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It was full 3" dual all the way back to the the mufflers. It just necked down to 2.5" at the muffler inlets. I figured that by then the gas had probably cooled enough (i.e. lost some velocity) that the neck-down didn't really create much additional backpressure.
Kind of how I feel about my 2.25s... Granted I still melted my rear bumper a little with them so how much it cooled IDK. Sounds fantastic though. More or less glass packs. Not great for flow but keeps it reasonably quiet until you stab it....
I love Engine Masters! I've thought about paying for MT On-Demand just for that one show. I'm honestly surprised the decrease was just 20hp with dual 2.5", and that was a looooong exhaust. Granted, there are probably diminishing effects after a certain length because of the exhaust gases cooling.
A single 3" is about comparable to dual 2.5", and my engine has about 2/3 the power potential, so I'd estimate much less power lost - not the ~40-50HP my backpressure measurements indicate. Granted, mine narrows down to a single 2.5" for a foot or two after the cat.
Originally Posted by MatthewMiller
It was full 3" dual all the way back to the the mufflers. It just necked down to 2.5" at the muffler inlets. I figured that by then the gas had probably cooled enough (i.e. lost some velocity) that the neck-down didn't really create much additional backpressure.
It was kind of loud, honestly. More than I would have predicted for a car that had cats and mufflers. Granted it had no resonator anymore. That said, it was one of the quieter cars at any competition where lots of musclecars gathered. I have friends with recent-year Camaros, Mustangs, and Corvettes that have aftermarket exhausts (some with headers) that were all louder. My car was considerably louder at WOT as it approached peak torque. That was where the "x" seemed to "come on song," and it made a wonderful wail. That was never an issue in normal street driving. What was an issue was drone on the highway at ~70-76mph. For whatever reason, the Corsas either didn't attenuate it at all or they didn't attenuate it enough. Once you get rid of a center resonator and add long-tube headers and such, you've changed enough things that the tuning resonance may move. That's what I assume happened.
I agree that exhaust has cooled enough by the mufflers to reduce the importance of pipe diameter.
Hmm. My car doesn't have a resonator either (I don't think it came with one from the factory?). Since I put the headers and 3" front Y on, it definitely screams when I get up to the torque peak. It sounds glorious, but there are definitely times I wish it were quieter. Man, I really don't want to have issues with drone. Being able to (relatively) comfortably daily this car is more important than eking out every last horsepower. If the issue really is just the size of my exhaust I might have to go with an electric cutout I can open when the speed is worth the noise. Although that will mess with my tune and make it run lean every time I do that... argh.
Edit: What trans and rear end do you have? Mine's got the ZF6 and 3.33, and I definitely notice some drone already at that awkward spot between 5th and 6th gear. Maybe 2000-2500 rpm and around 55mph. Especially when going up a slight grade.
Originally Posted by MatthewMiller
So the one flaw in that test is that they didn't adjust the carb jetting to have the same a/f for both exhaust systems. This would not happen in any car with a MAF because the engine would directly measure the amount of air going in and adjust fuel accordingly. I'm a little less sure about a speed/density setup, but I believe it would still account for the change in airflow using the MAP sensor. My point is that there would be less actual difference between the bigger and smaller exhaust setups than they found. The other point, of course, is that if a 620hp engine with 3" duals has virtually no power loss compared to open headers (as they found), then 3" dual is more than good enough for any 400hp (or in the OP's case a 300hp) engine.
Originally Posted by sstonebreaker
Actually, they state often on that show that they optimize the carb tuning for best horsepower on each run. That's their stated reason for usually running carbs instead of EFI - they're all old skool guys more comfortable with tuning carbs than EFI.
But as you noted, the main reason I posted that link was because if a 600 hp big block gets by just fine with 3" duals, then there shouldn't be an issue with his exhaust.
I still think he needs to address that cold cylinder, though.
Originally Posted by MatthewMiller
I just saw that in that particular episode they compared the a/f of the small exhaust to open-header-with-18"-collector-extension and with the small exhaust system it ran considerably richer (jump to 5:55 in the video). And that was the run whose power level they used for the comparison. So for whatever reason, they didn't optimize the jetting for that run, and that a/f difference would have accounted for at least some of the power loss.
Definitely. To drive that point home even further, they had a waaay longer exhaust than a C4 would have, so with a shorter exhaust system and proper a/f adjustment, they probably would have found smaller differences for the 2.5" system too. On a 400hp engine, even a 2.5" system would probably have little loss. But there's no downside other than cost in going with a 3" system, so you might as well go that route if you have the option.
Absolutely.
I noticed that too. I think that one may have slipped past them. I believe they've also said they run carbs to make changing setups quicker.
Speed density automatically compensates for changes in airflow to some degree, but it wouldn't catch that. It'll use the O2 to add/pull fuel as needed in normal operation, but it won't pull fuel at WOT - it will add it if it notices the car running lean in normal operation, but it will never pull it. That said, it's easy enough to tweak the VE tables to correct AFRs at WOT. The MAP is just used as a lookup (combined with RPM) to get a value from the VE table, which tells it how much air the engine is pumping.
As discussed above, I think loudness/tone is also a possible downside of a 3" system, depending on your personal taste.
I'll get some pics of the plugs and leakdown results as soon as I can. Possibly compression test too. I recall that a few years ago I ran it around the block with almost no timing for an experiment, and by the time I got home the headers were glowing. It's entirely possible I torched something when I did that, although the engine feels pretty healthy down low.
Last edited by C4ProjectCar; Aug 19, 2020 at 10:31 AM.
if a 600 hp big block gets by just fine with 3" duals, then there shouldn't be an issue with his exhaust.
I still think he needs to address that cold cylinder, though.
Are you saying his single set up is fine? He measured 7-8 PSI worth of back pressure. I think that is an issue. If that were my car? 7-8 PSI in the exhaust would be my first priority.
I recall that a few years ago I ran it around the block with almost no timing for an experiment, and by the time I got home the headers were glowing. It's entirely possible I torched something when I did that, although the engine feels pretty healthy down low.
Headers glow b/c more of the combustion event is happening IN the header. It's not making the combustion hotter or hurting the engine components. You may have a mechanical problem....but not likely from that.
Are you saying his single set up is fine? He measured 7-8 PSI worth of back pressure. I think that is an issue. If that were my car? 7-8 PSI in the exhaust would be my first priority.
The back pressure's definitely an issue, you're right about that. But I don't think it's primarily an issue of size or flow capacity. The charts say he's right on the edge of the system's capacity, so 7-8 psi seems excessive if his exhaust system were functioning at full capacity. I think a more likely issue for the majority of the back pressure is a partially clogged cat.
I think the cold cylinder needs to be addressed first. If his cat IS clogged, what clogged it? A screwed up A/F ratio could do it. So if he doesn't fix the cold cylinder, he runs the risk of torching his next cat.
Headers glow b/c more of the combustion event is happening IN the header. It's not making the combustion hotter or hurting the engine components. You may have a mechanical problem....but not likely from that.
No risk of a burned valve or something?
Originally Posted by sstonebreaker
The back pressure's definitely an issue, you're right about that. But I don't think it's primarily an issue of size or flow capacity. The charts say he's right on the edge of the system's capacity, so 7-8 psi seems excessive if his exhaust system were functioning at full capacity. I think a more likely issue for the majority of the back pressure is a partially clogged cat.
I think the cold cylinder needs to be addressed first. If his cat IS clogged, what clogged it? A screwed up A/F ratio could do it. So if he doesn't fix the cold cylinder, he runs the risk of torching his next cat.
I've been messaging with another forum member with a similar setup who's had the same issue with a cold #1 primary. He said he's tried swapping the injector, the plug wire, etc. without making any difference. I'll leakdown/compression test it, and if the car is still slower than it should be after fixing the exhaust issue I'll dig deeper. I'm not counting out an issue with the #1 cyl though - the driver side bank seems to be running a bit richer than the passenger side. I just want to sort out this exhaust issue to see where that gets me first.
If my cat is clogged, it's probably from me learning how to tune on this car I know for a fact my AFRs were super screwy. Especially with the crappy conditions the car's been exposed to: a 10min, 3mi commute every day for a year with a 20-30°F cold start every day throughout the winter. This engine has LIVED in open loop without me having tuned open loop fueling.
I don't think you'll burn a valve by running retarded timing....although I can't say I've ever held it WOT with deliberately retarded timing, either.
Originally Posted by sstonebreaker
The back pressure's definitely an issue, you're right about that. But I don't think it's primarily an issue of size or flow capacity. The charts say he's right on the edge of the system's capacity, so 7-8 psi seems excessive if his exhaust system were functioning at full capacity. I think a more likely issue for the majority of the back pressure is a partially clogged cat.
I think the cold cylinder needs to be addressed first. If his cat IS clogged, what clogged it? A screwed up A/F ratio could do it. So if he doesn't fix the cold cylinder, he runs the risk of torching his next cat.
Copy that. I got you now.
.
Last edited by Tom400CFI; Aug 19, 2020 at 11:40 AM.
I've been messaging with another forum member with a similar setup who's had the same issue with a cold #1 primary. He said he's tried swapping the injector, the plug wire, etc. without making any difference. I'll leakdown/compression test it, and if the car is still slower than it should be after fixing the exhaust issue I'll dig deeper. I'm not counting out an issue with the #1 cyl though - the driver side bank seems to be running a bit richer than the passenger side. I just want to sort out this exhaust issue to see where that gets me first.
If my cat is clogged, it's probably from me learning how to tune on this car I know for a fact my AFRs were super screwy. Especially with the crappy conditions the car's been exposed to: a 10min, 3mi commute every day for a year with a 20-30°F cold start every day throughout the winter. This engine has LIVED in open loop without me having tuned open loop fueling.
Well, a fairly cheap way to test if the cat is clogged would be to buy/fab a test pipe. Weld some flanges on a piece of exhaust tubing and bolt it in place of the cat.
Well, a fairly cheap way to test if the cat is clogged would be to buy/fab a test pipe. Weld some flanges on a piece of exhaust tubing and bolt it in place of the cat.
Fair enough. I'll pull the cat, and if it's not obviously visually clogged, I'll have an exhaust shop fab me up a test pipe to match. Don't have a welder sadly.
Why can't you just unbolt the cat flange, move it aside/out of the way and tie it w/coat hangars....go run the car for a 5 min test? Is the front of cat welded to the Y pipe? I can't remember....
Why can't you just unbolt the cat flange, move it aside/out of the way and tie it w/coat hangars....go run the car for a 5 min test? Is the front of cat welded to the Y pipe? I can't remember....
Front of the cat isn't welded, but my car doesn't have its inspection sticker so getting pulled over would be big $.
Why can't you just unbolt the cat flange, move it aside/out of the way and tie it w/coat hangars....go run the car for a 5 min test? Is the front of cat welded to the Y pipe? I can't remember....
With the test pipe, you could also determine how much of the back pressure is coming from the cat and how much is coming from the rest of the exhaust system.
If your an engineering student, you can probably do a better version of this than me.
One generic way to estimate the required pipe diameter at different locations is using horsepower, and a typical brake fuel specific and air fuel ratio to calculate the mass flow through the engine and exhaust.
You could actually use your own MAF gr/sec data if you have it?
Using temperatures at the various locations you can figure the cfm's at that location, and using a typical velocity, can figure the required diameter.
I just used the density of air, not combustion gases, because I'm lazy and can fit an equation to an air density/temperature graph.
I am ignoring the mass of fuel also.
Most values I have read for desired exhaust velocity are in the 300 to 350 ft/sec range, and most OEM exhausts I have looked at seam to be designed around 340 ft/sec.
Converters light off at about 1000 deg and combust more heat back into the outlet.
I've have used pyrometers in my own exhausts and the other temperatures are typical for what I have seen.
Using about 325 ft/sec exhaust velocity and 245 hp for the stock L98, the values I get are pretty close to the stock exhaust dimensions.
Using 325 ft/sec exhaust velocity with the 400 hp your engine may be capable of.
This isn't meant to be the perfect way, but a decent estimate
If your an engineering student, you can probably do a better version of this than me.
One generic way to estimate the required pipe diameter at different locations is using horsepower, and a typical brake fuel specific and air fuel ratio to calculate the mass flow through the engine and exhaust.
You could actually use your own MAF gr/sec data if you have it?
Using temperatures at the various locations you can figure the cfm's at that location, and using a typical velocity, can figure the required diameter.
I just used the density of air, not combustion gases, because I'm lazy and can fit an equation to an air density/temperature graph.
I am ignoring the mass of fuel also.
Most values I have read for desired exhaust velocity are in the 300 to 350 ft/sec range, and most OEM exhausts I have looked at seam to be designed around 340 ft/sec.
Converters light off at about 1000 deg and combust more heat back into the outlet.
I've have used pyrometers in my own exhausts and the other temperatures are typical for what I have seen.
Using about 325 ft/sec exhaust velocity and 245 hp for the stock L98, the values I get are pretty close to the stock exhaust dimensions.
Using 325 ft/sec exhaust velocity with the 400 hp your engine may be capable of.
This isn't meant to be the perfect way, but a decent estimate
Wrong type of engineer but I'd agree those numbers look ballpark. Now do it again with mixture properties. (as my professor would say on an exam...)
As promised. Even hits around 4k produce no visible difference in the values here. 3000rpm was the max. The first 8 seconds are 1400 high idle cold. Obviously there will be a tad more under load with higher egts but mine is not long enough to drive with. Lunch breaks are fun.
If your an engineering student, you can probably do a better version of this than me.
One generic way to estimate the required pipe diameter at different locations is using horsepower, and a typical brake fuel specific and air fuel ratio to calculate the mass flow through the engine and exhaust.
You could actually use your own MAF gr/sec data if you have it?
Using temperatures at the various locations you can figure the cfm's at that location, and using a typical velocity, can figure the required diameter.
I just used the density of air, not combustion gases, because I'm lazy and can fit an equation to an air density/temperature graph.
I am ignoring the mass of fuel also.
Most values I have read for desired exhaust velocity are in the 300 to 350 ft/sec range, and most OEM exhausts I have looked at seam to be designed around 340 ft/sec.
Converters light off at about 1000 deg and combust more heat back into the outlet.
I've have used pyrometers in my own exhausts and the other temperatures are typical for what I have seen.
Using about 325 ft/sec exhaust velocity and 245 hp for the stock L98, the values I get are pretty close to the stock exhaust dimensions.
Using 325 ft/sec exhaust velocity with the 400 hp your engine may be capable of.
This isn't meant to be the perfect way, but a decent estimate
Awesome idea! I love when people dig into the science and equations a bit more like this. I'm focusing on computational fluid dynamics, so this is kind of thing is right up my alley.
I started doing something similar a couple days ago but gave up after an hour or two. Couldn't find the surface roughness of exhaust tubing, and calculations were getting a bit complicated in Excel. No MAF, but I wrote a script that imitates the ECM airflow calculation as part of a tuning calibration tool.
I emailed a couple cat manufacturers to get a loss coefficient for a 3" cat, so maybe once they get back to me I'll try to make a script in Python.
Thanks for running the numbers for me. Is that 325 ft/s at the exhaust port? I assume it must be, and then you use density to adjust velocity along the system.
As promised. Even hits around 4k produce no visible difference in the values here. 3000rpm was the max. The first 8 seconds are 1400 high idle cold. Obviously there will be a tad more under load with higher egts but mine is not long enough to drive with. Lunch breaks are fun.
Interesting, thanks. I think being under load would make a big difference (higher EGTs and higher VE), but even still you're a far cry from 8psi. I'll have to rev mine to 3000 in neutral and see how it compares.
Interesting, thanks. I think being under load would make a big difference (higher EGTs and higher VE), but even still you're a far cry from 8psi. I'll have to rev mine to 3000 in neutral and see how it compares.
My light load vs open map VE values are very close and produce roughly the same AFRs. GM spec is at idle.
I just realized what I said may not make sense... so I am using a stock unported CFI manifold. Meaning port velocity through it is crazy high. Now my overall VE is substantially lower than yours but remains pretty flat through light tip in... it is odd. Seeing the port difference helps understand why it is so though. My upper RPM ve table stays flat from around 4k up to 6500... that is due to the mismatch of parts. Probably should have clarified that before I said that.
From: Las Vegas - Just stop perpetuating myths please.
Well your doing a lot of assuming for the gauge reading. Can't you use a fuel pressure gauge for a carburetor? Home Depot has a little 2" 0-15psi gauge for $9. I'm sure you could adapt it to your use for testing.
As for 4" connections on a cat I had a exhaust specialty shop cut and weld 4" connections on mine but this was in San Diego, CA. Actually they said open them up but I think they cut and welded the new connections on. I'm sure if you search your area you can find similar shops.
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Slideshow: A Jaguar designer's personal project imagines what a modern front-engined Corvette might look like if Chevrolet revisited the golden age of the Stingray.
I know for a fact my AFRs were super screwy. Especially with the crappy conditions the car's been exposed to: a 10min, 3mi commute every day for a year with a 20-30°F cold start every day throughout the winter. This engine has LIVED in open loop without me having tuned open loop fueling.
(as my professor would say on an exam...)