OK, regarding my circa 1980 Chevy 350 that I am coverting to TBI fuel injection.....

The real problem is that I don't know what's inside. And I'm not particularly keen on tearing it down to find out.

One of my biggest complaints is the noticably lopey idle. Of course this could be just poor carb tuning and/or poor timing, but I'm concerned it indicates a bigish cam.

I futzed around with it a bit - have the Holley 4bbl idle screw turned fairly rich, set timing by ear, and it now idles reasonaly well at about 700rpm. But at that - it averages only 12" HG vaccum (taken direct from the Edelbrock performer intake, just behind the carb), with frequent, erratic excursions between 10-14"Hg.

I'm a bit concerned this will be too low / unstable a vac signal for the MAP sensor in the TBI setup to be able to establish a good idle.

Are those numbers particularly low / fluctuating?

I have read of using a vac canister or vac line restriction prior to the MAP, but am concerned that this would have the opposite problem of not allowing the MAP sensor to be responsive enough?

Bill

From what I know you are right to be worried about the low Hg readings. I would say get a computer friendly cam but good news is that there are plenty out there that make some good power theses days.

Bill,

Most of my experience is with the TPI but TBI uses the same sensors, 12" is sorta low but I bet your PROM could be burned to accept the lower readings..

The flucuation from 10-14 sounds like a miss to me, I want to say I ran about 13-14 and the prom was able to be tuned that low.

Run your idle up to around 1000 and time for maxium vaccum and seet what you can come up with.

Sounds like your going to need a new PROM burned but I think it will work. The vac canister will throw the ECM for a loop, keep the canister for your power brake reserve...

HTH

If you put a stock TBI with a stock prom and the motor thats a little hot, doesn't take much, the motor will run lean. Less power and possible problems. You can add fuel by increasing the fuel pressure but the top end will always suffer. A Stock TBI can't flow enough for a 350 at higher rpms. Your power will start dropping off around 4k b/c of the relatively small TBI. TPI is better but can't do much over 4800rpm with out modes. Get a volt meter and connect it to the MAP sensor or messure the vacuum directly. Mess with the fuel pressure and see what happens to vacuum. Plus the seat of the pants acceleration runs to tune it in. This is a crude method but better then nothing.

Mike

Ps. Are you using the ESC system also? Could a problem with this be a timing issue producing the low idle vacuum, just another random thought.

My opinion:

just change the cam to a stock TBI cam..........they are super cheap and honestly have all the power (torque) you'd need, where you need it (down low).

...or if you dont want to go with a stock cam get one that has 114-116* lobe centers.. that helps the lowend vacume. You know about www.diy-efi.org right? You can find all sorts of software and tech info for reprogramming the eprom.

14 inches is the min you want for TBI. My big block has 10, but I have a MPI. Call Howel they know their stuff. Just pick the right cam.

If it's an early 80's block , then it should be a flat-tappet hyd. cam. If you want to use a stock TBI cam, ask for one for a 3/4 or 1 ton chevy truck w/ tbi up to about '92. They were flat-tappets. Cars used roller cams since about '87. Comp Cams makes a flat-tappet in the Extreme 4x4 line for the TBI trucks.

Thanks much for tips and info guys.

TPIJeep - I'll try that and report the numbers. I might actually get off my lazy butt enough to run a compression test and read the plugs too :D

Charly....now, you know I'm way too lazy to pull the motor out and swap a cam. And "just change..." translates to grief for me - I aint so smart about motors!

Yep - I've spent a bazzilion hours at diy-efi and Thirdgen. May just have to get my own proramming gear...it seems pretty cool. Haven;t read much on folks using the GM ECM in alpha-N though - I think that would be cool.

Bill, I have an '89 350 with TBI in a '68 GMC. Bumped the comp. up to 9.3 and have a 212 duration cam with 116 LDA and it makes 16 in Hg at idle. Idles slightly loppey. Everything else is stock. Won't rev above 4.5 k or so. Pulls real good down low. Only problem I have is the check engine light comes on if I let it idle while cold. Goes off and runs leaner once it warms up. I think that is caused by having my O2 sensor too far down stream. It is in the header reducer. I set up a buddies the same way only I put the O2 sensor as far forward in the collector as I could and he don't have that problem.