Help! Going Dual Exhaust

[krscholz]

As in most states, Classic cars are exempt from emission requirements. Given that I'm ready to take advantage of my exemption and scratch the y-pipe and cat and go 2/ 1/2 inch dual exhaust with magnaflow hideaway mufflers.

Here are my concerns for my 1975:

1. Getting rid of the A.I.R. pump - that's seems simple enough - any reason why I shouldn't remove it?.
2. What about carbureator settings afterwards?
3. What about timing settings - should I advance/retard it or leave it alone. I say advance it, if so how many degrees?
4. What about the heat riser on the right side manifold (passenger side) of the engine - stay or go? I say stay because the engine appears to take longer to warm up without it.
5. What about the spacer between the manifold and header pipe that is on the driver's side - stay or go?
6. Anything else that I should be aware of?
7. Does someone have a bulleting or article I can read or send me.

Thanks much for your help, Ken

 

[LVMYVT76]

I HAVE GONE TO 2 1/2 IN EXHAUST ON MY 76. I went to a speed shop that did exhaust, they did a great job , bending the pipes where they don't hang down but low enough to not cause problems with heat on the rear fiberglass spring and floor boards. The shop should be able to answer your questions, if not go to another shop! Ask others where they go, ask car clubs etc. where they go. Expect to pay a little more for a great job!
check about hangers and placement, ask questions before starting and not have problems afterwards. I aslo have put on headers before the exhaust was going on. The AIR pump won't make much difference . You must make up your mind if you want the car stock or not, in order to make your mind up on what to pull off or not.

 

[krscholz]

Need more help!

Any more ideas guys on my posting?

 

[Vette79]

After replacing my exhaust with headers and duals, I did not have to make any adjustments with the carb or timing. You do not need to re-install the heat riser. I also removed all of my emissions equipment ex cept for the fuel vapor canister and the connections to it from the fuel tank and carb. The exhaust job was pretty straight forward. I did have a custom exhaust shop install the exhaust tubes so that it didn't hang to low at the rear differential.

 

[krscholz]

I noticed that one time my linkage from the Heat Riser was disconnected and it took longer for my engine to warm-up - was this related to the Heat Riser or something else.

 

[Echo1]

I went with long tube headers. It got rid of all the junk and headaches of all the spacers. I did not have to make any adjustments after I pulled the smog pump.

 

[krscholz]

Going Dual Exhaust - Final Analysis

Thanks for all your input. Here is what I found. . if you go dual, you may or may not want to use the heat riser. If you choose not to use it, the carburetor may take extra time to warm up. This is critical in colder climates. If you choose to use it, the restriction on exhaust from the flapper is minimal. With freer flow exhaust, you should experience slightly enhanced horsepower. Remember the change to dual exhaust will not be like your car taking a Viagra pill. Keep in mind, the trade-off with freer exhaust and less back pressure will be less torque at the lower end. This trade-off is well worth the effort. You should also consider removing the air pump as it only robs horsepower. As far as resetting the timing and carburetor mixtures, some say they have changed the settings, however most have not and are quite satisfied with the performance.

For me I want the sound and the performance so that’s why I went dual.

 

[LVMYVT76]

I went dual exhaust so I could use the headders, need exhaust to keep any extra hp to engine, low restriction mufflers. why go dual without hedders?

 

[JohnZ]

The A.I.R. pump uses less than 1/4-hp; hardly worth the effort to remove it.

 

[krscholz]

Going Dual Exhaust - One More Thing . . .

I spent last weekend installing my dual exhaust system from Corvette Central. It took roughly four hours and the system fit perfectly and included everything I needed. The package included Magnaflow mufflers and the car seems to have more power but I haven't tested it yet. The sound is more fuller now but not overbearing that I can't hear the radio. If you're thinking about doing the same for your car, contact Gus at Corvette Central, he is a wealth of information and was very helpful to me. His e-mail address is as follows: mail@corvettecentral.com Any other questions contact me as well.

 

[Evolution1980]

Glad you are enjoying the new exhaust. A more pleasing exhaust note is one way for us to perceive the car as having more power.
Did you keep your original cast iron exhaust manifolds? If so, you'd likely get a bigger performance boost by dropping those and getting a nice pair of headers.

As for getting rid of the heat riser and any other emissions stuff to help the carb warmup faster, that's of no consequence to many of us as we don't drive our cars when it's snowing. I'm assuming you won't be either...?

 

[Jimdvette]

Converting from a catalyctic system from 75-79 you'll find if you use a complete system for a 74 will work perfectly . Just apply the 74 part #'s for your application. Small block . Sorry I posted after you did you're swap. maybe this will help someone else later.

 

[84Turbo]

Thanks for all your input. Here is what I found. . if you go dual, you may or may not want to use the heat riser. If you choose not to use it, the carburetor may take extra time to warm up. This is critical in colder climates. If you choose to use it, the restriction on exhaust from the flapper is minimal. With freer flow exhaust, you should experience slightly enhanced horsepower. Remember the change to dual exhaust will not be like your car taking a Viagra pill. Keep in mind, the trade-off with freer exhaust and less back pressure will be less torque at the lower end. This trade-off is well worth the effort. You should also consider removing the air pump as it only robs horsepower. As far as resetting the timing and carburetor mixtures, some say they have changed the settings, however most have not and are quite satisfied with the performance.

For me I want the sound and the performance so that’s why I went dual.

That's fiction. Improving the engine's VE improves torque and horsepower. It does not impair it.

 

[Bob Chadwick]

That's fiction. Improving the engine's VE improves torque and horsepower. It does not impair it.

It doesn't impair torque, it changes the torque curve raising it to a higher point and reducing low end oomph.

 

[84Turbo]

It doesn't impair torque, it changes the torque curve raising it to a higher point and reducing low end oomph.

I agree that it will change the torque curve. It will improve it. I dispute your claim that it will reduce the low end oomph.
Less backpressure results in several things. Reduced power lost at the crank due to the piston not having to push as hard to remove the burned gasses, and a reduction in residual EGR, allowing an increase in VE. Both of these items are efficiency and cylinder combustion pressure improvements, not torque impairments.

 

[Evolution1980]

I agree that it will change the torque curve. It will improve it. I dispute your claim that it will reduce the low end oomph.
Less backpressure results in several things. Reduced power lost at the crank due to the piston not having to push as hard to remove the burned gasses, and a reduction in residual EGR, allowing an increase in VE. Both of these items are efficiency and cylinder combustion pressure improvements, not torque impairments.

What is "VE"? Before I pipe in here, I want to know so I don't misspeak.

 

[Evolution1980]

I dispute your claim that it will reduce the low end oomph.
Less backpressure results in several things. Reduced power lost at the crank due to the piston not having to push as hard to remove the burned gasses

I think you are going to be in a very very small minority here. (And that's OK, as only a few people believed the Earth was round at one time.) It would seem that if we followed your logic, no exhaust manifolds/headers would produce more low RPM torque than any size exhaust manifolds/headers. That's just not the case.

Also, how exactly is the the piston encountering 'resistance' from pushing exhaust gasses when there's almost no resistance in pushing them out since it's the cams that open the exhaust valves? As well, you have exhaust gas velocities which effect scavenging of exhaust gasses. Smaller diameter exhaust manifolds/headers will create a higher velocity which will in turn 'pull' gasses out of the combustion chamber. Larger diameter headers will have less restriction and thus not scavenge exhaust gas as well.

I think you'll be somewhat hard pressed to find dyno results that show 'little to no' backpressure as having torque build up faster at lower RPM's.
Some backpressure is required for optimal engine performance when not running WOT at the end of the RPM range.

 

[Bob Chadwick]

I didn't have a chance to do a whole bunch of research but according to an excerpt from Car Craft Magazine

Note that the smallest diameter creates good midrange torque yet falls off at the top, while the larger primary header pipes add more high-rpm power at the expense of low-speed torque.

According to Magnaflow

As a general rule, you can enlarge the pipe diameter of your OEM exhaust system by 1/4- to 1/2-inch to increase your horsepower. However, any additional increase in pipe diameter is likely to decrease your performance; specifically, low end torque.

 

[84Turbo]

I think you are going to be in a very very small minority here. (And that's OK, as only a few people believed the Earth was round at one time.) I have no problem being in the minority. I've been there plenty of times before. It would seem that if we followed your logic, no exhaust manifolds/headers would produce more low RPM torque than any size exhaust manifolds/headers. That's just not the case. Yes and no. But that's an academic exercise, as we're talking real vehicles here that need to pass noise laws, or to prevent from asphyxiating the driver.

Also, how exactly is the the piston encountering 'resistance' from pushing exhaust gasses when there's almost no resistance in pushing them out since it's the cams that open the exhaust valves? Gas does not move by itself. There must be a pressure differential to move the gas mass. The more backpressure, the smaller the delta pressure, and the less willing the gas is to move by itself. Under high (or higher than necessary) backpressure, the only way for the gas to be extracted from the cylinder is to consume crank horsepower driving the piston up against the reluctant-to-move exhaust pressure. As well, you have exhaust gas velocities which effect scavenging of exhaust gasses. Smaller diameter exhaust manifolds/headers will create a higher velocity which will in turn 'pull' gasses out of the combustion chamber. Larger diameter headers will have less restriction and thus not scavenge exhaust gas as well. Yes and no, again. What is the gauge pressure level at the exit of open headers? Zero. That's why headers (and the tuning) works (at the track). Once you put eight feet of exhaust pipe and the restriction of mufflers at the end, the scavenging efficiency of headers plummets.

I think you'll be somewhat hard pressed to find dyno results that show 'little to no' backpressure as having torque build up faster at lower RPM's.
Some backpressure is required for optimal engine performance when not running WOT at the end of the RPM range.

What law of gas physics or combustion efficiency backs up your last statement?

 

[84Turbo]

I didn't have a chance to do a whole bunch of research but according to an excerpt from Car Craft Magazine



According to Magnaflow

We all agree about the small vs big pipe issue when it comes to open headers (exiting into zero backpressure atmosphere). Once you stick a length of pipe and mufflers on the end, the scavenging ability and efficency of the headers takes a dive.
Regarding Magnaflows claims, they seem a bit contradictory. In the graph area they state that their mufflers reduce backpressure, enhancing performance. I agree. Makes sense. But then later they state that increasing the pipe size will also increase performance, (makes sense again) but further increases will diminish performance. No data or reasons why that would actually occur.