Log Exhaust Manifold Tech
 

Most of us don't consider using the stock log-style exhaust manifolds for high performance. A set of headers or the factory HO/Ram Air manifolds are usually used in place of the more restrictive standard log exhaust manifolds. However on some of the milder closer to stock engine builds the log manifolds can work just fine.

Here's a little modification that can be done to the log exhaust manifolds to help improve them. If you have access to some high-speed grinding tools used for head porting you can work on opening up the outlets. You can go from the stock 1.850" opening which is same basic size as the inside diameter of a 2" exhaust pipe, to 2.150" which pretty much matches the I.D. of a 2-1/4" exhaust pipe.

Shown below are a pair of right side 4-bolt 1969 log manifolds, casting # 9796992-1 date coded E129 and J078. The upper manifold in the picture has been opened up to 2.150", the lower manifold has the stock 1.850" opening:

http://i137.photobucket.com/albums/q...7.jpg~original

The early-style right side manifolds have a larger opening than the left side ones, the right side was bored out to around 2.050" to accomodate the thermostatic flapper valve that diverts exhaust flow through the intake manifold crossover to activate the choke during engine warmup. In the 1968 model year the flapper valve was eliminated from the right side manifold if I'm not mistaken, but the manifold outlet was still bored out and was larger than the left side manifold outlet. By 1969 the right side manifold no longer had the provison for the flapper valve, the outlet casting was shortened and the opening size matched the smaller left side outlet.

Here's a look at the center manifold inlets at the head flange on a 1969 9796992-1 (at the right) and a 1964 545467 equipped with a thermostatic valve. Note the wider casting and longer bolt bosses on the earlier manifold coming off of the head flange:

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A look at the inside, note the smoother inlet on the early manifold and the slightly more restrictive opening on the later inlet with the bolt clearance notches cast in:

http://i137.photobucket.com/albums/q...9.jpg~original

It looks like using some of the earlier log manifolds might offer a little better exhaust flow. Removing the flap and shaft and plugging the shaft holes on the earlier manifolds would be the way to go.

http://i137.photobucket.com/albums/q...0.jpg~original

very cool post,as my budget doesnt allow 4-7 hundred dollar manifolds right now.i was wondering if there was something i could do for now with what i have.thanks a lot!

Bart, stop giving away all of the secrets!;)

A few years ago people were throwing away log manifolds, now it seems like everyone wants big money for them.

I think that the logs got a bad rap, in part, due to the restrictive compression bent head pipes. With the outlets opened up like b-man has done above and mandrel bent head pipes like Pypes offers, logs can work just fine for many moderate performance applications. I've got a set on the stock appearing 455 in my Lemans that is currently putting out around 425 HP.

Hey Bart, looks good. Did you open yours up on a machine? You said porting tools. Looks too good& straight for a hand grinder.

My understanding of the log's biggest drawback is the absence of chambers or dividers like found in the RA and Long Branches.

I remember looking a a cross section cut of the RA/HO mainfolds years ago and thought at least one of the holes was pretty darned small where they all turn into the "dump".

I did the work with a large hand air grinder that was equipped with a bullet-shaped stone that was about 1-3/4" in diameter or even a tad larger. If I wasn't careful it would start running around inside the walls of the opening, using a large stone inside a round opening was not easy. Even though I was using a fairly ideal grinding tool it was still a lot of work, but it seemed to work out well.

Unfortunately I no longer have access to that tool, but I did at least one set that I still have for future use on a '64 GTO Tri-Power 389 engine. I plan to keep this engine all stock with the exception of some sort of mild cam and a free-flowing 2-1/2" exhaust.

Of course after you have done the work to enlarge the outlets you now need some larger headpipes (or downpipes) to take advantage of that modification.

http://i137.photobucket.com/albums/q...1.jpg~original

http://i137.photobucket.com/albums/q...2.jpg~original

These headpipes were fabricated using a stub of 2-1/4" exhaust tubing that was flared on both ends. The 2-bolt flange is an original factory piece that's been opened up a little using a half-round file. It's always a good idea to make sure you put the 2-bolt flange on the pipe stub before making the second flare on the pipe. ;)

The bottom flare on the stub makes for a pretty decent transition from 2-1/4" to 2-1/2" tubing. The 2-1/2" 90-degree pieces were cut from some bare mild steel 180-degree mandrel bends that I bought from a parts house many years ago, today I'd make sure to use aluminized tubing instead.

http://i137.photobucket.com/albums/q...3.jpg~original

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As you can clearly see the old factory 2" crush-bent pipes can't even come close to flowing as well as the new bigger headpipes, the 2" pipe shown here is a rusty original from the right side on a '69 Grand Prix.

http://i137.photobucket.com/albums/q...5.jpg~original

Hey Bart, you know that Pypes has those ready to bolt on, don't you?
http://www.pypesexhaust.com/images/dga10.jpg

Yes I do, and I plan on buying a set to use on a '64 GTO 389 engine that's going into my '64 LeMans convertible.

I made the ones in the pics about 12 years ago, and they will still need a lot of work to make them fit the car correctly. Just thought I'd drag them out to take some pics for this thread. :)

Bart,

If you are planning on taking off the heat flapper, I'd buy it from you - need one for my 68.

B-man

looks good, nice work! but you knew that...

Chris, got a '68 manifold with flapper intact.

The exit modification does little for Hp...at least on a chassis dyno.

I have opened them up from the 1 7/8" to 2 1/8" then finally as much as 2.45" and only found that they hold peak Hp for 200 additional rpms before falling off.. I even cut the crushed neck off the pypes head pipe, and made my own entrances.

I'm still not done though...This year should be a record year for the "Log". Jeremy and I will make lemonade out of these lemons yet!

just an fyi, PY reproduced the log manifolds and did the exact modifications that Bart has shown you here.

What would be a good limit for HP. Mine makes 325 so I'm not worried I will out perform them, just looking for conversation.

Thanks
Dave

Well, Jay's old combo was around 575 hp...
Can't wait to see he does with his new combo this year!

It's only a matter of how hard you like to beat your head against the wall. Part of the fun though is doing what so many people say can't be done.:D

WOW!!!! 575HP. With those #'s headers almost don't make sense....

That number would be a LOT easier to get with headers. I think that a reasonable limit would be somewhere south of 400 HP.

It starts in the engine build... You need more exhaust duration and a few other goodies to trick the engine. Plenty of head prep as well. Jeremy was close but it only made 532/705 last year.. High 5's to low 6's is what I was looking for this year.

I have a lot more detail over on TAC under 455.

My new pair of PYPES stainless steel headpipes arrived today (p/n DGA10S-1 & DGA10S-2), these will sure make setting up my 2-1/2" dual exhaust with log manifolds a lot easier.

http://i137.photobucket.com/albums/q...7.jpg~original

I measured the inside diameter of the pipe at the flare and came up with 2.100".

http://i137.photobucket.com/albums/q...8.jpg~original