Performance Exhaust

[greg82255]

Just curious - Is there a noticeable difference in performance or fuel economy between the stock cast-iron exhaust manifolds and a performance set of headers like the new CMI E-top headers? If so, what is the power increase? I keep reading about how the stock manifolds are highly restrictive. This would be on a vortec 350 with a roller cam/rockers (315 hp). I know there are better ways to spend $1500-2000 and increase power but I'm just asking about exhaust here.

 

[Silverbullet555]

Re: Performance Exhaust

The good news is that with a merc there are a lot more options than a Volvo.

I was talking to Dustin Whipple at Whipple Superchargers and one of the things that came up as an early performance mod was headers. I have a big block and they tend to respond a little better to upgrades like headers and CMI headers are on my list of future upgrades as I want to get up to about 450HP from 375.

Don't know how much of an increase you are likely to get. CMI might be able to give you an idea.

 

[greg82255]

Re: Performance Exhaust

I guess I spoke too fast. Just found some Dyno data from CMI's website. It looks like they add somewhere between 10 and 15% (depending on the RPM) on a 383 stroker, so I assume its going to be less for a 350. Here's the site if anyone is interested.

http://www.custommarine.com/headerapps/dyno_data2.php

 

[TilliamWe]

Re: Performance Exhaust

Yeah Greg, exhaust manifolds, by themselves, do not bring a Small Block Chevy HUGE gains. But if you were to make a 383 and put in a higher lift cam, (or supercharge the engine) they would work in conjuction with that to produce more power. Big Blocks do indeed respond more to exhaust changes.

 

[Volphin]

Re: Performance Exhaust

Still, it seems worth it to me... but depending on what you get, it can get expensive.

V

 

[John_S]

Re: Performance Exhaust

Yeah Greg, exhaust manifolds, by themselves, do not bring a Small Block Chevy HUGE gains. But if you were to make a 383 and put in a higher lift cam, (or supercharge the engine) they would work in conjuction with that to produce more power. Big Blocks do indeed respond more to exhaust changes.

Agree

 

[greg82255]

Re: Performance Exhaust

TilliamWe, you make a very good point. I guess it doesn't really make sense to spend $2000+ for maybe 2 mph in the end. If I ever build a 383 I'll keep that in mind. Those headers do look awesome though.

 

[sschefer]

Re: Performance Exhaust

On the 350 Vortec you're looking at trying to improve torque to get a little better performance. You won't do much for that with a high end exhaust system unless you make a few other changes. It would probably end up costing a lot for little or no long lasting gain. Long lasting gain is the kind of gain that scares you everytime you get on it. Short term gain is the kind that you notice the first couple of times and then just get used to it.

 

[greg82255]

Re: Performance Exhaust

On the 350 Vortec you're looking at trying to improve torque to get a little better performance. You won't do much for that with a high end exhaust system unless you make a few other changes. It would probably end up costing a lot for little or no long lasting gain. Long lasting gain is the kind of gain that scares you everytime you get on it. Short term gain is the kind that you notice the first couple of times and then just get used to it.

Very well put. I really asked this question because I was reading another forum and I saw someone said that they took their 350 and put on a holley 850 cfm carb and a stainless marine exhaust, and they claimed to get 8 more mph (including a prop change). Is this even possible?

 

[TilliamWe]

Re: Performance Exhaust

Possible, but it really means they probably didn't have it propped right in the first place. And they don't tell you that they have to stand in the bow to get it to plane! "But it runs great in the top end!"
SBC's do not NEED that much carburetion. 650cfm carb will feed a 350 all day long, even one with a higher lift/longer duration cam. 750 is overkill, 850 is a joke.

 

[greg82255]

Re: Performance Exhaust

650cfm carb will feed a 350 all day long, even one with a higher lift/longer duration cam.

Is the stock carb for a Merc 260 650 cfm? Or is it something smaller and would I benefit from switching to a 650 cfm?

 

[sschefer]

Re: Performance Exhaust

Actually the Holley single pumper 850 will do O.K. if you change the springs in the accellerator pumps to the lighter ones so they don't dump everything all at once. Still, it's pretty much a waste of money and time. 600-650 is all a 350 can suck down, the best carb ever made for those things was the Carter Q-Jet with the plastic thermal plate. The problem with them was that the castings were junk and the most common problem was a sand hole which made most of them throw aways. I loved that little Holley 600 single pumper.

 

[John_S]

Re: Performance Exhaust

Is the stock carb for a Merc 260 650 cfm? Or is it something smaller and would I benefit from switching to a 650 cfm?

The Merc Q-jet and the weber that came on 350 MAGs and 5.7LX's was 750 cfm. But they both are vacuum secondaries, so the only open to what the engine needs. Some merc 5.7L 4brls had 600cfm webers.

No benefit for you to change.