Is there a formula anywhere to determine a roots style blower size vs engine size and boost created. I realize it depends on the speed of the blower and its efficiency etc. But I'm just looking for something general.

This is my application(so you can tell me it's impossible) .6 liter(600cc) motorcycle engine with probably 12:1 comp. and 12,000 max rpm. I realize the compression would be better lower. Anyways, say I'm looking for ~6psi boost, would I want a blower delivering/displacing like 1.0 liter of air or .8 liter etc.? I think this will run on 100 octane gas.

I've searched here and the general internet for a few hours already and haven't found anything that helpful.

 

I don't have a really good answer. I'm kinda out of my league here but let me tell you my seriously cheesy way of thinking about this, based on my limited experience with roots blowers:

THink about the smallest roots blower I'm personally familiar with: The Weiand 142. Fortunately, it's name is it's ci displacement (theoretical) per revolution. When the input shaft rotates 360* it moves 142ci of air.

Now here's the part I DO understand pretty well: a roots blower doesn't actually compress the air like a turbo or centrifugal blower does. It's a positive displacement pump. It builds boost by trying to shove 10 pounds of stuff in a 5 pound bag, basically. It tries to cram more air into the motor that it can normally take. Boost happens as a by-product, almost.

So here's where I make up my own numbers.......

Regualr atmosperic pressure is roughly 15 PSI. 6 PSI of boost is really 15 PSI atmospheric PLUS 6 PSI of boost provided by the blower for a total of 21 PSI. Basically, you're trying to shove 40% more air into the engine than it would normally take (21 is 40% more than 15).

Now let's assume that a 4 stroke engine ingests exactly it's stated displacment worth of air per revolution. HALF of it's displacement, actually, since it only inhales every OTHER revolution. Figuring out how many CI it wants to eat is pretty simple from there. i.e.- a 350 Chevy wants to inhale 175ci woth of air every revolution.

Figuring out how to get to 6 PSI of boost is now a matter of juggling pulley ratios to make the blower shove 40% more ci into the engine than it can normally take per revolution. Or asked another way, what pulley ratio do you need to shove 254 ci of air into the engine per engine revolution (245 is 40% more than 175). Answer: 1.72:1 blower-to-engine crank ratio.

SANITY TIME!!!!:

THis is NOT taking into account MANY factors like blower bleed-back, efficiency, cam timing, and about a billion other things.

I am NOT a rocket scientist. And my physics skills are sub-par even considering I'm a salesman by trade. But read on, it might still be of interest.

HOWEVER...........

I factored in a simple 10% "fudge factor" guessing that the blower isn't perfectly efficient, some of the boost will bleed out during cam overlap and generally to get something close to the right answer on a repeatable basis.

1.72:1 + 10% = 1.89:1

Wanna guess what the actual 6 PSI pulley ratio is on a Weiand 142 blower for a 350 engine? 1.94:1. I'm pretty damned close.

I have tried this formula out on several combinations, including my own 383 and come up within about .5 PSI of ACTUAL OBSERVED boost. I should have gone with a slightly higher fudge factor since bigger cams and such seem to "bleed off" more boost. Maybe closer to 12%. Seems the higher the engine's N/A volumetric efficiency the lower the boost will be once the blower is strapped on, which makes sense if you think about it.

Conclustions?

I'm NOT saying I'm anywhere near right.

I'm NOT staking my reputation on this.

I'm NOT trying to confuse or mislead anyone here.

These are almost RANDOM thoughts on the subject, so please don't rake me over the coals. I already know I'm wrong for a billion different reasons of physics and mathematics.

It's just that this simple approach has gotten me a reasonably close answer when applied to typical street driven V8 4-stroke automotive engines. Take it for what it's worth.

Hope that helps. Bet it doesn't.

 

Considering it comes out pretty close, it doesn't sound half bad to me. See what anyone else has to say?

 

Sounds ok, but what roots blower goes on a 600cc bike?

4-71? 2-71? some kind of Eaton?

 

I found some japanese company called agura that makes a roots supercharger that displaces 400cc(.4 L). I've just been doing some brainstorming for the SAE formula car competition, maybe a turbo would be better. The problem is that there is a 20mm restrictor plate and the supercharger or turbo must be after the throttle body (like a blower or draw through turbo). Most powerful engines make 80-90 Wheel hp and 0-60 mph in the mid to low 3 second range with ~600lbs race weight with driver.

 

Sounds good (well, close enough to get you there) except for this part. Most of the blowers that we deal with effectively just accelerate the air and hitting the restriction of the engine creates the boost. Generally positive displacement blowers are the only ones that are considered capable of building pressure in the case. Hell, you can blow right through a turbo or centrifugal blower without it spinning.

WRT to what will work on 600cc engine... maybe one of the small eatons? An M45 or M63? An M45 driven at a 1:1 ratio should put you at just about 10 psi boost…

 

hey Beast5spdGTA i am in fsae too, up here at michigan tech, i am on the chassis team this year.

the thing that i always wondered is how does the flow at the restrictor look? is it pulsed from the 4 banger or is it constant?
if it is pulsed, you could put a super or turbo to try and get the flow more constant. you may not build lots of boost, but every little bit counts. just my .02

 

Personally I don't know, but I believe it is pulsed from what I've read, making a need for a plenum of some sort. I was thinking of an engine using 1 or 2 cylinders since the restrictor seems to do a good job of killing high rpm hp.

I haven't done any theoretical research/math on it all, just trying to think of a different view than the normal high output 4 banger.

 

http://cgi.ebay.com/ebaymotors/ws/eB...category=35572


Check that thing out
I remember seeing a roots bower on an old body style 750 gsxr that made 10psi on ebay and the guy said it was some sort of eaton. the shaft almost touched his led when he rode it

Zac

 

what about finding a way to pressurize the area infront of the restrictor? maybe a HUGE *** ram air box. make it out of the entire side pod or something.

are you sure that 12,000 rpms is the highest you can go? i think our engines are hitting higher than that but i cant remember right.

 

Maybe they are, I've read a bunch of different things, seems like "most" (of the teams I read about) are only getting to like 8-9K rpm, but I know most of the engines stock make peak hp around 12K and rev to 14-15K without a restrictor.

I was also thinking of some sort of ram-air set up, I was thinking the positive displacement type compressor b/c I figure it would work better under a vaccum situation than a centrifugal type.

 

According to Turbocharged by Hugh MacInnes a 600cc engine at 12000rpm flows 150cfm. The book recommends at Warner-Ishi RHB3 for engines from .5 to 1.3 liters such as motorcycles.

 

Not to be a bummer to you guys, but you should really take fluids or some sort of cfd class before you worry about a power adder on a fsae engine. When you do use a huffer, the rules say you have to run a smaller restrictor than the 20mm, I believe its a 17mm. With the 20mm restrictor airflow becomes supersonic~10,000 rpm on a na engine creating all kinds of turbulent flow. UTA is about one of the only teams I know of that was succesful with turbo 250's for quite a few years. If you really want to find out about it you might want to contact Dr. Bob Woods there. I would just stick to figuring out how to make the 600cc high rpm engine behave more efficiently with the restrictor. Thats what I am doing with our car!

 

Maybe you're talking about a few yrs ago or different formula SAE comp. formula SAE rules 2003 All I see is the size 20 mm for gas, 19 mm for E85 and the order of T/B, restrictor, comp., engine.

That's kinda funny that for my school (FL Tech) ME don't have to take compressible flow(Fluids 2) or air breathing engines, both which would probably help a lot for this situation.

Anyways, 87Z-ya, gives a good reason to why most engines with the restrictor peak at a relatively low rpm. Kinda furthers my reasoning to research towards a 1 or 2 cylinder high torque/low rpm engine with a supercharger/turbo of some sort.

I also believe last yr (and other recent yrs) cornell was the winner with a turbo E85 engine which was also the most powerful.

 

damn cornell.

i am only a sophomore, and i am in thermo right now, fluids next year sometime.

 

Your right, they changed the rules again on me. You are correct that a low rpm single cylinder is the way to go. Torque is king in competition. Think of making a mini tpi engine! On our 2002 car (The only aluminum frame car) you can feel the f4i engine do absolutly nothing above 10000 rpm's. The dyno shows it also. The team last year modeled the intake and exhaust around the bikes, not taking into account what the restrictor does. Im changing all that this year. Hope to meet you there. Maybe I will bring my Z28 up to detroit to do some street racing while Im there if its done.

 

Forgot to mention that you guys should feel fortunate that your schools promote fsae. Our school actually kicked us out of the university last year and said that it was not a benefit to the school and a insurance liability. We were forced to use on of our sponsors juice machine facility as a shop. Just recently with a new dean we have gotton back on campus. Fsae is a benefit and incorporate what we do on paper (or computer) to real world like no other can!

 

I'm not sure if I'll get there this yr being a junior, but I know my school at least supports it, last yr they couldn't get their computer to run the inj. right so they didn't compete, which is weird since I thought stand alone systems were pretty simple to tune, but whatever.

Yeah, I could image the engine not working well since I've seen unrestricted N/A motorcycle appl. with individual T/B almost as big as the restrictor it's self, so I'm sure the engine would like about 2-3 times the amount of air it's allowed w/o vaccum.

Are you a grad student or what?

 

so what are you thinking about this low rpm high torque thing? would you have low rpms but then boost it?

 

Ace_Murdock, yeah that's my idea is a high torque single or 2 cylinder engine with forced induction of some type, and keeping rpms in the less than 8-9K range.

Realize I've never been ina vehicle that does 0-60mph in sub 4 sec range but that must be fast.

 

i want to drive one of these cars so much, but of course we have lots and lots of snow up here, so no traction.

and for some odd reason the team doesn't practice when there is no snow.

 

The reason boosting is good even on a restricted engine is that it increases the combustion efficiency, no pumping losses, it increases the volumetric efficiency of the induction system, fuel always burns better when the engine is spinning slower. The bottom line is unless you have a nozzle and an afterburner you're only going to flow so much air through that throttle body. Air = Power.

Now talking about engine configs is even more tricky. Piston force is piston area times pressure, so having a big piston is good. However because of displacement restrictions your stroke gets short and you lose your mechanical advantage because you have a smaller moment arm now. OK, so you lop off a couple of cylinders. Now you've created dynamic problems because you're only firing a couple times or once in your case per revolution. Here are some rules of thumb.

Smaller displacement engines are always more efficient. ie. Honda's I have more hp/L than you!

Multiple cylinder engines always run better because of a more smoothe firing cycle per revolution. Less dynamic losses from loading and unloading.

What you want to do is optimize your bore stroke ratio. You do this by first determining where the lowest BSFC of the engine is. This has the most to do with combustion efficiency. Then you determine what the mean piston acceleration that you want at that engine speed. Then you design the crank stroke around that, assuming the con rod lenght is constant. Sizing the turbine and compressor so that you don't need a waste gate will utilize all of the available energy. Then you still need to match the cam to the intake and exhaust so that the engine is tuned properly, because all blowing does is increase the density. If it won't flow right, it still won't make enough power.

The next thing you do is pick gear ratios based on your desired vehicle acceleration and engine performance. This makes it even more tricky. A high revving engine can use high gear ratios to multiply the torque and get better acceleration. A low revving engine must use lower ratios to get the desired speed, but makes up for that with high torque at the lower speeds.

Lucky for you a lot of these decisions have already been made. So all you have to do now is design around those decisions. Should be pretty easy. Have fun boys.