HOOKER CAST TURBO MANIFOLDS VS E-BAY TURBO HEADERS-BOOST PSI VS BACK PRESSURE. BACK PRESSURE VS HP!
2022 ж. 9 Қар.
98 579 Рет қаралды
DO CHEAP, EBAY LS TURBO HEADERS MAKE POWER? WHICH ONE MAKES MORE POWER, CAST IRON HOOKER LS TURBO EXHAUST MANIFOLDS OR E-BAY TURBO HEADERS? HOW MUCH BACK PRESSURE IS TOO MUCH? WILL BACK PRESSURE REDUCE HORSEPOWER? HOW MUCH IS A TURBO CAM WORTH? HOW TO MEASURE BACK PRESSURE. CHECK OUT THIS COMPARISON BETWEEN A SET OF HOOKER CAST-IRON, SINGLE TURBO MANIFOLDS AND THE TYPICAL, CHEAP, E-BAY TUBULAR TURBO HEADERS. WHICH ONE MAKES MORE POWER? WHICH ONE HAS HIGHER BACK PRESSURE? ALSO TESTED STOCK CAM VS BTR STAGE 2 TURBO CAM AND NA VS TURBO ON A 5.3L LS.
It is a pity the overboosting was not resolved for a more fair x psi comparison on both setups throughout the rpm range. And it would be nice to see a pair of factory manifolds with your usual fabbed up Y piece into the turbo to see how that compares with the merged Hooker setup.
Another successful testing session showing the facts! Good stuff and thank you for all your hard work over the years. It definitely gives guys a opportunity to make a informed decision without having to only listen to the guys selling the products, which would be the better choice for each individual based on what their needs or wants are!
This COMPLETELY makes me NOT regret selling my manifold turbo setup. On to better things now!! Thanks for the results!
Finally some more dyno videos! Keep em coming!!
I'd like to see the back pressure with one of the wastegates blocked off on the tubular system tested as well. The change in back pressure and HP in the Hooker system I think is due to the increased back pressure and the wastegate unable to blow off enough exhaust to bring it down. I also expected the cast iron manifolds to retain more heat energy and drive the turbo harder, and I thought we'd see a better boost number lower in the RPM range, and that didn't seem to happen. I'm going to have to watch this one again taking note of back pressure reading line location. Awesome info as always, thanks Richard for your dedication to excellence in automotive performance testing 👍
Very enlightening as always, thanks Richard
Hello like minded people
Wassup dude
When did hooker come out with these?
@@danlyons4602 they have been out. Id like to see the summit racing manifold tested with it dual gates and stainless steel cast
@@Fackler91 I realized after clicking they've been around for a few years. Pretty good option for someone going LS turbo route.
Lol .
Great test Mr. Holdener. The back pressure surely could be preventing the turbine from spinning as fast as it could. It could also be diluting the cylinder mixture and acting as a buffer.
Great information thanks for sharing. I ran a big block turbo car for years with a manifold on one side and a header on the other
Mave my 6.0 ls set up the same way
Higher back pressure results in less exhaust gases leaving the cylinder, which, in turn, results in less intake air and fuel entering the cylinder, and less power output. On a turbo setup, you need exhaust pressure at the turbo to spin it fast enough to create the boost you're looking for, but any other exhaust pressure increase results in loss of power.
CORRECT ✅
Purrrrrfect video...praying this test would go down..thank uuu
Awesome Tech! Thanks Richard!
Well done, and would not have guessed this
thank you mate, the exact vid i wanted to see!
Yeah Richard! you read my mind 👍this will be interesting! Of course my vote and preference is good fabricated long tube but bolt on cast stuff can be great for an easy street set up
I think the real interesting thing to look at would be HP/PSI on each engine across the dyno graph since the boost levels weren't even. That would probably somewhat normalize the results, although not completely. Just jam it into a spread sheet and calculate it out, if the dyno software doesn't.
They were both even at 11psi up top so no need to over think it. The free flowing bigger pipes won.
@@3800TURBO Yes, less backpressure obviously wins up top. That's not the only thing that matters. The boost curve on the manifold setup is screwed up. There's no way it should take 6000rpm to reach peak boost and be down 2-4psi until then.
No YT channels talk about back pressure except for Banks. Awesome data.
I'd be interested in seeing a comparison of those two exhaust setups NA and see how that compares. I'd love to see longtubes (whether fancy turbo headers for a race car or typical LTs) thrown in too, all 3 both NA and turbo.
Would also love to see the stock manifolds vs tubular vs Holley manifolds for a bottom line NA and with boost
Great test... Even thought the Hooker manifolds look good, the tubular headers create lower back pressure.. Tubular manifolds for me...
Thanks Richard, great product review / comparison. One question I have is price vs longevity.
Exactly. It was no surprise that tubular headers make more power, but they are nowhere near as durable as cast manifolds in a turbo setup.
drive pressure being lower or closer to 1:1 with boost pressure is just more efficient on the top end. I'd be interested to see cams with high overlap vs none on high backpressure setups.
Hey Richard, have you ever tested a turbo manifold vs a regular manifold (or headers), on a naturally aspirated engine? I think it would be interesting to see how much power the turbo manifold gives up, just so you can install a turbo.
Richard is the GOAT. Period. Testing and results. Nobody can compare.
Myself and many others running the hooker setup would really like to see these things pushed further to see data and their limits. I’m making around 960whp with them and I’ve seem to basically hit a wall for power when everything else is good for 1300+. Thanks for the videos.
If you've got an extra input it's not too hard to add a sensor for it. I'd be really curious to see that measured at various parts of the manifold, like at the turbo, before the crossover, etc.
If you've got an extra input it's not too hard to add a sensor for it. I'd be really curious to see that measured at various parts of the manifold, like at the turbo, before the crossover, etc.
Steve Morris has a vid making 1200hp using them so they will make more just need the right setup or base hp to start
Would adding an additional wastegate help, like the tubular system, or alternatively, adding a second turbo and wastegate with it, I'd think you might be pushing the bottom end to it's limits at 1500 HP 😀
I think they got a cork in there some where. Maybe a spot in the casting design that's restricting flow.
Bernoulli's principal. A decrease in pressure = increase in flow. I would guess the decrease in back pressure allowed more efficient spool which allowed the compressor to do I its thing more efficiently. Basically the entire turbo is working more efficiently as the back pressure drops.
Great vid showing how back pressure is not a good thing 👍. I knew it cost power and can hurt boost curves but I didn't know even that little bit of Bp could rob that much power. More air in=more air out=more air efficiently in = more air efficiently out = mo powa!
Now we need a trick flow turbo header test, on a 5.3 preferably
Very interesting vid for sure, keep them coming Richard. Would really like to read the real reason , since I’m also working on a turbo (gen 3 stroker hemi) build ….😉 Kinda new to all this stuff but I think it has something to do with exhaust pulses and wave tuning…. Probably would see the same affect NA then … .
I'm thinking about that time you did manifolds vs headers with a supercharger and saw more boost. That was a weird one. -- Here's my only hypothesis: Because the exhaust gas is under pressure which makes it more dense, the tune frequency for the header is still valid. You still have that good factory intake. The cylinder gases are still getting discharged to a primary tube that likely holds more volume than the cylinder, so the system is still able to resonate and function properly.
Richard had the best tech channel on KZhead
Solid info 💪
SUGGESTION: measure the BSFC of an engine during runs at various throttle settings. Use a Hellion sytem on a Hemi, which uses full-length headers before each turbo. Find which uses the least BSFC, has the lowest EGT and backpressure, etc. Then use cast log manifolds with the exact same turbos.
I would love to see how the cast turbo manifolds like the Summit offering stack up here.
Didn't this video come out a few months back, turbo headers were the best right ! Glad I have mine.
Shit, "common" knowledge in some circles was that the turbo was the biggest restriction and the pre turbo exhaust didnt matter much... Seems there might be some error to that. Very awesome test. I built my turbo setup with that info in mind... Im thinking i may have made that in error now.
Full-Race did a test on one of their 300HP Honda's, and going from Log to Tubular Manifold they picked up 40HP on a little 4 banger. It convinced me to change the header design on my turbo kit.
Thanks for sharing your wealth of knowledge with us.
Thanks Richard! I am curious whether the higher B/P was a function of the Hooker casting restrictions or waste gate (WG) size? Could a "Y" pipe & flange replace the single WG, then 2 WG's be installed on the branches of the "Y" to see if B/P would significantly drop with the increase in "window" of 2 WG's. Any thoughts Mr. R. H.? How smooth are the internal castings of the Hooker manifolds? Has anyone tried porting or Extrude Honing? Thanks, Paul from S. Central Tx.
Nicely done, I think it comes down to packaging and fabrication skill. I would also like to see a test done using stock manifolds as a budget builder would.
THEY WORK GREAT
@Richard Holdener I have a question. I have a silverado with a cammed lq4 that I want to turbo. Eventually I want to go 408 with a big cam. I'm only planning to run 15psi on both setups. The question, is there any benefit to running a vsr80mm or bigger vs 7875 at that boost level?
A twin scroll comparison would be interesting divided vs non divided, single and twin scroll and if it's out there twin turbo twin scroll.
It would need to be done on something other than a v8 where the two inlets can isolate all of the negative cylinder to cylinder interference during each's valve overlap. Having two divided turbos would work on a v8, but only if the runners crossed banks to properly pair them, which is why it's only really done on "hot v" setups with the exhaust in the valley where the intake manifold typically sits. 2 rotor, 4cyl and 6cyl can utilize a (single turbo) divided turbo setup but you won't find any tangible benefit with 8 cylinders since they can't all be meaningfully separated
I wonder if exhaust overlap has an effect. Would the air charge be more diluted at a given back pressure figure if theres more cam overlap? Im going to have to watch the turbo comparison video again. Not as apples to apples as it adds the compressor variable.
If you’ve watch any of these videos before it was pretty obvious which would win. Richard should run both hot sides N/A without a turbo on them. I think it’s obvious the headers would flow more and make more power than hooker set up N/A but it would be kinda interesting seeing what the difference would be. Anyway as always with all his dyno runs the more power N/A the more power under boost.
Would the back pressure on the manifolds come down if you were to make a bypass pipe parallel to the passenger side manifold. So the merge on the passenger manifold would not be a factor anymore
Hey Richard you should try the stock cast manifolds with 2 wastegates and compare to the tubular header manifold
For sure the back pressure made a difference. You reduced the pumping loss on the exhaust stroke for the same air flow on the intake. The question is why was the back pressure less. If you measured exhaust temperature and exhaust pressure at the inlet of the turbine that may help figure out why. A guess would be the headers are radiating more heat than the manifolds and lowering the specific volume of the exhaust gas which will equal less pressure for the same mass flow. Probably more to it, could turn into a science experiment real quick.
I'm wondering if the cast manifolds have a cork somewhere. May be one spot where it's just not large enough. Seen this difference even between different factory manifolds on turbo setups.
@@3800TURBO Thinking out loud - Definitely the contributing factor would be the flow capability of the manifolds vs the headers, but the turbo/wastegate are the governing flow devices, so trying to figure out how the pieces fit together. I did not see where the exhaust pressure was measured, but if it is just before the turbo, then up stream flow restrictions would not directly cause the pressure to be higher in front of the turbo. Since between 6000 and 6500 rpm boost conditions are the same, that would mean that compressor power required is the same and turbo shaft speed is the same. That would also imply that the turbine volumetric flow is the same since turbine speed is the same for both cases. To get the intake conditions to equal in the two cases, the higher pressure and likely higher temperature case would require less turbine mass flow to drive the compressor than the lower pressure/temperature conditions. For the turbine side it is an energy balance, in both cases turbo shaft power would be equal, but the higher pressure and likely hotter air will allow more energy extraction and require less mass flow to get the same shaft power. Long winded way of saying I agree that it is the flow restriction of the manifolds, just trying to figure out the impact to the devices that will most directly impact the back pressure at these levels of pressure and flow.
Thanks for redoing this video. So headers do work with boost just like cams. I would like to know if the NA engine sees a similar power difference with the headers vs the Holly cast manifolds.
THIS IS NOT AN NA EXHAUST
@@richardholdener1727 you could still run them both na tho, please lol
@@justin_parks yeah, He does that a lot. annoying...
Makes sense - back pressure is bad because during valve overlap you'll have dirty air flowing back into the cylinder and intake. Less clean air = less power. Would be interesting to see this test with factory cam with 0 overlap.
Virtually all cams have some overlap, including factory cams. Engines don't function very well with 0 overlap.
Wow .... Great info. It would be neat to see if the cfm was the same on both from 6 to 6.5?
I'm curious on if it's the type of merge they use. I'm running an action turbo manifold that's a stainless 3in pipe basically. Pass side dumps into the same pipe the driver side feeds into. It's on a 5.3 now but I'll be moving to a 403 soon and hoping this setup doesn't restrict my power output.
I would guess the increased back pressure makes the scavenging effect less efficient and leaving more inert gas in the combustion chamber for the next power cycle.
The pressure in the exhaust between the exhaust valve and the turbo is created by the piston pushing the waste gases out on the up stroke. This requires energy from the closed system, more pressure requires more energy meaning less is available for the output. It's a simple conservation of energy equation.
Hey I was thinking it would be interesting to see a comparison between a self tuning/learning system with just basic input like you can do in your garage vs a Dyno tune
Even though there was no change in AFR, is it possible that there was a greater air mass with less backpressure? Boost is a measure of restriction, not airflow. Could it be that the engine became more efficient and the given mass air was achieved with a lower boost number which would mean a greater mass air when boost was matched? I realize AFR didn’t change but is there enough leeway in the Holley controller that 1lb or a 1/2lb of boost was compensated for in the fueling?
I think back pressure accounts for about half of the 6% difference in power between the cast and tubular manifolds. The other half would come from increased airflow and the extra fuel that comes with it.
Great info! Great content, thank you! Any guess of how much more a natural aspirated aluminum headed BB Chevy would weight vs a single turbo inter cooled cast block LS package??
Not sure
Great, great video man! I have to say that area below 3500rpm that you pointed out is important for many of us too when it comes to a street car. At 78mph my vette cruises at about 2k rpm. On my Ford Lightning I love the low down power too. 2. I wonder also if scavenging is improved with the long tubes, helping pull out exhaust gasses using pulses from other cylinders, or is that less important on turbo engines because they already have pressurized intake air?
The higher the pressure of the exhaust gas, the longer the primary tube needs to be to emphasize the exact same RPM resonance, because pressure peaks and troughs travel faster in high-pressure air than in low-pressure air.
it would be interesting to see what it would be if you stuck a extra gate on the cross over of the hooker set up . but no back pressure results in bad turbo lag so the right size header and cross over is important.
I have a stainless steel version of the hooker cast iron turbo header. Got it on ebay, and can't find anymore like it.
Did the log manifold need less fuel per boost pressure than the tubular? Is there a direct correlation between fueling and back pressure? Also with less back pressure, can you run more or less timing?
Could you test the maxspeedingrods log manifold? $79 seems really cheap.
How would truck manifolds compair to those im curious now🤔
My theory: More back pressure means you're having to drive the turbo harder to make the same boost, which means the engine's ingesting the air with more difficulty because of residual pressure from the exhaust stroke where it couldn't escape as easily due to the flow restriction (or lack of scavenging without header primary tubes) a.k.a. back pressure... I talked in a circle.
And higher peak cylinder pressures(theoretically) although as someone else said, there may be a little more residual exhaust diluting cylinder air composition.
Great video and comparison. I'm curious what the heat differential was between the two exhausts closest to the turbine wheel. Possibly the cast iron sapping more heat versus the header transporting more heat to the turbine? Temp probe would be great to debunk that being that heat and pressure are relative.
THE REVERSE WOULD BE TRUE FOR HEAT
I'd also be curious to see the EGT differential between each setup, given that PV=nRT with more back pressure EGT should also rise I would think as long as timing, a/f, etc. stays consistent. Great video Richard!
I can only think this is directly related to pumping losses and the ability to effectively evacuate the combustion chamber. I wonder if less exhaust lobe timing would be better on the hooker setup
That’s a lot higher back pressure than I expected! maybe they aren’t as pretty on the inside lol but my guess the gate is probably the biggest problem and log manifold design’s in general can see some nasty high pressure points. how smooth is the flow to the single gate? I can’t imagine it’s dead consistent
What about temperature difference between the cast iron and the thin pipe. Did temp play a difference?
Yes
Adding a second wastegate on the crossover of the Hooker setup might drop the back pressure quite a bit.
I say this was a pretty perfect example of pumping losses.
Richard now what is the difference with the stock exhaust manifolds here compare to these doing the same tests. I ask because you have uses the stockers more then anything else when you do turbos with LS's. Thanks ✌ 🇺🇸
Yes again it’s an air pump more back pressure will effect flow be a good balance of both for high hp I think
Only thing OTHER than back pressure (and lower VE because less of the gas is able to leave) I can think of is if there was some more heat transfer from turbine to compressor on the cast setup? That seems like a stretch though. Maybe faster turbine speed due to faster boost response also led to a higher efficiency/cooler charge and better VE too? That’s still a knock on back pressure effect though.
NA test is needed with the cast iron manifolds and the turbo headers with Y pipe. I think you'll see a significant difference there and thus one of the reasons for the boosted difference.
How exactly do you run a turbo header na?
@@HerrPoopschitz take the turbo off and just run a regular exhaust pipe off the manifold or headers
@@HerrPoopschitz I seen guys run those turbo headers without turbos and just run the exhaust out the fender
@@jmullis78ify Ah. Goofy lazy people. Gotcha.
@@jmullis78ify You don’t even have to take the turbo off, just disconnect the intake and have the engine breathe atmospheric pressure air. That way the NA combo still has a bit of the back pressure, just not as much as it would under load
Richard, you're using a Holley EFI for the tests right? Why not grab a couple of MAC valves to get turbo control? Once you got the PID figured out you could probably use one of these to really get some some consistent boost levels when testing so long as the wastegates had enough authority. Couold even use shop air to better control it by adding pressure on the gate. Keeping the boost level even would really help on tests like this I think. Wish there was an easy way to measure the CFM into the intke too.
In my mind its the same concept if it were a N/A combo. And you've proven it in other tests where you ran extremely long exhaust tubing. Exhaust restriction robs power whether its attributed to manifold, muffler, or undersized tube dia. they're all causing additional back pressure.
Would be nice to see turbo RPM on each setup
I could go pull out my chemical engineering fluid dynamics book, but....not sure what I could really calculate without more data. Regardless, yes, I could see the pressure difference making up part or most of the power difference. Think about a pressure washer, put a big nozzle on it and it moves the same quantity of water as a small nozzle would, but the engine has to work harder to do it. It isn't a perfect analogy because water isn't compressible like a gas is, but you get the idea. Back pressure indicates restriction to flow, which means more energy required to move the same quantity of fluid.
Ever tested the speedway motors LS turbo kit on a 5.3. Same manifolds with VS 7875
no sir-but I'm sure it works
Pretty awesome test sir. Have you noticed if stock truck manifolds have similar back pressure results as the hooker setup?
THEY MIGHT BE LESS
Richard have you seen the newer Summit LS manifold? It's a true divided manifold with two wastegate locations. May be a good compromise.
Interesting!
Not in price
I think it's a reasonable assumption to conclude that the change in horsepower difference is attributable to back pressure change. To me, it raises the question of does the back pressure make it harder for the piston push against the pressure, or does the pressure make it harder for the turbocharger to push the boost into the combustion chamber, or both?
Fun video. I’m a bit confused about the back-pressure. Was it measured nearer to the head or nearer to the turbo? Ideally we would measure it at both places to evaluate the exhaust manifolds. Higher back-pressure should result in higher cylinder pressure when the intake valve opens. This is going to resist flow and reduce the total mass of air going in, or lower volumetric efficiency. The dyno doesn’t lie, there is less torque, so less air and fuel, assuming something else is not stealing power.
It looked like he had a tube right under the turbine inlet, so it was the pressure driving the turbo in these tests, not the pressure at the ports.
Sooooo. We have always said "just get all the exhaust to the turbo". S log manifold does that. I've Lay's said that if headers make sense N/a, they make sense boosted. So maybe improved scanning is resulting in less backpressure. Help me here
Headers were more efficient at flow. Like you say boost is relative. Air in and air out equals efficiency and power. Take your cake and eat it too. Fun test. Wish the hookers flowed better they're a slick setup. Curious how long tubes would compare to the short tube headers. Also curious if scavenging is still a thing with turbo or if it's mainly flow restriction. And if a collector extension pre turbo would make any change as well. I supose the y pipe counts as a collector extension tho. 😂 As always thanks for your efforts and keep up the good work!
Yes, more back pressure is the reason for less power
Why weren't you using your electronic boost controller? I still loved the test. Restriction/inefficiency is seen by the back pressure developed. I really like the design of the Hooker setup. Hopefully they will create a larger higher flowing version.
Yeah, he has an aftermarket EFDI and I thought he got an electronic controller that stood alone as well. Badly needed for this test :(
Great deal
I feel like back pressure is a large part of it, but with those pressure ratios of the tubular manifolds the turbo is going to be spinning much faster. I think that the tubular manifolds put the turbo more into its sweet spot and that made half the power gain. It could easily have shown LITTLE GAIN or even NO GAIN if a smaller turbo was used that has a different "sweet spot".
Help me understand, I thought that, like for like, compressor wheel RPM had a direct relationship with boost pressure. It seems like this is a cylinder filling/ pressure reversion issue.
@@ThomasCWiley compressor wheel speed is same as turbine wheel speed. The veloctiy (air flow) of the exhaust as it goes from manifolds to downpipe is what determines wheel speed. Backpressure is like boost, it is a measure of pressure and not necessarily airflow. You can have lots of boost and little air flow, and also you can have lots of backpressure and little air flow.
I see what you're saying about the turbo being in the sweet spot, but 11psi is 11psi, and the motor picked up 45hp. 40-50HP is actually a really common gain from what I've noticed, Full Race did a test back in the day comparing a log-style manifold to an equal length 4-1 manifold and their honda picked up 40HP. Gentle curves with merge collectors w/exhaust pulses is always going to be better than having the gases make sharp 90* turns. kzhead.info/sun/h8-IdqZqiWSppJs/bejne.html
More back pressure reduces air flow through engine due to less favourable pressure ratios and also causes higher pumping losses as piston works harder to push the exhaust out
I think the higher backpressure creates lower HP levels through charge dilution. The less backpressure, the more spent gasses you can get out of the cylinder, letting more of a fresh charge in. I also think the Hooker system is a great system for a street bound Turbo LS set up. It's much easier to package in a street car. Now if you're running at the track, and looking for all the horse powers, then you would want the tubular, but in a race car you're less worried about how it looks, cutting the car up to fit it, etc. One thing I wonder though, if you plumbed both into a full street exhaust system, how much would that of leveled the playing field for this test?
hello great content not sure if u have anything on manifold pressure vs turbo charge pressure and pressure drop from intercooler, would like to see power band differences on turbos with proper back housings but wrong front housings ive seen 8 psi of manifold pressure and 20 psi at the compressor housing..
how can you have a 12 psi drop across a core?
12 psi drop was not across the core only this was single turbo coyote with on3 turbo 2.5 inch pipe from turbo to intercooler i tried different intercooler still same pressure drop i was getting reading from port at compressor also waste gate 12 psi spring opened to early only giving 4 psi at manifold so i had to switch it to manifold signal i am still working on tuning this car on the dyno seems to have falling powerband.
One interesting comparison may be spool time, if the Hooker manifolds get a turbo moving from idle quicker (IE: gaslight stomp).
It will largely depend on the total exhaust system's volume, and a bit on how much heat is lost. You can size the x-over on the Hooker much smaller than you typically see (2-2.25") and quite possibly have better transient response. The headers also have much more surface area to lose heat, which Richard has shown will slow turbo response.
@@hydrocarbon8272 The cast will get hotter and hold way more heat then stainless tubular. Think of a cast iron skillet.
It is obvious that the tube headers are a more efficient design. You mentioned that the tube headers had two wastegates and the manifolds had one. If the manifolds had a second wastegate installed in the cross-over pipe, would this reduce back pressure some and bring the HP closer to that of the headers? There are two advantages that I think the manifolds should have, one is reliability, two the exhaust should retain more heat for better turbocharger function. But I realize that these two advantages may not overcome the flow advantages of the headers.
Seems to me they both controlled boost fine but the headers got up to pressure much faster and with much less restriction. More of the kinetic energy got to the turbo to spin it harder instead of turning into heat and pressure
What A/R exhaust housing was that Precision turbo? .81 or .96 ?
Yes back pressure is power
I have gone to a bigger sized AR and gained similar power with everything else being the same so I would say sure the power comes from the reduction in back pressure.
Backpressure definitely makes a difference, but not all that HP is gained just from that. I think probably 50% of the HP gains is from scavenging increase front the longer tubes and smoother air flow.
I think the lost hp could have been caused by the pumping loss of the piston pushing the exhaust out of the engine due to higher back pressure. But that is a pretty obvious reason so maybe I'm wrong.
Normally they would say send it to Richard but now, Mr Banks, has done this and calculated the hp that is lost by back pressure ratio.