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Old 06-14-2018, 03:51 PM   #1
svinkle
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Default B20 exhaust manifold swapability

Can I use this exhaust manifold on my 72-73 F Head (with SU's)?

IMG_0880

IMG_0881

The car currently has a junky aftermarket header that leaks, bumps into the right control arm, and is welded (not flanged and bolted) to the rest of the exhaust. I've got this manifold from my '69 parts car, but the internet has me confused with info about early vs. late oe manifolds and whether there's a difference. I also have the rest of the exhaust from the parts car, so ideally I'd just swap everything over (with some fresh studs of course).

In short: Will this manifold work well with my F head?
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Old 06-14-2018, 03:54 PM   #2
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You might need to drill out that bottom center hole - where there is a simple metal centering plug/stud.

I think that's bigger on the later heads?

Also be aware of the 'thickness' changes - carbed cars had thinner flanges, FI had thicker, it can matter because some bolts clamp both intake and exhaust manifold at the same time with HD washers. I've made up 'crooked' washers before that had 1/2 of the diameter welded up with some half washers.
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Old 06-14-2018, 03:58 PM   #3
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They all interchange. You may have to get the spacers right between the intake and exhaust manifold at the studs. On our 69 wagon we have a later exhaust manifold like that with the early aluminum intake for dual SUs.
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Old 06-14-2018, 04:04 PM   #4
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That's an F/I exhaust manifold so it was designed for an E or F head. The problem you'll have is with the difference in manifold thickness and the need for stepped washers where the manifolds share a stud. I had to make some rather large offset washers when I put the Stahl header on my '71 142E.

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Old 06-14-2018, 04:07 PM   #5
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And FWIW that manifold looks like a FI manifold, with the thicker flange.

I think B20's generally came with the one piece cast iron exhaust/intake manifolds. Warmed by the exhaust for cold weather, some of them had secondary valves that sent the intake through a longer. warmer path at low throttle/idle.
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Old 06-14-2018, 04:08 PM   #6
svinkle
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Quote:
Originally Posted by JohnMc View Post
You might need to drill out that bottom center hole - where there is a simple metal centering plug/stud.

I think that's bigger on the later heads?

Also be aware of the 'thickness' changes - carbed cars had thinner flanges, FI had thicker, it can matter because some bolts clamp both intake and exhaust manifold at the same time with HD washers. I've made up 'crooked' washers before that had 1/2 of the diameter welded up with some half washers.
Yeah iirc the smooth centering stud on my head is bigger.

Regarding thicknesses, that's actually one of the issues with the current header. It's thinner than the SU manifold at the shared stud spots:

IMG_0879

By my rough measurement, the thickness at shared bolt flats is 15mm on the SU intake manifold and 19 on the carb exhaust manifold. I've already got some stepped down big washers that I can use.

The consensus seems to be that the OE manifolds perform great compared to most headers. I just want to make sure this one is the type that work well.
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Old 06-14-2018, 04:14 PM   #7
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I think the consensus is that the stock header (and dual downpipe!) is really good, so good that you really won't "NEED" a header until you've got lots of other mods all lined up which all work together to support more power and higher revs.

And it's likely that, in a street driven car, with a relatively stock head, the stock exhaust manifold and dual downpipe exhaust might work a little better than the typical 4:1 exhaust header you see for sale.

I think the only one to avoid, even with a totally stock motor, are the *old* B18 single downpipe exhausts. Those actually do cork a stock motor up some. Well, maybe a stock motor with a D cam swapped in (zoom zoom).
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Old 06-14-2018, 04:15 PM   #8
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Ahh so much good info. You guys are awesome.

All of my searching told me that the '69 should've had one of the combined manifolds, so it makes sense that this one wasn't original to that car. And partially explains why I was so confused.

I'll prep the surfaces and grab some studs and a new flange gasket and get swappin'
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Old 06-14-2018, 06:04 PM   #9
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Quote:
Originally Posted by JohnMc View Post
And FWIW that manifold looks like a FI manifold, with the thicker flange.

I think B20's generally came with the one piece cast iron exhaust/intake manifolds. Warmed by the exhaust for cold weather, some of them had secondary valves that sent the intake through a longer. warmer path at low throttle/idle.
Yeah, that's an FI intake with a thin flange Stahl header. I was just showing an extreme example of the stepped washers.

The FI exhaust is thicker than the aluminum carb intake but the difference isn't that much. I've made washers for both setups.

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Old 06-16-2018, 08:10 PM   #10
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Why the focus on making offset washers?

I've always just ground away the extra 4-5mm of flange thickness of the cast iron exhaust manifold from any dual downpipe early B18 (separate) or later B18 or B20 (separated from intake), or FI...

Been doing this for decades, never had one crack...
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Old 06-16-2018, 08:39 PM   #11
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Sure, that works but what if you want to use that exhaust manifold on an FI engine again? I'd rather not make permanent changes to unobtanium parts.
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Old 06-16-2018, 09:05 PM   #12
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Unobtainium?
Not around here...

How many pounds you need?

"...but what if you want to use that exhaust manifold on an FI engine again?"
Offset washers?
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Old 06-16-2018, 09:09 PM   #13
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I'll hit you up when I need a few then. Uncracked FI exhaust manifolds are rare around these parts.
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Old 06-16-2018, 09:22 PM   #14
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Is the utility/desirability of the uncut FI manifold you allude to based on it's "originality" for an FI setup, or something else that makes it special compared to a non-FI one modified for FI use?

Certainly for use with separate SU intake, they're all interchangeable with minor mods...
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Old 06-16-2018, 09:41 PM   #15
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It's the originality that's desirable for the FI cars. They tend to crack and new replacements aren't available. That leaves cutting the one piece manifolds and grinding for hours to make them look presentable.
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