Purple 92 SS

Ive been contemplating switching over from a Carbed setup to a TPI setup. Heres what i have, some background about the car, and what not.


Have currently :

355cuin motor with comp Cam 292 H, Edlebrock Performer RPM 73cc heads, (aluminum), Edlebrock Performer RPM Air Gap Intake, and a Holley 750 Vac Sec. Carb.

Thinking of going to:

Stock 85-92 TPI setup, to be modded at later dates...

Pro's & Cons:

Heres what i need help with, would the cam be too much for the intake, and what effect would my cam have on this intake.

I figure i'd need everything from wiring harness, computer, full TPI setup, and even the Distributor possibly.

Is there anything that I am forgetting?

Obviously i want to go fuel injected for better drivability, and what not, but im still a bit concerned if the TPI will do waht i need it to do.Any Insight is appreciated, as ive never messed with TPI at all before..

History of car:
Was once born a fuel injected 92 v6... soo... thee computer is currently wired to think it is all there even though it isnt.

-steve

contactpatch

You should think about trying a reasonably sized carb,
such as a Edelbrock 600, or a q-jet.
[something with reasonably sized primaries.]
Use the big one, for the strip.
Perhaps you could borrow one, for a while.

Purple 92 SS

the car is not a daily driver.. a 600 would be ok, but if im doing anything to the intake setup at all, its dropping the carb.. dont want to continue to run carbed at all really.

-steve

TKOPerformance

Your engine revs too high for the TPI setup most likely. A TPI on a 350 basically craps out at about 5,200 RPM. With your cam you should be pulling a lot closer to 6,200.

If you're dead set on the TPI stay with the later SD setups, as they are easier to tune and more reliable.

In truth you'd be better off with a aftermarket programmable system.

KiLLJ0Y

you want the best bang for the buck, that revs higher than TPI, then do the LT1 intake.

contactpatch

...
Radical cam in a speed density system, can be tricky., to
get the engine to idle.

Purple 92 SS

ive thought about the Accel Superram but wasnt wanting to drop 2 grand to do it all..

the LT1 is interesting to me.. i dont know enough about it though to know the pros/cons of it and if i should do it... The car does pull high RPMS easy now, and if thats as high as the TPI goes then i guess i cant use it at all..

Thanks.
-steve

ChevelleFan

iTrader: (4)

I'm in the middle of swapping my carb for an EFI setup on my Chevelle. You said you didn't want to drop $2000 into an EFI setup. You should be prepared to spend at least $1000, by the time you are done. If you get it done for less, you must be a really good parts scrounger. There are tons of minor things that can set you back. It's more than just an intake swap. Ask me how I know..


Yes, you can buy complete intake setups on ebay for around $300, another $50 for a computer, a wiring harness for at least $100, a distributor for $40-50. Then you need to piece together a hi-pressure fuel system. EFI runs at 40-60 psi of fuel pressure. Plain rubber hose won't cut it. Then, you'll need an air-intake setup. And the throttle cable from a carb setup is too short. Did your used TPI setup come with all the bolts? Did it come with a set of plenum-to-runner and runner-to-base gaskets? It all adds up.

Everything you'll ever read about TPI will mention how badly it is choked over 5000 RPM. I can't personally verify it, but I've never read that TPI is RPM-friendly. Certainly your cam will produce it's peak power well over 5000 RPM. I decided to buy a Holley Stealth Ram, as I didn't want to discover that I had crappy high-end power after doing all this work. I have a 406, so I'm not too concerned about any lack of low-end.

And GM computers, while readily available, sound like a pain-in-the-*** to tune. I haven't done it, but I've read about it. The folks in the DIY-PROM board sound pretty knowledable and can point you in the right direction on how to tune the computer. I'm going with a MegaSquirt computer for mine. The Holley Commander 950 seems like a popular, capable computer as well.

Going to EFI ain't cheap. I'm just writing this to point out the big picture. It's an involved swap, but not difficult. I just hope it's worthwhile! :-)

-Dave

TKOPerformance

I've got to agree with Dave on this. EFI is many things, but cheap isn't one of them.

The problem with the TPI and RPM is that the runners are just too long. Long runners are designed to build torque, but they don't flow enough air at high RPM. This is why shorter runners and "siamesed" runners are so popular for the TPI system. All are a crutch to try and get it to flow better at higher RPM. The LT1 intake is a polar opposite, with very short runners, capable of supporting 6,500 RPM +, but at a tradeoff in low speed torque. The LT1 is a good swap for someone that already has a running TPI on their car, but for someone looking to convert you'll be happier and bucks ahead to just go with a good aftermarket system from the get go.

The GM computers are quite primative, and to properly tune them you need a couple hundred bucks worth of tools (wideband O2, ect.). These featuresa re already built in to many aftermarket systems, and don't cost you anything extra.

The Megasquirt system looks really cool, but you need some serious technical knowledge to make it work. If you're looking for a turn key system I would say try Holley, FAST, or the like.

contactpatch

TKO'...
>The problem with the TPI and RPM is that the runners are just too long.<
.
I disagree.
The problem with TPI, is that the runners [include the 'base']
have a cross section that is too small.
The runners, are surely longer than typical, but still are not
resonant, even at 5000 rpm. The 22 inch, .or whatever runners,
are resonant somewhere near ten grand.
.
I have a disagreement with the word 'tuned', but i'll
leave that to another post.

TKOPerformance

There are several problems with the factory TPI system. First, the runners are too long, and take a very convoluted path into the heads. Air doesn't want to make turns. Any change in direction over about 3% and the air will simply slide rigth past the curve, hit the back wall, and create a ton of turbulance. This isn't good at any engine speed, but at higher engine speeds airflow becomes a real problem. Look at a better designed EFI manifold, like the one on a 5.0 Ford. The air enter the throttle body, feeds the plenum, and then is basically rammed into the cylinders. The design is a lot like a Holley Stealth Ram.

The TPI system is an outgrowth of GMs carbureted intake design. The concept of "tuned" runner first appeared on the old Z28 manifold. This filtered down into the unsuccessful and short lived cross fire fuel injection system, which is at its core frighteningly similar to the TPI system. The design of this manifold system is very dated. It was dated in '85 when they first started using it.

Ultimately it doesn't matter why it doesn't work. Sufice to say that it doesn't work. Shorter runners give you the ability to make more top end power. Longer runners bolster low speed torque. Think about single plane vs dual plane intakes for carbureted engines. It's the same concept, except that you are only flowing air, so you don't need to worry about suspension and atomization. You can crutch a TPI system along, but for the saem amount of money you could just upgrade to an LT1 or other shorter runner intake system.