: Dual Exhaust



DopeStar 156
05-31-05, 12:33 AM
All right guys, convince me to do it. Reasons why and why not. I was thinking about it and other than looking cool I'm sure it has to have other good purposes too. I think it provides an HP boost.

davesdeville
05-31-05, 01:33 AM
Well... It looks cool and provides a HP boost.

sizanjdf
05-31-05, 02:58 AM
It looks cool and proivedes a HP boost. I think it would sound pretty cool too...if you want it do it, you'll probably be glad you did. Make sure you go with true duals though!!

DopeStar 156
05-31-05, 04:06 AM
It looks cool and proivedes a HP boost. I think it would sound pretty cool too...if you want it do it, you'll probably be glad you did. Make sure you go with true duals though!!
Oh of course I'd get real duals! No question there. Anyone here have it done? Was it expensive? I want it to look "stock." Meaning that if Caddy made this car dual exhaust, this is how they'd have made it look. I think it's a shame this car was only single. It really deserved dual in my opinion.

davesdeville
05-31-05, 05:07 AM
I'll try to be a bit more helpful in this reply. Expect to pay around $300 and up. Sometimes you'll get a deal but this seems to be the going rate for a good system. If you plan on leaving a stock or close to stock 307 in the car for years to come you won't want anything bigger than about 2.25" pipe IMO, if it's not mandrel bent then 2.5" You have to pass emissions? If so for true duals you'll have to buy at least one new cat.

Sometimes there's a problem clearing the crossmember. This can be solved by just running the pipe under the crossmember raising the chances of it scraping or using a crossmember from a later model Caprice that came with dual exhaust.

sizanjdf
05-31-05, 07:29 AM
ive heard of a few people cutting the cross member to do it, and a few who simply ran it side by side through the indention already in it....I think some one had pics of there duals on here a while back...if you can get away with out the cats dont use them!!!

caddydaddy
05-31-05, 10:10 AM
You can also get the crossmember from a Caprice/Impala to get the dual indentations.

DopeStar 156
05-31-05, 11:27 AM
Yeah I definately have to pass emissions here in NJ. I'll have to look into this for the future.

N0DIH
05-31-05, 05:00 PM
Or get a crossmember from a 73-74 A or G Body. They were dual indented too

But why spend the $$, the 307 exhaust lends itself to duals on one side (with no cat) very easily. See if you can take a look at the hooker super comp dual cat back for a GN Regal, that pipe is an excellent peice for an example. 2 2.5" pipes will support a 455 to 5800 rpm and 11.30's, so there is no reason to ever go fatter except for looks. And 2 2.5" pipes will sit side by side where the cat was and connect up to the 307 nicely. Get a diesel dual exhaust block for the hole in the pass side manifold. That is available from Detroit Diesel now, as they took the 350 Olds diesel parts from GM.

That is an exhaust to look at too, the 78-85 diesel B/D Body cars. Option code NM8 is the "leaded fuel exhaust" that has no cats....

Do they do a visual inspection? Often a good running 307 will pass without cats.... But it can be risky so close to DC...

cadillacmike68
05-31-05, 06:31 PM
I had my 1968 DVC converted to dual. Everything after the manifold was replaced. complete stainless - cost me about $500.00. They ran the right side pipe under the crossmember, it's a tad lower than the left side, but not noticable unless the car is on a lift. I used "classic" cadillac turndowns, barely visible from the rear - chrome of course.

They installed a "balancer pipe connecting the two lines - I don't see how much good this will do - my 1995 doesn't have any such balancer pipe and it's quite as a mouse. I can always have it removed...

Only real problem is that you can't fit 2 huge resonators back there - fuel tank is in the way (it's offset to RT to make room for the single resonator), so the shop used 2 very small "resonators" that don't really quiet the car enough! That and their initial muffler choice was too loud, so I had to buy 2 more mufflers.

Overall it was a class job, and if only I can get it quieter...

N0DIH
05-31-05, 08:18 PM
Might be able to put a large 3 to 3.5in pipe and oval it to make it have additional clearance.

Get some reducers to keep the flow good and smooth. Not perfect, but a 307 will still respond well to it.

Fleet
06-01-05, 01:18 AM
I had my 1968 DVC converted to dual. Everything after the manifold was replaced. complete stainless - cost me about $500.00. They ran the right side pipe under the crossmember, it's a tad lower than the left side, but not noticable unless the car is on a lift. I used "classic" cadillac turndowns, barely visible from the rear - chrome of course.

They installed a "balancer pipe connecting the two lines - I don't see how much good this will do - my 1995 doesn't have any such balancer pipe and it's quite as a mouse. I can always have it removed...

Only real problem is that you can't fit 2 huge resonators back there - fuel tank is in the way (it's offset to RT to make room for the single resonator), so the shop used 2 very small "resonators" that don't really quiet the car enough! That and their initial muffler choice was too loud, so I had to buy 2 more mufflers.

Overall it was a class job, and if only I can get it quieter...
The former owner of my '69 Fleetwood Brougham had duals installed. I'm glad he did it because I would have done it anyway. Increases horsepower and even helps fuel mileage a little.

The balance tube increases low-end torque. For some reason, it was not put on mine, but I'll take care of that someday.

There are two resonators and the car is quieter than I thought it would be, which was a nice surprise.

shreddedtires
06-01-05, 09:27 AM
Dual exhaust is a mod that will be coming soon for my fiancee's car, as soon as we replace the stock Cadillac hubcaps with something chrome that spins... It's her car, but I do all the work on it. We're gonna probably go with chrome dual sidepipes which should look and sound great. Dual exhaust has been known to increase hp and lower MPG... So I guess that COST is the only downside. But of course, everything worth having is either difficult or expensive.

davesdeville
06-01-05, 10:51 AM
Dual exhaust has been known to increase hp and lower MPG...

That makes it sound like it lowers gas mileage. Which I don't think is the statement you were going for...

Destroyer
06-01-05, 02:38 PM
Or get a crossmember from a 73-74 A or G Body. They were dual indented too

But why spend the $$, the 307 exhaust lends itself to duals on one side (with no cat) very easily. ..Or just look at mine. Mine is a 91 with an Iroc 350 but it would look the same on your car. I'm running Magnaflow mufflers and even though I loved them at first, I feel they are way too loud. Gonna put some generic mufflers on.

N0DIH
06-01-05, 02:45 PM
It should not lower gas mileage at all. If it lowers mpg there is something else at work.

Now, with a non computer carb, the fix mixture will be changed, as the flow is increased, the pressure drop through the carb is changed. So often it needs to be rejetted richer. But, for the computer controlled cars (EFI or carb) this is not needed. Might introduce lean fuel pinging, or poor drivability. Which leads to---

What also changes is if there is a significant backpressure change, the EGR can open at the wrong time or not open at all, increasing the potential for pinging or poor drivability. Experientmenting with a negative backpressure EGR or a ported EGR will often aleviate this problem. Although it is hard to get as good drivability as a tuned factory EGR. Look for a car that has duals factory with a similar or the same engine and use EGR if it fits on your engine. Generally GM uses the same EGR valve flanges on most engines. There are around 2-3 different ones for non Digitial or Linear EGR engines.

N0DIH
06-01-05, 02:51 PM
That is precisely what I was thinking of doing to mine (85 Cutlass w/Olds SB). Looks great. I had Hooker Super Comp mufflers. Quiet inside the car, but fairly loud outside under power. No droning!!! I liked them a lot. Best $300 I spent on the car!

What size is the exhaust? 2.5?


Or just look at mine. Mine is a 91 with an Iroc 350 but it would look the same on your car. I'm running Magnaflow mufflers and even though I loved them at first, I feel they are way too loud. Gonna put some generic mufflers on.

Destroyer
06-01-05, 03:33 PM
That is precisely what I was thinking of doing to mine (85 Cutlass w/Olds SB). Looks great. I had Hooker Super Comp mufflers. Quiet inside the car, but fairly loud outside under power. No droning!!! I liked them a lot. Best $300 I spent on the car!

What size is the exhaust? 2.5?
Exhaust is 2 1/4. I was going to go with a 2.5" but since I'm not going too crazy with the motor I figured I could use a little more back pressure. Wish I had the time to finish my carb conversion on it......... :rant2:

shreddedtires
06-01-05, 06:03 PM
True, I meant to say you have BETTER miles per gallon because of... something... My mind just totally went blank...

N0DIH
06-01-05, 07:04 PM
There is never a condition where you need to ADD backpressure. There is MORE than enough in the ports, you do not want more. Ever.....

That is some really odd myth that has circled the automotive world that is totally false.

Now for headers, there is some other things at work, but never do you want to add backpressure.


Exhaust is 2 1/4. I was going to go with a 2.5" but since I'm not going too crazy with the motor I figured I could use a little more back pressure. Wish I had the time to finish my carb conversion on it......... :rant2:

brougham
06-01-05, 08:47 PM
There is never a condition where you need to ADD backpressure. There is MORE than enough in the ports, you do not want more. Ever.....

That is some really odd myth that has circled the automotive world that is totally false.

Now for headers, there is some other things at work, but never do you want to add backpressure.

Actually that's not exactly true. If you don't have enoguh of it you can loose some power.

Destroyer
06-01-05, 09:47 PM
There is never a condition where you need to ADD backpressure. There is MORE than enough in the ports, you do not want more. Ever.....

That is some really odd myth that has circled the automotive world that is totally false.

Now for headers, there is some other things at work, but never do you want to add backpressure.
Well I wasnt actually trying to "add" back pressure but I didn't want to "remove" more than I had to. If the cam was lopier, had more compression, etc.. they I would have done 2.5". :D

N0DIH
06-02-05, 12:19 AM
It isn't backpressure for small pipe, that is the myth. It is a pipe that matches the pulse size (think of it as an egg going down the pipes) to allow for sufficient scavenging. It is much more complicated.

But as we have manifolds on most of our cars, and not headers, it is a non issue. There is no scavegening, there is too much backpressure and lack of flow in the manifolds to support it. Maybe if you have a set of Pontiac Long Branch manifolds on a Pontiac engine yes. The key is equal length pipes on each port, long enough to support a good scavange (like around 36+ inches, prefer longer for lower rpom torque) merging into 2 pipes then into the other 2 pipes to make a single pipe going back (on each side, 4 to 2 to 1) all properly tuned.

I have seen many pipe comparissons, and with MANIFOLDS, pipe size was a non issue, with headers it was a small issue, not near as big of a deal as people think. The header primary pipe was the single biggest factor. Once you get past collectors, it really doesn't matter much.

Hence why Jim Hand runs 11.30's on 2.5" duals with headers with only 1.75" primaries. And with a 4000# car and 3.55's. And 473 cubic inches.

davesdeville
06-02-05, 04:07 AM
It's not exactly "pulse size" either, you're looking to balance exhaust velocity with backpressure..

BluEyes
06-02-05, 02:26 PM
I'm pretty sure I have seen dual in/dual out cats for sale - that would help routing for two pipes on the same side and passing emissions.
Plus, some states require the cat to be in the original location, so duals with dual cats (especially duals on each side ofthe vehicle) can sometimes cause you to fail emissions on that stupid technicality. A dual in/out in the stock location should solve this problem if NJ requires this.

Anyways, for routing, here's what I'm planning for my '70 SDV, see if you like it. I have a complete exhaust off another '70, so what I plan is to leave one copy of the stock exhaust in place, then branch another copy of it off the original exhaust right before the y-pipe where the two head tubes join. This will give me duals, AND a crossover pipe. Both pipes would run through the single hump in the x-member and back out on the same side. Not the most optimal system, but my engine is stock, and it seems I can do this on the cheap. Being a '70 my car never had cats, so I don't have to worry about them :D

N0DIH
06-02-05, 05:20 PM
ANY backpressure will cause dilution of the A/F mix in the chamber, it will prevent the raw A/F from coming in as fast, will prevent filling up of the cyl to the max potential, which means volumetric efficiency is lost, power is lost. This will cause a much more serious power loss as RPM increases. You need the draw from the escaping gasses to help pull in fresh fuel to be burned, else it will have to rely soley on pumping from the piston going down, which will drop your volumetric efficiency to below 80%, if not worse. A good well designed exhaust/intake system can, at peaks, help deliver over 100% volumetric efficiency.

Any WHY do you think backpressure is good? Scientifically, please explain, someone.

Backpressure is bad. Even GM FSM's note it as being a problem and to measure, if it above like 3psi, they attack the backpressure problem. That little amount is enough to be a problem.


It's not exactly "pulse size" either, you're looking to balance exhaust velocity with backpressure..

davesdeville
06-02-05, 09:37 PM
So stick dual 3.5" pipes on your Fleetwood and see if it helps.

Destroyer
06-02-05, 09:39 PM
Any WHY do you think backpressure is good? Scientifically, please explain, someone.

.Uhhhhhh...........just what I heard and have kinda accepted as fact over the years. I was told a 2.5" exhaust would be too much. I'm no expert so you may be right. Still, I noticed a very nice difference with the dual exhaust, no cat, and magnaflows over the stock system.

N0DIH
06-04-05, 12:06 AM
Considering the point I am making is you don't NEED to put on a big exhaust, but you don't want to put one on that ADDS backpressure in any way shape or form. There is properly sized exhaust, but that only applies to headers, not log manifolds like most of our cars have. For those, as big as you want, it doesn't matter. You won't gain anything significant because the manifolds are the choke hold on the engine. So I can put 5in exhaust or my factory stock and it will run the same. Or I can run tiny exhaust and choke it even more.

Now, add headers, and the primary tube length and size DOES matter. This is likely where the myth started. A car with 1 5/8" primaries like a 5.7L will likely slow down some in the low end if 2in primaries are installed. But take a 2in primary pipe and couple it to 42 or longer primary tubes, and the 1 5/8" with say 36", and the longer tube car, despite the smaller size, will run better in the low end, but may start to have some power loss at higher rpm.

It all comes down to velocity. Backpressure slows velocity. So avoid it at ALL costs. The higher velocity on the intake and exhaust the more power you will make.

In reality, dual 2.5" mandrel bent pipe is sufficient for a 473 CID Pontiac at 5800 rpm to push 4000 lbs down the 1/4mile in 11.32 seconds. Do you NEED more? Jim Hand has tested his 2.5" pipes with 2.25" tailpipes and found little, if any loss when he was running low 12's, with a 455 and Ram Air IV cam with Rhoades lifters. So, for our puny little small blocks, 2.25" in likely the most we will ever need. As Jim would point out, there are design exceptions to every rule, so this is rule of thumb, not gospel in granite....