Hey guys,
I am curious. I know that with NA cars that some back pressure is needed (usually for lower and mid range power). I also know that in turbo cars that back pressure is the enemy. The less the pressure the better off the turbo is, and HP goes up. Given that is also considering that the turbo runs off the exhaust.

My question is what about on S/C's
It is still force inducted (boosted), but it isnt exhaust driven. So I would believe that it would behave more like an NA car when it comes to exhausts and back pressure.

So I am wondering what would happen to the low and mid range power if I had an open exhaust?
Does back pressure help or hurt these applications?
I want drivability, not just all out HP.

 

No engine likes being choked anymore than you would.

Anyone who says an engine needs backpressure in the exhaust is just plain wrong.

 

thanks, you beat me to the answer!

Remember that ANY engine needs an exhaust system that is properly sized for the displacement and power output. If the system is too large the exhaust gases can "stack up" inside the system and become a restriction - I.E., you wouldn't put a dual 5" system on a 283 2-barrel. You also wouldn't want to go the other way and run 2" exhaust on a 1000 hp 500" big block.

It's just like any other engine mod - right parts to suit the application.

 

Thanks for your answers
I guess I am missing what you are saying, or in general maybe I know what you are saying to a point but I am not understanding completely.

Basically I guess what I am asking is if an NA car has an exhaust tuned for its performance (H or X section, proper diameter pipes, equal length long tube header, and etc), then usually the car picks up the low and mid range. If the car just had open headers (ie. drag cars) then it is looking for the max hp. Turbo cars of course like free flowing exhaust since the turbo itself tends to act like a restriction. So you open the exhaust and the turbo is happy. Of course, you tend to loose power on the low side, but once that turbo is spooled up, then you are happy

So with all that said, how does an S/C act? What does an S/C like? Does it loose low end and mid range if you just put a straight pipe on it? Do you really see the gains of a straight pipe on a S/C car as you would with a turbo? Or is it better to keep an S/C car with an exhaust and muffler designed for performance?

(I am asking because currently I have S/C's on both my cars and performance exhausts on both my cars, and I was thinking about taking off the exhausts and making straight pipes for them)

--- (also how can a system be too big, and gases stack up?)

 

You will make more power with less exhaust backpressure with a fi setup. You could go with the largest diameter piping that will fit and it wouldnt hurt anything. The engine size, cam and turbo or supercharger itself will determine the powerband.

Carl

 

You are completely missing it.

An engine does not want a restrictive exhaust. Fact, and end of story.


Your reference to a turbocharger causing a restriction is one of the bad aspects of a turbo setup. However the gains are more than worth it The engine certainly does not want or need to be strangled anymore than you would.

 

(also how can a system be too big, and gases stack up?)

Think about the properties of a gas. When the exhaust pulse leaves the head it's extremely hot, goes into it's own header tube then into the single larger exhaust pipe for that bank. Now what happens to the temperature of a gas as it expands, it cools. The larger expanse it has to go into the more the cooling effect. The way your A/C system works with an orifice tube. As the exhaust gasses cool down, they tend to exert less pressure on the system. Given this, the larger the pipe and the longer the distance the more the exhuast will tend to cool, and the less velocity it will tend to maintain. Exhaust systems for street cars always have to be a compromise of maintaining enough velocity at low rpms but still allow enough flow at high rpm's to keep the engine happy across most of operating driving range. Actual backpressure is always bad from a performance standpoint. Backpressure is simply resistance to flow, meaning flow is being resticted, so the engine isn't pushing out of the exhaust it's potential. The perfect exhaust system is one that would adjust to always keep maximum flow and as high a velocity as it can without effecting flow, at any engine operating range you would want to put it. Remember back when everyone was punching out there cat convertors and placing them back on the car expecting some big power increse but never got it. The simply created an expansion chamber in the exhaust system. The smart guys put a piece of straight pipe through the middle of the old convertor.


Patrick

 

So considering that I am running an exhaust (and possibly converting to a straight pipe) is there a formula or something that I can use that gives me the proper diameter of tubing to use for the amount of power I am making/ flow I am creating?

 

I'm confused.

Are you trying to achieve something out of the ordinary ? Why not just buy one of the many tried and tested systems that fit your car ?

And as for wanting driveability. How do you think pretty much any exhaust is going to hurt a large V8 to make it undriveable ?

You're worrying/thinking over nothing.

Just buy any reputable system and you'll be fine.

 

Ok, the thing is I am looking for the best performance and the best sound for both my cars. Something that some of the aftermarket exhausts made for my car have failed to do. Also I am looking to maintain the low and mid range performance from my cars (from which I understand that a good exhaust will increase the low and mid range power curves too). So I have two, as you put it, reputable systems on my cars, but I am thinking of something different.
Both my cars have S/C's. One has a Maggie, and the other has a Centri type. One car has full exhaust from the block back ( having after market headers), and the other is just basically a cat back (or it starts at the end of the stock headers - but those soon will be swapped out too). I am trying to find out that if I put a similar sized tubing (if not a little bigger to match the headers end diameter) and then run it through as straight as possible (with maybe an H pipe or X pipe in between) then out the end with no mufflers or resonators (except MAYBE a glass pack), will this harm my performance numbers over what I have now. Given I want max hp, but I also want the low end and mid range to still be there too (and if i remember correctly this is why H pipes and X pipes are put in). Basically I want my numbers to go up, while still keeping the curve the same through out the rev range.

I know I have seen some people that end up with a shift in their power curve. Basically, lets say that 100 hp is achieved by 2500rpm, and after a mod or so, their power curve has shifted and showed that 3000-3500rpm they are making 100hp now. Usually this happens when adding in a bigger turbo (and since thats where alot of my performance knowledge is based.......)

So, I am trying to avoid the above happening, where instead I would like to make a change and see 150hp at 2500rpm (instead of the original 100hp at 2500 rpm).

So thats why I am asking if I did a setup like I mentioned, would it not give me or change the outcome I am looking for?

 

You could shove a small child up the exhaust and it wouldnt have a massive impact on performance.

Any differences between any reputable aftermarket exhaust will be minimal in terms of performance. In fact, I'd almost go a far as to say there is no way you'd actually feel it when driving.
The noise may make you think there are changes, but they will be too small to feel. Many other things will have a big impact, like heads, cam, tune. But I doubt there is an aftermarket exhaust out there that is so ****, it would hurt performance

I'm pretty sure some members on here have made close to 4 figures on almost stock exhausts with cats fitted too.

If you want a noise, which is purely down to user choice. The only way to find one you like, is to try them all.

About 3 years ago I ran 198mph over 7/8th mile ( event **** up ). Little did I know at the time, my two centre Moroso Spiral flows had totally collapsed inside.
Basically there was only about 1.5-2" of an orifice in each tube for exhaust gasses to escape through.
At the time I was logging around 14psi just after the collectors. Which I thought very odd. But as it was first time ever measuring backpressure, didnt know what to expect.

With them fixed, that dropped to around 4psi maximum.

When driving etc I had no idea how badly restricted the exhaust was, you simply couldnt feel it.

 

Flow is the only thing that matters velocity restrictions etc affect flow but in the end flow is all that matters. back-pressure can help flow (really pipe sizing that causes a little back-pressure) if it increases flow like properly sized long tube headers will increases back pressure over open exhaust ports but they promote flow thru the scavenging effect. The primary tubes in a header are a specific length and the length relates to how long it takes the pressure wave exiting the exhaust port to get to the merge point this is timed so that the pressure wave and velocity of the escaping gas pulls some of the gas from the other primary tubes at the merge point this creates a partial vacuum that then travels up the other primary tubes from the merge to the port (taking a specific amount of time/ distance) to help pull out gas from the port when that valve opens. Another example is a turbo where the pressure wave is used to drive a turbine to boost the intake pressure and increase flow.
Going too large with an exhaust primary tube can decrease the velocity hurting the scavenging effect too small will hurt top end power (too small or too large is relative to the amount of gas flowing so too small is great for low end power and bad for top end and vice versa). After the merge at the collector the flow most depends on least back-pressure, back-pressure can come from too small of a pipe or even changes in the pipe (think hollowed out cats) where the velocity of the gas is reduced and then has to speed back up. After all of the gas streams come together other than noise there is no reason for any back-pressure but there are also not many chances to improve flow so the c5 with its efficient cat back (for stock displacement and rpm) normally do not benefit from improved cat back systems.

 

Steveturbo, thanks for that input. I never thought about measuring the back pressure.I assume you just put a pressure gauge tapped in with a sensor?

MTO, there is my SC brother :-) So, basically what you are saying is if I keep my long tube headers on the car, anything after those headers will really not effect the performance of the engine or the car? If so, then I could put on a straight pipe or even leave it open headers and it wont effect anything? Is this what you are saying?

steveturbo, is that what you are saying too?

 

Very clear explanation!

 

I wont say it has no effect. But it certainly isnt going to have such an impact that you will hate the car.

I tapped and fitted a piece of brake pipe to dissipate heat then routed a line to a pressure sensor to log via my ecu. It was attached just after my merge collector.
But you could just hook it up to a gauge too.

 

Ok guys now here is a thought I was just curious about (though I am sure it has been covered a few times in the other fourms), and it isnt about any doubts, but more of just understanding and making sure that after my new knowledge I use it correctly. Anyways, what about the C6 stock exhaust? Doesnt that car have an exhaust that adjusts (opens and closes) to the way the car is being driven? I know that on my older Yamaha R1 (japanese sports bike - aka rice rocket for those who arent familiar with it) had the EXUP system, which allowed the bike to adjust the exhaust to actually have a better response through the dreaded low and mid range that alot of sports bikes have problems with.
So, just wondering, if this the case (that they are using exhaust adjusters) to optimize performance, then if I open up the exhaust (basically my idea was to run without a muffler, or just a straight through muffler like a glass pack) would it hurt the low and mid range power?

And if adjusting the exhaust is better for the performance with the low and mid range, is there a way to make something like this (though I am sure the one on the C6 is computer controlled, but I am pretty sure the one on the Yamaha R1's EXUP was throttle controlled - by wire)

Thank again for the education.
As you can tell, I dont look at things 1 or 2 dimensional, but try to get the whole picture, looking at it from every possible angle. This way I understand the whole thing, and not just 1 application.

 

less back pressure is better in general but flow is the goal and once you get past the merge at the collectors you could just dump the gas and be good (noise would be an issue) my post is in responce to "do they need back pressure?" and the answer is no, not any more than is required to get good exhaust scavenging. what confuses most people is that the primary tubes that are optimal for lower rpm scavenging tend to create more back pressure at higher rpms due to design considerations. so a lot of people will try to tell you that more back pressure makes more low end power where the truth is that a opimal system for making low end power is more restrictive and creates more back pressure. the result is that adding back pressure for no reason is never good for any engine so free flow as much as you can afford/stand.

 

Thanks MTO.
And thank you to all that have responded.
I think I have a very clear understanding of everything
(and even about the C6 adjustable exhaust based off the info MTO just posted)