Exhaust manifold choices

Logan

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I'm new to this sight and have never joined a forum till now. I figured it was time that I ask for help from fellow FMJ drivers and perhaps offer some of what I have learned from driving these old cars for the past 17 years. My situation is that I am having a dual exhaust system made for my 78 town and country wagon and I'm trying to chose the right manifold. The engine that will be in the car is the rebuilt motor from my 87 Fifth Ave. The motor will be mostly stock with the exception of an improved intake and a Holley 390 CFM 4BL. Originally the 87 318 had a air pump but I would rather not transfer that mess. What I'm thinking is to use the 87's stock manifold and create an air aspirator system for each bank of exhaust using a check valve on each side. I'm thinking about this because I'm going to install 2 new catalytic converters before 2 glasspacks that will dump out in front of the rear tires. I drive this car in the city a lot and I'm thinking that the aspirator system will help the converters work better under low RPM where they might otherwise struggle for oxygen. My other manifold option is from a 74 318 and has no air inlets, just better flow. To use that one I would have to block the small exhaust/air holes in the 302 heads. Has anyone out there been down this road or have anyadvice.
 

Logan

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The cats are completely voluntary. Aside from cutting greenhouse gasses they reduce the noise of the dual exhaust system to a level that is tolerable to my neighbors and doesn't draw much unwanted attention from the fuzz.
 

AJ/FormS

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The cats are completely voluntary. Aside from cutting greenhouse gasses they reduce the noise of the dual exhaust system to a level that is tolerable to my neighbors and doesn't draw much unwanted attention from the fuzz.
Well in that case, I got my trusty sawzall right here. Cats are not mufflers. Cats are inventions of evil men.
lol
 

slant6billy

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Manifolds from a 90-93 truck 360 pre magnum would give you the air tube you seek. I have a set from a 92 3/4 ton truck. Now the manifolds from magnums could get you some nice subtle gains. Just stay away from the ones with the egr. My thought is to get a passenger side from a 93-98 Jeep Grand Cherokee with a 5.2/ 5.9 Magnum V8. Driver side from any ram truck 94 and up to 01. The exit banks away from the engine by using the jeep/ ram parts. Those magnum manifolds flow better than log style 340 manifolds. I'm putting my ram back to manifolds from headers. As for the cats and glass pack combo: It is not going to deter noise like you think. Cats actually take the high frequency pulses out. So you get a low drone outboard motor boat result sound. I did this with my van and it only sounded "ok" at idle. Part throttle and full throttle was droning badly. My recommendation is a balance pipe either H or X, but the X has to set further back after the trans tail. The balance pipe can go before the trans where the Y pipe usually goes. Glasspack need to get worn in to sound right. The Thrush mufflers are a better choice for a sound quality: Thrush Turbo Mufflers 17704

A hollow cat with an H pipe and thrush type mufflers won't drone you out of the car like a real cat and glasspack. Trust me, that droning will fatigue you. Noise is not fun after a while. That real cat and glasspack is going to annoy your neighbors too.
 

Logan

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Thanks for the manifold ideas, I really appreciate the thoughtful suggestions. I'm going to look further into the 360 manifolds with the air tubes as they look like they would flow better than the stock 87 318's. The cat/glasspack combo is one I have done before and the sound grew on me. My 87 Fifth Ave with that combo would idle extremely low and it sounded just perfect. I hope my neighbors will still talk to me when this is all finished.
 

Aspen500

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As Slant6 said, an interior drone from the exhaust will drive you insane. I used to have a drone above 50 mph that made it impossible to talk to a person in the other seat, much less even begin to hear the radio. Fought it and fought it (t-pipes exit at bumper) ended up changing to different mufflers and problem solved.
 

Logan

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I appreciate the advice on the exhaust components beyond the cats. What I'm less resolved on is the manifold. I'm curious as to weather or not anyone has tried running a separate aspirator valve for each bank of the engine. The valves would be piped into the original air pump inlets on the back of the manifolds.
 

89.Fifth

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I appreciate the advice on the exhaust components beyond the cats. What I'm less resolved on is the manifold. I'm curious as to weather or not anyone has tried running a separate aspirator valve for each bank of the engine. The valves would be piped into the original air pump inlets on the back of the manifolds.

Never tried an aspirator. Is there an off the shelf part that you can modify? Or an example of it used in another application?
 

Logan

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You can buy these aspirator valves for about 20 bucks at many parts stores and they function much like the original valves. It's basically like a check valve for the exhaust that will allow burst of low idle vacuum to draw in fresh air for the oxygen deprived cats. There will definitely be some modifying to complete this puzzle but nothing drastic. The example I'm basing this idea on comes from the original emissions components on my 78 318. Once this project is complete I might see if I can have the exhaust gas measured with the aspirators hooked up then disconnected.
 

4speedjim

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Welcome to the forum. Good luck with your set up. These guys know their stuff here. You should consider applying what they said to you build. They wont steer you wrong.
 

AJ/FormS

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Aspirators only work when there is a low pressure pulse in the exhaust, lower than atmospheric pressure. Some how, I'm having a hard time imagining your exhaust having lower than atmospheric pressure at any time but at idle, or unloaded.
But I suppose you could plumb a pressure gauge into it and see. I have done that on several occasions, and usually see pressure in the 2 to 4psi range. 4psi is already a horsepower thief.
 

4speedjim

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Good point AJ. Specially with the length size cats bends. I never thought about that. Is that why they added the air pump? To overcome the backpressure?
 

Logan

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I'm pretty sure that at Idle is the only time you need those aspirator valves to let oxygen into the system.
 

BudW

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Cats are not mufflers.
Very True - but a stopped up cat will make exhaust more quiet (and restrict the exhaust flow, more so).

Cats are inventions of evil men.
they will also do a number on your car (and lawn) if you drive through tall dry grass.
They will also give you a number of burns if trying to work by them (when hot).
BudW
 

BudW

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I’m going to make a new post about exhaust manifold options (with pictures) – shortly.

The A.I.R. ports is the big difference (or issue) and does limit what you can use in your application more than anything does (for either exhaust manifold or header interchange).

Personally, I prefer exhaust manifolds over headers for three huge reasons.
One is the cast iron is more quiet.
Second is cast iron doesn’t require constant maintaince (exhaust leaks, loose bolts etc.) that headers have.
Third is cast iron has better fitment issues (ie: can’t access starter with headers on, or spark plug wire routing, etc.).
Not a reason, per se, but in emission states, headers might have a tendency to tell Emission Inspection personnel to look your car over using a finer tooth comb – possibly,

That said, headers will flow more than exhaust manifolds will – so in all out air flow numbers – you can’t beat headers.
BudW
 

BudW

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I'm pretty sure that at Idle is the only time you need those aspirator valves to let oxygen into the system.
I slightly disagree here - but not enough to argue about it.

Being the A.I.R. ports are at an almost 90’ angle to the exhaust flow, there is a suction from the airflow past the ports (venturi effect). As long as a person is not “on the gas”, then there is air flow through the aspirator valves – even when cruising.

I have seen factory built cars with aspirator valves going to exhaust manifolds as well to the rear cat converter.
I think as emission laws got stricter, Chrysler had to install an A.I.R. pump with a diverter valve switching air flow to the exhaust manifolds when cold, to rear cat when hot.

With a car at idle, and with air cleaner off (ie: the hose to aspirator is disconnected – you can feel the suction of air going into the aspirator.
Also – which is very annoying to me – the aspirator valve is quite noisy (when disconnected) – and needs the air cleaner to stay quiet.
BudW
 

Oldiron440

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Back in 95 I bought a new Ram1500 with a 360, a day or two after driving it off the lot I ordered a cat back system from Mopar. After removing the exhaust from the truck I started it up and was surprised how quite it was, but the thing that surprised me the most was the truck was louder with the full exhaust hooked up.
To this day it makes no sense.
 
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