tbi will make the same power. What is the cfm to a tbi thrittle body, maybe 500cfm. A tpi with an air foil is around 700cfm. More air=more power(with the needed fuel increase) This not what i think, it is just the facts.

 

True, but that is beside the point.

In that case you should not turbocharge a TPI either ... because the LTI / LS1 flows more and will make more power ....

Can you turbocharge a TBI? - Yes you can!
Would a TBI make more power with boost? - YES it would make more power!

When tuned an engine will make about 50% more power at 8 psi boost.

Stock 1988 L03 (305) TBI = 170 hp
With 8 psi boost 170 *1.5 = 255 hp

Stock 1988 L98 (350) TPI = 230 hp

A junkyard turbo 305 TBI will leave a stock 350 TPI way behind and it could be done cheap.

 

I never said you cant do this. It just would not be as effeicent as a tpi. And my car with a catback and an air foil, ran 13.8 @98. My freinds 305 tbi formula runs 16.2@79. Thats almost a 3 second and 20mph differance. If a 5lb turbo kit makes a 16.2 car leave a 13.8 in the dust, my car now, would run 9s with 15 lbs of boost. LS1s are insane. There in a class of there own. This proves my theory even more. LS1=lots of power, LS1= lots of air flow. Go to ls1motorsports.com and look at the trans am project car, Stock LS1 with a single turbo, 600+ rwhp! It has 130,000 miles on the motor too.

 

Another bad thing about tbi is that tbi's dont distribute fuel very evenly. Having a fuel injector at every cylinder gives you perfect fuel distribution. On a car, especially turbo because of the extra heat, you dont want any lean cylinders. Not trying to be a jerk, just trying give the guy some info on turbocharging his car. If he is puting a turbo on the car, he probubly wants to go fast, right? So why not save money and do it the best way the first time.

 

Huh, never seen this… I know what they're doing but I'd be interested in seeing some exact details. Are they online somewhere or someone have a scan?

Given. In most cases you could get away with a relatively small inline pump or even a stock TPI (or replacement) intank pump, no big deal.

One of these days I'll figure out how to transpose map tables for this kind of stuff. As far as the FMU goes, it won't take much, something in the 1.5-2.5:1 ratio will work.

Lets assume that these numbers are right (not that they have to be, I've opened up TBI TB's to 54mm, making them right in between the big and biggest TB available for the TPI, and it's much easier upgrade the manifold to something that flows that well with the TBI, not so with the TPI), 500cfm is more then to feed a 305 at 100% VE at 5000rpm, making the assumption that the engine is actually at about 80% VE at redline that's enough flow to not be a restriction till 6200rpm. BTW, either intake/TB will outflow what the stock TPI heads will flow.

Boost becomes the equalizer anyway. Put the same heads and cam on the TBI engine, you'll have an engine which should respond faster then the TBI and tolerate slightly higher boost, but otherwise breath the same.

eh, close… (assuming 70% adiabatic efficiency) it will be about 30% more or about 46% more with a 70% efficient intercooler.

Cheaper then with a TPI.

Evenly? Fuel distribution differences with a good intake are typically less then the flow differences between injectors (some of the new aftermarket injectors can have flow variations % in the teens, right out of the box). Add to that that until the LS1 intakes, GM v8 MPI intakes were much more flow distribution challenged then most "wet flow" intakes that you'll run across. Add hot air from a turbo vaporizing the fuel at the TB + the pressure eliminating some of the pressure drop when you open the throttle blades (eliminating some of the need for a pump shot…) and you've got a very sweet setup for a boosted application.

"best way the first time…" what would that be? Personally, for a street car done on the cheap I'd rather start with a TBI… but I'm weird that way (and before you comment on what I own… currently the f-bodies that I own are TPI and LT1…)

 

What ever. Build a tbi turbo, push it to its max. airflow and then move up to a higherflowing intake. I couldn't care less what you think. I just think it would be stupid to turbo a tbi. Thats just me.

 

It is very hard to tune any TBI system with forced induction. When you burn your own chips you will understand. TBI is not worthless. Although not the best, people who don't understand it knock it; and they are ignorant.

 

This is what I did:

http://www.joby.se/corvette/mods//2001-0X01_tbi_mod/


It is in the Vortech installation manuals
http://www.vortechsuperchargers.com/support/

You can see it in the " 1990-1995 5.7 TBI Truck/SUV ", page 6
http://www.vortechsuperchargers.com/...s/4gb218im.pdf

 

Jonas, thanks for the links… that vortech manual has some rather interesting ideas. As far as your TBI modification… 2 comments:
- it won't work on an L03 or L05 TB, there is no passage below the throttle shaft like on the crossfire (for that matter, my crossfire TB has been opened up to the point where that passage is barely there)
- drilling that hole there doesn't accomplish what you want it to do. You want to route pressurized air without fuel to that part of the shaft, so what gets inevitably blown out is just air, no fuel. The air that you're forcing around your shaft is coming from below the injector and has fuel in it. The vortech setup works because they add the spacer which is drilled with a passage that you connect to the outlet of the supercharger that pressurizes the shaft with "clean" air.

If it really bothered you I'd probably try something like drilling from the sides of the casting at an angle. There is room to do it from the top also if you haven't bored your TB.

I suspect that most people woudn't worry about it, (TBI or Crossfire) though technically it could cause a fire/explosion under the hood.

Huh, I wonder, is there room to run a spacer between the TB and manifold on a L03? I know on the crossfire cars it's tight (I managed on mine, but I don't think that the spacers that I ran would fit on a 'vette), then you could just run something like I made for my crossfire but with 2 bores and drilled for the pressure passage.

 

Huh, I just had another thought… looking at the gasket pattern of the 2 bore TB, I bet that it could be drilled from the outside from the front of the casting. Maybe if I get the chance I'll take a look at it on my truck.

 

You can do two things, or both.

1) Apply 'dry boost' to the trottle shaft to keep the fuel from blowing out.

2) Apply engine vacuum to the throttle shaft. The vacuum will be high when the throttle is closed. Both dry outside air and wet air from above the throttle will be sucked into the manifold. This is what I did.

The best solution would be to apply boost near the end of the throttle shaft, and vacuum between that port and the throttle bore.