LS (LM7) w/ Carb and OEM PCM. - Third Generation F-Body Message Boards

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LS (LM7) w/ Carb and OEM PCM.

Old 03-25-2018, 12:45 PM
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LS (LM7) w/ Carb and OEM PCM.

I've a question regarding a potential swap that is in the works.
I've an LM7 from my 06 Tahoe. What I'd like to do is set it up with a carburetor and perhaps a cam upgrade.
My question is whether the factory computer is capable of running this engine package considering the EFI has been removed? I suspect not but I'm asking those that have been there and done that.
Thanks in advance.
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Old 12-06-2018, 07:55 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

Ok I feel like I have glanced over this and just was wondering a couple of things? Can stock radiator be used if not what did everyone use as replacement? Same thing for the fan and shroud? Sorry if this was already addressed
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Old 12-06-2018, 08:36 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

There have been zero replies to my initial query. Maybe this will bring it to the top again.

Last edited by skinny z; 12-07-2018 at 10:38 AM.
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Old 12-07-2018, 08:25 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

The carb part is easy. All you need is a manifold to mount it on.

The harder part will be to create an electrical signal that looks like what comes out of a distributor.
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Old 12-07-2018, 09:47 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

Edelbrock makes a "kit" with a manifold and stand alone ignition controller.
But my question was regarding whether the OEM PCM can be used IF the fuel injection part of it's circuitry isn't being used. I suspect not.
If it can be used in that configuration, then that saves the expense of an aftermarket controller. The manifold will be easy to source.

Last edited by skinny z; 12-07-2018 at 10:41 AM.
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Old 12-08-2018, 03:52 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

There is a procedure to downgrade a fuel injected thirdgen to a carburetor. The check engine light will be on, but the PCM stays in the car.
https://www.thirdgen.org/carbswap/

There is also this thing if you want to use an HEI distributor:



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Old 12-11-2018, 08:42 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

My question was directed at the donor engine's PCM (not a 3rd gen computer) In this case it's a '06 LM7.
If I were to substitute the truck intake and EFI with a carb and manifold, will the stock PCM still be viable as a means to control the ignition? As I pointed out in post #1, there is a kit available which includes the manifold and stand alone ignition controller but I've found nothing that supports being able to use the '06 PCM while making the move to carburetion.





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Old 12-12-2018, 11:25 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by skinny z View Post
My question was directed at the donor engine's PCM (not a 3rd gen computer) In this case it's a '06 LM7.
If I were to substitute the truck intake and EFI with a carb and manifold, will the stock PCM still be viable as a means to control the ignition? As I pointed out in post #1, there is a kit available which includes the manifold and stand alone ignition controller but I've found nothing that supports being able to use the '06 PCM while making the move to carburetion.
The ignition controller eliminates the need for a PCM. If you need electronic transmission control, you will need to get a stand-alone TCM.

Iím not a fan of carbíd LS engines. I maintain you can spend less money overall by getting an aftermarket self-tuning system. With improved drivability to boot. I even converted my tuned factory system to self-tuning aftermarket.
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Old 12-12-2018, 11:31 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

I may be that I swap in the LM7 in its entirety. Truck manifold and all (just as a starting point). But an intake and carb suit me just fine. If I can use the '06 PCM sans the EFI, I can avoid the expense of buying an aftermarket controller. The manifold is easily sourced used.
So, the question remains.
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Old 12-12-2018, 11:56 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

Most functions of the ECM require some sort of workaround. Functions that I can think of...
fuel mixture, ignition timing, emissions control, OBD, fuel pump, engine cooling fan, charging system, transmission, A/C, vehicle security. Some of that can be deleted from the programming. Iím not expert, so take this for what itís worth. The later ECMís arenít so easy to reprogram. It canít be programmed to control A/C in the manner we want (A/C off at high RPM). It canít be combined with a cable throttle. Itís an all or nothing thing. So to answer your question, get rid of the 06 ECM.
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Old 12-12-2018, 12:16 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Taking into consideration all of the points made previously, that may be the best course of action.
This swap may happen sooner rather than later once I've determined
the costs involved compared to a new Gen 1 shortblock. I've too many solid supporting parts to abandon the old platform ( ported heads, valvetrain, ignition, oil pan) but with an entire running LS engine waiting in the wings the swap may make economic sense.

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Old 12-12-2018, 04:25 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

I doubt that you could get an LS ECM to run it without the FI on it, but if you could get a TPS on the carb, that may help? Alternatively, you could run it fuel injected, stock ECM with the 4150 carb manifold, get the TB adapter and put a standard 14" air cleaner on it. Holley just came out with the adapter plate to do this, and I think MAST has done it in the past
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Old 12-15-2018, 12:11 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Seems to me that may be problematic as well. I'm guessing that involves removing the 8 injectors and substituting them with a TB style injection arrangement?
But as was pointed out by:
Originally Posted by johnw999 View Post
Most functions of the ECM require some sort of workaround. Functions that I can think of...
fuel mixture, ignition timing, emissions control, OBD, fuel pump, engine cooling fan, charging system, transmission, A/C, vehicle security. It canít be combined with a cable throttle. Itís an all or nothing thing. So to answer your question, get rid of the 06 ECM.
It may be that trying to make it work might be impossible. Nothing from the donor would be used other than the engine and computer. The transmission and all other components are all old school 3rd gen. The 700R4 only has the TCC and that's already taken care of.
In all likelihood I'll go with something like this.


Either a "kit" as pictured above or sourcing the parts independently. I don't care much for either of the Edelbrock manifolds above but then again, I haven't researched that option too deeply.

Last edited by skinny z; 12-15-2018 at 12:14 PM.
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Old 12-15-2018, 12:30 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

If you have a complete engine, wiring harness, and controller why not just throw it in the car?

If you really want a single-plane intake then put one on the engine that has fuel injector bungs and add a throttle body elbow.

The point is stay EFI the whole time.
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Old 12-15-2018, 12:38 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

That's also something I've considered. It may be that the with my glass cowl hood, I've enough room for the truck intake and high centre mount alternator. And all things considered it might not be a bad starting point.
I'm still working out trying to balance the cost of a new Gen 1 shortblock vs this LS swap so I can back to racing again. It's been two full seasons since one thing or another has kept me from the track. Logistically, the shortblock is simpler and I've all of the supporting parts. The LS swap would invite all of the purchases and fabricating that folks before me have endured and while probably resulting in better performance, would consume more resources. Time as well as money.
I'm sort of working it through with this kind of dialogue.
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Old 12-15-2018, 01:45 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

If you're drag racing then I don't think the LS swap is worthwhile for a naturally aspirated 5.3L. It would make a nice driver though.
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Old 12-16-2018, 12:46 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by QwkTrip View Post
If you're drag racing then I don't think the LS swap is worthwhile for a naturally aspirated 5.3L. It would make a nice driver though.
Taking into consideration what you have engine-wise, I'd say that's a fair comment. But that said, I have a modest goal regarding the drag strip and that's an 11 second time slip. And doing that with my less than drag racing friendly chassis. All indications where that another 60 rear wheel HP will get me there with the current vehicle weight at 3600 lbs, gearing, transmission/converter and tire. Time slip and online calculators suggest I've an average RWHP of 320. Bump that up to about 375 and the math says 11.99 at 112 MPH. The RWHP calculation is a little skewed to the low side as the transmission and converter aren't well suited for 1/4 mile racing. I cover over 3000 RPM of engine speed in a pass and consequently have reduced performance because of it. Arguably this last 350 (with ported heads and poor cam choice) made 400 CHP. I'm calculating that if I can manage 450 CHP, it might put me over the top.
Here's the dilemma. Is that more easily accomplished with a new Gen 1 shortblock (and possibly a stroker) and better spec'd cam or with the 5.3 and a few upgrades like intake and cam? I'm doubting that 450 HP is possible with this LS unless I added better heads as well. An over the counter LS3 might do it but not this LM7.
Of course a 427 of any engine family would also get me there.
So much for my Sunday morning muse.
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Old 12-16-2018, 05:02 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Sorry, that wasn't meant as a knock on the 5.3L. I genuinely believe the SBC is a solid platform and can do better in drag racing than a warmed over 5.3L LS. You said you had quite a bit of good SBC stuff so I was figuring there were some decent heads in your inventory and maybe a 383 stroker possible.
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Old 12-17-2018, 10:06 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

This raises an interesting question and one I'm sure has been asked before (though I haven't seen any direct comparisons).
Take a 5.7 L Gen 1 SBC with 255 CFM @ .500" (intake) heads vs a 5.3 L LS with stock LM7 heads and cam. Throw a similarly suitable cam and intake manifold on both and who comes out on top? Based on the heads, I'd say the Gen 1. Info I can find on the stock LM7 head is about 215 @ .500" (pretty much OEM Vortec numbers). You'd have to get into bigger valves and some pocket porting to reach the 260 number with the LS. Not unlike the RHS Vortec which I've had ported (and flowed). Any meaningful HP upgrade when going from the Gen 1 to the LM7 would at the very least need better heads and cam.
When I look at it this way, and as suggested here, there's isn't much to be gained with the truck engine swap. Now having said that, providing what's needed for the swap just paves the way for a better iteration of the LS but that kind of draws the line in the sand given my goals.
Been an interesting conversation to this point and one which has provided the answer I was looking regarding the PCM and also contributed to the selection process.
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Old 12-17-2018, 11:07 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by skinny z View Post
This raises an interesting question and one I'm sure has been asked before (though I haven't seen any direct comparisons).
Take a 5.7 L Gen 1 SBC with 255 CFM @ .500" (intake) heads vs a 5.3 L LS with stock LM7 heads and cam. Throw a similarly suitable cam and intake manifold on both and who comes out on top? Based on the heads, I'd say the Gen 1. Info I can find on the stock LM7 head is about 215 @ .500" (pretty much OEM Vortec numbers). You'd have to get into bigger valves and some pocket porting to reach the 260 number with the LS. Not unlike the RHS Vortec which I've had ported (and flowed). Any meaningful HP upgrade when going from the Gen 1 to the LM7 would at the very least need better heads and cam.
When I look at it this way, and as suggested here, there's isn't much to be gained with the truck engine swap. Now having said that, providing what's needed for the swap just paves the way for a better iteration of the LS but that kind of draws the line in the sand given my goals.
Been an interesting conversation to this point and one which has provided the answer I was looking regarding the PCM and also contributed to the selection process.
The stock LM7 706 heads flow just under 300cfm at .500 lift. That's enough to support almost 600hp NA. The stock LM7 also, with headers, dynos around 350hp at the crank. A $150 OEM cam (LS6, LS2, LS7) and spring swap can add 70hp. There really isn't a comparison in my opinion in dollar spent per HP. Not to mention engine idle, longevity, and strength. After the higher cost of entry, for a typical LS swap that is, the 5.3 is better. The LM7 is also the lower end of 5.3's. A newer L33, LC9, or LMG gives you a near 400hp motor with cast iron manifolds for under $2k. IMHO it crushes any "basic 350 SBC" build. Or spend pennies more for a 6.0. Lots of LS motors making 430rwhp with a cam in the .22X intake duration range with stock heads (early LS 706, 853, and 241 heads flow about the same).
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Old 12-17-2018, 11:14 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by ShiftyCapone View Post
The stock LM7 706 heads flow just under 300cfm at .500 lift. .
That alone is enough to tip the scales of favour of this LM7 I have. (i've yet to sheck if Hhave 706 heads. Do the 862 flow as well. I've only recently read an article of the stock heads and that's where the 215 cfm @ .500" came from. Based on that number, what I have now for a Gen 1 beats it. If you've a link to some kind of testing I'm interested. Maybe Stan Weiss has it in his data base.

Last edited by skinny z; 12-17-2018 at 11:17 PM.
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Old 12-17-2018, 11:21 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by skinny z View Post
That alone is enough to tip the scales of favour of this LM7 I have. I've only recently read an article of the stock heads and that's where the 215 cfm @ .500" came from. Based on that number, what I have now for a Gen 1 beats it. If you've a link to some kind of testing I'm interested. Maybe Stan Weiss has it in his data base.
https://www.google.com/amp/s/www.hotrod.com/articles/ls-cam-test-comparison/amp/

This is an old article, and take it with a grain of salt but snarf around on LS1 tech and check out the 346 guys running these heads and also look at flow tests of 706 heads from all the big LS head porters (advanced induction, Texas speed, TEA, etc) and you can see the potential of the oldest of the cathedral port heads. There are plenty of other sources of evidence to back this testing (YouTube etc).
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Old 12-17-2018, 11:37 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

I didn't see flow numbers with a quick scan through that article posted above. The HP numbers were similar to the modified LM7 in the link below. Heads, cam and intake.
What I did find with a quick online search was another test with the 706 heads coming in at 220 cfm @ .500".
https://www.hotrod.com/articles/ccrp...-block-engine/
That falls in line with what I read earlier. That said, I'm in agreement regarding the potential of those heads. In the same article I linked, said with some bowl work (called stage 1) and bump up to a 1.94" intake pushed the cfm to 256. Coincidentally the same CFM as my current heads. I'm sort of ahead there already.
If you want to follow the magazine test stuff, or take a contributor like Vizard, a 350 making 450 HP (Gen 1 or LS) isn't a big deal. Getting the Gen 1 or LM7 there takes some work obviously. An LS3 has it done for you though doesn't it.
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Old 12-17-2018, 11:45 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by skinny z View Post
I didn't see flow numbers with a quick scan through that article posted above. The HP numbers were similar to the modified LM7 in the link below. Heads, cam and intake.
What I did find with a quick online search was another test with the 706 heads coming in at 220 cfm @ .500".
https://www.hotrod.com/articles/ccrp...-block-engine/
That falls in line with what I read earlier. That said, I'm in agreement regarding the potential of those heads. In the same article I linked, said with some bowl work (called stage 1) and bump up to a 1.94" intake pushed the cfm to 256. Coincidentally the same CFM as my current heads. I'm sort of ahead there already.
If you want to follow the magazine test stuff, or take a contributor like Vizard, a 350 making 450 HP (Gen 1 or LS) isn't a big deal. Getting the Gen 1 or LM7 there takes some work obviously. An LS3 has it done for you though doesn't it.
Indeed vizard is the man. The numbers I reference are advertised from the afore mentioned porters. They have published data. However, flow testing, like chassis dyno is just a tool so to your point, there will be a difference. Even if they flow 220. They are aluminum and $50 used, with serious potential with minor port work.
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Old 01-07-2019, 12:56 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by ShiftyCapone View Post
The stock LM7 706 heads flow just under 300cfm at .500 lift.
Stock? Only numbers Ive seen like that was out of ported 706's...
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Old 01-07-2019, 01:04 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

To the OP, go 5.3 and factory EFI. Get a cam, port the heads. You'll make AT LEAST 400 RWHP. I have a 5.3 in my garage and a set of LS3 style heads also. Turbo project is on hold indefinitely so I plan no getting a 6.0. porting the LS3 heads and building a lil stick-shift fun-mobile. I also tune with HP Tuners so tuning will be a snap for me. Should be fun...
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Old 01-07-2019, 11:32 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

Yeah, there are a few options regarding the 5.3. Heads, cam and intake seems to be the way to go.
As far as heads go, what's the best factory head for the small bore 5.3? No porting or other mods (other than springs). Just bolt on and go.
The truck intake I think isn't particularly well suited for what I want to do. That said, probably a carb style throttle body with port injection so I can keep the factory injector harness. Tuning will be tricky for me though. I haven't invested in HP Tuners yet and I'm not sure of the capabilities of the Tech 2.






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Old 01-07-2019, 12:10 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by skinny z View Post
As far as heads go, what's the best factory head for the small bore 5.3? No porting or other mods (other than springs). Just bolt on and go.
The truck intake I think isn't particularly well suited for what I want to do. That said, probably a carb style throttle body with port injection so I can keep the factory injector harness. Tuning will be tricky for me though. I haven't invested in HP Tuners yet and I'm not sure of the capabilities of the Tech 2.
799 or 243 heads are the best factory heads for the 5.3 small bore. Tech 2 isnt quite the same as HP Tuners. Tech 2 can do factory stuff but HP Tuners can do performance stuff. And if you know how to adjust a carb and a distributor, you can learn how to tune with HP Tuners. Buy some books and READ READ READ. Im STILL learning new tricks everyday.
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Old 01-07-2019, 08:30 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

I've no concerns regarding tuning or adopting some new software. It's part of what I do for a living although in an industrial automation sense and not vehicular. The Tech 2 is used for general maintenance on the trucks (although I'll admit I'm not fully acquainted with it yet.)
That said, it looks to me that an LS swap would take more hotrodding than it would doing the actual swap. Now if I had the LS3 that's currently on sale up here, I'd certainly move the swap up on the priority list. But swapping the 5.3 and adding heads, cam and intake is beyond what I'd need to do to get roughly the same performance out my current Gen 1 top end and related parts plus the needed shortblock (unfortunately).
Ball park budget figures for a fesh 383 forged shortblock is about 5k. And no additional parts for installation. I couldn't touch the 5.3 swap for that money. Not if the HCI upgrade is on the table. And the LS3, with all things considered would be about twice the Gen 1 entrance fee.
I think...

Last edited by skinny z; 01-07-2019 at 08:34 PM.
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Old 01-07-2019, 09:11 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

I wouldn't with an LS3 unless you want to relay on the harness for the relocated knock sensor and that 58x to 24x cam sensor converter box. I know 2 people that had that box fail, leaving them stranded. You could run an E38 computer but that's DBW. Stick to the Gen 3 stuff.
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Old 01-07-2019, 09:25 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by five7kid View Post
Iím not a fan of carbíd LS engines. I maintain you can spend less money overall by getting an aftermarket self-tuning system. With improved drivability to boot. I even converted my tuned factory system to self-tuning aftermarket.
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Old 01-07-2019, 09:57 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by skinny z View Post
I've no concerns regarding tuning or adopting some new software. It's part of what I do for a living although in an industrial automation sense and not vehicular. The Tech 2 is used for general maintenance on the trucks (although I'll admit I'm not fully acquainted with it yet.)
That said, it looks to me that an LS swap would take more hotrodding than it would doing the actual swap. Now if I had the LS3 that's currently on sale up here, I'd certainly move the swap up on the priority list. But swapping the 5.3 and adding heads, cam and intake is beyond what I'd need to do to get roughly the same performance out my current Gen 1 top end and related parts plus the needed shortblock (unfortunately).
Ball park budget figures for a fesh 383 forged shortblock is about 5k. And no additional parts for installation. I couldn't touch the 5.3 swap for that money. Not if the HCI upgrade is on the table. And the LS3, with all things considered would be about twice the Gen 1 entrance fee.
I think...
Skinny, you said your goal was drag racing. Doesn't articwhiteZ still have that 427 Chevy short block still for sale? It's a steal.
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Old 01-07-2019, 11:16 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

A quick search didn't turn up anything here. FWIW, it would present a few changes to my engine platform. Most notably would be cylinder heads. I've barely enough to satisfy a 383 (although I could confidently build something to make peak HP around 6000 rpm). For a 427...I doubt it. Interesting thought though. If you come across the sale, let me know.
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Old 01-11-2019, 07:54 AM
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Re: LS (LM7) w/ Carb and OEM PCM.

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Old 01-12-2019, 02:38 PM
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Re: LS (LM7) w/ Carb and OEM PCM.

Originally Posted by NoEmissions84TA View Post
Skinny, you said your goal was drag racing. Doesn't articwhiteZ still have that 427 Chevy short block still for sale? It's a steal.
His sale technique is a little cryptic but I left an inquiry just the same.
The thread is all about the intake that's for sale then this pops up.

Originally Posted by articwhiteZ View Post
BTT

this is all up for sale cheap..427 sbc $5200
NOTE no water pump power steering alt. or brackets.. you get every thing..
And I'll admit that it is a steal.
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