4-7 swap cams? - Team Camaro Tech
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post #1 of 9 (permalink) Old Jan 24th, 10, 08:33 PM Thread Starter
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Jeremiah
 
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4-7 swap cams?

In the process of doing a rebuild on the wifes 69. I found two cams I like and cam quest said they were both good choices. Both cams are approx in size but one is a 4-7 swap.

What is the benifit of the 4-7 swap and should I consider it over a standard grind?

69 Z/28, red/white stripe, black houndstoot with docs.
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post #2 of 9 (permalink) Old Jan 24th, 10, 08:46 PM
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Re: 4-7 swap cams?

Hot Rod did an article on it a few years back. Sometimes it works, sometimes not. When it does it is usually on a full-race engine and gains maybe 15 hp. No one knows why.

William

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post #3 of 9 (permalink) Old Jan 24th, 10, 09:12 PM Thread Starter
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Jeremiah
 
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Re: 4-7 swap cams?

Found this info on another site. Sounds like it could only do good?

V8 engines have paired cylinders. What this means is that you can take the firing order 1-8-4-3-6-5-7-2 and split it down the middle.
So you have 1843 & 6572. Now take the first digit of each set and they are a pair. 1 and 6 they are paired cylinders. Then take the second digit of each set, which means 8 and 5 are paired cylinders and so on.

What this means is that when 1 is at TDC (top dead center or at the top of the bore) then 6 is in the same position. The difference is that that one is ending the compression stroke while the other is ending the exhaust stroke.

Now when you by 4/7 cam it swaps the two cylinders in the firing order. The reason for this is that the engine is supposed to run smoother. Also it is easier on the main bearings and the crank because the loads of the crank are more evenly distributed. It is supposed to also assist in flattening the torque curve.

Look at the firing order. 18436572. Now look at a crankshaft. It has 4 journals, each with 2 rods on it. The 1 & 2 rods are on the same journal; likewise, 3 & 4, 5 & 6, and 7 & 8 are all "pairs" on journals. 1 fires immediately after 2; 3 immediately after 4; and 5 immediately after 6. That means, each of those 3 journals, gets 2 shots of power applied to it in rapid succession. The 4-7 swap fixes ONE of those 3 instances of successive firing on the same rod journal. There are still 2 of them. Note that the 4-7 one is right between the other 2. Eliminating that one allows the crank to return to torsional "rest", so to speak, and helps keep from breaking.

The 5,7 fuel distribution was in succession of each other and the #5 cylinder supposedly robbed fuel atomization from the #7 cylinder on open plenum/single plane intake manifolds.

69 Z/28, red/white stripe, black houndstoot with docs.
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post #4 of 9 (permalink) Old Jan 24th, 10, 09:35 PM
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Michael Gekko
 
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Re: 4-7 swap cams?

That seems like a reasonable explanation. Makes sense.

But I wonder why the 4/7 swap wasn't done originally in the first place?

1968 Camaro NHRA/IHRA SS/C 'Super Stocker' 396/Jerico
1968 Street/Strip Camaro 350 w/PG
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post #5 of 9 (permalink) Old Jan 24th, 10, 09:50 PM Thread Starter
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Re: 4-7 swap cams?

I wondered the same thing?

Seems like it would sound a little different also?

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post #6 of 9 (permalink) Old Jan 24th, 10, 10:58 PM
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Sean
 
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Re: 4-7 swap cams?

That's the reason they did it I beleive, but in a driver or even a street strip car you will never know the difference. It's one of those every tenth of a horsepower counts deals, like pro stock which I believe is where it started.

We put a 4/7 swap cam in one of the race cars 632's for the reasons stated, and it needed a cam anyway so why not? We thought it sounded smoother but noticed no difference anywhere else. This was a nitrous motor in a 7 second pro street car.

Bottom line is pick the cam with the specs that suit your motor best, if it's the 4/7 swap cam then fine, but don't pick it for the 4/7 swap alone, imo.

Sean

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post #7 of 9 (permalink) Old Jan 24th, 10, 11:17 PM
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Re: 4-7 swap cams?

Sean, you are right I think it started with Pro Stock, maybe even WJ from what I remember.

1968 Camaro NHRA/IHRA SS/C 'Super Stocker' 396/Jerico
1968 Street/Strip Camaro 350 w/PG
1968 Caprice NHRA/IHRA Stock ELiminator O/SA 327-250 HP



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post #8 of 9 (permalink) Old Jan 25th, 10, 01:30 AM
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Re: 4-7 swap cams?

Standard Chevy firing order 18436572
4/7 swap 18736542
LS firing order (swaps 2/3) 18726543

I think Olds using the 4/7 swap firing order in the early 60s. 4/7 swap on a street car MIGHT sound a bit smoother, but I highly doubt you would notice any difference in HP.

In my opinion, if you are going to do the swap, might as well change 2 and 3 while you are at it and go to the Chevy LS firing order.

Here is an interesting article. It might be the one William mentioned.
http://www.hotrod.com/techfaq/113_07...ing_order.html

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post #9 of 9 (permalink) Old Jan 25th, 10, 07:10 PM Thread Starter
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Re: 4-7 swap cams?

interesting reading for sure!

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