exhaust size

[Bruceynz]

Hi Guy's

My Cordoba had a 318 in it, I had a nice 2.5" single exhaust built for it, be lucky if it got 30 miles on it before I pulled it off the road. Now has a mild 360 in it with headers, there is very little work to do to use the existing 2.5" single system. Just need to feed headers into a Y and then into muffler and its done. Is a 2.5" single system to small or will it be ok?

My engine is a stock 360 with a bit better cam and edelbrock intake.

Thanks
Bruce

 

[Aspen500]

The single 2.5" system should be just fine with a basically stock 360.

 

[slant6billy]

2.5 is ok, but lets look at what you have. So 2.5 on each side, then a Y pipe and then 2.5 single. So you get a mid pipe reduction going on. Now, that maybe ok with a mild engine. Your Y pipe keeps your bottom end torque. However, it may rob some top end. I'm not saying step up to 3" after your Y, but my Ram SS/T has 3" from the cat back (and that was from the mopar performance SS/T package new). Exhaust is hot and expanding by design. Giving it a big pipe to leave will help. Crossover H pipes and X pipes work with different positions placed from the motor. Depending on the motor specs, an X pipe might not work as well an H pipe. New 5.7 hemis work beter with h pipes. Mustangs need the x pipe moved further back from the h pipe area. True duals are great for an all out dragster, but a street car needs back pressure. Where you put the reduction or y pipe is key to make it just right.

 

[Bruceynz]

What HP is your SS/T?

 

[slant6billy]

What HP is your SS/T?

Based on the sales brochure either 250 or 290 stock. With the headers, Volant CIA, Hypertech box and pcm reprogramming she puts 307 at the rear wheels on the dyno. the power doesn't start out low, but comes in at 2300rpm and goes to about 5700 (rev limiter disabled). It fares well against my Volare, but the volare has a much more and weighs lighter.

 

[jasperjacko]

My first thought is that your exhaust will hurt you. My car has a mild 360 with headers and single exhaust. It has 3" collectors-2.5" into 3" y outlet, with Borla pro xcess straight through style muffler and 3" exhaust out the back. It runs as good or better than it did with dual turbos and turn downs at the rear axle using 2.5 pipe. I did have a Flowmaster delta 5 on my current config. and it was terrible above 4000 rpm. i measured over 4 psi of back pressure in the y section. That caused the engine to hit a wall. Flowmasters may sound good, but they do NOT "flow". There may be a lot of cars using them and running good, but that is in spite of the bad muffler design. What muffler are running?

 

[Bruceynz]

Hello,

Ok I don't think a 360 with mid 70's heads, A bit better than stock cam and edelbrock intake is going to make anywhere near 300hp at the wheels. In their day they were rated at 175hp SAE net. So I am thinking low 200 maybe 220hp net if I am lucky. The muffler is a local one made in nz by autobend 2.5"/2.5". I am just trying to work out if I should ditch $600 worth of exhaust system for .5" of pipe diameter. Are we talking .5" will rob 100hp or 5hp? Or is it more the muffler will rob power? Take into account this is a road car, I want to drive it 250 miles and I don't want to be deaf when I get there, I want to drive it to work every day, I doubt my car will ever see a 1/4 mile drag strip. My aim was to increase the power, get rid of all the emissions, I didn't intend to build a super sleeper road rocket having said that if I need a 3" out the back I will change it to 3". Can't beat you guys experience.

Thanks
Bruce

 

[jasperjacko]

Well then I would try what you have. It might work great for your purpose.

 

[Bruceynz]

Hi,

So do you think it was the muffler causing the problem not a pipe size?

There is quite a bit of extra cross sectional area between 2.5" and 3"

My headers are 2.5" out, I think 3" out of the Y would be better than 2.5" but was wondering how much it was going to hurt performance. If its going to rob a heap of HP then I am best to change it but if we are talking 10hp I might stick with the 2.5" its not like I drive my car around at 5000rpm I just drive normally so would be lucky to see 2000rpm in normal traffic town driving.

Thanks
Bruce

 

[jasperjacko]

The muffler in my experiment,

 

[Bruceynz]

My engine is 360ci it fires 4 times per rev, does this make it look like a 180ci engine to the exhaust?

 

[jasperjacko]

no
--- Post updated ---
exhaust has no eyes.lol

 

[Bruceynz]

Hahaha true
--- Post updated ---
I know a certian size pipe can shift so much cfm, at light throttle I am sure 2.5" will be fine. I doubt 2.5" will hurt my initial take off speeds but more like top end of the rpm. I talked to dunnuck racing, he writes articles about mopar engines and we talked about carbs, he said his dyno muel 360 responds really well to a smaller carb, the smallest carb he had on hand was a 625cfm and it loved it, I have a brand new 600cfm to go on mine, he thought that would be good. The cfm calculator on summit says 513cfm for a 360 at 5800rpm which is inline with what dunnuck racing was saying. A 2.5" pipe can flow 509cfm. If the engine sucks in 513cfm how much does it expand when it is heated up?

Thanks
Bruce

 

[jasperjacko]

Flowing 509 cfm at what pressure? Higher pressure hurts h.p. Not a problem at normal driving rpm's