Back pressure in N/A cars
08-31-2009, 05:11 PM
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Back pressure in N/A cars
So hi there, after talking about getting a dual resonated midpipe with my friend. He told me becareful because i might loose Back pressure since i have a N/A car. I have no idea what he meant, so i googled it. Some say back pressure is good, other say its just a ricer gimmick. Hence the question will a dual resonated with a borla catback makes me "loose" back pressure?
08-31-2009, 06:07 PM
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So hi there, after talking about getting a dual resonated midpipe with my friend. He told me becareful because i might loose Back pressure since i have a N/A car. I have no idea what he meant, so i googled it. Some say back pressure is good, other say its just a ricer gimmick. Hence the question will a dual resonated with a borla catback makes me "loose" back pressure?
09-01-2009, 12:36 PM
#7
to the OP: we have side exhaust ports here and no overlap, back pressure and helmholtz resonances shouldn't play a big role. Just remove any kind of restriciton
09-01-2009, 02:18 PM
#11
Getting a 3in header as well is another story
Anyway the OP was talking about back pressure but Ross' answer is the one that fits better imho.
On a street driven car it usually ends to be a matter of personal preference!
Anyway the OP was talking about back pressure but Ross' answer is the one that fits better imho.
On a street driven car it usually ends to be a matter of personal preference!
09-02-2009, 01:15 PM
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You NEVER want backpressure! Show me a race car that utilizes it and I'll show you a car that's never won. Here's what's going on and why people get confused.
As we all know different rpms and loads (which effect VE) will move different amounts of air. As a generalization the higher the rpms, the more air we move in and out of the engine. Any given pipe size can only flow so much air per any given unit time before pressure inside it rises to the point that flow is being severely impeded. When this happens, a larger pipe size is needed. At idle there isn't as much air flowing through the system and there is at 9000 rpm. This is pretty obvious. We want the pipe to be as small as possible while still being able to do it's job without restriction.
At idle, since there is relatively little flow, we really don't need a large pipe. You could have a very small one and have no appreciable "backpressure". However our engines don't stay at only idle speeds. We can see that a pipe size that is small that has good exhaust velocity for idle speeds is going to have problems when we raise the rpms. We need a larger pipe. At 2000 rpm it needs to be larger. Depending on the pipe diameter at this rpm, it too will be too small at some point. The same holds true for higher and higher rpms. We typically size our exhaust based on the absolute max flow through it which means at redline. Everywhere below that is the compromise.
High exhaust gas velocity allows for greater scavenging (Typically. The Renesis is different). Greater scavenging allows for a higher VE which means more power and/or better economy. The generally accepted max gas speed is .6 mach. This is an average speed though as gas flow is not a constant but rather a set of pulses. The above number is a subject of debate though.
If we have a pipe sized for high gas velocity at 9000 rpm, we can see that anywhere below this rpm is going to mean slower and slower velocities. At some point there is no gain whatsoever. When people say to reduce the size of the pipe to give backpressure so you can pick up low end power, what is really happening is that you are raising the velocity of exhaust gasses at lower rpms. The downside is that you choke off the top end. Yes you are increasing exhaust pressure but it's not the pressure that's giving you low end power. It's velocity. Pressure is killing power up high. Now when someone says backpressure is needed for low end power, you can correct them.
Keep in mind I have greatly simplified things. There is so much more to exhaust than just this. I've ignored lots of stuff such as tuned lengths, wave reflections and what kinds there are, temperatures, etc...
As we all know different rpms and loads (which effect VE) will move different amounts of air. As a generalization the higher the rpms, the more air we move in and out of the engine. Any given pipe size can only flow so much air per any given unit time before pressure inside it rises to the point that flow is being severely impeded. When this happens, a larger pipe size is needed. At idle there isn't as much air flowing through the system and there is at 9000 rpm. This is pretty obvious. We want the pipe to be as small as possible while still being able to do it's job without restriction.
At idle, since there is relatively little flow, we really don't need a large pipe. You could have a very small one and have no appreciable "backpressure". However our engines don't stay at only idle speeds. We can see that a pipe size that is small that has good exhaust velocity for idle speeds is going to have problems when we raise the rpms. We need a larger pipe. At 2000 rpm it needs to be larger. Depending on the pipe diameter at this rpm, it too will be too small at some point. The same holds true for higher and higher rpms. We typically size our exhaust based on the absolute max flow through it which means at redline. Everywhere below that is the compromise.
High exhaust gas velocity allows for greater scavenging (Typically. The Renesis is different). Greater scavenging allows for a higher VE which means more power and/or better economy. The generally accepted max gas speed is .6 mach. This is an average speed though as gas flow is not a constant but rather a set of pulses. The above number is a subject of debate though.
If we have a pipe sized for high gas velocity at 9000 rpm, we can see that anywhere below this rpm is going to mean slower and slower velocities. At some point there is no gain whatsoever. When people say to reduce the size of the pipe to give backpressure so you can pick up low end power, what is really happening is that you are raising the velocity of exhaust gasses at lower rpms. The downside is that you choke off the top end. Yes you are increasing exhaust pressure but it's not the pressure that's giving you low end power. It's velocity. Pressure is killing power up high. Now when someone says backpressure is needed for low end power, you can correct them.
Keep in mind I have greatly simplified things. There is so much more to exhaust than just this. I've ignored lots of stuff such as tuned lengths, wave reflections and what kinds there are, temperatures, etc...
09-02-2009, 01:22 PM
#16
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BHR is a great company and Charles has done a lot for the enthusiast rx8 community. Wether it be extensive research and development of better products to helping out the club here with support and wisdom.
I agree his prices are a little on the high end side of things, but the mans gotta make a living some how.
I agree his prices are a little on the high end side of things, but the mans gotta make a living some how.
09-02-2009, 03:07 PM
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You can slap whatever pipe you want on your car, but for me, BHR!!!
09-02-2009, 03:13 PM
#18
Why is re amemiya expensive?
Build quality is important to some of us, that's why we like even BHR's expensive product.
09-02-2009, 03:46 PM
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I am all about quality, so when you say "build quality is important to some of us," please don't think you're excluding me...
I think its great that Charles has done for the rx8 community, really offered specialized product and whatnot, its just the pricing of their mid-pipe that has me wondering why anyone would pick one up. No, I haven't heard it in person, for the record. Personally, a slight tone change in something designed to quiet the exhaust as much as possible, would not be worth an increased price.
How much are BHR's midpipes?
"$425+"
&
"$575+" for the shiny version
Racing Beat offers a dual resonated pipe for $339. Their unresonated pipe is $248. Not only will the quality of product be great and fitment be superior, but you can trust a company who is renowned for excellent products with a history greater than 30 years.
A midpipe is not something that takes a whole lot of development, like many other companies. A simply designed midpipe will net roughly the same amount as a more expensive one. I agree some of the other products are great and prove to be well thought out, it just strikes me that the midpipes seem expensive. Not bashing, I just can't see myself spending $425 (let alone $575) on a pipe that does nearly the same thing as some other products.
I think its great that Charles has done for the rx8 community, really offered specialized product and whatnot, its just the pricing of their mid-pipe that has me wondering why anyone would pick one up. No, I haven't heard it in person, for the record. Personally, a slight tone change in something designed to quiet the exhaust as much as possible, would not be worth an increased price.
How much are BHR's midpipes?
"$425+"
&
"$575+" for the shiny version
Racing Beat offers a dual resonated pipe for $339. Their unresonated pipe is $248. Not only will the quality of product be great and fitment be superior, but you can trust a company who is renowned for excellent products with a history greater than 30 years.
A midpipe is not something that takes a whole lot of development, like many other companies. A simply designed midpipe will net roughly the same amount as a more expensive one. I agree some of the other products are great and prove to be well thought out, it just strikes me that the midpipes seem expensive. Not bashing, I just can't see myself spending $425 (let alone $575) on a pipe that does nearly the same thing as some other products.
09-02-2009, 03:58 PM
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Oh it gets even cheaper, some midpipes can be found for even cheaper. I just used RB in my example because there is no doubt in any informed individual on this forum that Racing Beat is synonymous with quality.
09-02-2009, 04:30 PM
#22
You are right. Anyway you need to understand that some midpipes are "sound" tuned while others are not. Raspyness etc must be all kept into account and producing a single resonated midpipe that is as quiet as dual resonated one means some testing.
I personally built with another mate here a single run of 5 midpipes entirely out of 316 steel (instead of the classic 304 one). Guess what? We ended up paying around 400€ per item, meaning that we had to sell them for at least 500€ to ensure any profit.
Add the bells and the whistles such as the gaskets and the new bolts and you get a further idea of why a midpipe is slightly more expensive than another one of about the same quality
I'm not defending BHR even if do like their products, it's just that at time the production mechanisms can be tricky and hard to understand. I'm sure that Charles will vouch in if it is worth an extra explanation!
I personally built with another mate here a single run of 5 midpipes entirely out of 316 steel (instead of the classic 304 one). Guess what? We ended up paying around 400€ per item, meaning that we had to sell them for at least 500€ to ensure any profit.
Add the bells and the whistles such as the gaskets and the new bolts and you get a further idea of why a midpipe is slightly more expensive than another one of about the same quality
I'm not defending BHR even if do like their products, it's just that at time the production mechanisms can be tricky and hard to understand. I'm sure that Charles will vouch in if it is worth an extra explanation!
09-02-2009, 04:33 PM
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Heh, I know they aren't gouging 8 owners with loads of profit; their margins are probably very humble at that. Just some input considering I see so many people on here bowing down to their products.
09-02-2009, 04:37 PM
#24
Yes sure, is that at times this H4t3rz vs fanbois stuff gets to a point where the main focus is lost
We had some users break their agency powerės welds for instance. Buying 2 pipes is more expensive than buying one! In that case i think that both BHR and RB are on the same quality boat. Also remember that people kept buying Mazsport's dual resonated midpipe, an ugly fitting-poorly assembled ensemble of a midpipe with 2 rb silencers sold at an insanely high price! At least this isn't the case with said products!
We had some users break their agency powerės welds for instance. Buying 2 pipes is more expensive than buying one! In that case i think that both BHR and RB are on the same quality boat. Also remember that people kept buying Mazsport's dual resonated midpipe, an ugly fitting-poorly assembled ensemble of a midpipe with 2 rb silencers sold at an insanely high price! At least this isn't the case with said products!
09-02-2009, 05:05 PM
#25
the OE exhaust manifold outlet is approx. 2-5/8" OD
It's correct that the Renesis performs best with zero backpressure, but there is no real scavenging in a stock-block Renesis due to the zero overlap intake/exhaust port timing. You can't be too big given this fact, but you can be too small ...
the standard piping choices are 2.5" OD or 3" OD, this is why you rarely see anything inbetween them unless it's an imported metric part
It's correct that the Renesis performs best with zero backpressure, but there is no real scavenging in a stock-block Renesis due to the zero overlap intake/exhaust port timing. You can't be too big given this fact, but you can be too small ...
the standard piping choices are 2.5" OD or 3" OD, this is why you rarely see anything inbetween them unless it's an imported metric part