1970 318 duster exhaust question???

-
91r/t

91r/t

Well-Known Member
Joined
Jun 17, 2012
Messages
94
Reaction score
13
Location
Colden, NY
I have a quick question to run by you guys. I have a 70 duster 318 3 speed that is bone stock and I want to add dual exhaust. I have been looking at having an exhaust guy near me do it up in 2in. with the stock chrome tips, but then I found TTI's 2.5in kit with the turn downs. I want the sleeper ok and don't really want the chrome tips coming out the back so I was thinking of doing the TTI option. Do you guys think the 2.5 exhaust with an H pipe and stock manifolds will reduce the back pressure too much and actually hurt my performance on my little 318 2 barrel? Like I said its stock with manifolds and a single exhaust right now. Looking for a better sound not really any huge performance gains, but the TTI system is so nice with the stock appearing turndowns.
 
I'm thinking you specify to them and they make them either way. At least that is how the big block B-Body systems work. It says manifolds back so that's what I am assuming. I'd think I'd have to give them my casting numbers to make sure they weren't assuming 340 manifolds.
 
Yep....just looked and it said give them your casting numbers to ensure the correct bend on header pipe.
 
I had dual exhaust set up on the factory manifolds of a '71 Dart, they worked and sounded fantastic (Mel & Sons, Concord, CA), I don't remember what mufflers I used but they were a common manufacture and not too loud, just loud enough for people to notice. I thought you couldn't get anything but exhaust systems for headers though, my 16 year old is looking for something similar.
:burnout:
 
when u switch from 2" to 2.5 it moves the power band higher...it would probably be quicker with the 2"
 
Your local exhaust guy should be able to give you the stock looking turn downs if you ask.
I thought I had a better pic, but you get the idea.
C
 

Attachments

  • DSC01670.jpg
    45.7 KB · Views: 484
First off, contrary to belief there is no such thing as backpressure. Second, it is all about the exhaust gas velocity. If you are just looking for a bigger sound go for a larger pipe,but you will hurt your low rpm velocity. Especially if your driving this car on the street. You will have to wind the engine up to get it to produce good torque. As doug 71 said, the larger the pipe you will move your power band higher. This is because when you have a smaller pipe you don't need as much air to keep velocity at a decent rate, but when you use a bigger pipe you create a dead spot in velocity which you feel as a loss of torque. When your rpm gets higher you start sucking more air which in turn makes your velocity higher for that given pipe size that's when you feel the power come on. But most 318 will be happy with a dual 2 inch if your driving on the street. I use dual 2 inch on my 360 pickup, and I get 18 mpg highway because I don't turn high rpm and I need the lower speed torque aka velocity. But exhaust pipe size also goes hand in hand with cam size as well. Obviously a low lift cam will have higher torque at a lower rpm so you will need a smaller pipe.
 
First off, contrary to belief there is no such thing as backpressure. Second, it is all about the exhaust gas velocity. If you are just looking for a bigger sound go for a larger pipe,but you will hurt your low rpm velocity. Especially if your driving this car on the street. You will have to wind the engine up to get it to produce good torque. As doug 71 said, the larger the pipe you will move your power band higher. This is because when you have a smaller pipe you don't need as much air to keep velocity at a decent rate, but when you use a bigger pipe you create a dead spot in velocity which you feel as a loss of torque. When your rpm gets higher you start sucking more air which in turn makes your velocity higher for that given pipe size that's when you feel the power come on. But most 318 will be happy with a dual 2 inch if your driving on the street. I use dual 2 inch on my 360 pickup, and I get 18 mpg highway because I don't turn high rpm and I need the lower speed torque aka velocity. But exhaust pipe size also goes hand in hand with cam size as well. Obviously a low lift cam will have higher torque at a lower rpm so you will need a smaller pipe.
 
Ok sounds to me I am going to have the exhaust guy do the 2in pipe. It is a stock 318 so it won't be making any power up high on the RPM scale. Thanks for the input guys!
 
Are you gonna leave it a 318? I am a big proponent of a good HP single exhaust on a smaller cube engine. IMO, even a 340 built mildy would show bigger gains with an HP single exhaust. Especially using something like a Flowmaster Y section and a large single pipe like a 3 or 3.5". It's been proven on the dyno.
 
I put dual 3 inchers on my totally stock long block,73 318. Far as my a$$-dyno can tell,I didnt lose nuttin. And that LC 318 sounds so mellow at full song,pulling through my favorite carb, the mighty TQuad.
EDIT
-I keep hearing this over and over; about your teener with headers,making more torque with smaller exhaust than big. Can somebody point me to the science behind that? Id like to know if my S would go faster and quicker with smaller exhaust than with open headers.And just how far up the cid scale would that apply. For example, If I put a 418 ci or a 518 cid.Would they also go faster/quicker with a smaller exhaust than big?
-And taking that to the extreme,just what size exhaust should the super-stockers of yesteryear have installed to go even faster/quicker than they did?
-Heres my science; go find 2 straws, a regular one, and a supersized one. Blow through them, one at a time. Through which one can you blow more air through, per unit of time? Explain to me how this does not apply to my teener.
 
Yea I am leaving it a 318. The dual exhaust is worth a few ponies lost to me for the cool factor that is achieved :)
 
Alpha13 did a pretty good job of putting down the science. If he had added that air has mass, would have been perfect. The straw analogy is overly simplistic and omits too many present and contributing factors to be valid.
 
I put a dual 2 1/4" exhaust with an H-pipe on a 70 Swinger with a 318, and it really woke it up. the stock exhaust on a 318 is very restrictive. It's the first thing to change for increased performance as it gives the most benefit.
 
Teeth & Tires said:
Alpha13 did a pretty good job of putting down the science. If he had added that air has mass, would have been perfect. The straw analogy is overly simplistic and omits too many present and contributing factors to be valid.
Click to expand...

Air has mass. Lol .I agree about the straw analogy. It's like comparing apples and oranges.
 
krazykuda said:
I put a dual 2 1/4" exhaust with an H-pipe on a 70 Swinger with a 318, and it really woke it up. the stock exhaust on a 318 is very restrictive. It's the first thing to change for increased performance as it gives the most benefit.
Click to expand...

Krazy kuda is right about his experience. More than likely though he was running it with a 4 barrel and flooring it Alot lol. In my opinion if I was driving a 318 2 barrel with 2.76 gears, in the city nonetheless, it would be getting the 2 inch exhaust. Like I said before torque and velocity go hand in hand. Besides 2 inch is easier to install anyway given the small space to work with around the tank and rear end. I'm sure some will argue though...
 
alpha13 said:
First off, contrary to belief there is no such thing as backpressure. Second, it is all about the exhaust gas velocity. If you are just looking for a bigger sound go for a larger pipe,but you will hurt your low rpm velocity. Especially if your driving this car on the street. You will have to wind the engine up to get it to produce good torque. As doug 71 said, the larger the pipe you will move your power band higher. This is because when you have a smaller pipe you don't need as much air to keep velocity at a decent rate, but when you use a bigger pipe you create a dead spot in velocity which you feel as a loss of torque. When your rpm gets higher you start sucking more air which in turn makes your velocity higher for that given pipe size that's when you feel the power come on. But most 318 will be happy with a dual 2 inch if your driving on the street. I use dual 2 inch on my 360 pickup, and I get 18 mpg highway because I don't turn high rpm and I need the lower speed torque aka velocity. But exhaust pipe size also goes hand in hand with cam size as well. Obviously a low lift cam will have higher torque at a lower rpm so you will need a smaller pipe.
Click to expand...

I'm callin BS! Don't get me wrong here alpha13, your argument is well worded and all, but it doesn't make sense to me. I'm no expert on the topic, lol, but this goes against common sense. BTW, yes, there IS something as back pressure. And to illustrate my point, try running your manifolds thru some 1 1/2" exhaust. I think the effects of back pressure may be noticeable.

Yes, velocity will be greater with smaller diameter pipe. This is true, but does not equate to better performance. It's actually robbing potential horsepower.

Thirdly, again, not my own experience here, but I have always heard that the larger diameter pipes will make your exhaust quieter, not louder.

Yes, the smaller pipe is going to be easier to work with! That may be the only advantage with it though!

If I'm completely wrong with everything I've said, and you are correct, my apologies! Either way, no offense intended!
 
Yea my car has the 8 3/4 rear with 3:23 gears, not that that would make me think it's a horsepower monster haha. I am leaning towards the 2in exhaust due to non performance aspect of this car. I know I have 2.5in on my roadrunner and that woke that up....but that car is a whole other monster.
 
bighammer said:
I'm callin BS! Don't get me wrong here alpha13, your argument is well worded and all, but it doesn't make sense to me. I'm no expert on the topic, lol, but this goes against common sense. BTW, yes, there IS something as back pressure. And to illustrate my point, try running your manifolds thru some 1 1/2" exhaust. I think the effects of back pressure may be noticeable.

Yes, velocity will be greater with smaller diameter pipe. This is true, but does not equate to better performance. It's actually robbing potential horsepower.

Thirdly, again, not my own experience here, but I have always heard that the larger diameter pipes will make your exhaust quieter, not louder.

Yes, the smaller pipe is going to be easier to work with! That may be the only advantage with it though!

If I'm completely wrong with everything I've said, and you are correct, my apologies! Either way, no offense intended!
Click to expand...
See this is where people are wrong BigHammer. There is no such thing as back pressure. I have asthma so it's pretty easy for me to understand. Your analogy with the manifolds and small pipe size simulate restriction, not backpressure. And yes you are right, if you use too small of a pipe you will restrict your engine, but velocity and volume go hand in hand as well. If you take that same 1 1/2" pipe and put it on a small 3 cylinder smart car it will have a sufficient amount of pipe size to flow it's particular volume. Now performance exhaust is typically bigger so you can flow more volume, but to achieve a reasonable exhaust velocity you must create more volume. When you rev higher you create more volume thus allowing the velocity to remain at a decent rate.
I'll give you an example. Take a stock 318, put a dual 2 inch exhaust on it. (I'm guessing on this next part for comparison.) Exhaust gas velocity will peak at around 2500 rpm. Now take the same 318 and then put a 2 1/2" dual exhaust on it exhaust gas velocity will now peak at a higher 3800 rpm.
Why you ask? As engine speed increases, volume increases, and power increases. But limit(restrict) the amount of air exiting the engine and it will hit it's peak velocity at a lower rpm. As I've stated before, obviously if you restrict the engine too much it will hurt it's amount of airflow thus decreasing power. Alot of engines come from the factory with insufficient flowing manifolds and exhaust for performance use. That's why putting a larger exhaust pipe,within reason, on a stock engine will make more power. But it will also make more power in a higher rpm. How much power is debatable. Thats why when running a higher rear gear(numerically lower) you need alot more low speed torque A.K.A velocity because you spend more of your time in lower rpm. Now when you move your torque curve higher because your velocity peak is higher due to your particular engine combo you need a lower gear (numerically higher)as a crutch to make up for the loss of low speed torque. That's a whole nother aspect though.lastly, a 1 inch pipe and a 3 inch pipe can achieve the same velocity,but they would also require two different volumes to achieve it. The 1 inch pipe having a lower volume and the 3 inch having a higher volume. I could keep going on and on about this, but if you don't understand by now there really isn't much more I can do for you.:violent2: of course in no way am I trying to come off rude.
 
Alpha13.. I'm sure you are right with your explanation in regards to the pipe size argument. But In my ignorance, it just goes against my limited concept of how this stuff works! I always heard that, to make more power, let the engine "breathe" better. Usually this is done in the first stages with a better intake and exhaust manifolds / headers.

The way an engine works has been likened to the way an air compressor works. To get the most efficient air production, the air hose itself (exhaust pipe) needs to be of adequate size, otherwise performance drops off significantly.

Don't get me wrong, I'm not saying that you're incorrect. I'm just trying to wrap my brain around the idea of the smaller pipe being a better option. I'm getting ready to put dual exhaust (2 1/2") on my Dart... Hoping for an improved performance factor!
 
There is such a thing as back pressure. It's called a restrictive exhaust. LMAO
It's more in the way people think and the went understanding of the terms used.

The slower velocity of a to big of a exhaust pipe will slow a car down. The high velocity of a smaller pipe will work. But the problem most people have is with the wrong size for the combonation.

For a stock 318, a good size is 2-1/4 for dual exhaust or a single 3 inch to maximize everything. Going larger can be hurtful. I did do a 2-1/2 many many moons ago. It just seemed loud. Not fast. 2 inch seems small t me even for a sock engine.
 
I run 2 1/2 from my headers all the way out the back, and I think it performs very well.

With that said, I originally had 2 1/2 going back to the muffs with turn downs in front of the diff. Since i ran it all the way out the back, the performance difference is very noticeable, for the better..
 
You must log in or register to reply here.
-

Back
Top