DZAUTO

A few days ago, I was inquiring about installing a FULL 2 1/2in exhaust system on my son's 73 SB car. THERE AIN'T NO SUCH THING AVAILABLE. The 66-later cars with a hi-perf SB DO HAVE 2 1/2in pipe, BUUUUUUUUUUUUT, the front few inches of the head pipe are reduced down to 2in to mate with a 2in manifold!
This is the 2 1/2in head pipes available from Corvette Central


We wanted to install 2 1/2in manifolds---------AND----------head pipes that start out with 2 1/2in for a 73 with auto tranny. As I said, no such thing exists.
So, I ordered a COMPLETE 2 1/2in system for the 73, including head pipes with 2in up front (for an auto tranny, which is a Turbo 400).


Everything arrived today. In the next day or two, I'll take the head pipes, flanges, sleeves and "home made" manifolds to my muffler guy (that I've been using for 30+yrs). I've already discussed this with him on the phone. He will cut off the 2in from the head pipes (top picture), fabricate and weld on a piece of 2 1/2in pipe so that the head pipes will mate to 2 1/2in manifolds.
In the back of my mind------------------WHY DON'T THE CORVETTE VENDORS MAKE 2 1/2IN HEAD PIPES FOR 66-LATER CARS???? I can't help but be sure that there are others besides me that want to install 2 1/2in manifolds/pipes on their 66-later cars.

MANIFOLDS
I bought aftermarket 2 1/2in (L&R) manifolds (the ones made in China) sold by Dorman or Graywerks (Amazon sells Graywerks manifolds).
This is a Chinese 2 1/2 manifold before I cut off the alternator bracket (although the one shown below is an actual GM manifold).



This is after the bracket is cut off and grinding done to the end of the manifold.



This will leave an empty bolt hole in the manifold. So, I installed an appropriate size bolt (7/16 right manifold and 3/8 on left manifold) and tightened the snot out of the bolts. Then I ground down everything with a heavy grinding wheel and smoothed out the grinding marks with a die grinder and small sanding disc. Last, I used a carbide rotary file in the die grinder in an effort to create a cast finish. I will use some kind of finish on the manifolds to provide an even cast finish (haven't decided what to use yet).

Finished manifold.






Here you can clearly see where the bolt was installed and ground off in the hole. Hopefully, when some kind of finish compound is applied to the manifolds, this will not be so easy to see.

68hemi

Great work!!

I have to take a look at the original 57 manifolds and the repo 2 1/2" manifolds to see if I am going to have to cut anything off of them to keep an original look.

I had it easier for my 57 for the 2 1/2" exhaust. I ordered the repo manifolds and a
2 1/2" exhaust system for a 1962 340 h.p. with 57-2" tail pipes from Corvette Central. They were very accommodating.

GUSTO14

Tom, I seem to recall you demo'd this solution a while back and I thought the, that this is why I appreciate the Forum. It's a terrific place to share solutions to perplexing problems, thanks to folks like you.

I do believe that they sent you the wrong exhaust tips however for your son's '73. Those tips are for the 70 -72 Corvette with the offset outlet. On the '73 they centered the pipe on the outlet.

Good luck and thanks again for you help to the rest of us on the Forum.

GUSTO

Black_Magic

I've done a couple of sets of those exact same manifolds. I left bolt hole because it is hidden under alternator. One set on my '65 & 1 on my '69. They work just fine.

Did more grinding/finishing the below pic shows.

George

DZAUTO

That is correct, the tips are 70-72 style.
I had an extensive discussion with Dave (the exhaust guru at CC) at CC before ordering the system. After explaining what I wanted to end up with, he STRONGLY recommended using 72 pipes/mufflers/tips. He also said the 72 muffler hangers are different from 73, so those were also included.
BUUUUUUUUUUUUUUUT, bottom line, we'll find out when we install everything (or at least attempt to install it).

Oh ya, by the way, I posted this whole issue over on the 68-82 section.
I KNEW, I KNEW, I KNEW that my best information and advice would come from right here on the 53-67 section (even though I'm going to be working with a 73). It seems that once you get beyond the 67 cars, fewer and fewer 68-later people are true mechanics and innovators.
Today or tomorrow I'll go to my muffler shop to have the 2in sections replaced with 2 1/2in sections, then post pictures of the change.

jrs 427

My exhaust guy makes up any combination I want. No inspectors under the car to tell if it is factory. Main idea is to remove waste gasses so I can add more fuel.
Good luck Tom.

jimh_1962

I did the same thing... I ordered 2 inch exhaust not measuring out my exhaust manifolds which were 2 1/2. I had a replace the flanges with 2 1/2. The welds were not the greatest but still fine with me for now. The main thing was to keep the geometry so that the exhaust hung the same. My mistake when I ordered the exhaust.

DZAUTO

Here are some more pictures.

This is looking at the top of a bone stock GM left side 2 1/2in manifold (3814901) after coating (this is what an original should look like).


The most recent Dorman and Grayrock 2 1/2in manifolds that I have bought have had SEVERE grinding done on the tops of the manifolds (I have no clue why they would do this!!!!!!!!). So, using my angled die grinder and a 3in sanding disc, I remove all the grinding marks. After that, I use a carbide rotary file, letting it bounce along the surface, to sorta kinda return a cast appearing finish.






On the right manifold, which comes with a choke heat riser tube installed, I knock out the tube, and use a 1/8in pipe thread tap to cut threads in the top and bottom holes. Then I install 1/8in pipe plugs VERY TIGHTLY, grind it down and finish it like the rest of the manifold.



My whole intent with this project (roller SB400/true 2 1/2in exhaust) is to make it look as if it was assembled at the St. Louis plant.

And oh ya, after talking with folks here, and doing lots of measuring, I think we will have sufficient hood clearance so that we can use an LT1 manifold (which has been up in the attic for several years).
This actually came off of a (totally destroyed) 69 Z28. Notice it has the hole up front for an oil fill tube. I will add a second gasket to the chrome oil fill cap which goes on one of the valve covers (to assure a VERY TIGHT seal), and then install a chrome oil fill tube and chrome oil fill cap in the manifold (you have MUCH LESS chance of an oil spill when using the tube up front). I did the same thing to the Z28 intake on the 400 in our 70 Chevelle and our youngest son's 68 Chevelle -------------sure makes it nice to pour in oil!






Our youngest son's 68 Chevelle with a 350 with a 68 Z28 manifold (67-68 and 69-72 manifolds are slightly different).

Westlotorn

Have there been any tests done to see how much power is gained moving from the 2 inch to 2.5 inch? I know you are using a 400 block which will put out more exhaust than the 327. Nice work on the manifolds.

DZAUTO

There has been LOTS of discussion and debate about the 2in vs 2 1/2in manifolds/pipe. FOR THE MOST PART, I feel the persons who are adamant in their opinion of using 2in manifolds are only considering their installation on a 327 or 350.
I have never really subscribed to this view point, so I've never become involved in the controversial discussions.
BUT, since we're building a roller SB400, it will be near impossible to convince me that the 2 1/2in manifolds and FULL 2 1/2in pipe will not be beneficial.
But, regardless, that's what we're going with!!!

GUSTO14

Tom, I realize it's a moot point now, but I know that Allen's will bend a set of header pipes for anyone that requests it if they want to us 2 1/2" manifolds with their dual exhaust system. All the customer has to do is tell them they want to use 2 1/2" manifolds with the system they order.

I just spoke with them to verify that they still do this.

GUSTO

jimh_1962

Yeah, thats where I made my mistake... Oh well a little modification does not hurt anything.

DZAUTO

OK, it's done and ready to bolt on to the car when we get the engine built and installed. Notice my muffler guy expanded the existing 2 1/2in pipe so that the new piece of 2 1/2in fits inside and makes a good seal and unrestricted flow after being welded together.

jrm5657

That is correct. We have a 2-1/2" system coming this week for a 72 small block that has been upgraded with 2-1/2" manifolds. The head pipes will be 2-1/2" all the way from the manifolds to the muffler inlets.

DZAUTO

A little late for me now. I've already had the head pipes modified which I got from CC. We'll use those.

MarkC

I changed out my 2 inch manifolds with sidepipes on my 66 L79 to Brzezinski ported 2 1/2 inchmanifolds, mandrel bent 2 1/2 inch pipe and 3 inch vette packs. There was a big change in power, especially over 3500 rpm's. Latter I also changed the intake and carb and plan to put the car on a dyno this Friday. One reason I changed out my exhaust was the very restrictive 2 inch pipe that was 1 5/8" inside diameter tn two turns before reaching the Sidepipes. I think all these mods helped the engine breath while still keeping the original look. I have to say the 650 AED carb on the Z/28 manifold with the exhaust crossover blocked performs very well. I have no choke and the engine starts fine, runs great when hot and the carb accelerates like my Z06, well almost. It was money well spent.

HilltopClassicCars

Tom, nice project you've got going on there. If you have access to an air/pneumatic needle scaler, it might make those ground down areas of the exhaust manifold look more like the original rough cast look?

DZAUTO

Just for review, before and after.





Eliminating the 2in bottle neck HAS TO IMPROVE FLOW!

GearheadJoe

Hi Tom:

Your exhaust manifold re-work is impressive. Those Dorman manifolds are a bargain compared to the available repros, although the Dormans need more port matching than the repros.

For my '67 383 project, I determined that there were '63-'65 factory 2.5" manifolds that looked exactly like my stock 2.0" manifolds in terms of configuration and the appearance from above.

For '67, the passenger side manifold is a symmetric design that has no additional provisions for an alternator, while the driver side manifold has an alternator mounting boss that first appeared in 1965. I'm away from home right now but I can dig out the 2.5" casting numbers if anyone is interested.

Rather than pay $$ for rusty '63-'65 originals, I decided to pay the same $$ (about $230 each) for the very nice reproductions sold by Paragon.

To solve the problem with the 2.0" head pipes used in '67, I simply used the 2.5" head pipes for a '65. These fit perfectly on my '67.

It seems possible that the '65 2.5" head pipes *might* fit a C3, since the frames were very similar to the C2 frame in that area. However, I really don't know whether they would fit.

In any event, below is a photo of the 383 installed in my '67. The exhaust manifolds are 2.5" and so are head pipes, but you really can't tell unless you get under the car.

DZAUTO

You are totally correct about EVERYTHING---------------------------EXCEPT, EXCEPT, EXCEPT------------------my son's 73 is an automatic (Turbo-400), and the 2 1/2in pipes for a 63-65 WILL NOT work with the T-400. That has been the ENTIRE problem in regard to installing 2 1/2in manifolds/pipes on my son's 73. Thus, the reason for the modifications to the L82 style 2 1/2in pipes which are 2in on the end where they connect to the manifold.
I went over to the 68-82 section inquiring about this very same thing------------------THEY JUST DID NOT GET IT!!!!!