I began an explanation to my freind about why there is more lag on boosted stick cars than on boosted autos when used in a drag application. I tried to tell him that without load there is not really significant boost. his reply was that he could understand why turbo vehicles wouldnt have boost off the line and that he was unsure about anything on a centrifugal application, but he was sure that a roots application would have zero lag and would be fully boosted on the starting line even while freewheeling with the clutch pushed in so that when the clutch released there would be ZERO lag of boost. I said to him that a guy who had never had an auto roots car might believe that he had boost instantly. but a glimpse at a boost gauge on the starting line would show no boost ( or almost none ) until the car was launched. it would rapidly build boost but an auto car would have boost before launch and keep it because there was load. his question to me was simply, "why" or "why did I wrongly think this to be the case" I was short on ways to properly explain other than to give him an example of a vacuum gauge. a car without load at 4000 rpm would have a completely different vacuum reading than a car fully loaded and lugging up a hill at 4000 RPM. I know this is the case but a did not have a more in depth answer, especially one that the average car guy could understand. does any one have a simpler explanation that does not end in, "well, it just does"

 

are you talking abotu lag on turbos or superchagrers?
on turbos, there is lag because the turbo spins by exhaust gasses, so it takes more gasses going through (I.E. more RPMs) , to fully boost the turbo.
on SCs, it doesnt need to spool up as much, but it still spools better at higher RPMs because the belts are turning faster.

 

A roots blower is a positive displacement pump which means that it moves the exact same amount of air every revolution. You don't get boost without WOT because the throttle is closed.

A centrifugal blower is entirely RPM dependent. It works off of the principle that air accelerated away from a central point into a smaller chamber and then rapidly expanded will create boost. It works better the faster you spin it.

A turbo works off of a pressure differential and a heat differential. The more you boost the more heat/pressure you make so you can boost more and make more heat/pressure etc... So an auto car can lean against the converter and start boosting off of the line. You can do the same thing with a manual with an ign stall box wich blows hot unburnt fuel into the exhaust housing where if it is hot enough will auto ignite and increase the turbine efficiency. Not as good as an auto, but does ok.

Side note: WRC cars have an extra injector in the exhaust to keep the turbo spooled up under decels and shifts.

 

so what your trying to say is that on two identicle roots blown vehicles, one being auto and one being stick both at wide open throttle at high RPM with a stutter box, both will have the same amount of boost on the gauge at the starting line before the light drops. I have seen that this is just not the case. I am just curious as to a clarification of what you mean? because in my experience the stick car has nearly no boost until it builds just after the clutch is dumped (lag) and the auto when the trans brake is released has boost both before launch and after launch.

 

his reply was that he could understand why turbo vehicles wouldnt have boost off the line and that he was unsure about anything on a centrifugal application, but he was sure that a roots application would have zero lag and would be fully boosted on the starting line even while freewheeling with the clutch pushed in so that when the clutch released there would be ZERO lag of boost. I had trouble explaining to him that even on a roots application (which is what he is interested in) when you kick in the clutch and rev it to 6000 rpm the boost gauge will read almost nothing, or nothing, and sometimes even a tiny little vacuum. do the same against a 6000 rpm stall converter and full boost or near full boost will appear. because with load boost can occur.

 

holding the clutch in just reving the motor is like putting an auto car in neutral and reving the motor......there is no load on the motor and therefore there will be NO boost being built on the line NO MATTER what you use (ie roots, turbo, centrifugal, screw, etc etc)

Use a two step (stick car) or a trans brake (auto car) and you will be able to build boost on the line because *most* trans brakes hold first/reverse so that there is load on the motor.

how that works with a stick, i'm not quite sure....but it does

That is why Harlan from www.ls1tech.com invented a two-step for the LS1 motor so he could build boost on the line with his stick twin turbo car....

 

I understand how a two step could help build boost on launch in a stick turbo car (still no boost until motor is loaded) but how does a two step build boost on a stick roots car without load.

 

OK, now I'm confused. You say that you've personally revved a roots blown engine wide open with nothing but a stall box and made no boost? HHHMMMM...... That's very interesting to me. Superchargers are only RPM dependent. They could really care less about what the rest of the engine was doing. All a stall box does is fail to burn the mix in every other cyl. A turbo is the only device that operates off of engine operating conditions.

The only reason in my mind that a blower would make more boost under load is that it would be spinning faster under loaded conditions that no load condition.

I would like a more in depth explanation of this phenomenon. I'm not trying to be a smart-a$$ just curious.

 

my only comparison to this phenominon was an analogy: a regular asperated car/truck without load at 4000 rpm would have a completely different vacuum reading than a car fully loaded and lugging up a hill at 4000 RPM.

 

That's because manifold vacuum has to do with cylinder evac and scavenging effect. If you're not making any heat then the exhaust won't draw air as well into the cylinder.

 

vacuum and boost are both measurements of atmosphere in the intake plenum. with and without load on normal and super asperated engines will produce different results (4 different results) the relation is load. I need a better answer to why.

 

OK, let me try this. A supercharger is just a pump. If you hook it up to a machine and run it at a certain RPM Air in will always equal air out. If the volume of your container doesn't change, the pressure inside the container will not change either. Do we agree on that?

On an engine the volume of air that is being displaced by the cylinders and the intake manifold at a certain RPM is constant.

At that same RPM the air being pumped by the super is also constant.

That means that the pressure inside that volume will be constant. The only thing I can think of is that under load the cylinders might not be able to evacuate properly causing an amount of residual exhaust gas to be present in the cylinder. This decreases your volume which will increase your manifold pressure. That's the only thing I can think of.

During natual aspiration a dynamic gas flow is occuring where intake air velocity is directly related to exhaust gas air velocity. When exhaust gases are hot they travel much faster than when they are cold. Hence intake valves being much larger than exhaust valves. An engine under load will be operating under higher temperatures than an engine not under load. Therefore you get different gas dynamics.

 

you are so very close and I know it has to do with the resistance pushing down on the piston because that is where the resistance is as far as how the engine could see it. but I know there is a better, simpler analogy, or example, explanation to explain this although we are addressing it correctly. when the engine is under load the boost has the resistance of the piston not wanting to be pushed down causing boost to register on the gauge, when there is no load this resistance is lighter and no or little boost registers on the gauge. have I answered my own question with some help from ATOmonkey or am I way off.

 

Lemme jump in here (been a while)...

A Roots blown car held wide open against a two-step, converter, or against a rev limiter will show FULL BOOST (been there, done that) whether its a stick car or an automatic car (transmission that is...). Ask anybody who owns a Roots blown car (including me). Hell.. Even you snap the throttle on a Roots blown car, you WILL see FULL BOOST!

Now a turbo car with a stick WILL NOT make boost on the line unless you rev it up against a two step OR the rev limiter (if someone is actually crazy enough to rev their engine to redline and hold it there...). An automatic car WILL show boost if held against the converter (well... depending on the size of the turbo and the stall of the converter). Even then I wouldnt hold it there long due to the fact that the transmission fluid will get REALLY hot and cause it to break down early. Dont believe me? Grab the trans cooler line and see how hot it gets after someone (you trust) stalls the car up for a second or two...

Lastly... due to the fact that centrifugal blowers are HEAVILY dependant on RPM to make boost, you wont see full boost at the line UNLESS you hold the engine against the redline limter with the throttle WIDE OPEN. If you hold the engine wide open against a two step, you will see less boost on your boost gauge due to the fact that the two step will be set to a lower RPM than the redline limiter (stick car). If held against a converter (automatic car), you will definately see less boost (unless your stall speed is a few RPM less than redline ).

Hope this answers your question.

 

ATOmonkey your thinking in the wrong direction. I know little about turbos, but roots is my catagory. I have experiemted with alot of them, and have lots of results to share, but it would take the entire night to type.
Lets start simply, true and false.
roots blowers pump a fixed amount of air every revolution while off the car, sitting on a table: TRUE
roots blowers pump a fixed amount of air every revolution while bolted to an engine: FALSE
The reason? Theres a carb sitting on top while its on an engine! This simple fact has so many people confused... At idle the throttle blades are CLOSED thus, hard as it may try, the blower can only extract so much air from the outside world, thus creating a vaccum inside and under the blower... add that to engine vaccum and you have your unusually high vaccum reading for such a high duration motor (this is assuming the engine HAS a high duration cam) Thus my 303/313 duration cam pulls 14" of vaccum at idle, instead of the 9" it did while Naturally aspirated.
Simpler: In order to create boost (pressure) inside the intake manifold we need to flow <i>more</i> air into the <I>cylinders</i> than would normally be accepted in a N/A application, thus at idle when the cylinders are hardly being filled, and the carb is closed, the engine and blower must "work" to get air, thus creating vaccum, but we all knew this already.
Now lets look at WOT(load): we have the carb opened up all the way, to its maximum CFM potential... and we have the engine, lets say right off idle @ 1000 RPMS... now the blower has the entire area of the carb to work with, and instantly! bam! its <b>very next revolution of its rotors</b> takes in nearly its entire displacement of air and forces it into the manifold, thus instantly creating an uneven situation: 350 chevy, 360* of revolution requires 175 Cubic inches of air. 671 blower: 1 revolution produces (6 * 71 =) 426 Cubic inches of air. thus, we have boost. your 671 blower is forcing 426 cubic inches of air into an engine space that only requires 175 cubic inches of air, and this produces extreme/instant pressures in the manifold! ever see a blower manifold? really small, small as they could make it! Why? Well, look at the situation above, but consider this: right when you stab the throttle, the blower makes boost right? TRUE, BUT, before it can fill cylinders, it must fill the manifold, thus the first few pumps of the blower are aimed at pressureizing the manifold so that it may produce more boost for the cylinders... this only takes a few seconds however, and less at higher rpms. This also assumes a 1:1 blower drive ratio (most people under-drive for the street to lower the boost)
lets look at load more indepth: Simply (over simply) load is a situation where the engine is trying to "rev up" but cannot because somthing is holding it back (i.e. a car ) thus the engine must take in as much air as it can get (carb position / cam profile mostly affect this) to remedy the situation... a blower on the other hand can only do one thing: take in as much air is available to it (carb position / size + blower size affect this) and toss it down the intake manifold, if this is more air than the engine would normally take in, then you have leftover air. the leftover air cannot escape UP, theres a huge blower blocking its way out. and it cant escape DOWN, the engine will only accept so much. thus it becomes pressure/boost. the next time a valve opens, it rushes in and lowers the amount of pressure in the manifold by attempting to equalize pressure with the cylinder. this creates gobs of cylinder pressure and downforce on the piston which creates gobs of torque at extremelly low rpms, and gobs of horsepower at high rpms. Load is simply the situation in which the blower will respond with extra air because it is available at that time only. An engine cruising the highway at 3K rpms will not be under boost because the throttle position is not open enough for the blower to pressureize the manifold, if it was then the engine would be accelerating, thus the driver would back off on the throttle to a point where less air is available to the blower and it only takes in enough to keep the car moving... <b> at that rpm / load situation --- I am NOT saying throttle position DIRECTLY affects boost, but I AM saying it plays a major role in how much air is available to the blower. </b> we also see this as increased vaccum when not under load (better throttle response, better gas mileage) we see better fuel atomazation -gas mileage (the mixing action of the blower's rotors does this well) and when under load we see boost which means MORE POWER! forced induction rocks!
this could go on for days, here are some examples and combos I have found to work / not work. my tests were all on small block chevys, mostly 355 / 383's, and alot of this information comes from tests my friend has done as well...
High duration / high stall defeats the purpose of roots blowers. My best results with any blower have been with moderate cam profiles, 212/218 @ .050 cams (instead of 250/260~ N/A cams)
and moderate stalls, somtimes near stock stalls... 2000 RPMS / 2400 RPMS work well, anything beyond is a waste of gas. Manual trannys will benefit from a numerically LOWER first gear ratio, 2.43:1 instead of 3.53:1 this allows the blower to build more boost in first gear and pull longer, harder down the track through all gears. shifting before boost really gets "rolling" causes it to fall off to fast. Also, lower rpms result in a more efficient engine. Spinning these things above 6K can only cause problems...
The higher a blower spins, the more heat it generates, the less effective it gets... at higher rpms the rotors can actually warp and scrape the outer walls... and thats BAD.
example combo:
350 engine, 671 blower, Th350 tranny, 2000 Stall, 3.23 Rear gear, 8% underdriven blower. Advertised duration: 272/282 .050 @ 212 / 222 Duration. .550/.570 lift, 114 LCA.
2.02 / 1.60 decent aluminum heads, 2 steps colder plugs.
Initial advance: 15* Centrifugal advance: 25* total advance: 40*(in by 2400 RPMS) BTM boost retard 1.3*~ Per PSI,
Idle speed: 600 Rpms. 92 octane gas. Street tires. Street exhaust. Street car.
*Results as seen: 2000 RPMS we see immediate boost, 6~7 PSI range. also making 450 Ft. Lbs of torque @ 2000 Rpms. the current advance with boost retard is around 30*~. At 2600~Rpms we see max boost, 10~11 PSI, with timing at 30* due to boost retard. making 500 Ft. Lbs of torque @ 2600 as well. as we near 3500 Rpms things top out with 600 Ft. Lbs of torque, and horsepower is closing fast in the 400 Range. At 4500~5000 Rpms horsepower is near peak, 580~ range, boost begins to falls off, and torque begins to fall off. 5500 Rpms, horsepower peaks ~600 Horsepower and torque falls drastically the manifold is heating up and so is the blower, its time to shift.
As we hit our goal in 2nd gear, 65 MPH, we shift again and let off the throttle, and begin cruising the highway. boost falls off and timing advances to the full 40*. Cylinder pressures / filling is small, and load is very light. now we see the efficient blower motor doing its job, running slightly lean because the power valves have close, as well as the secondaries, thus the front jets are doing all the work and the engine is running hotter and getting 16~18 MPG as we cruise. its also cleaning the plugs because we ran rich under boost, the now the lean / hot condition disposes of unwanted carbon. then, we see in our mirror, a camaro closing fast. he pulls along side you in his highly modified LS-1 model, convertable with the top down, leather interior, radio blasting, and revvs his loud engine. he cant see or hear your blower, sitting below your nice fiberglass hood. and your car is pretty quiet, (street exhaust remember) so he thinks hes looking at a sorry thirdgen with its stock L03 powerplant because you have the RS stickers on your bumper. the dude laughs and pulls away, just as you planned. right as his car passes yours, you Stab the gas. 5 things happen at once: your 50CC accelorator pumps flood the volume of the blower, your secondaries open, your blower sees that its time to go and pumps its volume into the manifold, your power valves see boost as a no-vaccum situation and flow the extra gas, your BTM sees boost and retards your highway 40* advance to a reasonable 30* total, the blower makes its tell-tale, g0d-awful whine and your car starts pumping out 600 horsepower. the little 400 horsepower camaro gets passed like hes standing still. the guy takes off his glasses as you look in your rear view mirror and you see him shake his head in shame.

I hope this has been a valuable lesson.

 

i think that was the craziest post i've ever seen

 

Hey, Kingtalon you need to get a publisher for that post. If I'm backwards then so are you because I said the same thing.

 

actually it didnt help too terribly because of what I have seen. because the few roots cars that I have experience with I witnessed while holding the clutch in no matter how high you rev the engine the motor would show no boost unless you did snap the throttle. all day you could sit there at 6000 rpm even and the boost gauge would show 0, until you dumped the clutch then the boost would build rather quickly. I noted this because I had previously only had auto trans car roots boosted experience. It made me curious of the phenomenon and why it occured. when on a long road trip with a freind (who has no personal blower experience) I discussed this with him and why I believed he should keep his project car an auto if he is going to buy a 172-174-177 roots blower to street/drag race with.

 

ATOmonkey I mean no disrespect, you are VERY Knowledgable but from what I read you are on the right trail, but off to a side path a little....

<b>If the volume of your container doesn't change, the pressure inside the container will not change either. Do we agree on that? </b> NO I do not agree, As i said in the beginning of my super post, the amount of air exiting a 426 cubic inch blower will not be 426 cubic inches ALL THE TIME.... the pressure WILL change when the blower is allowed more air.

<b>On an engine the volume of air that is being displaced by the cylinders and the intake manifold at a certain RPM is constant. </b> Do not agree, RPM has NOTHING to do with displacment! the displacment of an engine is always constant, but the AIR displacment changes CONSTANTLY depending on engine load! more load = more displaced air, thus when you begin to back off the throttle at 4000 RPMS you may sustain 4000 RPMS but engine air displacement drops off considerably, taking in just enough to keep it going rather than accelerate.

<b>At that same RPM the air being pumped by the super is also constant. </b> Disagree again, At any given RPM, the air being pumped by the blower is directly related to throttle position, it is definetelly NOT constant. What would happen if a 426 CI blower ALWAYS displaced 426 cubic inches? no matter what throttle position? the engine would be under 10 PSI of boost ALL THE TIME even under IDLE! that would have dire consequences.

<b>That means that the pressure inside that volume will be constant.</b> I think I made my point that nothing is constant with these things, but as I said before, you have 99% Correct idea, your just typing it a little off-key so people may get the wrong idea, even though you know exactly how it works. Thus my super-long post. ATOMonkey you have helped me before to understand and now I wish to help you a little. Im not picking on you! All Hail ATOMonkey!:hail: :hail:

 

<b>no matter how high you rev the engine the motor would show no boost unless you did snap the throttle. </b>

Simply put, holding a high RPM in neutral does not take much throttle opening, and the air displacment of the engine is still very little even at 6000 rpms in neutral! reason: no load. now, snapping the throttle open creates a boost situation: nearly wide open throttle + sudden light engine load = boost. for a second or 2

 

I meant with a rev limiter at WOT. the three cars I have noted this with 1 was a chrysler (and we had it at wot and only 4000 rpm) 2 was a ford (only around 3800 and wot) and the third was a 350 it acted that way at 3000 rpm so I put a 6000 chip and did the same for a moment or two (the owner revs it to like 7500 commonly) and I noted that in all cases the boost gauge would make and ititial showing and would not exactly read zero but there was little or no significant boost like a pound or two unless snapped.

 

Well Anytime a wot condition is present there isnt always a significant amount of air flow taking place. the fact a rev limiter was involved explains alot. I have limited experience with rev limiters, but any sustained RPM, whether the throttle is open or closed, rev limiter or no- your going to see a decrease in airflow demand on the engine. Remember when I said throttle position doesnt always control boost level? This is one perfect example. we have the blower seeing WOT and pulling its cubic inches forcing air into the manifold... to a point (0 vaccum condition) but once the blower tries to make boost pressure, the extra air simply exits the exhaust due to the scavenging effects of a sustained RPM. Notice when the throttle is SNAPPED you get a little boost? the engine has room to work.. from IDLE to The rev Limit, it can use the boost pressure to make power and spin the engine up. Once it hits that wall, that rev limit, sustained RPM, the boost "falls off" just like in a turbo charged car, or during a highway cruise, your valves only let so much by, the rest exits the exhaust in a waste. the difference between a rev limit and a highway cruise however, is primarily the air demand of the engine. WOT + blower = high air demand, WOT + blower + rev limit = high air demand, Lots of wasted gas and air, lots of heat. highway cruise is nowhere near WOT, thus the air demands are much lower, but manifold pressures remain the same, and gas mileage due to increased efficiency takes over. whenever I try a rev limiter on my blowers it seems to over-heat the manifold and blower, thus making me less horsepower. I find better results powerbraking with street tires and then easing into the throttle. I never hit WOT until 2nd gear, otherwise my tires would go up in flame and i would never make it to the end of the track. Slicks however, I simply snap the throttle and hope my drivetrain stays together... Low end boost can create way too much cylinder pressure, especially with a roller can and a wide LCA. somtimes 250-300 PSI can be seen, and thats more than most normal headgaskets can take.

 

Hey B4Ctom...

Let me give an example...

I own a 98 Pontiac GTP. In park or neutral... if you hold the throttle WIDE OPEN, the car WILL NOT rev higher than 4000 RPM (GM programmed the car with a "SAFETY" rev limiter, dont ask why...) and the boost gauge reads 12 PSI (not stock... I repeat my car is NOT stock... B4 I get flamed by those that know that stock boost for a GTP is 8 PSI... I have a "SMALL" pulley ). Now... a buddy of mine has a 72 Camaro with a 383, 4 speed, B&M 177 blower (I think thats the model...) and when he goes FULL THROTTLE on the two step, his gauge reads 7 PSI (READ... WIDE OPEN THROTTLE!)

Now if you hold the throttle at 3000 (or any rpm) without a two-step then no you will not see boost, just like KINGTALON said.

PS... I did say WIDE OPEN in my last post, didnt I?



Oh BTW... both vehicles I referred to are equipped with Roots blowers...

 

this post has become a favorite it seems. Sorry for all the long posts I get carried away somtimes. well, usually.
Like JAYDUBB said, and I will re-say, it all depends on engine demand.

Rev limiters are funny things, and when combined with funnier cam patterns things get real funny.

A blower will take as much air in as it can, and depending on throttle position and engine demand, it may or may NOT lead to leftover air in the intake manifold (boost).

The original subject was "please explain boost lag..." Let me say this about turbo / centrifugal applications:
the amount of air these types produce is based on their speed. Turbos, as everyone knows, get speed from exhaust pressure / heat. when engine output is low, such as on a highway cruise, the turbo isnt getting enough exhaust heat / pressure to spool up and cause boost, it just sort of free-wheels like a windmill would from the passing exhaust. When the car is put under a load, such as during acceleration, the exhaust gasses become more forcefull, due to a bigger intake charge, and that spools the turbo to a point where its output exceeds that required by the engine under a N/A format, thus producing boost. the added intake charge from the turbo then produces MORE exhaust, thus spools the turbo even MORE creating MORE boost... and so on... The only way to regulate this before it gets out of hand is the wastegate, which controls and amount of exhaust that forces its way pas the impeller (windmill). Thus, by controlling the wastegate, you can control the boost, thus the name "Boost controller" The device that changes how the wastegate operates under these high heat / exhaust situation. this is an over-simplified example I fear my knowledge is limited on turbos.
Turbo and centrifugals are a "blow through" while roots blowers are "draw through" the difference is obvious, turbos force air through a pipe to the intake and blow the air in. Blowers however, bypass the need for wastegates and blow-off valves and simply operate on engine demand as to how much air they will supply to the manifold. blowers are also much simpler... despite the price of a nice 671 setup its actually really simple how they work... as everyone can see.
But, lets cut it short before I go on all day, "Boost lag" for a turbo is the time from when exhaust pressure begins to build, to the time the turbos output exceeds that required by the engine... the bigger the turbo, the more exhaust output required to spin it fast enough to bring it to an acceptable output level... but the bigger the turbo, the more boost we see once this level is reached. bigger engines Generally output more exhaust thus make it easier to spin bigger turbos, but bigger engines also require more air thus requiring bigger turbos... its all a big catch 22 and eventually we are going to blow ourselves up with the bigger is better notion.
another note on "boost lag" situations in turbo applications, Ive noticed cylinder pressure plays a huge roll in how fast a turbo spins up. Even at idle, a turbos air output can exceed whats required by an engine, thus causeing the wastegate to hang open while the car idles. By altering the cam profile with a earlier intake valve duration and a later exhaust vale opening, we eliminate the need for the car to draw air in by itself. Of course, if you remove the turbo, the car will not run. but under high load at low RPMs, the turbo spools REALLY quickly and reaches max boost really fast. Some factory equiped turbo engines already figured this out, such as the Talon TSI Eclipse cars, the engines are MADE for turbo applications, remove the turbo and they will not run at all. this is due to a super early intake valve opening and a super late exhaust valve opening. the longer the boost can push down on a piston the better the results, and the more cylinder pressures you can see the more exhaust will be "forced" out of the cylinders when that valve opens....
<b>Disclaimer: everything here is experimental and above all else I am NOT an expert on this stuff, and I do NOT know everything! For all intents and purposes these things are my OPINIONS and nothing here should be taken as factual information unless you research it for yourself. </b>
Moral: What works for me might suck for you.

 

thanks guys even those who got flamed for making big responses but this has armed me with the info needed to explain why in more ways than one, one way or the other.

 

hey B4C no one flamed me! At least not that Im aware of... Say did you flame me and I dont know about it?

You dare flame the great post-master!?

 

maybe you should call yourself the "resident post expert"

 

I like that. I think I will. -say, are you mocking me mr. b4c?

 

how about "post master general". Oh, no, nevermind that ones already being used. P.S. is it me or does System of a down ROCK!