Another Exhaust Posting

[Jason S]

Last week I was driving the 1973 GMC to work and I began to hear the awful tick-tick-tick sound of a exhaust manifold donut failure...  When I got home I proceeded to remove the passenger side exhaust pipe to replace the the offending donut gasket.  As it had been well over a decade with this exhaust system, I was surprised to find that I could remove all three nuts from the exhaust manifold without breaking a manifold stud.

After I got the nuts off and fought with getting the flange off the pipe, I had to cut the pipe to get to the donut.  With the pipe off and seeing inside, I remembered the muffler shop had pressed the dunut onto a short piece of pipe that was then inserted it into the down pipe.  The donut came out and part of the pipe insert tore off.  I decided at that point the whole shooting match should probably be replaced instead of a patch job to get me "down the road".

I did some researching for a dual exhaust kit like I had installed in the '74 K10. It originally came from Whitney's (don't laugh) and only cost about $113 delivered (10 years ago).  What I got from them was a 2 1/4" dual kit that installed fairly easy and has held up very well.  But I couldn't find the same kit from them now for anywhere near the same price. 

What I wound up getting was the Dynomax dual kit for a small block 73-86 4x4 GM truck, an exhaust crossover kit and a pair of Thrush Turbos.  I went with the Thrush because I had been running Mac mufflers and was kinda tired of the drone at highway speeds. 

The install was not as quick as when I did the '74's kit.  I think the main reason I had so much trouble was that I was trying to install the H-pipe kit and keep the tail pieces in the stock location.  However, after many hours of trial and error and tweaking and re-tweaking it's finally in and leak free. 

Overall, I'm not sure if I like the Thrush Turbo mufflers.  They definitely don't have the drone of the Mac mufflers, but they remind me of the General Lee when the truck is idling.  I'll give them some time. 

Seat of the pants dyno has not indicated a significant gain or loss.  The performance with the 2 1/4 all the way through seems to be about the same as what I had before, which was 2" with H-pipe to 3" into mufflers to 2 1/4".  Overall, I think the $250 for the dual kit, crossover and mufflers plus about $35 for additional clamps and a couple pieces of pipe (about $285 total parts) plus my time was worth it. 

SO, here are some photos of the project. 

The first photo is what was on the pickup since around 1997 (how time flies).


The second photo is a close up of the passenger side down pipe with the worn donut removed and residual gasket and centering pipe debris remaining.



The third photo is the new Dynomax kit laid out with the crossover kit and mufflers.



The fourth photo is of the Mac mufflers (they may return to use).

[Jason S]

Additional photos of the Dynomax kit, Thrush mufflers and crossover kit installation.

Right out of the box I had to modify the Dynomax parts.  The exhaust downpipe flanges that go over the exhaust manifold studs were not the correct flange or the holes were cut incorrectly.  They are very nice, thick steel plate with laser cut holes; however, I had to remove material to get them to fit the exhaust studs.  As it was late in the day when I started, the hardware store that has Dremel bits would've been a 30 mile round trip and closed; I decided to file the excess material out.  The first photo shows the before on the left and after on the right.


The second photo is the Fel-Pro sintered metal donut gasket.  If you run stock manifolds that use a donut, get this type if you have to replace your donut gaskets.


The passenger side manifold required a donut gasket between the manifold and the exhaust down pipe.  The driver side manifold on my pickup has a "ball" type mounting arrangement so did not require a donut gasket.  I had replaced the original manifold (cracked) with N type manifold and think that the one I have may be a later style. Nevertheless, I used the sintered metal gasket on the passenger side so that I wouldn't ever have to worry about crushing donut gaskets.
The third photo is the modified crossover pipe.  The kit I bought had two crossover pipes included with it: one had about a slight bend in it (maybe 22.5 degrees) and the other was straight.  There was no way to use the straight or slightly bent crossover in my truck.  So I took the straight pipe to a muffler shop and had it bent into a 90 degree and then trimmed to fit.



The forth photo is looking down from about the back of the transfer case.  There are a serious amount of clamps on the system right now. I'll weld up the connections as soon as I am sure the pipe alignment is going to work and stay and figure out for sure what mufflers I want.

[Jason S]

Some more photos of the final install. 

The fitment is okay.  While a real exhaust shop would have made a much better install, I think this will be a good setup until I tear this truck down completely. 

The issues that I have with this dual exhaust kit include: instructions (or lack thereof- a "line drawing" and no details), having to tweak pipes bends (matching the left and right tailpipe angles), it is not a factory replacement exhaust and does not exit at factory locations. With that being said, I can say that being patient and tweaking and re-tweaking can get you a decent 2 1/4" mandrel bent exhaust system. I really like the dual exhaust setup of the '80's GM trucks, this is not a "true" replacement (but it works).

The first photo is looking up the passenger side near the exhaust manifold.



The second photo is looking toward the front from behind the transfer case.  There is a close clearance to the transfer case support brace, but I just couldn't seem to avoid that. Either the pipe went near the brace, near the transfer case or near the cab floor.  I figured that located near the brace was better than near the other two options.



The third photo is a bad shot of the driver side pipe to manifold. 


The fourth photo is the clearance between the driver side pipe and the transfer case crossmember.

[Jason S]

Final touches to the exhaust install.  After I had everything together, the passenger side came up with a leak at the manifold.  I torqued the manifold to down pipe a bit more but found that didn't help. So I loosened a few clamps, loosened the manifold nuts and adjusted and tweaked some more.  After I was satisfied the manifold, donut gasket and downpipe were situated; I tightened the manifold connection, started the pickup and let things warm up and tightened the remaining clamps and retightened the manifold connection.  So far all seems well.

As I was really wanting a factory type install (or at least a factory look installation), the final touch was the GM exhaust cut on the end of the tailpipe.  The majority of the system that was on the truck was a factory GM setup from a 1985 C-20 and was added to the then new 2" run up to the transfer case area.  The Mac mufflers replaced factory 1985 GM mufflers in about 2000.  I duplicated the factory GM cuts on the new pipes as best I could; essentially it's a 90 degree cut for length and then a 45 degree cut starting about 1/4 of the way into the pipe.   

The first photo is a poor shot of the passenger exhaust manifold to downpipe connection with the sintered metal donut gasket.


The second and third photos are of the factory GM pickup dual exhaust bends at the exit location and the cutoff at the end.



The forth photo is my replication of the GM cutoff at the tailpipe.

[bake74]

     So how long did it take to do the total install ?

[Jason S]

Thursday:  about 2.5 hours to remove manifold nuts, flanges, cut pipes and decide to replace system. About another 3+ hours of researching and ordering.
Friday:  about 1.5 hours to remove remainder of exhaust system.

Monday:  About 4 hours to file out flanges, clean off manifolds, fitting pipes, checking and looking and looking and fitting.
Tuesday:  About 8 hours of fitting, tweaking, cutting, helping with childrens, fitting, tweaking, feeding childrens, looking for tools, etc.
Wednesday:  About 4 hours of final fitting, cutting, tweaking and tightening.
Thursday: About 1 hour of loosening clamps, tweaking passenger side manifold connection, re-tightening and re-checking. 

So I figure about 21 hours of work removing, figuring, ordering, modifying, reinstalling and tweaking.  Compared to the last setup I diid (i.e., '74 K10), this exceeded that truck's total exhaust time by at least 100%. The '74 was a ground up rebuild and restoration, but I seem to remember that I only spent about 6 to 8 hours on the installation of the exhaust system.   

[westsidek20]

Is that the best place/design for the "H" setup?

[Jason S]

Probably not the best locale for the H-pipe, but with this system it's about the best fit possible.  I understand that the H-pipe should probably be a the hottest spot in the exhaust under normal operating loads and that probably would have been further toward the front.  But I think that any crossover is better than no crossover.  My opinion is that it can help with exhaust pulses stacking up prior to the muffler to ease some exhaust "congestion". 

[zieg85]

Nice job.  21 hours wow but at least you can say you did it.  Personally I let the exhaust shops do mine.  Having worked for one I know what it takes to do it right.  I also know what my time is worth and what one trip to the emergency room or the eye doctor when something happens.  I have only had a couple done by one guy I trust over the years.  He knew that I know what is going on.  My first visit was a fact finding quote getting visit.  I told him I was referred to him by a friend, which I was.  I told him I have done custom pipe work before and expected perfection which is easy if you know what you are doing.  I wanted the pipes routed exactly the way the factory had done it using factory hangers, tailpipes even in height and how they looked from behind the truck.  I also told him to quote fairly using the thicker pipe.  I was extremely happy with the outcome.  I will use him on my next one for sure.

[Jason S]

Thanks Zieg.

Yeah, I spent a whole lot more time on the project than I anticipated, but I had expected the install to go like it had on the '74.

I concur about the time aspect and medical aspect.  I forgot to add the $5.00 cost of the Ibuprofen...

Overall, I'm fairly pleased with the results except that it really did take a lot of time with trial-and-error fitting (whereas a muffler shop would've been able to just make a new bend), having to modify flanges, etc.

If you don't have an exhaust shop nearby to do the job correctly and not break the bank and are willing or able to invest the time, a kit like this may be worth your time.

[Psycho71]

Yeah, 4wd's can be fun when it comes to locating the h-pipe. Mine is right under the tailshaft of the transfer case. it probably would work better a little farther forward.  I was concerned about having mine under the x-case, but there are a couple of things that hang lower, and should drag before it will, so I call it good until I crush it.

I ran mine w/o an h-pipe for a while, and definitely like the sound better with it. The magnaflows I put on mine had a funky sound under load, but the h-pipe noticeably reduced that. I was ready to yank the mufflers, but decided to keep them now.

I did mine myself as well. I bought the flowmaster 2.5" kit (universal dual). I turned mine over the rear end and stopped there. it took me about 6 hours to gut the old system and install the new one, and that included a trip to the muffler shop for a couple of bends. Having headers likely made it a bit less time consuming though. The toughest part for me was some of the welds, and trying to get to them while under the burb with the helmet on. Some of it just had to be done blind.

[westsidek20]

Is there a difference in performance or sound if you  turn the pipes behind the wheels or if you just lead them straight to the back?

[Captkaos]

Sound.

[ehjorten]

That sintered metal exhaust donut should last you a while.  If you use the factory style metal/fabric woven ones you have to have that short section of pipe inside the Passenger's side down-pipe.  If not the donut will eventually crush down and fall into the exhaust pipe.  The factory down-pipe was expanded in that area and a pipe was fit into it.  If you just press in a short section without that formed feature in the pipe to hold it, the heat of the exhaust will expand the pipe and the pressed-in one will move down in the pipe to where it isn't doing anything.  Most aftermarket setups will not have that extra detail so using the sintered metal donut is the best option!

[Stewart G Griffin]

Would you happen to know the stock downpipe size?   Also, can i use the sintered metal donut on both sides?