Replace leaking manifold with headers?

Fisher67

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Nov 5, 2015
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Got the rusty OEM muffler replaced yesterday at AA Performance and they did a great job. Unfortunately, the ticking I have been hearing was confirmed to be a leaking left manifold. They suggested a guy who can remove the OEM one and repair it form much less than replacement. Any thoughts on this, and is it worth the additional cost to step up to a Doug Thorley header? Any performance gains in either power or improved MPG?

Thanks,

Todd
 

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AimCOTaco

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I and a few others have the DT headers. The general consensus is that the performance gains on the 100 are not big enough for most people to justify replacing the stock manifolds if they are fine. In the case the stockers are already leaking I think it's a great option as a lot of the cost is in the installation. (I installed my own so it was easier to justify the cost).
That said, my non-leaking stockers got traded for beers so finding used stock manifolds is another option, just don't pay too much.

If the manifolds are coming off, its a great time to replace the 2 forward O2 sensors as well if they are old (might pick up a little economy and performance there also).
 

DaveInDenver

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I've personally had mixed results with repaired cast manifolds staying fixed. It's possible to do well but requires ideally brazing and it's best if you can remove it to prep and do right. Welding in place isn't likely to last.

Thing is if you're going to the trouble and risk of removing a manifold then going with headers starts to make sense. I did that with the '91, went to headers rather than repairing the manifold. Did it make any difference in performance? Nope.

Did make it hotter under the hood and especially right under my left foot in the cab, so I went back to a manifold. But luckily when I pulled the manifold originally I only had to repair one hole and none of the studs had seized the second time around.
 

AlpineAccess

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Loveland
A few guys on mud have successfully removed the manifold from the vehicle, ground out the crack and rewelded them with no issues - repairs sound like they've lasted. The cracks I've seen them document are on the inside of the flange where it mates with the outlet "tube" and is a pretty straight forward spot to weld if you get it good and clean first.

This is unlike the exhaust manifolds I had repaired on my old work truck (chevy provided by the company) where it cracked in the casting. @DaveInDenver is correct that welding cast like that is really hit or miss. The tick returned on that after a full winter of heat cycling, but certainly wasn't nearly as bad as it was prior. It would tick when really cold and then heat up, expand, and be ok. Before that I was straight up mad max everywhere, sounded horrible and I got a lot of awkward looks from people.

I have a very small exhaust tick at my passenger side header on my 100, and I plan to pull the OEM manifold and fill in the crack. Will try to document that when I get to it.

Totally agree on new O2 sensors, that's a great recommendation by @AimCOTaco to knock them out while in there. They look like a piece of cake to get to with the manifolds out.
 

Stuckinthe80s

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I don't know much about the DT headers but I do have a bad taste in my mouth from all of the headers I've put on a bunch of old hot rods. It seemed like no matter what, I could never keep a good seal on the flange that would stay for more than a couple of months and I tried everything except lock wire: proper bolts, studs, studs with lock nuts, those super cool looking locking nuts they use on race cars (which did work the best but not 100%) Every method always ended up with the ticking. Granted, it wasn't like it was that big of a deal to go back in and tighten everything up but it was still annoying. To be fair, I never used super expensive headers so that was probably part of it. $200 Hookers were always my go to. (take that how you want to)

If Scott at AA Performance has a guy he's recommending I'm sure the guy knows what he is doing. Scott is such a good guy and really stands behind his work so I would trust any recommendations he gives.

Either way, I'm looking forward to what you are going to document because I know it is just a matter of time until I'll be dealing with the same issue.
 

DaveInDenver

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Doug Thorley have a reasonably good reputation for mini trucks. It was between them and LC Engineering (I went with LCE). I used factory studs, nuts and gasket and it never leaked. The downtube connection to the exhaust, now that was a constant PITA.

I just used one gasket, LOL. If you want to test the stacking two gaskets theory I might suggest a quick way would be to forget to check if the o-ring came off with your oil filter before you spin the new one on. That's a mistake you make just once. And I haven't again in over 30 years...

Now I will say that some people decide to use high temp RTV on their header gaskets and I can see an argument compensating for a *small* amount of warp that way.

The key IMO is that LCE uses almost ridiculously 1/2" thick flange like it would be if it was a cast manifold, welded I assume by a shop who has some clue how to deal with heat warping. It's not rocket surgery though when you're TIG welding 0.65" wall tubing to a hunk of 1/2" steel to hopefully keep the flange flat enough that it doesn't require a planer to make it flat.

So that's what I'd look for on the Thorley, pretty sure they have those thick flanges, too.
 

CORunner

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Dec 30, 2018
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Denver CO
I had a tick on my 100 that would not go away, so I pulled the trigger on DT headers during a black Friday sale at Summit. Had them installed by Exhaust Pros Northglenn. They charged a flat fee of $600 to do the install (in 2019). Really good price. Some shops quoted me over $1000 for the install. They also did a cat back magnaflow system.

Couple of pointers if you go this route.
- Throw away the DT gaskets that come with the headers. They are junk and they will eventually fail (few threads on IH8MUD).
- Buy and use a new set of OEM gaskets.
- Replace all the studs with OEM. Mine was a AZ/CO land cruiser, but still had issues trying to re-use studs.

I did notice a gain in performance. More pep everywhere.

Thread on IH8MUD did dyno runs stock and with DT headers. They got a peak 20hp gain and 10 ft-lbs. Increases across whole RPM range too. Not just peak.

https://forum.ih8mud.com/threads/dt-headers-dyno-result-thread.514381/

Having said all that, if my stock exhaust manifolds weren't leaking, I wouldn't upgrade to the DT headers. Since I had to fix the leak, DT all the way!! In the time I had the DTs, they never developed a tick.
 

AimCOTaco

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I'll add that I did use new oem gaskets, new oem nuts, and a few new oem studs (only those that backed out or were damaged) and haven't had any leaks after 5+ years. The DTs have been cool 😎
 
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