What's the pros and cons for the 4/7 firing order swap in a bigblock chevy ?

Me and a friend , who has a dragracing only chevelle -69 , have tryed to figure this out.
And we decided to ask this question here. Can you guys help us out with that?

He (my dragracing friend )has to tune/upgrade his engine this winter and is curious about this "firing order swap thing".

I have also made a short video (about 1 meg.) on his chevelle when we were testing at the strip last summer. Use this link to the homepage and click (or right click for downloading) on Burnis, on the left side of the page, between the flags.


web page (http://hem.passagen.se/amcar)

Stefan B

do a search on here or read the tech talk on reher morrison's site.

Excellent question!
Which answer would you like? The truth is I do not really know the answer but,,,,,
The answer I have been given that seems the most plausible to me is: Moving the cylinders that fire consecutively towards the rear reduces crankshaft twist. 6&5 fire consecutively as do 4&3. so,,,,, if you switch 4 & 7 then you have 6&5 and 8&7 so in theory I guess there is less twist along the crank.
I have also been told it is worth a small increase in horsepower. I intend to find out the answer to that for myself at the first opportunity as it seems everything you buy is going to give you that "magic free horsepower"!!
I intend to discuss this in more detail with some of the cam grinders that do these cams to get their opinions in the near future
I do know it is being done by some NASCAR & Busch teams but these guys also have cams ground with differing timing on different cylinders to tune the engines! They also have the ability to measure individual cylinder pressures as well as crank twist among other things!

Thanks for the reply's and the explanation Wolfplace.
I think we have to look more in to this before getting on to the 4/7 firing order swap cams.

Kind regards
Stefan B.

im no mechanic but would'nt this be benificial because of 5 & 7 firing be side each other when using an open plain intake :confused: rob graemlins/waving.gif

Originally posted by 82 silverado:
im no mechanic but would'nt this be benificial because of 5 & 7 firing be side each other when using an open plain intake :confused: rob graemlins/waving.gif With the swap, 4 and 2 fire sequentially.

I have no experience with this, but one other item to consider:

With 5 & 7 firing consecutively, there is additional heat load at that corner of the engine.

With 2 & 4 firing consecutively, the "double whammy" heat load is at the front of the engine...right next to the water pump.

That's true, a bit cooler. Still doesn't address crossfiring, which is more prevalent than most realize. However, this can be controlled with proper wire spacing.

I think it was mentioned it is worth about 10hp on a 1000hp motor. But there is some info out there.

Later
Steve

I have posted this a couple of times on another board but here it is again ;)
You might see 10 HP in a certain hi hp hi rpm engine combos but in most cases I doubt it will be close to that.
I am always a bit skeptical of these "magic bullet" deals.
I have also heard about the fuel distribution fix & maybe there is some truth in it but the answer I gave about crank twist or distortion comes from some pretty sharp folks & this is what is suposed to be the major benifit.
I feel a good head guy can help the fuel distribution in the intake by changing the port runners & modifing the plenum better than by changing firing orders.
And in the case of Pro Stock I think the intakes they use are about as close to an individual runner deal as you could get & still have a plenum so I don't think fuel distribution is much of an issue.
As I said before,,,which answer would you like?? :D

Maybe "Professor Harold" will see this & comment as I am sure he has played with the 4/7 swap before & would have some excellent input from which we all can learn graemlins/thumbsup.gif