3.4 DOHC OBDII vs. OBDI

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Linlor
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Joined: Sun Oct 29, 2017 6:11 pm

3.4 DOHC OBDII vs. OBDI

Post by Linlor »

All;
I have been on here a few times in the past looking for some info and been long enough that I forgot my userID and password..... I tried a search with various keywords to try to get more info before posting but I haven't been able to find the info I am looking for.

The long story is below and I hope someone can help.....

I am finally at the point where I am installing my 3.4 DOHC swap that has been sitting for way too many years. A bit of background. I bought a '95 DOHC off a fellow Calgary Fiero owner and commissioned a Penock's member to make me a 3.4 DOHC harness. I used a '91-'93 ECU with EGR delete. All made sense when I was looking at this back on '08-'09..... (Been a while....) Harness is great and the service was great during the build as well.

After getting all the pieces, I ended up being able to buy a crate '96 DOHC so ended up selling off the '95 DOHC. Again, this was all eons ago in '08-'10.

I am now at the point of installing the '96 DOHC with a Getrag 5 spd into my project car. I laid out the wiring harness this past weekend found that very little was the same for plugs etc. That is when I started digging and remembered the info I put above.... (Again, really has been a while...)

Where I am today and a long way around to asking my questions.

- '96 DOHC full engine with all sensors bought and installed for a '96 install
- the difficult 24x crank sensor is already on the engine as well.... (apparently that is an issue with getting OBDII setup on most swaps?)

- '91-'93 ECU and engine harness

- 5 spd Getrag transmission
- I was reading that it was only possible to do an auto swap with OBDII?

- Updated fuel pump in tank and ready to be put back into the car.

- Battery relocate up front with 1 gauge cables coming back into engine bay ready to power up system

All sitting on the cradle ready to go.

Questions:
1. Does it make sense to get a new harness built that allows for ODBII as now it is much more refined than back when these were first talked about?
a. In reading old posts, the '96 has SFI not batch injector operation but only up to 3000 rpm when it switches to batch
b. I have two O2 sensors and several other additional sensors that would not be needed with a '91-'93 ECU
2. If I am to get a new harness, who makes one and what else do I need to do to the car/engine to install the OBDII vs. OBDI?
3. Are there significant issues that crop up when doing an ODBII install compared to the OBDI?
4. Just to muddy the waters a bit more, would OBD 1.5 be a better option for me since I already have the sensors that would connect to that setup and still give me SFI???

Realistically, staying with the '91-'93 harness would be a bit cheaper but I do want to do this the right way all along as this project car has been a work of love for a while now....

Hopefully someone can help me out here. I am agnostic on which way to go as I have to spend some money to either get a new harness or go return and purchase my '91-'93 senors that will work with the harness.

Thanks
Don

A pic of the car the engine and trans are going into a pic of the engine trans setup.

[img]https://i.imgur.com/Iynl7On.jpg[/img]

[img]https://i.imgur.com/y5k19Kz.jpg[/img]

[img]https://i.imgur.com/3FBpjeS.jpg[/img]
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neophile_17
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Location: Southbury, CT

Re: 3.4 DOHC OBDII vs. OBDI

Post by neophile_17 »

Hi Don,

I don't have an OBDII setup but I've had this debate several times myself. My take is that it matters a lot if you're going to keep the motor mostly stock.

OBDI works very well if tuned extensively by a masterful expert like GM or a very small group of guys. Let me explain a little more. I have two set ups- a basically stock 3400 (running a polished tune) and a modified 3500 (LX9). Both are running on OBDI and the 3500 should make significantly more power (cam, porting, headers) but it just isn't right. This is despite contracting a few hours of the same expert but OBDI is time intensive to tune. As a result the 3400 has been great and the 3500 is disappointing in comparison.

On the other hand OBII is not fully decoded (to my knowledge) which makes me very nervous. Small improvements are easily compensated for by the MAF sensor, but go too far and you're off the reservation without a map (pun vaguely intended).

I just can't stomach the idea of tricking the OBDII ECU into thinking I have an automatic and having little recourse if that doesn't work. Color me paranoid.

I don't know if this is actually helpful but maybe it's another data point. If you're planning to stay close to stock and don't have a tuner friend I'd go OBDI but if you've got bigger plans I'd search out a tuner you trust and can afford and go with his advice. Good luck!

Sam
85 GT LeMons Car LA1/LX9 Hybrid
85 SE LZ4 Pending...
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Series8217
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Re: 3.4 DOHC OBDII vs. OBDI

Post by Series8217 »

Your situation is going to require a custom reflash either way. The 91-95 ECU will have the wrong map for the 96-97 heads and intake, but the 96-97 OBD II ECU doesn't natively support a manual transmission.

Check with Ryan at gmtuners.com as he may have done the swap this way before, or tuned for it.
The Dark Side of Will
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Re: 3.4 DOHC OBDII vs. OBDI

Post by The Dark Side of Will »

neophile_17 wrote:As a result the 3400 has been great and the 3500 is disappointing in comparison.

On the other hand OBII is not fully decoded (to my knowledge) which makes me very nervous.
An OBDII V6 computer supported by HPTuners would be fine for all the applications mentioned in this thread. You may have to pay a local tuner for several hours of work up front to dial it in, but after that it will be fine.

One more vote for Ryan Gick at Sinister Performance ( www.gmtuners.com )
He set up the Shelby Series 1 .bin that I'm currently using in my Northstar car. He got the basic parameters in line, and I had a local tuner spend a few hours dialing in the MAF tables and specifics of the calibration.
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Shaun41178(2)
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Re: 3.4 DOHC OBDII vs. OBDI

Post by Shaun41178(2) »

Can you use the sensors from the obd1 on the 96 engine so you don't have to change the harness?
Linlor
Posts: 11
Joined: Sun Oct 29, 2017 6:11 pm

Re: 3.4 DOHC OBDII vs. OBDI

Post by Linlor »

Thanks for the great feedback folks.

I did contact Ryan and he can't do an OBDII harness but can probably help with a reflash.

I heard that the II setup was only autos from OEM but I had read the info from quite a while ago so thought it might be more fully sorted now. That may have been a poor assumption. I will be keeping the engine stock so the OBDI may be the way to go.

I have the ODBI harness already. Just may need to redo the flash with Ryan to confirm the 96-97 heads etc. as the burn would have been for a '95.

I appreciate all the help.

Cheers
Don
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