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:

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

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

http://i137.photobucket.com/albums/q...4.jpg~original

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

Pypes
 

Mine arrived today as well B man. I did have a Question About log Manifold.
I decided to use 1423 felpro gasket. When I held gasket and manifold up to head
it looks like you see a lot of gasket when installed. Does anyone every cut gasket
once installed ? Just did not look right to me. Thanks again !





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No gasket needed if both surfaces have been machined. I have never used one. But,those logs look like someone has tickled them a bit.

The steel shim manifold gaskets do have some additional material in between and around the ports but I've always left them alone. The gaskets came looking that way from the factory, the factory original look is always fine by me.

I've used the Felpro gaskets and have always had good luck with them. I usually trim them down with snips so that you can barely see them.

Yea, I got to looking around and I thought the 1423 was just for headers. Felpro does have another gasket Fel-Pro QMS9499SH , looks to have less material outside sealing area. But I think I will stay with 1423. And yes I have been on these logs for about a week but they already had attention when I got them. They are ready to install !

I'll bite since I don't know. What is the butterfly valve for and do I need it if I install the logs?

Thanks
Dave

The butterfly valve was intended to improve drivability when the motor was cold. The flap was closed until the motor warmed up there by forcing exhaust through the crossover under the carb, thus getting the motor up to temp quicker.

Pontiac eliminated the flapper by 1969 so no you don't need to run it for proper operation.

heat under the carb helped atomise the fuel on cold days. they were still using them through 76. If your car is in a high state of tune you may not need it but let your car warm up (Takes a bit longer).

On What? Every '69 and newer pontiac I've owned and seen didn't have them.

My wifes 77 Firebird with the stock 400 had a flapper.Its a seperate piece that goes between manifold and headpipe. The manifold is flat,the flapper valve has the bell for the exhaust pipe.It is the 3-bolt type.It is in the service manual. Its called E.F.E (early fuel evaporative) system.
The 2-bolt manifolds stopped using the flapper valve.
Craig

This TATA I am working on had a flapper on the driver side. Since the 79' TA Motors were held over from '77 model year (see date codes)they had them.

I took out the flapper though and welded a bolt into the hinge tunnel to seal it up.

Did you use a MIG? I have a mig and soon to have an ARC welder. I was curious if I could use my mig on this.

Thanks
Dave

I used a well fitted bolt and welded it with a Lincoln Electric MIG welder.

Thanks!! I have the same welder...

Hmm... I've got to learn the new jargon... what I've always called "heat risers" became E.F.E. systems!

SteveOw- any "theory" behind your removal of the center ports divider?

Well, you learn something new everyday. So the heat riser flapper thing went away in '69 and came back again as part of the emission control system in the mid '70s?

Jack, that center port divider is another thing the Pontiac engineers changed their mind on- I've got a bunch of sets of logs and only a couple have the divider. I don't recall the dates on the ones with them or without.

The set on my Lemans right now are '70 castings that don't have the divider.
BTW, my lemans ran a 12.76 at Lebanon Valley yesterday with logs... and G70-14 Polyglas tires. Those log manifolds aren't the hinderance that most people believe!;)

My 70 GTO got one of each!!:(:confused: Early Nov. 69 car,....... manifolds weren't that much earlier from memory.

Thanks for the thread too b-man.

Well I would assume that it would be easier to seal the gasket with the divider removed. I guess that I will cut them out when I open up the out port.

Thanks
Dave

Nope, no theory. In my case the center was flaking off, like it was really catching heat, so when I started to polish things up it would just flake off like rotten metel. So I took it out. Now looks great, all polished up and I think things will flow better with it gone. One bad note, I am sending my pypes down pipes back tomorrow, the coller will slip right over the bell at the end of the pipe. When I tighten the coller to the manifold the pipe will just pull loose. It just pulls it over the bell when tightened. Anyone ever have this problem ? Very Disappointing.

Nice thread Bart. You often come up with some interesting and instructional topics.

I have a largely original '66 WT 389 with the original heat riser valve still moving freely.
I also had a '69 Buick LeSabre some years ago that had that separate insert piece with the heat riser in it. Kind of a neat idea considering how those things always seemed to get stuck. With style, you could just toss it and get another one.

Good thread going here.

It looks like Pypes sells two types of downpipes.

One with the flanges (DS10S) and one without(DGA20S).

The set without flanges are not necked-down and would have less restriction but it looks like they will only work with RA manifolds:

http://www.pypesexhaust.com/downpipes.html

I suppose we cannot get these for Logs because the sharper radius requires the flanges to be pre-installed.

Maybe they were defects.
Did they replace with new ones with a larger bell?

I just installed DS10S and DGU11 . They were not that tight of a fit. But I had to remove and reinstall them together for the correct geometry. They sound great. Manifolds wil quiet mufflers and tailpipes and it is still pretty loud outside of the car. Inside it is nice and quiet. A small amount if any of get-up from the start and no lose of top end with manifolds.

Granted I am less then 400 HP. I would recommend the Pypes numbers above and manifolds.

Dave

correct..i do not know the exact date they stopped with the heat riser valve, but i know that 72 thru 74 pontiacs do not have them..they returned in 75 with the cat. converter. i personally dislike these, not only for the restriction, but for the fact that they can seize up and stick closed, severely damaging your engine.

unless you are doing a 100 point resto, id leave it out, and even if you do a 100 point resto, id tack weld it open..;)

i think opening up the log outlet and extrude hone polishing sure cant hurt..plus with good mandrel pipes that dont neck down you should be good up to 1hp/cu in....:)

Yeah I tack welded mine open.

Removed mine and put a pipe plug in the holes.

manifold
 

This is an excellent thread full of good info. My question is this, do the original
FB manilfols have a heat flapper in them. the reason I ask is because i don't recall
mine having them when I removed them during resto 10 years ago. I just sandblasted
them and coated them with exhaust calyx.

I do plan to remove mine again and clean them ports up and resize exhaust exit
hole.

Gerry C

Glad to see this thread.

Good tech, B-Man!

My Firebird was a california car and did not have a flapper assembly.

Is the heat riser necessary with an automatic choke?