I posted this under Engine Swap also. I've done a search and I'm not finding enough info on 90-92 SD 305TPI to 350 SD TPI swap. I have a 91 GTA 305 TPI A/T and a 350 TPI A/T out of a 91Z28. Things I know that need to be changed, knock sensor, injectors, prom and EGR. Is the ESM different from a 90-92 SD TPI 305 and a 90-92 SD TPI 350? Anything else I'm missing? I was reading about 305 heads on a 350 and someone said you needed to change the ESM. Thx. in advance for any help, Rob

 

Go to this thread to see what I'm finding out: https://www.thirdgen.org/forums/engi...-sd-305-a.html

 

The computer is identical only eprom. What really matters is the amount of modifications on your 350 vs a stock L98. If you are using the stock 305 Intake (including the injectors), all you have to do is change the Engine Displacement in your stock eprom but leave the injector size as is. The stock 305 injectors can easily support a stock L98.

I would not worry too much about the Knock Sensor - the L98s are quite prone to knock retard anyway and most guys tune around that.

If you are changing the cam (and I would), then you will need to make more changes to the eprom to get maximum power. You may also have to consider bigger injectors, especially if you change your intake. But, if your retain the stock TPI and don't go too big on the cam, the stock 305 injectors should be adequate with a little extra fuel pressur.

 

Thx. Grim. For right now I'm sticking with the stock 350 out of the 91 Z28. I have the 350 knock, injectors, prom and EGR so I might as well use them. I'm putting a 3:42 disc posi rear end in. What cam would you recommend for a stock 350 TPI with A/T and a 3:42 rear? I have a 350 4 bolt 1 piece RM that I'm slowly going to build for this car down the road. I want to spend the money on 4th gen seats, coverting to black interior, paint, all new suspension. The 4 bolt engine, I will go radical on.

 

The LPE 219* is as big as i would go with the stock TPI. I find stock TPI are better served with a single duration cam (Intake & Exhaust the same) than a dual duration (Exhaust duration longer than Intake). Check our Crane or Comp Cams for a roller cam with a near single duration pattern, about 112* LSA and a duration of 212-216* at .050".

The one fly in the ointment will be installing the proper speedo gears for the 3.42 rear end. There was never such a combo made. You may wish to post on the Transmission Board and ask for the combo of speedo gears that closest match the 3.42 rear.

 

Grim, thx. for the cam info. Gears are not a problem do to have a electric speedo. I will reprogram the prom for the 3:42 gears.

 

If you're using the same 700R4 you can just get the drive and driven gears for the 3.42 from a GM dealer. They fit the same as any other gears for that trans. Just remove the VSS and swap the driven gear, then remove the tailshaft housing, remove the retaining clip and slide the old drive gear off and slide the new drive gear on (may be a tight fit). Install a NEW retaining clip(trust me, don't reuse the old one), install tailshaft housing with new o ring and driveshaft seal if you like, reinstall VSS and you're done. To my knowledge you can't correct for different gears in the PROM.

 

Thx. Mongoose. Are you sure you can't just reprogram the prom for the gear change. Remember we are talking about 90-92 Speed Density setup and not the 89 and older MAF setups. I think it was 90 that they removed the cable driven speedo and replaced it with an Electronic speedo also. This will be my first gear change so any advise on this is greatly appreciated. I'm doing this to a 91 GTA.

 

Some info on what I'm doing or what I have: All this is going into my 91 GTA, I have a 91 Z28 350 TPI A/T parts car that was wrecked but a lot of upgrades were done to it. The 3:42 drum rear end was in the Z, I swapping out all the disc brake parts onto that rear end. I still have the 700r4 from the Z and I just removed the VSS and tail housing it has a red gear on the shaft. I have not checked the rebuilt 700r4 one I have or the 700r4 from my GTA. Does the gear color indicate gearing?

 

I just pulled apart 3 stock 700r4 all removed from 91 TPI cars and I pulled apart the 91Z that had the 3:42 gear, The only difference was the Z28's VSS gear is bigger than all the stock 700r4's. The gear on the shaft's of all 4 tranny's was exactly the same. It looks to me you just change the VSS gear.

 

U may, u may not.

It ultimately matters what the gear ratio was originally to what your now putting in.

If they are close together, u usually end up changing either/or, if u go from a 2.52 to a 3:42 your gonna end up chaning both.

later
Jeremy

 

Been doing my homework again and it looks like with the red 17 teeth shaft gear I just need to change the VSS gear to a 45 teeth for my 25.5" tall tires. The ones in the stock 700r4 were all 38 teeth and the one from the Z28 was a 44 teeth, 44 will probably work but might as well get the 45. Check out this site it was very helpful: http://www.transmissioncenter.net/sp...n_______va.htm

 

The ability to correct the speedometer in the ECM was never an option in a third gen, even with the electronic speedo. Firebirds have had electronic speedo since 1986 or 87 and the Camaros got them '90 or '91 I think. When I swapped out my 3.73 drum 10 bolt rear for a 2.77 disc 9 bolt I had to change both the drive and driven speedo gears but even if I didn't have to I'd have done it anyway just to have a matched set start together from day one. Not that's anywhere near the same level of importance but you wouldn't change the ring gear and not the pinion gear would you?

 

Mongoose, the shaft gear with clip is only $10.00. The VSS gear is the expensive one. I agree with the rear end theory. A red 17 tooth gear is a red 17 tooth gear, in my case rear end gear, tire size and the red 17 tooth shaft all I really need is the 45 tooth VSS gear. When it comes to mechanical parts I don't like meshing new and used parts together, so I will be buying both gears new.

 

I called the link that I posted above and was told to be dead on I would need a 15 teeth shaft gear and a 41 VSS gear due to my tires being 25.25"-25.5" tall. If my tires were 26" tall the 17 teeth and 45 teeth would have worked. BTW, GM only carries a 44 teeth gear and less. They also said use the calculator to figure out what you need exactly. Just click on the spinning odometer.

 

Actually, there is SOME ability to correct the speedo in the ECM for later models, but NOT an adjustment where you have changed rear-gears.

There is a "divisor" in the SD Programming, but it is only for BIG ADJUSTMENTS such as if you switch to a T56. If you just swap rear-gears, you must do it through the speedo gears in the tranny.

Also, though a "calculator" may say a particular combo will provide proper correction; it does not mean you can still buy those gears.

For example, the proper speedo gear combo for a 3.73 rear gear setup has been discontinued by GM for a number of years now and not available to my knowledge (unless someone has found a place that now reproduces them).

Five years ago, I was warned by a GM Mechanic to pickup the proper speedo gears for a 3.73 combo - I wish I had bought their entire stock.

 

Grim, if you check out the site I posted above it sounds like they carry the gears your talking about. I talked to them on the phone earlier today and they were very helpful and sounded like that had a lot of different gears(even gm discountunied gears) and alternate combos you could use.

 

I was aware of the divisor but as you said it's not for gear changes. Up until 2000 the gears for a 3.73 from GM were still available. My local Pontiac dealer found the high pressure inlet hose for my TPI swap through a "vintage parts" company they use. Ask your dealer if they have access to one. Sometimes the needed parts show up on a dealer inventory search as well. My original 3.73 speedo gears came from SLP back in '92, they may still have them.

 

Good, I was just clarifying in case you (or someone else reading) were not aware.

Funny thing, you CAN adjust for a gear-change, for the ECM's own internal caclculations that it uses throughout the calibration/bin. If you changed gears, but did not change the speedo gears, you can make an adjustment so the ECM calculates everything properly as if you had installed the proper speedo gears. Why GM didn't take it "one step further" and made the ECM give the proper display to the speedo, I don't know.

Possibly it has to do with a "carryover" from the electronics from the earlier ECMs. I know the PCMs with flash memory in 4th Gens can be adjusted properly in the PCM and have the speedo properly display...(though I am not certain about 1993-94 model years that still used a Memcal ... as I have never played with one of those to verify).

 

Dokken, I'm glad you are still able to get them from the after-market. Again, I am just clarifying in case you or someone else reading this post, knew that they just can't pop down to their local GM dealer. It seems more & more parts are starting to show up "discontinued" for our cars. Hopefully, the aftermarket or NOS supplier will start to fill that void.

Be glad that 3rd Gens do have a certain amount of popularity - unlike other GM (and non-GM) cars where a "discontinued" part becomes a major scavenger hunt involving phoning wrecking yards all over the country.