Yeah, that should be under ECU or System Configuration I believe..... "View" menu drop down.

Chuck

 

Thanks for the information Rob. Now that I think about it, both are meant to drive LS1 coils, so I assume that the connector for the coils is the same on both harnesses, but the pins just differ. Tricky. :

I'll have to dig out my eDist and check.

I haven't, as yet, had the pleasure of using my FAST configuration software...

 

BTW, the FAST SEFI-8LO no longer controls the firing of the coils after the introduction of the eDist, does it? I mean, the eDist doesn't get a signal from the SEFI-8LO, it gets it from the cam sync stub, right? So configuring the firing order in the SEFI-8LO doesn't impact anything and wouldn't unless you had a sequential system anyway, would it?

 

Somewhere at the beginning of this thread it was mentioned that you have to use the 4/7 swap cams with some sort of single plane manifold. I believe I know why that was said, but I don't believe it's true. If you run the "altered" firing order (18736542) and check which side of a dual plane intake it's sucking mixture from you'll see that it's still going left, right, left, right evenly.

One thing I haven't seen in this thread is WHY it makes more power. Both firing orders have 2 cyliners in the same bank firing consecutively.

On a standard cam it's 8 & 4, 5 & 7

On the 4/7 swap cam it's 7 & 3, 4 & 2

I can understand perhaps less crank stress from a firing order change but how the heck could this possibly generate more power? It's like stealing from Peter to pay Paul.

 

 

Still missing something here, how does switching firing order result in additional horsepower. Can someone simplify the entire equation for us ametuers here. Thanks

Johnny

 

I offered one possible explanation just above, but maybe not too clearly: Better airflow into the two front and two rear cylinders because the are working 90 degrees apart. That could boost the Volumetric Efficiency of those cylinders and produce a little more power. Normal SBC firing order only has front two 90 degrees apart.

Any takers? naysayers? other opinions?

 

That's my guess (and I do mean guess) also.

The 4/7 and LS1 firing orders fire cylinder #1 and then immediately fire #8 and #7 in succession, pulling air to the rear of the intake. In the standard SBC/LT1 firing order, #8 is fired after #1, but then the four center cylinders are fired and I suspect that cylinder #7 isn't nearly as efficient as it could be in the alternate 4/7 (or LS1) firing order.

Again, just a guess. I know that people have complained about the LT1/LT4 intakes having trouble filling the rear cylinders. My guess is that switching the firing order solves that problem which is where the extra horsepower comes from.

 

Ive done alotta research in this as i run a solid roller with 4/7 swap in my 383, basically the LS1 firing order can be done in an LT1 without a different crank, but the only hp benefit came from a 4/7 swap, the other half of the LS1 order change actually ends up causing a loss for most people and is intended for a smoother engine.

 

Do you have any data to back that up? It'd be interesting to see what just a firing order change did to power in an engine that was previously running with the same cam profile and a different order, but I don't expect that many people (if any) have those "back to back" comparison numbers.

For example, I'll be changing duration at the same time I change firing order, so who can say what power gain (or loss) is attributable to just the change in firing order? Would a cam with the same profile and a standard firing order make the same power? More? Less? How about an LS1 firing order? We'll never know, because I don't have any interest in buying or playing with two or three cams to find out.

Obviously, that makes it difficult to attribute gains and losses solely to a change in firing order. However, I suspect that GM did their homework and the LS1 ended up with more than just a 4/7 swap for more than just smoother operation. Of course, "just because GM did it" doesn't always mean it's the right thing to copy either.

 

I've read that it stresses the crank less, which results in less twist, which results in more accurate valve and ignition timing events.

Wall-2-Wall Engines did some dyno testing and found around 15 hp with the 4/7 swap in a 670 hp, 7,500 rpm SUPR dirt track engine.

Mike

 

I think the O2 thing could be an issue but if you keep everything in proper working order it should be OK. One thing I think should be addressed is the cylinder balance, I believe LT1 Edit has tables for this and since we are talking improving the VE of a few cylinders the cylinder offsets should be redone. This would mean dyno time with 8 EGT probes in your headers, but would help squeeze every last bit of power out of it while insuring safety.

 

I want to say LJ did a bunch of testing with the cylinder balance and showed that it worked good for idle and off idle but made no changes at WOT.

He was working on the split BLM issue

 

Engineermike,

I think that you're on the right track with your theory. I've got several friends at Comp Cams and I've asked a couple of the more knowledgeable ones about the benefits of the 4-7 swap. One told me that the swap results in slightly less monkey motion (harmonics) and that there may be some benefits in valvetrain stability because of this reduction.

Another one told me that there is a slight reduction in cylinder head temperature in cylinder 7 since the firing events are no longer consecutive. This could be the case, but since there are still the same number of combustion cycles per rev, it would seem that the head temp would end up at the same point regardless of firing order.

FWIW, one of them told me that they typically saw power increases of 0-5 hp.

Kevin

 

According to the GM SAE engineering paper, on the LS1 engine, the firing order reduced crank stresses by 7%, and caused an increase, in the bearing oil film thickness, of 13%.