Is this Factory? 307 exhaust headpipe restriction?

[andymarkv]

Hey guys!
I just replaced the driverside exhaust manifold outlet studs on a '71 C20 Longhorn with 307/4sp.




I found this reducer/gasket/adapter in the end of the head pipe? Its basically a donut gasket on a flanged piece of pipe that fits inside the end of the head pipe. The issue is the pipe starts at 2" diameter and necks down to 1 1/8"!!!
Has other people seen this? Is it also in the passenger side?



Before I put it back together I used a cut-off wheel to trim it to 1 3/4" diameter, from the tiny 1 1/8".

Now here is the surprising part, it really runs better. I mean its VERY noticeable. Back in 2010 I put in a small comp cam, a Edelbrock performer 4 barrel intake/edelbrock 600cfm carb, and recurved HEI dist. Usually that combo really perks up any engine, but this felt really lame. You could hear the 4barrel open, but not much happened. I just accepted the fact that its "just" a 8:1 307 and there isn't much I could do about it.
But man, when I took it out tonight, it feels really strong after opening the exhaust!!!
I know more flow is always better, but I didn't really think it would yeld real seat of the pants power.

Heck it even sounds louder! I was disappointed after the new duals, it has short magnaflow knock-off mufflers and 2.25" pipes, that it didn't have a little more bark. I was also pretty disappointed that the exhaust guys just stubbed the new pipes onto the old head pipes.....they didn't want to deal with old rusty manifold studs either.

I was supposed to return the truck to its owner Sunday. But now I feel I should pull the passenger side down and look in there.

I'd like to hear if other people have seen this too? I'd feel like an idiot if I hold up giving the truck back, have to drill and tap three more exhaust studs if its common knowledge that only the driverside had this weird restriction.

Pic of the truck. Its just a rough farm truck, but it would be nice to have the old 307 put the power down!

[Palf70Step]

I have never seen anything like that myself. Good job on "tuning" up your truck!

[cdowns]

there were no dual exhaust systems of any type from the factory oonly on big block trucks were there dual exhaust systems/// looks like a typical let bubba at it with a torch an some piping project

[PanelDeland]

It's not uncommon for an exhuast to have a tapered slighly smaller pipe in there.It was an old school way to make the donut last longer.Usually it was tapered from stock flare to a pipe that would just fit in the exhuast pipe.It was put in on top of the donut so that the donut could crush/seal but have a liner (of sorts) to prevent it from burning out.Usually they did not restrict the exhuast like the one you posted.

[andymarkv]

Originally Posted by PanelDeland

It's not uncommon for an exhuast to have a tapered slighly smaller pipe in there.It was an old school way to make the donut last longer.Usually it was tapered from stock flare to a pipe that would just fit in the exhuast pipe.It was put in on top of the donut so that the donut could crush/seal but have a liner (of sorts) to prevent it from burning out.Usually they did not restrict the exhuast like the one you posted.

Thanks! That sounds like a reasonable explanation. It does seem weird that they let it restrict that much.... But when it was stock with tiny two barrel and single exhaust, maybe it didn't matter as much.

So it also sounds like there is a good chance that the passenger side has it as well.... I guess I better take the time to pull it apart and look.
Man drilling and tapping the studs is a pain laying on your back!!! But if I can gain almost 50% more flow by trimming the insert.....I think it will be worth the effort.

[screamin_c10]

My 307 had the exact same restrictor...

[andymarkv]

Originally Posted by screamin_c10

My 307 had the exact same restrictor...

Was it on both sides?!?!?!??! I don't want to do the passenger side unless I have to.....

[andymarkv]

This is what I cut out of the drivers side.....

[screamin_c10]

Originally Posted by andymarkv

Was it on both sides?!?!?!??! I don't want to do the passenger side unless I have to.....

Nope, just one side. Come to think of it, I was kinda curious about that myself, why one side and not both?

[andymarkv]

Originally Posted by screamin_c10

Nope, just one side. Come to think of it, I was kinda curious about that myself, why one side and not both?

Dang..... Was yours restricted on the driver side as well?
I guess I really need to get in there and see what is there. I would be pissed if the passenger side was restricted as well. But I'm going to be pissed if I pull it all apart just to see its straight through.....

Oh, the fun of wrenching on old trucks!

[MikeyMan]

Hmmm.... Got me thinking about my 71 C/10 307 w/3 speed.

I figured the poor performance was due to the dinky 2bbl. A restricted exhaust would not help matters. I'm not in the mood for breaking manifold bolts right now, so I'll wait until the pipes need replacing.

[Legolas894]

Originally Posted by MikeyMan

Hmmm.... Got me thinking about my 71 C/10 307 w/3 speed.

I figured the poor performance was due to the dinky 2bbl. A restricted exhaust would not help matters. I'm not in the mood for breaking manifold bolts right now, so I'll wait until the pipes need replacing.

Not to brag but I just replaced my exhaust manifold on the driver's side with one I got off a board member. The "new" one had the bolt holes so I could finally mount my alternator up top. Anyway, believe it or not, I got all the bolts off with just a wrench. Exhaust manifold off the head no problem AND the three nuts off the yoke as well. I couldn't believe it. I was expecting some to snap off. The studs on the "new" manifold were also in good shape so it was a quick job.

No insert in the pipe though. Just the donut.

[71swb4x4]

A lot of times there is a flapper valve of sorts on the passenger side, I wonder if that pipe on the drivers side restricts the exhaust in a similar fassion as that flapper valve on the passenger side to keep the exhaust even???? I don't remember what the passenger side thing is exactly, but I think it helps the truck run better when cold.

[FirstOwner69]

Originally Posted by 71swb4x4

A lot of times there is a flapper valve of sorts on the passenger side, I wonder if that pipe on the drivers side restricts the exhaust in a similar fassion as that flapper valve on the passenger side to keep the exhaust even???? I don't remember what the passenger side thing is exactly, but I think it helps the truck run better when cold.

The "flapper valve" you refer to is called a heat riser. However, they were on the driver's side between the manifold and downpipe just like the restrictor noted in this thread. The role was to help warm the carburetor to prevent carb icing in cold weather and speed engine warm up. It contained a bi-metal spring which allowed the heat riser to open as the engine warmed up. They were notorious for seizing in a closed position and/or rattling. I still have a can of GM lubricant to help prevent valve from seizing. I am wondering if the restrictor described by the thread author was someone's attempt to serve the same function without the problems associated with the heat riser.

[FirstOwner69]

I see andymarkv beat me to it. However, it WAS standard equipment on my 1969 350. I'm not sure what the first year was. My 1965 Impala SS also had one as I recall. It rattled like mad and drove me nuts. Also, the cars may have had them on the passenger side (I don't recall for sure), but they were definitely on the driver's on the trucks.

[andymarkv]

Interesting!! So maybe because the heat raiser is missing means this is a replacement exhaust? And the whoever did the replacement just stuck in these donut insert gaskets at that time?

The headpipes have a factory looking flat spot on both the driver side and the passenger side. The flat spot on the driver side gives clearance for the clutch linkage, (you can kind of see it in the first pic) the flat spot on the passenger side gives clearance for the starter. This is probably also another reason why the exhaust shop that installed the duals just stubbed on to the existing head pipes.

Because of that I ASSUMED it was factory, but it could have been replaced at sometime with a factory style y pipe and the insert donut gaskets installed then?

[FirstOwner69]

It would be extremely unusual to find an original exhaust system on one of these trucks. Those were the days of leaded gas. In my experience it was normal to replace exhaust systems somewhere around 30,000-40,000 miles if not sooner, especially in cold climates. Not to mention the 10K between tune ups. The heat riser added to that problem since the system would not get dried out on short trips. There have definitely been some advantages to unleaded.

[PanelDeland]

Go to the help section of the parts store for new stud kit.They have brass nuts and don't rust to the studs and cause problems.

[andymarkv]

Originally Posted by 71swb4x4

A lot of times there is a flapper valve of sorts on the passenger side, I wonder if that pipe on the drivers side restricts the exhaust in a similar fassion as that flapper valve on the passenger side to keep the exhaust even???? I don't remember what the passenger side thing is exactly, but I think it helps the truck run better when cold.

Yep, your talking about the heat riser flap on the passenger side manifold of the later trucks. It closed and forced more hot exhaust through the intake manifold crossover to help heat the intake manifold and heat the exhaust manifold to provide warm incoming air through the intake snorkle.

But this truck is too old for that stuff.... I think that came in the late 70's when 1/2 ton trucks started getting cat converters. Warming the engine faster was important to light the converter off quickly.

Originally Posted by PanelDeland

Go to the help section of the parts store for new stud kit.They have brass nuts and don't rust to the studs and cause problems.

Yep, that's exactly what I bought! Its just going to suck to yank the starter and break off all three manifold studs just to find a free breathing mainfold opening! The manifold studs are so rusty there aren't even threads left!! My only hope is they turn out of the manifold instead of breaking off.....I'm not holding my breath!

[old Chevy guy]

If you take the other side apart just heat the nuts up one at a time till their cherry red with a torch and turn them off. when they start to move just work them back and forth. if they stop moving heat them again. I get 99 out of a hundred off without breaking them. Never seize is a wonderful thing also. I put it on almost everything I take apart and put back together.

[Legolas894]

Originally Posted by old Chevy guy

If you take the other side apart just heat the nuts up one at a time till their cherry red with a torch and turn them off. when they start to move just work them back and forth. if they stop moving heat them again. I get 99 out of a hundred off without breaking them. Never seize is a wonderful thing also. I put it on almost everything I take apart and put back together.

X2 on the never seize.

[andymarkv]

Originally Posted by FirstOwner69

It would be extremely unusual to find an original exhaust system on one of these trucks. Those were the days of leaded gas. In my experience it was normal to replace exhaust systems somewhere around 30,000-40,000 miles if not sooner, especially in cold climates. Not to mention the 10K between tune ups. The heat riser added to that problem since the system would not get dried out on short trips. There have definitely been some advantages to unleaded.

Funny you should say that! This truck has just turned 50,000 miles! (It never left the farm)
I totally forgot about the leaded gas back then! Its very possible that this is a replacement y pipe.
The Y back exhaust before the duals was definitely custom. It was 2" with a single glasspack turned out infront of the driver side rear tire.

You also reminded me to get off my butt and really look for the build date! It is 12/70, which means it should have the 9:1 307. I had been told it was a very late '71 model and I wondered if it had the '72 307 with 8:1. All that to say with 9:1 the cam I picked should feel pretty torquey compaired to 8:1

Originally Posted by old Chevy guy

If you take the other side apart just heat the nuts up one at a time till their cherry red with a torch and turn them off. when they start to move just work them back and forth. if they stop moving heat them again. I get 99 out of a hundred off without breaking them. Never seize is a wonderful thing also. I put it on almost everything I take apart and put back together.

I hear you!! Except there are almost no threads above and below the nut on the studs! They've rusted away.... Maybe I can heat the ear of the manifold and get lucky and turn the whole stud out?

[andymarkv]

Welp I yanked the starter and busted two out of three studs only to be greeted by a straight through 1.75" ID donut gasket insert!

Damn it!! But at least now I know for sure and the studs are changed if I ever need to get back in there for some reason.

[425HP409]

The exhaust restrictor is definitely factory. They were used in many engine combinations and I found one recently in a 72 402. I know that the V8 Chevy have had heat risers on the passenger side exhaust manifold since they were introduced in the 1955 265cid engine. I don't know what the exact reason for the exhaust restrictor was.
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[andymarkv]

I'm reading the restrictor was a "passive" heat riser for vehicles without the flap?

But man that doesn't seem to make sense to restrict the exhaust all the time just to quicken cold starts.....
Heck, I've got the exhaust crossover mostly blocked now. The truck is mostly a summer runner and the aluminum intake heats up quicker than the old cast 2bbl intake.