Chip???? or????? Help.

[CAMAROSS]

Hey guys, I am having my factory 350 TBI engine replaced with a 406 smallblock (mild cam, ported factory heads, etc.). We are going to re-use the factory TBI unit. Will I need any computer work? Chip? Help me please. Thanks.

[chevyrestoguy]

I was going to do a TBI set-up a few years ago and I researched this subject heavily. Here's what I know about TBIs. Any cam with a lobe center of less than 112 degrees and a duration @ .050 of more than 220 degrees will drive the MAP sensor nuts because it's looking for an optimum vacuum signal of around 19 inches. I heard that you can get the ECMs re-flashed, and that should settle the MAP down. The other concern is that your stock TBI unit is designed for an engine with a whole lot less cubic inches. I've heard that the big block TBI units are larger, and they'll be better suited for the 406. Plus, you can do all the usual TBI tricks (injector spacers, etc) to get even more performance out of it. I would love to hear how this works out for you because I was going to do the exact same thing (406 engine).

[gchemist]

Call this company.
http://www.wait4meperformance.com/

[CAMAROSS]

Thanks a million. I will keep you guys posted.

[rfmaster]

Hey Camaross

1) You can always DIY tune your new combo if you are willing to step back and learn capabilities and limitations of a stock TBI ECM. Before you put your 406 together you have to do some reading about what can be done with TBI (and how much its going to cost)

2) A good start can be found here:
http://www.thirdgen.org/techboard/tbi/

3) While it is true that a stock TBI can not readily handle large overlap cams with appropriate tuning, perseverance and time it can be tamed. However, for every day driving a large overlap cam with low idle vacuum is ill advised. Look for a cam with LSA 114deg, but since you have 406 additional overlap can be handled by extra CID. Do you have a roller or flat tappet cam?

4) A stock SBC TB has 46mm diameter throttles which flow around 490 CFM. It's enough for up to 350 CID mill running up to about 5000 RPM. Around 220 to 250 HP can be expected from a good combo. By increasing fuel pressure from 13psi up to 30 PSI stock 62lb-hr (orange-black) injectors can support 300+ HP engine output. Naturally, such modifications can only be had with VE and SA tunning (EPROM of the ECM).

5) BBC TB has 52 mm diameter throttles and can flow close to 640 CFM (depending who's numbers you believe). Stock BBC injectors 90 lb-hr operating at 13 psi can support about 280 to 300HP, but with increased fuel pressure higher HP can be achieved. Appropriate changes to VE tables and engine constants in the ECM EPROM are required.

The biggest issue with a stock ECM setup(for example 1227747 with ASDU calibration) is that engine idle area is set in 30 to 40 KPa region (about 16 to 20 in-Hg). Once MAP reports lower vacuum (higher KPa) ECM assumes non idle operation and will add fuel (based on VE tables) This causes unstable, surging idle and rich condition. In addition large overlap cam allows some intake mixture to flow through during overlap period, which causes O2 sensor to see a fake lean condition (unburned fuel and plenty of oxygen) confusing ECM to add more fuel which engine does not need at all. Hence, large overlap cam are PITA to deal with at idle!

Above is a short snippet of what you may need to overcome unless you chose your part combination carefully.

//RF

[CAMAROSS]

Lots of great info! I appreciate the help!!!!!!!!!

[78 Chevyrado]

I would definitely have a chip burned. I did that swap in my 87 and used all the stock stuff except i put 454 injectors (higher lbs/hr rating) in my 305 tbi unit (they swap out) and got a 350 chip from a suburban and it ran good and got good mileage, but at certain times it ran rich, telling me it needed a chip. The original 305 injectors made it so lean all it would do was backfire, but i never tried the 350 injectors (i had 454 injectors laying around). you might try the 350 injectors and see what they do. also my 400 was stock except for a mild rv cam.

[gchemist]

No rookies here: http://www.cfm-tech.com/GM_tbi_tuning_tips.htm