Free flow exhaust. How much do you need?
12-03-2002, 01:25 AM
#1
Free flow exhaust. How much do you need?
The PO of my 81'auto has removed both mufflers, and possibly knocked the core out of the cat. Not sure of the latter, as I haven't checked yet. The exhaust system consists of the cat and a tip with some type of sound deadning core. The car is not excessivly loud in my opinion with this set up. I know these cars are not known for their bottom end performance. My theory is that with more back pressure than I have at this time I may gain some bottom end grunt.I have heard that you can loose bottom end torque if you have no back pressure in the exhaust system. Any one have experience in this area?
12-03-2002, 04:01 PM
#2
Inventor
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I just finished exploring this topic!
I would bet that your cat is still there; without the cat and only a resonator tip, your car would be unbearably loud.
These cars are slower off-the-line because of the numerically low final drive ratio (2.75:1 for 81) - especially with the automatic. They are heavy too, 3385 lbs with the auto. I also found that the vacuum limiting devices on the intake rob the car of snap acceleration (with the vane Air Flow Meter) and with the oxygen sensor in the loop, the car runs too lean for real performance.
{Warning: Long winded self-back-patting treatise follows
}
On my 81, I first removed everything I could that wasn't being used, and dropped ~150 lbs. I removed and capped all the vacuum robbing devices of the intake, especially the 'Vacuum Limiter', and the vacuum retard to the distributor. I advanced the timing about 6 degrees. I grounded the oxygen sensor and richened the mixture control on the AFM. These steps all helped quite a bit.
Too rich a mixture will melt the cat so to save it (for emission tests) I replaced the entire exhaust from the intersection of the 2" pipes with 3"ID pipe, a 40 series Delta Flowmaster, two 12" 3"ID resonators and a Monza twin tip resonator.
Highway performance was greatly improved, at 80MPH I barely have the accelerator depressed. It has a real cammy feel, after 3 grand the car just rockets away. The buttometer said, however, that off-the-line was slower.
It was fricken' loud too! Especially when starting from a dead stop. I was really sad after adding resonator after resonator and still too loud.
Then I dreamed up these gizmos here:
Now it's only slightly louder at idle than stock. Only a bit louder when starting from a stop, reasonable slow speed power. And wicked fast to boot. Now I need a limited slip for when it's dry too!
Next up: I want to try replacing the AFM with a MAF conversion kit.
I would bet that your cat is still there; without the cat and only a resonator tip, your car would be unbearably loud.
These cars are slower off-the-line because of the numerically low final drive ratio (2.75:1 for 81) - especially with the automatic. They are heavy too, 3385 lbs with the auto. I also found that the vacuum limiting devices on the intake rob the car of snap acceleration (with the vane Air Flow Meter) and with the oxygen sensor in the loop, the car runs too lean for real performance.
{Warning: Long winded self-back-patting treatise follows
}On my 81, I first removed everything I could that wasn't being used, and dropped ~150 lbs. I removed and capped all the vacuum robbing devices of the intake, especially the 'Vacuum Limiter', and the vacuum retard to the distributor. I advanced the timing about 6 degrees. I grounded the oxygen sensor and richened the mixture control on the AFM. These steps all helped quite a bit.
Too rich a mixture will melt the cat so to save it (for emission tests) I replaced the entire exhaust from the intersection of the 2" pipes with 3"ID pipe, a 40 series Delta Flowmaster, two 12" 3"ID resonators and a Monza twin tip resonator.
Highway performance was greatly improved, at 80MPH I barely have the accelerator depressed. It has a real cammy feel, after 3 grand the car just rockets away. The buttometer said, however, that off-the-line was slower.
It was fricken' loud too! Especially when starting from a dead stop. I was really sad after adding resonator after resonator and still too loud.
Then I dreamed up these gizmos here:
Now it's only slightly louder at idle than stock. Only a bit louder when starting from a stop, reasonable slow speed power. And wicked fast to boot. Now I need a limited slip for when it's dry too!
Next up: I want to try replacing the AFM with a MAF conversion kit.
12-03-2002, 04:09 PM
#3
Nordschleife Master
Why remove the vacume retard to the distributer?
If you do that, the distributer won't function properly, and I'd be afraid of eighter haveing detonation at the top end, or screwing up my idle, depending on how the system is set up.
Also, with such a thin little line, why would it effect your snap so much?
If you do that, the distributer won't function properly, and I'd be afraid of eighter haveing detonation at the top end, or screwing up my idle, depending on how the system is set up.
Also, with such a thin little line, why would it effect your snap so much?
12-03-2002, 04:23 PM
#4
Inventor
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The vacuum retard is for emissions created when decelerating.
I removed the line to have more advance at part throttle/idle, which makes the engine run cooler and accelerate better. Not having the retard makes the idle speed increase, so I was able to turn in the idle screw, thus increasing the vacuum.
The early cars can run on 87 octane (USA method). So I with my advanced timing, I run Super, but I could probably just as well run Plus.
I removed the line to have more advance at part throttle/idle, which makes the engine run cooler and accelerate better. Not having the retard makes the idle speed increase, so I was able to turn in the idle screw, thus increasing the vacuum.
The early cars can run on 87 octane (USA method). So I with my advanced timing, I run Super, but I could probably just as well run Plus.
12-03-2002, 05:31 PM
#5
Racer
Jim, N/A engines in general benefit from a “certain” amount of backpressure like Ken stated. The only way to figure out how much your car needs, take many hours of R&D and many hours spent at a Chassis Dyno. The numbers are out there for our applications. However, the brilliant individuals that are in possession of them are not going to given them away for free as it cost them big dollars to come up with them in the first place. So don’t feel put off if someone doesn’t just through a number out there at you.
If you have a little money burning a hole in your pocket, you may be better off buying a custom made set-up from one of the 928 exhaust gurus. Using the trial and error method in this case could easily cost you more than if you would have just bought one pre-engineered.
Ken, I like your dual tip set-up. I see a cap nut on the end of an all threaded rod, a washer, the birdcage off of a safety light with a nut welded to it, another washer, a spring and a galvanized fender washer. Do you have any other pictures?
Cheers
Max
If you have a little money burning a hole in your pocket, you may be better off buying a custom made set-up from one of the 928 exhaust gurus. Using the trial and error method in this case could easily cost you more than if you would have just bought one pre-engineered.
Ken, I like your dual tip set-up. I see a cap nut on the end of an all threaded rod, a washer, the birdcage off of a safety light with a nut welded to it, another washer, a spring and a galvanized fender washer. Do you have any other pictures?
Cheers
Max
12-03-2002, 07:47 PM
#6
[quote] On my 81, I first removed everything I could that wasn't being used, and dropped ~150 lbs. I removed and capped all the vacuum robbing devices of the intake, especially the 'Vacuum Limiter', and the vacuum retard to the distributor. I advanced the timing about 6 degrees. I grounded the oxygen sensor and richened the mixture control on the AFM. These steps all helped quite a bit.
<hr></blockquote>
Very interesting. COuld you describe this job in a bit more detail? My 81 auto is laid up for some serious going-through, and I will be in the engine area for a FPR change anyway, so....
Oh, and the 3 spd is going for a 4spd! Thats should help a bit with off the line *go*
Thanks,
Brendan
<hr></blockquote>
Very interesting. COuld you describe this job in a bit more detail? My 81 auto is laid up for some serious going-through, and I will be in the engine area for a FPR change anyway, so....
Oh, and the 3 spd is going for a 4spd! Thats should help a bit with off the line *go*
Thanks,
Brendan
12-03-2002, 08:30 PM
#7
Inventor
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Max - Here's the exploded view. The cage is a 2" gutter downspout filter, trimmed and hooks bent on the ends. The Monza exhaust has a lip just inside the hole, the cage pops in and is held by the lip. The other parts are stainless steel springs/hardware from the hardware store. It's developing a nice patina from the exhaust, it's not as shiny as in the first pic.
The 'pop-off valve' is a stack of 1/4" fender washers with 6MM washers on either end, tack welded together. This is all a test rig, I'd like to properly machine this setup, with an easily adjustable spring tension device.
Brendan - vacuum related items removed:
-Vacuum limiter device (5/8" & 3/4" heater hose plugs)
-Carbon canister, hoses, gas tank vent line disconnected at rear wheel well after safety valve with 3/8" Fram gas filter as plug.
-Venturi Y-connector, direct hose from plenum to brake booster
-Retard hose to distributor
-Air pump and vacuum controls
-Cruise transducer
Green wire from computer grounded to relay frame, adjusted AFM for very light brown color on plugs.
I would very much like to find a 4-speed with limited slip eventually!
The 'pop-off valve' is a stack of 1/4" fender washers with 6MM washers on either end, tack welded together. This is all a test rig, I'd like to properly machine this setup, with an easily adjustable spring tension device.
Brendan - vacuum related items removed:
-Vacuum limiter device (5/8" & 3/4" heater hose plugs)
-Carbon canister, hoses, gas tank vent line disconnected at rear wheel well after safety valve with 3/8" Fram gas filter as plug.
-Venturi Y-connector, direct hose from plenum to brake booster
-Retard hose to distributor
-Air pump and vacuum controls
-Cruise transducer
Green wire from computer grounded to relay frame, adjusted AFM for very light brown color on plugs.
I would very much like to find a 4-speed with limited slip eventually!
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12-03-2002, 08:47 PM
#8
Racer
Ok, I see. The spring allows your disk (fender washers) to open as needed and the cage is used as a support as well as a seat.
It looks like in order to adjust, you simply change the spring.
I like the idea, great job.
It looks like in order to adjust, you simply change the spring.
I like the idea, great job.
12-03-2002, 10:46 PM
#9
Inventor
Rennlist Member
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The whole point is to slow the exhaust gas at low rpm's so that the resonators can do their job. Also it may help with reversion/overlap, but that's getting into a theoretical arena.
12-03-2002, 11:13 PM
#10
Sorry to dwell on this Porken, but:
Brendan - vacuum related items removed:
-Vacuum limiter device (5/8" & 3/4" heater hose plugs)
...Sorry. Don't get what you mean here.
-Carbon canister, hoses, gas tank vent line disconnected at rear wheel well after safety valve with 3/8" Fram gas filter as plug.
... The vent line that has vaccum back into the engine manifold to burn gas fumes? Sorry, please tell the back of the class.
-Venturi Y-connector, direct hose from plenum to brake booster
...On my 81, the Y connector gives vacuum to the AC controls...Is this what you removed? The area with the blue one way?
-Retard hose to distributor
...Two hoses there. Which one? I assume the one disconnected to check timing at 3k.
-Air pump and vacuum controls
...Got that off my 89. Haven't touched the 81.
-Cruise transducer
...got it. You totally removed?
Brendan - vacuum related items removed:
-Vacuum limiter device (5/8" & 3/4" heater hose plugs)
...Sorry. Don't get what you mean here.
-Carbon canister, hoses, gas tank vent line disconnected at rear wheel well after safety valve with 3/8" Fram gas filter as plug.
... The vent line that has vaccum back into the engine manifold to burn gas fumes? Sorry, please tell the back of the class.
-Venturi Y-connector, direct hose from plenum to brake booster
...On my 81, the Y connector gives vacuum to the AC controls...Is this what you removed? The area with the blue one way?
-Retard hose to distributor
...Two hoses there. Which one? I assume the one disconnected to check timing at 3k.
-Air pump and vacuum controls
...Got that off my 89. Haven't touched the 81.
-Cruise transducer
...got it. You totally removed?
12-03-2002, 11:56 PM
#11
Inventor
Rennlist Member
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Jeez,
Vacuum Limiter
This is the saucer shaped thing on the right* side of the throttle body. It burps air past the throttle on deceleration. 5/8" & 3/4" heater hose plugs replace this item. If you do the other things listed here, and the VL is still connected, your idle will go up and down, up and down.
Carbon Canister
Located in the right hand wheel well above the washer reservoir, weighs a ton. Stores gas vapors from tank. It has a bunch of hoses that run through the fender wall of different diameters (most/all of which are likely leaking air). One of which goes to a steel line that runs back to the gas tank. I removed the carbon cannister and all the hoses, disconnected the gas tank vent hose from the steel line in the right rear wheel well (forward of the wheel) and installed a 3/8" clear plastic gas filter (Fram F21111) onto that hose after the safety (tipover) valve. The gas tank is now direct venting, as cars did up until ~1970. There is also small vac line from the TB that goes through a coolant switch (under the cold start air valve) then to the valve forward of the washer fill, which regulates the flow from the cannister.
Venturi Y-connector
Plastic Y-connector attached to plenum, on the left side, between the third and fourth leg; one line goes to brake booster (where there is a T-connector for the AC), and another goes to the throttle body with a restriction. I have no idea what the heck this is for. I replaced it with a hose that goes directly to the T-connector/brake booster and capped the throttle body connection.
Retard Hose to Distributor
Connects to the inner/hard-to-reach barb, closest to the distributor. The retard vacuum hose goes to the front of the TB.
The advance hose comes from the back of the TB and connects to the outside barb. This is a good hose. Part throttle response will suffer if you disconnect the vacuum advance.
Forgot this one:
Idle speedup for AC
This is the valve that has a line to the front of the plenum, attached to the bar across the engine with some wires attached. This speeds up the idle if the AC pump is going. No AC = no valve needed.
*Right = USA passenger side
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
Vacuum Limiter
This is the saucer shaped thing on the right* side of the throttle body. It burps air past the throttle on deceleration. 5/8" & 3/4" heater hose plugs replace this item. If you do the other things listed here, and the VL is still connected, your idle will go up and down, up and down.
Carbon Canister
Located in the right hand wheel well above the washer reservoir, weighs a ton. Stores gas vapors from tank. It has a bunch of hoses that run through the fender wall of different diameters (most/all of which are likely leaking air). One of which goes to a steel line that runs back to the gas tank. I removed the carbon cannister and all the hoses, disconnected the gas tank vent hose from the steel line in the right rear wheel well (forward of the wheel) and installed a 3/8" clear plastic gas filter (Fram F21111) onto that hose after the safety (tipover) valve. The gas tank is now direct venting, as cars did up until ~1970. There is also small vac line from the TB that goes through a coolant switch (under the cold start air valve) then to the valve forward of the washer fill, which regulates the flow from the cannister.
Venturi Y-connector
Plastic Y-connector attached to plenum, on the left side, between the third and fourth leg; one line goes to brake booster (where there is a T-connector for the AC), and another goes to the throttle body with a restriction. I have no idea what the heck this is for. I replaced it with a hose that goes directly to the T-connector/brake booster and capped the throttle body connection.
Retard Hose to Distributor
Connects to the inner/hard-to-reach barb, closest to the distributor. The retard vacuum hose goes to the front of the TB.
The advance hose comes from the back of the TB and connects to the outside barb. This is a good hose. Part throttle response will suffer if you disconnect the vacuum advance.
Forgot this one:
Idle speedup for AC
This is the valve that has a line to the front of the plenum, attached to the bar across the engine with some wires attached. This speeds up the idle if the AC pump is going. No AC = no valve needed.
*Right = USA passenger side
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
