N11 Off-Road Exhaust System Numbers

[Mike Grady]

About 20 years ago I got a good deal on some Corvette parts from a dealer that was going out of business. Among the parts was (supposedly) an N11 Off-Road Exhaust System. Today I got the pipes out of storage with the intention of installing them on my '66 L79 convertible but I am puzzled by the part numbers I am seeing. Hopefully someone with more knowledge than I about these systems can help.

The system physically appears correct. The front pipes are 2" at the flange and then expand to 2 1/2". The rear pipes are welded to the mufflers. The GM parts sticker on the front left pipe shows #3877209. The right front pipe is #334926. The parts sticker is gone from the left pipe/muffler assembly and #3845737 is molded into the muffler shell. However, the number 5 has been stamped over the last 7 in the part number. From what I've been able to research, #3845735 is the correct number for the left side off road exhaust muffler. Since the pipes have been in my possession since leaving the dealer, someone involved with GM parts must have stamped the number. The right side pipe/muffler assembly has the # 7 1 022 5 A molded into the muffler shell. It also has the words OFF ROAD ASSEMBLY stamped in ink on the muffler. This stamp appears to be the same as the one shown in the photograph on pg. 366 of Noland Adams book.

Hopefully someone can help me determine that what I have is indeed an off-road exhaust system. I am not concerned with show appearance, I am primarily concerned with performance. I especially want to avoid putting on a system where one side is more restrictive than the other. Thanks.

[Joe L.]

Re: N11 Off-Road Exhaust System Numbers

Mike-----

I believe that you have the correct parts for your application.

First of all, the front pipe GM #3877209 is the correct left front pipe for a 1966 model with N-11. This is a 2-1/2" front pipe configured for a 2" outlet manifold which all 66 327 cid engines used. Earlier 63-65 models with 2-1/2" outlet manifolds used a different left front pipe configured for direct connection of the 2-1/2" forward exhaust pipe to the 2-1/2" manifold outlet. This all assumes, of course, that you have the correct exhaust manifold. For the left side for 1966, this should be GM casting and part #3846559.

Your front pipe, GM #334926, is also the "correct" right front exhaust pipe for your application. This is also a 2-1/2" front pipe configured for connection to a 2" outlet exhaust manifold. This is not the original part number for the right side, but a SERVICE replacement part number. The original part number was GM #3877210. This also assumes that you have the correct right side manifold which should be GM casting and part #3747042.

I also believe that you have the correct off-road muffler assemblies. The left side assembly was GM #3845735 and the right side GM #3845736. You can't go by the numbers embossed on the mufflers, though, even if there are numbers present. Usually, these numbers represent the part number of just the muffler portion of the assembly and the muffler portions of the off-road exhaust assemblies were never available seperately as a SERVICE part. It sounds like in the case of one side, someone altered the muffler part number to the ASSEMBLY part number to avoid confusion on the consumer end.

Basically, and regardless of part numbers, you pretty much have to have the correct muffler assemblies for several reasons. First, the 63-67 N-11 systems are all the same, although there may have been a predecessor part number. All are functionally equivalent and will interchange(not side-to-side, though). Second, N-11 mufflers were the ONLY systems which GM ever serviced for a Corvette that came complete with welded-on rear exhaust pipe(not tail pipe). Third, the only differently configured N-11 muffler/exhaust pipe assembly which GM produced was the 1968 system. This system differed only with respect to the mounting brackets welded onto the rear of the muffler and used the 68-72 style brackets instead of the 63-67 type. These 68 parts are long-since discontinued and I doubt that you have them.

[Wayne M.]

Re: N11 Off-Road Exhaust System Numbers

Mike -- Agree with Joe that you have an N11 system. Never have seen the embossed 3845737 (7 overstamped w/ 5, though). My N11's have the embossed part #'s (and other #'s) on both upper and lower surfaces of the muffler. My numbers are: (left) GM (top line), 3845735 (second line), 1 1 323 1 A (bottom line). Right side: GM, 3845736, 6 1 281 7 A.

Ink stamp may indicate original factory installation (new car take-offs ?) While these N11 #'s were used in '64 thru '67, data for 1963 is inconclusive. There would have been interference with underbody fibrerglass on the early cars, and maybe this is why there are no production quantities shown for N11 in '63.

[Joe L.]

Re: N11 Off-Road Exhaust System Numbers

Wayne-----

Apparently, there are differences in the part number debossments on these mufflers assemblies. I just went out and checked my set which still have the GM part number stickers on each unit. The GM #3845735(part label) corresponds to a debossment of "GM 3845737" on the muffler portion of the assembly. The GM #3845736(part label) corresponds to a debossment of "GM 3845738" on the muffler portion of the assembly. These numbers are on both sides of the mufflers and there are also other numbers similar to the ones you describe. There has been no attempt to change the last digits as Mike described, though. And, there is no "off road only" ink stamping.

As I mentioned in my previous post, I believe that the numbers on these mufflers are the part number for the muffler only portions of the assembly, which, for these assemblies, were never available as a seperate part. In fact, I did confirm that GM #3845737 and GM #3845738 were never SERVICE part numbers.

[Tom B.]

Re: N11 Off-Road Exhaust System Numbers

For whatever it's worth, the N11 (mufflers only) numbers from my 68 are, left side: 3845737 embossed, with 3845735 ink (or paint) stencil and for the right side: 3845736 embossed, with no ink (or paint) stencil. The ink stamping "off road only" does not appear on mine either. TBarr #24014

[Wayne M.]

Re: N11 Off-Road Exhaust System Numbers

Tom, Joe, Mike et al --- Well, this IS interesting; so far, I'm the only one reporting # 3845735 embossed, (on the left assembly of a set I bought from GM in 1980). Anyone out there still have FACTORY original N11 muffler assy's on their 32 to 36 year old cars ? Are yours the originals, Tom ?

I notice that 1968 was the last year for this low-restriction / under-car exhaust system option, and 3931803 /804 were the L/R part #'s. Could it be that they took the same muffler (as in '64-67) and welded it to a different rear pipe ? Was was there an internal change in the muffler ? Was the routing of the left-side pipe in '68 the only thing that changed from the C2 era ?

Joe's interpretation that the 3845737 is the muffler part of the assembly does make sense, except, as he points out, this # was never available (for service) as a separate component. And I don't think there are many examples of this occuring in the GM parts numbering system.

[Joe L.]

Re: N11 Off-Road Exhaust System Numbers

Wayne-----

I believe that the muffler portion of the 63-67 N-11 assemblies and the 68 assemblies are the same, and that would be supported by Tom's report. There may be a slight difference in the exhaust pipe portion of the assemblies, but the difference that I know exists is the rear muffler hanger bracket. 63-67 and 68-72 use a different bracket.

[Tom B.]

Re: N11 Off-Road Exhaust System Numbers

Wayne,

I can't be positive if my 1968 N11 system is original, but to the best I can tell it is. It's supported by my build sheet and the exhaust pipe weld to the mufflers are correct. The system is worn and aged, but in good condition and the car has 63,xxx original miles. There are more embossed alpha/numeric characters on my mufflers that are similar to what has been reported already. One additional ink mark is a well defined "bar" or "stripe" inked over the length of the embossed 3845737 on the left muffler.

I don't have a 63-67 undercar system to compare to, but I easily agree with Joe that the 68-72 rear muffler hanger bracket would be one (if not the only) difference for the "muffler" between the 63-67 and 68-72 years.

My information from here can only be speculative, which is I believe there must have been a clarification needed at one time for the part/assembly piece identification during factory production or service replacement. Hence, the additional ink number or 7 to 5 alteration, etc. Also, since the numbers (even for 68) have so far been significantly the same, then I believe there were no internal differences or configurations for the mufflers through then. I wouldn't want to mention all that to be misleading, but I could be mistaken as well. TBarr #24014

[Joe S.]

Re: N11 Off-Road Exhaust System Numbers

I have two sets of NOS, N11 pipes. The first set with shipping tags from the mid 80's is as follows: Right side pipe is embossed "GM 3845736" on one line and "21 273 5A" on the second. Left side pipe is embossed "GM 3845735" on one line and "81 221 5A" on the second. The second set with shipping tags from the early 90's is as follows: Right side pipe is embossed "GM A" on one line and "113451" on the second. Left side pipe is embossed "GM A" on one line and "121911" on the second. Neither set has ink stamp "off road" ID. By the way Joe, I finally solved my exhaust sytem dilema; with two sets of pipes, one goes on the car and one goes in the attic!