Does a turbo restrict your engine's rev??

Hey, does a turbo kind of restrict your engine from revving smoothly and quickly?? because since its in the exhaust path and needs to be spooled wouldn't make your revs slower instead of going super-fast like a race car??

no, since turbos produce boost, and therefor increase power they give better acceleration, of course, there is added resistance to the exhaust gasses that leave the engine and enter the turbo.

for a completely smooth climb through the revs you want a supercharger, since they are crank-powered they increae boost in direct relation to the crank's speed, so at 6,000rpm a supercharger will produce twice as much boost as it would @ 3,000rpm.

turbos however can reach full boost at low rpm, this is why you get turbo lag, as you floor the accelerator, you are making the engine accelerate and it produces more exhaust with more revs, so when there is enough exhaust gas pressure to get the turbo's exhaust turbine spinning fast enought it creates boost, as it hits full boost the engine's acceleration climbs much more steeply, if you look at the dyno chart of a turbo engine you'll se where in the rpm range the turbo kicked in, after that point the line indicating bhp becomes much more steep.

so a simple answer to your question is, no, not on the whole. you'll get lag where the engine is outside the boost threshold (the revs where the turbo does not create boost pressure) but after that the engine will accelerat much faster, but it wont feel as smooth as the power delivery of a natually aspirated engine.

No, Simply No.

however, 2.2striaghtsix...
a supercharger produces the same boost at all RPM's, because as it flows more the engine flows more.

Turbo's dont get their energy from exaust flow. they get it from exaust heat.

turbo lag is due to the ammount of time it takes for the engine to produce enough hot gasses to spool the turbo to full boost. turbos dont limit the RPM either. that is what wastegates are for, to keep the turbo from over spooling, however there is a point where a turbo cannot keep the required flow to make boost so a bigger turbo must be used which will spool at a higher RPM...but we are talking 10k RPM plus where a turbo that can spool at an acceptlby low RPM at boost for a daily driver would no longer be able to flow that same boost at extremely high RPM's...and aside from modded rotaries (which the 13BTT is a sequincal setup) there arent many daily drivers that can sustain 10,000 RPM.

No, Simply No.

however, 2.2striaghtsix...
a supercharger produces the same boost at all RPM's, because as it flows more the engine flows more.

then wouldnt you get the same amount of boost @ 3,000rpm as you would at 9,000rpm ? yet looking at dyno charts of s/c'd cars you can see the supercharger's deliver is in a constant relation to the engine revs.

what i mean is the gradient stay pretty much (only tiny amounts out) with an equal gradient throughout the rev range.

so for example: @ 3,000rpm it produces 5psi
@ 6,000rpm it would produce 10psi

because unlike a turbo it is powered off the crank, so the ratio of crank rpms to s/c pulley rpms would stay the same through out the rev range. like a 1:3 crank-to-pulley ratio.

so if the crank does 10rpm then the pulley (an s/c) does 30rpm
if the engine does 20rpm, the pulley does 60rpm and so on.

right ?

yes but at 3k rpm an engine flows X amount and at 6k rpm it flows 2X amount. So even though the SC is pushing twice as much air, the engine is consuming twice as much air.

I dont know a lot about sc's so i may be wrong, but that is what logic would dictate.

yes, correct. however you have to remember that the supercharger flows, say 100lbs/min of air at 1000 RPM, at 2000 RPM it will flow 200lbs/min (not nessicarily but hypothetically speaking if you have a completely linear power curve). The engine will inturn take in 100lbs/min at 1000 RPM and 200lbs/min at 2000 RPM. so boost stays linear.

Flow doubles (hypothetically) pressure stays constant.

you are thinking as if the engine where flowing no air, inwhich case you would be correct. but making power isnt about making boost, its about flowing air and burning fule.

Turbo's on the otherhand, flow acording to turbine RPM and what the engine is flwoing at that moment. all boost is is air that has built up and has no where to go.
once enough continuously expanding and then rapidly colling exaust gasses are being spent by the engine the turbine begines to turn fast enough to make a difference in manifold presure. (rapidly cooling causing more delta pressure drop through the exaust which is why post turbo exaust should be as large as possible, causing a greater pressure drop and pre turbo should allow for adiquite flow and maximum velocity)

once the turbo is completely spooled to your X ammount of desired boost the turbine RPM is held at a constant with a waste gate (or in the case of a vaible geometry turbo the turbine rearanges itself to not take maximum advantage of the exaust passing by it).

but lets say the engine's throttle is snaped shut, causing the engine to no longer take in the flow it was before, but theturbine is still spinning at 100,000 RPM or w/e it was to create the boost before.
This is where one of two things happen.
1 (idealy), you have a BOV or recirculation valve. the excess pressure that was suddenly built up keeps the turbine spinning as fast as possible for complete spooling to be as quick as possible when the engine goes under load agian and starts flowing more air.

2 (the way to fuck your turbo up, excuse my french), you have no way to vent excess pressure so... the built up air stops flowing completely and goes the other direction...causing turbo surge...the turbing spins in the opposite direction makking spooling when the engine goes back under load take twice as long.

superchargers CAN use BOV's... makes RPM drop much slower. the throttle plate restricts flow and excess pressure is built up. this excess presure makes it harder to turn the supercharger, if vented, the load stays constant and the RPM's drop more slowly making the engine stay in the power band longer during shifting to make for a smoother, more linear acceleration.

Turbos use all forms of energy released from the engine out of the exhaust to generate flow. AKA exhaust velocity, heat, noise, anything that can turn that impeller.

Superchargers produce a set amount of boost dictated by pulley size. The supercharger woudl have to spin a percentage faster at all RPM to create more boost. By spinning faster, they only create more flow which is inhaled by the engine. So the same amount of boost exists in the intake manifold at all times due to set flow from teh compressor and set consumption by the engine. You have to increase flow at all RPM to increase boost at all RPM. No way to do that by just raising RPM on both of them. RPM has to increase on teh compressor only. Simple gearing/pulley sizing. Mechanics 101

The revving question depends on what size turbo, exhaust design, ect. Generally a turbocharged engine will be laggy down low on revving but it will at one point in the RPM range rocket quicly.

thanks^

---My understanding (and personal experience) is that-- with a given cam, a properly sized turbo, compared to a N/A induction, will widen the powerband. The power will come in a bit lower, and it will continue a bit higher in the rev band.
---My possibly defective understanding about blowers (Rootes type) is that the boost as seen on the manifold pressure gauge stays more or less constant, at wide open throttle, thru the RPM range.
---Centrifugal belt driven superchargers, like Paxton and Vortech use a drive that is off the crank. This type of supercharger makes boost as follows: doubling the engine speed increases boost pressure four (4!) times. So, if at at 6,000 RPM you are targeting 8 lbs. boost, then, at 3,000 RPM you're only going to get 2 lbs. boost. This is assuming there is no blowoff valve in use to dump boost over a certain value. You'll also never see this truth mentioned in "Hot Rod" or the like, as they are only interested in hyping their advertisers product. These work well at the drag strip, or at the car show, in amateur built big engined muscle cars that don't need any more midrange power anyway, and spend all their time at high revs. Nobody has used this type of supercharger in big money racing classes since the early '60s (the Novi at Indy), since they are not even close to being competitive with turbos.

---My understanding (and personal experience) is that-- with a given cam, a properly sized turbo, compared to a N/A induction, will widen the powerband. The power will come in a bit lower, and it will continue a bit higher in the rev band.
---My possibly defective understanding about blowers (Rootes type) is that the boost as seen on the manifold pressure gauge stays more or less constant, at wide open throttle, thru the RPM range.
---Centrifugal belt driven superchargers, like Paxton and Vortech use a drive that is off the crank. This type of supercharger makes boost as follows: doubling the engine speed increases boost pressure four (4!) times. So, if at at 6,000 RPM you are targeting 8 lbs. boost, then, at 3,000 RPM you're only going to get 2 lbs. boost. This is assuming there is no blowoff valve in use to dump boost over a certain value. You'll also never see this truth mentioned in "Hot Rod" or the like, as they are only interested in hyping their advertisers product. These work well at the drag strip, or at the car show, in amateur built big engined muscle cars that don't need any more midrange power anyway, and spend all their time at high revs. Nobody has used this type of supercharger in big money racing classes since the early '60s (the Novi at Indy), since they are not even close to being competitive with turbos.

with the s/c that would broduce 4 times the boost over twice the engine speed, i would imagine that uses the same sort of pulley system as constant vaiable gearing.

its used on scooters (like moped things) that are "automatic" theres a pulley which is conical in shape, as the engine speed increases so does the belt's speed, and with that it moves further up the cone (towards the wider end) so it changes the pulley/crank ratio.

CBFryman, yea i get what you're saying now, i was thinking along the lines of volume of air flowed an not thinkin' about the pressure, yea but i got it now. thanks.

wait so lets say you install a twin turbo kit on a engine, and you properly size the turbo and tune it, use intercoolers, etc this would actually make your powerband better?? Because I always thought that to some extent N/A engines will sound louder and better, have a smoother constant torque, post better track times, have better dirveability, and less chance of stalling

all depends...no one engine is the same, N/A engines can have a peaky powerband or one flat as a board, if its peaky boosting at a low RPM can flatten the torque curve and therefor the powerband, on the otherhand, an already flat powerband can make its peak jump tramendously and if its flat all the way to below where the turbo spools it will seem like it got peakier but really it just jumped up when the turbo spooled.

ive never heard that about cyntrifical superchargers, and i doubt its true. id research it but im too lazy, care to post some references?

Centrifugal s/c's do exist. lol theyre just like a turbo execpt they power off of the crank. and also like a Turbo they are more focused toward higher end rpm here's a link to sum article about themhttp://www.coloradocobras.com/whipple/superchargers/centrifugal-superchargers.html

also can you tune your turbo or twin turbos to make full boost a 2000 rpm and all the way upto redline(lets say its 8,200 rpm)??

ive never heard that about cyntrifical superchargers, and i doubt its true. id research it but im too lazy, care to post some references?
Sure:
STREET SUPERCHARGING (S/A Designs) Page 77
"Let's say you want ten pounds of boost at 6,000 engine rpm, and you gear the blower accordingly. No problem. But at 3,000 engine rpm the blower speed will be cut in half(since the blower is geared in direst proportion to the crank, regardless of the overdrive ratio), and the boost level will be reduced by FOUR TIMES, to only 2 1/2 punds.---"
He made a couple more examples of this, in case the reader missed the point the first time. And--
"Thus the centrifugal blower is very peaky, or speed-sensitive."
----------------------
TURBOCHARGERS (HP Books) Page 9
"The biggest disadvantage of the centrifugal compressor when used as a supercharger is: Pressure output varies considerably with speed."
"A rule of thumb is: Boost pressure increases as the square of the speed of the compressor."
----------------------
Probably some more references floating around, but you get the point.
But hey, think what you want.

also can you tune your turbo or twin turbos to make full boost a 2000 rpm and all the way upto redline(lets say its 8,200 rpm)??
almost, about 3000prm usually, but it is possible, this is usually using one or two smaller turbos, installing a single huge turbo like you see on 2JZGTEs or RB26DETTs (or in this case you could call it "DET") (this is when tuned, you can get both twin-turbo factory stock) then since it takes more time to spool it might be around 4500-5000rpm to really hit good boost, there are lots of factors, balancing the turbine/compressor impellor, ball bearings etc..

it'll usually be just around, maybe a little under full boost at 3000rpm, but you can hit full boost, depends on the power you're after.

an centrifugal superchargers are basically turbos cut in half, if you remove the turbine housing an replace it with a pulley that joins to the shaft (between impellor/turbine) thats pretty much it.

lets say I have a 7.0L V10 that I want 800 hp from it, so I twin turbo it. Now is there a way to get the turbo hitting full boost or just under it at 2000-3000 rpm and continue the boost all the way upto the redline(about 8,500 rpm)???

yes, it can be done. mainly because with such a big engine there's plenty of exhaust flowing out to get the turbos to spool up fast. you might need some proper boost control at high revs to keep it all in good order.

wow thats awesome. If the 2 turbos spool at 2,000 rpm and give full boost until redline thats hardly any lag at all. some cars idle at like 800 rpm. Also would this make the powerband wider??

yup, it should do with the right tuning. the lag comes from big turbos an smaller engine, but the bigger the turbo the bigger the lag (compare turbo size to exhaust flow to get that)

so basically
bigger engine= less lag with bigger turbos
smaller engine= more lag with bigger turbos

also wats the diffrence between a flat and wide powerband??

WTF is the point of full boost at 2000rpm on an engine designed to spin 7000rpm+ anyways?

so theres no lag lol

so basically
bigger engine= less lag with bigger turbos
smaller engine= more lag with bigger turbos

also wats the diffrence between a flat and wide powerband??

yea, matching turbos to engine size/exhaust flow will relate to turbo lag.

flat isnt to do with the powerband, the powerband is the range in revs at which point the engine is at its peak power:

e.g. you have an engine making a maximum power of 400bhp

a wide powerband means there is a big range at which the engine is making maximum/almost maximum power within the two point on that given range.

so, it makes ~400bhp between 6500 to 7500rpm, then the maximum powerband is 1000rpm (6500-7500=1000) but this varies between engines and setups.

the "flat" refers to the power/torque curve (for definitions of bhp an torque search for the thread on it) a flat curve means its not very steep, or has a shallow gradient. a tall curve means its steep, or has a tall gradient.

hope this is right an makes sense.

Centrifugal s/c's do exist. lol theyre just like a turbo execpt they power off of the crank. and also like a Turbo they are more focused toward higher end rpm here's a link to sum article about themhttp://www.coloradocobras.com/whipple/superchargers/centrifugal-superchargers.html

also can you tune your turbo or twin turbos to make full boost a 2000 rpm and all the way upto redline(lets say its 8,200 rpm)??


i know what a cyntrifical superchargers is...i was talking about boost drop.

you mean the boost drop where halving the rpm cuts the boost down to 1/4 ? i cant see that as being true, i mean if its spinning twice as fast it can draw in twice as much air. so how would the boost drop to 1/4 of the amount ?

i think that if you double the rpm, you double the boost. if you halve the rpm, you halve the boost.
so its in a constant ratio.

basic mechanics says boost stays constant as airflow dobles when RPM doubles causing engine air consumption to double.
however im not a cyntrifical compressor expert so depending on the impellers compression map it could be true though i doubt it.

so theres no lag lol

First your definition of lag is wrong.

Second, you don't honestly think a powerband can last for 5k on an average street car do you? Factory cars make full boost by 2k. And they suck for top end power.

That rule only applies to positive displacement compressors.

On centrifugal compressors, boost increases as the square of the speed of the compressor wheel. Flow can increase phenominally without a large increase in wheel speed. AKA dynamic compressor.

I'm pretty sure a turbo sized too small can cause flow restriction (backpressure) at high speeds, causing a drop in redline.

oh, yeah. too small a turbo will create back pressure an wont do the engine any favours. it wont just cause a drop in the redline, but without anywhere to go (if the wastegate isnt up to the job) then you could blow something up, either having exhaust forced back into the engine, or blowing exhaust manifold seals/gaskets...

..i dunno. i haven't tried over-boosting recently.

yea thats tru too

i spoke to my physics teacher about double speed, square boost, an he said it sounds right an told me is was part of some famous phyisist's law or something like that, so i guess its true.

i spoke to my physics teacher about double speed, square boost, an he said it sounds right an told me is was part of some famous phyisist's law or something like that, so i guess its true.
I just can't tell you how relieved I am to hear that!
To find out that what has been common knowledge in the industrial and automotive world for over 90 years, was wrong, would have broken my heart, and forever left a deep scar on my soul (if I had one).

i didn't really think you were wrong, i was just thinking in my logic, that double speed double boost. but i was wrong. i would nver have expected that to be honest though.

No problemo.
And I was just teasing you also.

lol