EFI firing order (302 vs HO-5.0L)

[xcntrk]

Need some input from the experts. A lot of back and forth opinions on this matter and need some accurate information.

I have a 70's era carb'd 302 that I'm converting over to 5.0 HO Mustang EFI setup. Which firing order do I use?

The 302 firing order is 15426378. This matches my cam but if I wire up the mustang TFI distributor in this fashion, then the PCM injector and timing will be off. Meaning when the PCM tries to fire what it thinks is cylinder 3, cylinder 5 will fire instead. Wouldn't this cause misfires as combustion is occurring at undesirable piston stroke position? Granted the piston stroke and valves would all match with the 302 cam.

If I use the HO 5.0L firing order 13726548, the PCM would be happy but risk combusting cylinders early/late with the cam not in the correct position. Which theoretically is the same issue as above...

How do folks get around this when doing this conversion? For what it's worth I'm also doing the Explorer Serp swap as part of my EFI conversion so I will have the entire front of the motor torn apart should a (unplanned!) cam swap be required.

?:??:??:?

 

[welndmn]

You're over thinking it.
The firing order should always match the camshaft.

Running a non "HO" firing order cam with the computer is no big deal.
(There so PMS quirks but not help by major)
It pretty much changes the mustang SEFI to over to the batch fire like the trucks used. The fuel now just waits at the valve instead of being sequentiay injected when needed.

Changing your cam is the best option, but searching here wil tell you a lot of people run it this way.

 

[Broncobowsher]

The layout of the crank give several options for firing orders. With a piston at the top of the bore it can be the top of the compression stroke or the top of the exhaust stroke. The lobes on the cam make for the actual firing order. A few of the lobes can be changed to alter the firing order without changing the throws on the crank. Thus the ability to change the firing order with nothing more than a cam swap.

Now the cam is what dictates what the actual firing order will be. The spark plug wires MUST follow the firing order of the cam.

Now for the ECM and the different firing order. It has been done all different ways. Some just drop it on as is and it works fine. The injector puts the right amount of fuel in the right hole. It is sucked in and burned. Think about when you are getting deep into the injector duty cycle. When it is open 85% of the time it is open and spraying even when the cam is closed most of the time. It still works just fine. Any time you are above about 25% duty cycle on the injector you are spraying fuel onto a closed valve.

Or you can change the injector wiring to swap things around to match the firing order. this may help with idle as you are misting the injector more during the intake. Although with a warmed up engine any fuel on the back of the valve will vaporize anyway.

Getting into a custom computer tune there is usually a setting to change the firing order and which injectors are monitored by which O2 sensor.

And changing the cam to match the injector firing order.

They all work. Some report minor issues. The swapping injector wiring is most likely to piss off the computer as it thinks it is richening one bank and the other bank gets a couple of cylinders of richer mix. That may let the computer try an lean the other bank which goes back to leaning the first bank, which then tries to richen some more. Now you are throwing off the bank to bank.
Spraying the back of a closed valve will still get the fuel into the cylinder. The right amount into the right hole that the computer can watch. It probably is the best simple answer. Unless doing a cam swap anyway, then get the 351W firing order version of the cam you want (which is the same as the HO firing order).
Or if you are going into the brain of the computer play there, if that is where you are going anyway.

 

[Steve83]

Early camshaft:



Late camshaft:

 

[jw0747]

Let me keep this discussion very simple. You don't need to change cams in a stock carbed 302 to run it with Mustang efi. Mine has been like this for about 5-6 years and no problems whatsoever.

 

[xcntrk]

Thanks for the feedback all.

Sounds like this isn't a big issue in the first place. Also looking at the early camshaft diagrams Steve83 posted, it's a non-issue altogether as my ECU is from 89'-93' which still uses the early firing order anyway. So it should be a straight match for my 302 cam.

 

[WheelHorse]

Glad to see that you did in fact ask on here.

 

[Viperwolf1]

... my ECU is from 89'-93' which still uses the early firing order anyway.

Not if it's from a mustang. HO motors used 13726548. Still not an issue though.

 

[Broncobowsher]

Still not an issue though.

That's why I just let it go...

 

[xcntrk]

Thanks all!!