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Old 08-02-2011, 12:40 PM   #1
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'91 RS ECM and TBI 350 PROM?

I have a simple question that I have been searching for the answer to for a few days and am running out of time so I was hoping for a quick answer. I have heard that in order to do the 305 tbi to 350 tbi swap I will need several items, one of them being a PROM from a 350 tbi car. I have seen them on ebay for cheap but I wanted to make sure that it is directly swappable on my ECM out of my '91 RS before I buy one.

Any info is appreciated.
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Old 08-02-2011, 01:24 PM   #2
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Re: '91 RS ECM and TBI 350 PROM?

I have a 92 RS TBI and that fuel injection does no do any better with TBIs and it won't do any better with a 350. If you have a 350, you have to run it with either with a 4 barrel carburetor or With a TPI.
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Old 08-02-2011, 01:48 PM   #3
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Re: '91 RS ECM and TBI 350 PROM?

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I have a 92 RS TBI and that fuel injection does no do any better with TBIs and it won't do any better with a 350. If you have a 350, you have to run it with either with a 4 barrel carburetor or With a TPI.
I appreciate the info but I don't want to downgrade to a carb and I can't afford to completely redo my FI so I'm going to stick with my tbi 350 for now.

I just have a question with the computer aspect of it.
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Old 08-02-2011, 01:57 PM   #4
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Re: '91 RS ECM and TBI 350 PROM?

No problem, remember TBIs are designed only to give higher mileage. Most high performance parts you add to your engine will only give higher mileage but little performance. My Camaro showed that. Go to summit racing and buy a TBI spacer for trucks and goes between the TBI and the intake manifold. Is called Trans-Dapt Performance TBI Spacer. I recommend the swirl torque TBI spacer part number TRD-2533. It cost less than $50.00.
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Old 08-02-2011, 03:14 PM   #5
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Re: '91 RS ECM and TBI 350 PROM?

Quote:
Originally Posted by Blade09 View Post
I have a simple question that I have been searching for the answer to for a few days and am running out of time so I was hoping for a quick answer. I have heard that in order to do the 305 tbi to 350 tbi swap I will need several items, one of them being a PROM from a 350 tbi car. I have seen them on ebay for cheap but I wanted to make sure that it is directly swappable on my ECM out of my '91 RS before I buy one.

Any info is appreciated.
The Caprice's that used the '8746 ECMs have PROMs that will swap right in. Just check that it is a 350 under the hood.

As long as you use the 350 injectors the stock RS 305 PROM can be used.

Check the TBI board here for a sticky on a 305 to 350 swap. It will also mention the knock sensor and filter. But again you can get away with using the 305 filter with the 350 sensor. I say it that way as I am assuming a knock sensor will be in the 350 as delivered.

Otherwise just use the 305 sensor for now. Although, being in the water jacket, it can be better to put the correct sensor in right away.

RBob.
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Old 08-02-2011, 03:43 PM   #6
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by RBob View Post
The Caprice's that used the '8746 ECMs have PROMs that will swap right in. Just check that it is a 350 under the hood.

As long as you use the 350 injectors the stock RS 305 PROM can be used.

Check the TBI board here for a sticky on a 305 to 350 swap. It will also mention the knock sensor and filter. But again you can get away with using the 305 filter with the 350 sensor. I say it that way as I am assuming a knock sensor will be in the 350 as delivered.

Otherwise just use the 305 sensor for now. Although, being in the water jacket, it can be better to put the correct sensor in right away.

RBob.
Okay, I read somewhere that the 305 PROM would make it run rich if I was using the stock 350 injectors.. However, that prompts a different question. Can I use a stock prom at all if I am going to run a mild cam etc? I know that is getting a bit off track but I want to ask while I have your attention!
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Old 08-02-2011, 06:30 PM   #7
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by Blade09 View Post
Okay, I read somewhere that the 305 PROM would make it run rich if I was using the stock 350 injectors.. However, that prompts a different question. Can I use a stock prom at all if I am going to run a mild cam etc? I know that is getting a bit off track but I want to ask while I have your attention!
On the 5.0L and 5.7L TBI engines GM based the injector flow rate on the displacement. It was a ratio between the two. So the fueling is similar as the ratios are close.

Although some of the later 5.7l TBIs did get larger injectors.

55#/hr and a 305 = 5.54:1 ratio

61#/hr and a 350 = 5.73:1 ratio

A 3.3% difference.

The other side of this is that a Caprice is a heavier vehicle. So a 5.7l Caprice PROM isn't always the best answer either.

As for a mild cam, once you start any mod's, best to get into tuning.

RBob.
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Old 08-02-2011, 06:59 PM   #8
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Re: '91 RS ECM and TBI 350 PROM?

Quote:
Originally Posted by RBob View Post
On the 5.0L and 5.7L TBI engines GM based the injector flow rate on the displacement. It was a ratio between the two. So the fueling is similar as the ratios are close.

Although some of the later 5.7l TBIs did get larger injectors.

55#/hr and a 305 = 5.54:1 ratio

61#/hr and a 350 = 5.73:1 ratio

A 3.3% difference.

The other side of this is that a Caprice is a heavier vehicle. So a 5.7l Caprice PROM isn't always the best answer either.

As for a mild cam, once you start any mod's, best to get into tuning.

RBob.
I hear ya loud and clear and clear on tuning... But can it survive run decent for a little while? I need to wrap this up before I go back to school but I can tune while I'm at school.
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Old 08-03-2011, 07:25 AM   #9
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by Blade09 View Post
I hear ya loud and clear and clear on tuning... But can it survive run decent for a little while? I need to wrap this up before I go back to school but I can tune while I'm at school.
As long as the 350 is an L05, I don't see why it wouldn't work. Did you hit the 305/350 swap thread sticky on the TBI board?

RBob.
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Old 08-03-2011, 08:50 AM   #10
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Re: '91 RS ECM and TBI 350 PROM?

Not exactly sure what it is but its an older block we had sitting around already. Can't tell ya much other than its a 350 2 bolt main. And I have read the swap thread a couple times but there is a bit of contradiction between posts and it gets difficult to get straight answers.

Side question, how different are the older 350 heads and the 305 ones? The 350 heads I have don't have the right accessory mounting holes so I was gonna slap on the 305 heads for now.
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Old 08-03-2011, 09:37 AM   #11
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by Blade09 View Post
Not exactly sure what it is but its an older block we had sitting around already. Can't tell ya much other than its a 350 2 bolt main. And I have read the swap thread a couple times but there is a bit of contradiction between posts and it gets difficult to get straight answers.

Side question, how different are the older 350 heads and the 305 ones? The 350 heads I have don't have the right accessory mounting holes so I was gonna slap on the 305 heads for now.
Yea, I just reviewed the first page of that thread. Once past the first post a lot of misinformation in it.

Well, then it is a mix & match. Most likely a two-piece RMS, if so need a different flex-plate.

The old school heads are non-swirl port. So the SA calibration won't be correct with those heads. And putting the 305 heads on is likely going to raise the compression ratio. How much I'm not sure. Get the casting numbers from both the 305 & 350 heads and do a 'net search for chamber volume.

It may end up that the stock 350 compression ratio is so low that the 305 heads will work. Also depends upon which pistons are in it.

RBob.
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Old 08-03-2011, 10:49 AM   #12
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by RBob View Post
Yea, I just reviewed the first page of that thread. Once past the first post a lot of misinformation in it.

Well, then it is a mix & match. Most likely a two-piece RMS, if so need a different flex-plate.

The old school heads are non-swirl port. So the SA calibration won't be correct with those heads. And putting the 305 heads on is likely going to raise the compression ratio. How much I'm not sure. Get the casting numbers from both the 305 & 350 heads and do a 'net search for chamber volume.

It may end up that the stock 350 compression ratio is so low that the 305 heads will work. Also depends upon which pistons are in it.

RBob.
Could you point me towards the proper flexplate to get the job done?
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Old 08-03-2011, 01:31 PM   #13
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Re: '91 RS ECM and TBI 350 PROM?

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Get the casting numbers from both the 305 & 350 heads and do a 'net search for chamber volume.
If you are not successfull doing that search consider sending "Fast355" a pm. I think he may have the answer.
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Old 08-03-2011, 05:28 PM   #14
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by Blade09 View Post
Could you point me towards the proper flexplate to get the job done?
First you need to check that it is a two-piece RMS. If so then grab one from an '85 Camaro with an LG4 engine.

RBob.
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Old 08-03-2011, 05:34 PM   #15
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by RBob View Post
First you need to check that it is a two-piece RMS. If so then grab one from an '85 Camaro with an LG4 engine.

RBob.
Alright I confirmed it is a 2p piece. I am usually the exact opposite but this time i'm in a hurry. Is there a part number or description of what i'm looking for as a new item?
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Old 08-04-2011, 10:40 AM   #16
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Re: '91 RS ECM and TBI 350 PROM?

Check rockauto.com. Good prices.
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Old 08-04-2011, 12:09 PM   #17
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Re: '91 RS ECM and TBI 350 PROM?

Rbob, is this the right part? According to rockauto this is the OEM replacement for the '85 lg4 flexplate. *DORMAN Part # 04326*

Please let me know as soon as you can as I need to order it today if possible.
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Old 08-04-2011, 01:39 PM   #18
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by Blade09 View Post
Rbob, is this the right part? According to rockauto this is the OEM replacement for the '85 lg4 flexplate. *DORMAN Part # 04326*

Please let me know as soon as you can as I need to order it today if possible.
Looks correct from here. There is a Rock Auto discount code in the "Aftermarket Vendor Review" board sticky's.

RBob.
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Old 08-04-2011, 01:51 PM   #19
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Re: '91 RS ECM and TBI 350 PROM?

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Originally Posted by RBob View Post
Looks correct from here. There is a Rock Auto discount code in the "Aftermarket Vendor Review" board sticky's.

RBob.
Alright, I just wasn't sure if there was a difference between the 305 and 350 flexplates. Thanks for the info Rbob and company!
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Old 08-04-2011, 01:51 PM
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