Chevy Impala SS Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
531 Posts
Discussion Starter · #1 ·
I was just wondering if anyone ever tried the 4-7 swap on the LT1. Is it possible with the stock PCM. Just a thought i had.
 

·
Registered
Joined
·
7,024 Posts
I was just wondering if anyone ever tried the 4-7 swap on the LT1. Is it possible with the stock PCM. Just a thought i had.
What is the 4-7 swap?
 

·
Registered
Joined
·
4,073 Posts
Is this where the firing order is changed by using a cam set up for it?

If so, then yes I believe it's possible with the PCM, just move the injector wire, along with the plug wire. I would think it could be done by simply swapping the appropriate pins on the PCM for those 2 injectors. However, the O2's would have some difficulty adjusting the fuel without some programming, if it's possible to program something like that.
 

·
Registered
Joined
·
194 Posts

·
Registered
Joined
·
531 Posts
Discussion Starter · #5 ·
What is the 4-7 swap?
This is actually where the cam is ground that 4 and 7 are swapped in the firing order. You have to have the cam made for it. The LSx motors use the 18736542 order. Smokey Yunick came up with this many, many years ago and it actually has about a 15hp gain and the engine runs much smoother. I've used it many times on Gen I carbed motors and now that the ImpSS is back in the scene, it made me think about it. Motocross season is coming to an end and the wife is back into drag racing now that the fuel pump issue is fixed.
 

·
Registered
Joined
·
4,993 Posts
The LSx motors use the 18736542 order.
No they don't. GM swapped 4-7 and 2-3 for the Gen 3 and newer V8's. The Gen 1-2 firing order is 1-8-4-3-6-5-7-2, the Gen 3-4 firing order is 1-8-7-2-6-5-4-3
 

·
Registered
Joined
·
7,024 Posts
This is actually where the cam is ground that 4 and 7 are swapped in the firing order. You have to have the cam made for it.
Rumor has it that the stock LT1 can can be ground to the GMPP 845 spec as well? Food for thought.
 

·
Registered
Joined
·
1,643 Posts
GM swapped 4-7 and 2-3 for the Gen 3 and newer V8's. The Gen 1-2 firing order is 1-8-4-3-6-5-7-2, the Gen 3-4 firing order is 1-8-7-2-6-5-4-3
A) Gen 1 & 2 SBC firing order: 1-8-4-3-6-5-7-2

B) Swapping '4' & '7' : 1-8-7-3-6-5-4-2

C) Gen3 & 4 / Ford: 1-8-4-2-6-5-7-3
Though a bit off-topic, it appears an official, as in referenciing a GM service manual, ruling is warranted.

The Gen3/4 firing order is as Mike posted, 18726543.

BTW, it amuses me that as much thot goes into adapting mods I consider 'race only' for street use. That and another favorite, longer rods. There a lot of pro 'race only' teams that do not even go those routes to be competitive which should speak volumes in the way of their questioned superiority. More effort should be focused on flame travel, quality of the mixture, and an equal A/F distribution. Amateur tuners contemplate those hard part mods and then consider it acceptable to run just one (or two) WB sensors or even none at all. Focus on the 2X4s and not the saw dust. FFT.
 

·
Registered
Joined
·
4,993 Posts
Though a bit off-topic, it appears an official, as in referenciing a GM service manual, ruling is warranted.

The Gen3/4 firing order is as Mike posted, 18726543.

BTW, it amuses me that as much thot goes into adapting mods I consider 'race only' for street use. That and another favorite, longer rods. There a lot of pro 'race only' teams that do not even go those routes to be competitive which should speak volumes in the way of their questioned superiority. More effort should be focused on flame travel, quality of the mixture, and an equal A/F distribution. Amateur tuners contemplate those hard part mods and then consider it acceptable to run just one (or two) WB sensors or even none at all. Focus on the 2X4s and not the saw dust. FFT.
Holy crap...it's like the good old days (2005-2006) when I used to agree with yours posts!!!
 

·
Registered
Joined
·
1,729 Posts
Rumor has it that the stock LT1 can can be ground to the GMPP 845 spec as well? Food for thought.
i recorded my cam grind. ground to 845 spec. yes it can be done and much cheaper.



7 videos total, once you click on first one the rest should show up on youtube.
 

·
Registered
Joined
·
531 Posts
Discussion Starter · #11 ·
Not arguing what you think or I think or anyone else. I was just asking if anyone has done it. I guess Smokey's idea that the more efficient a motor runs, the more durability it has and more power it makes.
But taking advice from an expert has no chance vs. know-it-alls.
 

·
Registered
Joined
·
4,993 Posts
Not arguing what you think or I think or anyone else. I was just asking if anyone has done it. I guess Smokey's idea that the more efficient a motor runs, the more durability it has and more power it makes.
But taking advice from an expert has no chance vs. know-it-alls.
Don't get all defensive...everyone is just pointing out WHY it'd be so difficult to do.

The way GM handles firing order differences (like on say a 2002 Express van with an old school Vortec 350) is to change which wires go to which pins at the PCM...this changes the firing order for the injectors, and it would work on an LT1 PCM as well. They also then set the spark plugs on the distributor in the correct order, again, not a huge deal if they will reach on your opti...and it will work.

What the LT1 PCM can't do, that the 2002 PCM that GM used for 2 different firing order V8's can...is reassign which O2 sensors in the exhaust apply to which injectors. The newer PCM's have a table called "Injector Bank Select" in HPTuners, it's probably called something similar but not identical in EFILive and OBD2 TunerCat...however, that table as far as I know, does not exist in the LT1 PCM. That table allows you to assign injectors to bank 0 (driver side) or bank 1 (passenger side), and the fuel trim data applies properly.

IF your engine ran perfectly, and your O2 sensor feedback was essentially doing nothing, it'd never matter anyways...also IF you want to run the car in open loop 100% of the time (it's possible, it's what GM does on vehicles that ship to countries that still have leaded fuel in their gas pumps, as that kills O2 sensors) then it'd also never matter anyways...but in a typical car, it's very difficult to achieve closed loop, and achieve fuel trims that are ALWAYS at +/- 0%. SO...without being able to assign which trims apply to which injectors, when you swap the wires for 4-7 and 2-3, the O2 sensor will be measuring what 4 is doing it, and applying it to the bank that 7 is on, measuring what 7 is doing, and applying it to the bank 4 is on...and so on with 2 and 3...so if you have one bank a little leaner than the other due to airflow (very common with the LT1...search for "split BLM"...airflow has a hard time being dead even to every cylinder) or say maybe due to a couple injectors flowing a little bit less than a couple others...you'll get a run away situation...it'll measure lean on that bank, and add fuel to the other bank, at which point the other bank will show rich, and the computer will measure that and pull fuel from the first bank that was already lean...making it leaner, so it will then add fuel to the other bank still...eventually one fuel trim will go full positive, and the other will go full negative, and the engine will run like crap...and MAYBE even damage itself.

IF you want to run open loop and spend time with 8 widebands, or 8 EGT probes...or both...and then spend time in the cylinder balance tables as well as messing with the port geometry and injectors in the engine to get EVERY cylinder running the same...then yeah, it won't be a problem, go have the cam custom ground (you'll have to start with a blank...re-grinding a stock cam to new specs reduces the base circle a bit, no big deal, but also can't really change the firing order as too much material is already missing from the lobes...it'd be a welding festival followed by a heat treating festival...wouldn't be worth it)...OR if you REALLY want to run closed loop...get the cam ground, call up EFI Connections and order their 24x conversion kit, plug in a 12200411 PCM, put the correct data in the injector bank select table and misfire detection tables (use a 2002 Express Van operating system as an example) and start tuning.
 

·
Premium Member
Joined
·
20,353 Posts
Not arguing what you think or I think or anyone else. I was just asking if anyone has done it. I guess Smokey's idea that the more efficient a motor runs, the more durability it has and more power it makes.
But taking advice from an expert has no chance vs. know-it-alls.
1. It's kinda pointless to ask (and receive an answer) IF something been done without being willing to accept HOW it can be done, as well.

2. AFAIK, Smokey Yunick never applied his work in this area to computerized FI applications. While it may work, the real questions are......how well will it work and at what 'cost'?

3. These guys are trying to answer these questions for you as well.....so.....lighten up a bit and accept insight where it's provided ;) .

Just my $.02 :cool: .

KW
 

·
Registered
Joined
·
531 Posts
Discussion Starter · #14 ·
I'm not getting very defensive and it just kinda irritated me a little. Mike just explained exactly what I needed to know. I have done many, many 4-7 swaps on Gen I's and most dirt trackers prefer it. But for years I've never seen it brought up in the EFI world so I thought I'd ask here since there are actually more knowledgeable and mature guys on here. Other forums will have an adbundance of readers who would not have a clue or guess at the answer.
So, basically what we're looking at is the LT1 PCM can't support the change.Damn this EFI is a love/hate relationship!!!

Sorry, been out of the seriousness of the cars for about 3 years due to military, retirement and kids racing mx. So now I'm back in the business of cars again, in partnership, owning a speedshop with a good friend, the car bug is biting again and the wife is racing again. She's happy running mid 7's... but to me, if something doesn't run, sound and perform to 110%...I ain't happy.
 

·
Premium Member
Joined
·
20,353 Posts
......So now I'm back in the business of cars again, in partnership, owning a speedshop with a good friend, the car bug is biting again and the wife is racing again........if something doesn't......perform to 110%...I ain't happy.
1. Your wife is cooler than most.

2. You job is cooler than most.

3. You're a perfectionist; this can be a good thing......and a bad thing. Just don't let it get your blood pressure up :p .

KW
 

·
Registered
Joined
·
531 Posts
Discussion Starter · #16 ·
3. You're a perfectionist; this can be a good thing......and a bad thing. Just don't let it get your blood pressure up :p .

KW
LOL, I'm pretty laid back now that I'm retired and not getting shot at anymore.
Plus...medication helps. lmao
 
1 - 16 of 16 Posts
Top