cusz28

Does anyone know how i can reprogram firing order on a 95 LT1 computer to accept a 4-7 swap cam?

AL SS590 M6

Why not just swap the where the wires plug onto the opti? Plug the 4 where the 7 was and vece versa. Might need new longer wires or not.

97FormulaWS-6

Can't "reprogram" a firing order... it's handled by the opti-crap.

You will have to mess around with injector firing, but that can be fixed the same way as the firing... just swap the connectors/wires for 4/7.

cusz28

ss590, you have it nailed on what to do with the wires, but other things need to be changed, and i don't know how to do it. If i just swap the injectors for 4 and 7, the computer will freak out. lets say 1,3, and 5 are running normal, but 7 is running lean. since 7 is now connected to 4, on the opposite bank, it will make 4 run rich. the computer doesn't know that 4 is now 7, it will lean 7 even more. i can goof with the opti and change the firing order there, but i need to find a way to change the firing order in the pcm itself.

Injuneer

What do you think you will gain with the 4-7 swap? Do a "search" on this forum. The subject has been discussed before.

Two possibilities on the injector swap problem - run an open loop tune. Get a 93 ECM and run batch-fire.

cusz28

very interesting idea about running batch fire. I know real world makes no difference from batch to seq, but i want to retain the ability to really fine tune everything. I love making things difficult, its a challenge. I've also only heard of one person successfully doing the swap, although i don't know how much or what i would gain.

96capricemgr

I would leave the injector wiring alone and have just those two out of synch. The 94+ pcms go into batch fire at higher rpms anyway.

cusz28

wouldn't that create problems because of the runners being so short?

cusz28

if there isn't a way to change the firing order, is there a way to delay the injector enough to get it to fire closer to the intake valve opening?

Injuneer

Sure.... buy one of these - total control of firing order and full sequential injector timing vs RPM using end of event or start of event reference, with indivudual cylinder trims:

http://www.motec.com/m880/m880overview/

The other trick to timing the injectors to intake valve opening is to use very large injectors, so the pulse width is short enough to (almost) fit into the intake duration at high RPM.

96capricemgr

Fred touched on something. At WOT the injectior is open much longer than the valve anyway, so you are getting all worked up over injector timing for almost no reason. OEMs went sequential more for idle emissions than anything else.

cusz28

97formula, your absolutly right. I did a lot of searching and calling around and the firing order isn't programmed in the computer. it's in the slots of the opti's optical wheel. Injun, if i wanted to spend that much money on an ecu, I wouldn't be having this discussion, i would have paid someone to do it for me. I was talking to efi connections, and I'm thinking of just ditching the opti and going with a ls1 computer and coil per cylinder.

rskrause

Just switch the inector wires and the plug wires. Of course, you will need a new cam and all this effort will not give you a gain. But go for it if you want to try!

Rich

cusz28

just switching the two injector wires will make the computer freak out. if 4 and 7 are swapped and 4 is running lean, the computer will sense that bank is lean and try to make 4 rich to compensate. not knowing 4 is now 7, seven now runs rich and the computer tries to lean seven, which is now four. you see what i mean? Now, the guy from efi connection said the firing order is made up by the optical disk's low res side, and there's really no way to change that. but, with the ls1 computer and coil swap, not only is the opti removed completely, but all i have to do to change the firing order is to swap the connections for coil and injector, then change the bank assignments for those two cylinders. basically, instead of changing the firing order directly, the computer sees the engine's cylinders as 1354 on drivers side, and 2768 on the passengers side.

cusz28

i think the main point of the 4-7 cam swap isn't as much for crank stress as it is to keep 5 and 7 from generating too much heat too far from the water pump, where they get cooled less than up front. all that happens is the cooling problem is moved closer to the front of the engine, where it's easier to cool. if general motors changed the firing order of 4 and 7, and 2 and 3 in the ls1, there must be something we don't know. ford had the same firing order for it's earlier small blocks, just with different numbering, and they ran like scalded dogs.