Replacing a cracked Exhaust Manifold
 

I purchased my C1 as nice "driver" that I am very happy with. And I knew that there would be a few things here and there that would need to be addressed.

The car was running a little ragged, and it was pretty obvious that the timing was off a bit. At idle, it ran rough. When starting in 1st gear, it sounded like a tractor, but smoothed out a bit when it got up in the RPMs.

https://cimg9.ibsrv.net/gimg/www.cor...3355a9f031.jpg
You can see the crack bottom edge of the manifold.

https://cimg4.ibsrv.net/gimg/www.cor...90f3801fca.jpg
3749985

https://cimg6.ibsrv.net/gimg/www.cor...6c3485cc34.jpg
3750556

The manifold bolts look like they are going to be a bear to get off.

I took the car to a mechanic that my wife and I trust, and asked him to perform a complete tune up, and take care of a couple of leaks that needed to be addressed.
One was a coolant leak. Loose hose.


Another leak had to do with the steering box. I'm not sure if I got the full answer to that one, but, it was fixed.

The steering does not seem to have to much play back and forth, and it feels good and centered as you drive along.


A couple of plug wires had melted onto the rear of the passenger side exhaust manifold. These were replaced.


There was also a couple of bad spots in the mufflers. So we replaced them as well..


We found an exhaust leak in the upper section of the down pipe at the manifold flange. (passenger side) So that was replaced.

I picked the car up Friday morning, and the result was like night and day!


We went through all the work that was done, and he pointed out that the Right Side Exhaust manifold had a hairline crack just above the flange.

Not sure why he didn't call to tell me about it, so it could be replaced, but that's ok. It took me a bit to get to the casting number to find the correct part to order. So it may have been a good thing that he waited.


The engine in my car is a 350 ci. from a 1970 car. The manifolds appear to have come from the original engine.

As far as I can tell, the replacement manifold(s) would be:

3749965 (L), and 3750556 (R) for 300 Horsepower with a Powerglide Trans.
I have a 4 spd Manual Transmission.
These are 2"

3797901 (L), and 3797902 (R) for 300 & 340 Horsepower with M/T.
These are 2 1/2", and were apparently used on 1962 and 1963 327 engines, so they could have been on the original engine for this car.


My car has the generator that is bolted to the manifold, so these part numbers seem to work for that.
I would love to switch the generator to an Alternator, but that's a different post for the near future...

So what I have is a 1962 Corvette, with a 1970 350 ci replacement engine, that has 1962/63 manifolds...

My question is whether or not I should switch from the current 2" manifolds, to the 2 1/2" set to match the 4 spd manual transmission?
Or does it really matter in the grand scheme of things?

I've also considered switching to headers, but I would be left with how to mount the generator/alternator correctly.
But that would be down the road a bit.


When I picked the car up, the engine ran so much better at idle and at highway speeds. Like Night and Day.
Even with the cracked manifold.


Right now, we're just enjoying the car. We both love it.
So I just want to replace the cracked manifold and take my wife for another drive. :)


You're help with these questions would be appreciated.

Thank you,

Mike

Personally I don’t think it matters if it’s 2 or 2 1/2. I would stick with the stock manifolds instead of headers. They usually leak over time and you’ll have to change the exhaust hook up at the header.

Soak the exhaust manifold bolts with shots of PB-Blaster spray over a couple of days...then just take the thing off - no biggee; IIRC repros are available...

C1 steering boxes leak like mad if they had the original thin lube still in them; a Service Bulletin was issued to replace the thin stuff with good chassis grease and vendors sell a kit with a zerk fitting to go in the upper steering box bolt hole so you can shoot the heavier grease in there. If your mechanic didn't deal with this, then he didn't do you any favors...

Michael, changing to 2 1/2" manifolds isn't really going to matter much in any event if you don't also change out the exhaust system to match. At the very least you would need to change out the head pipes and neck them down to the remainder of the exhaust system. If you do that, the farther back you go with the 2 1/2" pipes the more beneficial the change would be.

You really haven't said what the engine in your Corvette is other than a 1970 350. What induction system are you using and what cam does it have, if you know? These will really determine what benefits you might gain from a change to headers or to the larger manifolds.

I'm also having difficulty seeing the crack you mentioned. Can you circle it or point to it in the picture?

Good luck... GUSTO

I don't see it either.

Manifolds are generally trouble free, unlike headers. Are you racing? Headers will offer HP advantages in certain RPM ranges.

Not sure what that hose clamped heat riser gizmo is.

2 1/2" won't matter unless you are racing.

Do you stop light street drag race much?

No? Don't worry about it.

Doug

The intake is a standard intake manifold for a 350 ci / 300 hp. The Engine ID decodes to a 1970 350 for an Impala / Bel Air.

I am not sure whether or not it was brought to "Corvette" specs when it was re-built a few years back. So I'm not sure what cam, pistons, Compression Ratio, etc., it might have

I ordered a replacement from Eckler's Corvette.

Here is a picture with arrows pointing to the crack.

https://cimg7.ibsrv.net/gimg/www.cor...09f2e918ad.jpg

Mike

I think its a homegrown choke stove...

That is a choke stove for vehicles that use a hot air choke but have lost their source. They were pretty widely sold back in the 60's and probably into the 70's for folks that installed headers on a vehicle that previously drew hot air through their exhaust manifiold. I installed a set of headers on my '63 back then and found them at some of the hot-rod shops to address the lack of a hot air source. It turned out that I didn't need one as my 327 ran just fine without a choke if I just took a minute or two to let it warm up. JEGs still has them...
https://cimg3.ibsrv.net/gimg/www.cor...00bd1cad28.jpg
https://www.jegs.com/i/Dorman+Produc...CABEgLEOPD_BwE

The need for them pretty much ended with the advent of electric chokes. By the mid 70's many cars already had them and Holley sold a retro-fit kit to replace a hot air or mechanical choke with an electric one. These worked just fine and were much simpler (and less obtrusive) looking to use.

GUSTO

Well it reads like you are fairly new to the small block chevy (sbc) and you will soon find out the options for swapping and replacement are nearly endless. Lots of good suggestions already but maybe I could add another. For a "frequent driver" car I think those tight fitting shorty hugger headers make good sense. First is they are cheap in even stainless on eBay. Next they give you great room for plug wires. Finally they do add power due to individual tubes provide some amount of exhaust scavenging w/o reversion.

You can find them made from SS on eBay in the $150 range. To bad you all ready paid for your replacement as I'm sure it was more than $150 - though you could return it. Then you have a original to repair (weld) and restore. This may take more work though as you have to replace the other manifold also.

Something you/we probably don't know is the actual power rating of the 1970 motor. But for 300HP the 2" system should be good enough and save you time and money from upgrading to 2&1/2". I really don't think you would ever see a difference driving on the road or even the dyno.

BTW I wish I had a '62! :)