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Old 03-05-2009, 09:19 AM   #1 (permalink)
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Default Does the HO/351 firing order really make a difference?

I was wondering, given the exact same cam specs (same lobes and separation angle), if there is any real difference between the old firing order (1-5-4-2-6-3-7-8) and the HO/351 firing order (1-3-7-2-6-5-4-8).

I don't really want to get into a discussion about which is better: flat tappet vs. roller tappet. Just want to know if there really is a difference in either power, street manners or some other aspect (vacuum, perhaps?) between the two firing orders.

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Chuck
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Old 03-05-2009, 09:39 AM   #2 (permalink)
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Default Does the HO/351 firing order really make a difference?

IIRC it's supposed to make *a* difference in most cases, how much seems to be a subject for debate, but the uber-high HP builders can make it work for them and show a definite advantage. The Dave Vizard-type folks who like using every trick in the book do it.

It's *supposed* to idle a little better, I think, but I think one of the main reasons its' done is that it's easier on the crank, this has been well proven and it helps in super high HP situations. The engine certainly sounds a little different if you listen hard enough.
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Old 03-05-2009, 09:53 AM   #3 (permalink)
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Default

Also, I think most performance parts (mainly camshafts) are made for the 351 firing order (aka, the 5.0 HO firing order, but the 351 was first with it). So IF there is an advantage to the firing order itself I dont know... but I do know that the 13726548 firing order is much more common in the aftermarket. Crank is the same either way if I remember correctly

It isnt a big deal... just swap the plug wires on the distributor around to the new firing order if you get a 13726548 camshaft.
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Old 03-05-2009, 12:36 PM   #4 (permalink)
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Default Load on mains

The early firing order puts a little more load on the front mains by firing cylinder 5 right after 1 in the sequence (both are up front). 5.0 HO and 351W firing order better balance the main bearing loads. It probably doesn't make a huge difference in power or stress on the crank but I suspect Ford thought it was worth something in the HO motors. Its obviously more important to get a cam with the right lobe profile than firing order. I currently have an early 302 with a mild performance cam that has the 351W/5.0 HO firing order. FWIW, it does seem to run very smooth in my opinion.
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Old 03-05-2009, 12:48 PM   #5 (permalink)
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Thanks guys. I had posted this to another forum (corral) and they seem to agree that it was mostly an engineering move to reduce stress on the front main bearings. Yes, the crank is the same for both firing orders. 5/4 and 3/7 are just exchanging their exhaust and compression strokes. You still have two sets of same-bearing firing (3/7 followed by 2/6), but you have mains on either side of those to help distribute the force. For 1/5, you only have one helper main.

With that in mind, I can see why HP applications would like the 351/HO firing order, especially at high RPMs.

Thanks again!
M/C
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Old 03-06-2009, 03:26 AM   #6 (permalink)
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An older repair book I have states the new firing order was used for the 351 because Ford engineers found it made a much smoother running engine, they just couldn't get the longer stroke 351 to run as smooth with the older 302 firing order. I don't think it was a real noticable difference on the street with a carbed engine (at least not for power), because prior to the 5.0 HO coming out the majority of after market cams available had the 302 firing order and you had to switch the plug wires around to use the 302 cam in a 351. However, there has to be something to that smoothness improvement with the newer order because Ford saw fit to use it on the HO.
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Old 03-06-2009, 08:52 AM   #7 (permalink)
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The first 5.0 HO engines from the early eighties used a 351W marine cam, which obviously had the alternative firing order. I guess they saw no need to change back to the 302 firing order when they switched to roller cams a few years later.
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Old 03-06-2009, 11:00 AM   #8 (permalink)
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Quote:
Originally Posted by Mustang Chuck
Thanks guys. I had posted this to another forum (corral) and they seem to agree that it was mostly an engineering move to reduce stress on the front main bearings. Yes, the crank is the same for both firing orders. 5/4 and 3/7 are just exchanging their exhaust and compression strokes. You still have two sets of same-bearing firing (3/7 followed by 2/6), but you have mains on either side of those to help distribute the force. For 1/5, you only have one helper main.

With that in mind, I can see why HP applications would like the 351/HO firing order, especially at high RPMs.

Thanks again!
M/C
I thought I saw this over there I don't do a lot of posting over there, I could do with out some of the attitudes. Does anyone know how switching firing orders effects header design? With a 4-1 probably not much, but if I was going to use Tri-Ys it might make a differance which 2 cylinders are paired up. I don't know for sure.

Rusty
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Old 03-09-2009, 12:00 PM   #9 (permalink)
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The same two cylinders are pared together. They are just compressing instead of exhausting (and vice versa). I would imagine that it wouldn't make any difference.

M/C
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Old 03-23-2009, 03:47 PM   #10 (permalink)
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hey mustang chuck, I've been looking for you so that i can find out what year gt wheels are on your 69. I have a 69 mach 1 also and would like to fit some of those to my car, I ab going to try and buy them and would like to know what you had to do to make them fit
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