All I've really read are ballpark figures based on under or overdriving the blower. Exactly what variables factor into boost? And with the possibility of clearances being different between blowers (I ***ume this has an effect) is the only true way to measure it with a gauge? And last question. If you had a gauge, where would it go?
boost is the reverse of va***. the only way i know how to mesure it is with a boost guage. it needs to go on a manifold va*** port. it should read va*** untill boost is produced by either a turbo or a super charger.
and i dont belive clearances have any thing to do with it on a blower i belive its more rpm dependent. but im a turbo guy when it comes to boost. and thats a whole nother ball of wax
You can get a rough idea by knowing the displacements of the blower and the engine, and the drive ratio. Here's an equation I stole from the internet, it's amazing how fast you can find this info using google.
with a roots type blower (positive displacement air pump) the boost curve is pretty much flat over the whole rpm range, as long as the blower is making a moderate amount of boost, and the engine can handle the additional airflow.
Buy a gauge. It might look cool mounted on the motor,especially if it is liquid filled and small. Definately live longer mounted under the dash,or on the steering column.
And make sure you get your reading from below the blower in the blower manifold not under the ****erflys in the carby manifold i know its simple but no one mentioned it
If you're a safety nut you won't want a tube with pressurized air/fuel mix entering the p***enger compartment. Something many guys don't consider.
you said "calculate boost" are you doing software simulation and need boost figures? a gauge will show you the boost, but i dont see how it will help you calculate boost. Fred
Grimlock, I agree with all of the above comments for the most part. In my experience with the blower on my Willys I did all of the calculations and they were all wrong. Leakage in the Roots style blower is greater at low speeds than at higher rpms. I have experimented with advancing and retarding my camshaft as well and am able to change the boost at a given lower rpm. Also I have noticed boost changes at a given rpm due to restriction in the exhaust (with or without mufflers) I even went as far to hook a manometer in the exhaust and compare the reading with the boot gauge at given rpms. As stated above boost also works the opposite of vacuum more throttle plate opening the more boost till it flatlines. The best ****ogy I have heard is calculating boost it is like trying to tell how long it will take to air up a tire with holes in it only knowing the cfms out of the air compressor and the volume of the tire.
I have also heard people talk about the air density in making boost "its not making any boost there is no air" not sure about that although it sounds like it could play a part in the outcome
The equation might get you in the ballpark, but you need to get it running and get a gage on it to know for sure. as he said, there are too many variables.
the things that effect boost on a blower engine size cam size intake restrictions belt slippage blower clearances carb size engine rpm air temperature / pressure piston blowby exhaust restrictions compression ratio anytime you remove a restriction, boost will usualy drop, but power will go up. this includes porting the heads or installing a larger exhaust if you have 9 psi in denver, youll have 10 or 11 at sea level because of air density
Oh and one more thing you will not get boost in the shop revving the motor and waiting to see the boost gauge work like a tacho the engine must be under load on the street or track
Hey Grim , ask this guy about it . He doesn't make the systems but he makes them work . He is a huge source of info in setting up the electrical components that enable these systems to work and may lend some great insight to your project . Turbos are mostly his gig but he is not shy about performance . Trust me . Billy Glidden (yes , Bob Gliddens son )wouldn't let him touch his blender if he was a dolt and he has wired both his cars . At least call him . All he does for the most part is compression aspirated race cars . He could change everything. G'luck . swaZZie His name is Mike but he goes by Sparky. http://www.wiresandpliers.com/index.html
free reving will usualy give you a zero reading, no boost, no vacuum, my buddies 2000 eclipse turbo will get up to 2 pounds in a free rev a brake torque in the driveway, without spinning the tires usualy give you 1/2 of full boost if the cars set for 8 or 9 psi. example, my sisters 91 talon turbo with the autotragic, its 9psi when you hammer on it in teh streets, a brake Tq in the driveway gives almost 5 psi boost is load sensitive, which is why you wont have boost when cruising at a steady speed, but under load, even if its not wide open throttle, you have boost
Alright. Ths is starting to make more sense. I didn't figure it would be exact without running a gauge, but after reading this it's sounding nearly impossible. I'm not doing any sort of software simulation. Just looking for a realistic ratio to drive the 4-71 on my flathead. Plenty of machine work going into the pulleys and I don't want to end up with completely unusable parts...or something that will destroy my motor. I was thinking 1:1 but after re-reading a few old articles and hearing Flatdog is using a 3-71 I'm wondering if mine should be slightly underdriven? So is a boost gauge just an air pressure gauge?
Grimlock, go to dyersblowers.com. They have a chart with the different size blowers and have figured out how much each blower would boost each motor by the cubic inches. They will tell you that any modifications will increase and decrease boost also.
Grim ,I run a 4-71 1to 1 8lbs I started at 28 t bottom now up to 30 b 28 t pullies.Lot of missinfo out there.just ask will set you straightDon't type or speel good..
Cool. I had planned to call you to double check my info. I was pretty sure you were runing a 4-71 but then heard differently - can't remember where. So this is good. You'll probably see lots of general questions out here on the HAMB but I will definitely be in contact with you soon since what you've already done is so close to what I want to do. And I'll be sharing the build with everyone as I go. You might have to forgive some strangeness on my part since I might not do some things the best way - I'm going for performance within a certain era vs. an all out quarter mile attack like you. I have huge respect for what you've done with that motor. Especially in a full fendered car.
Just for the record - it is fun as hell to hit the throttle and try to keep your eye on the Boost Gauge.
Grim, Most boost gauges are compound gauges; they read vacuum, in inches of water, and pressure in pounds/square inch. Zero is about at the 11:00 position. Mine is a 268 cuin flatty with a Weiand 142 Joe Abbin kit. I think this equates to 142 cuin/revolution of the blower drive. My boost gauge runs at about -18" of vacuum at idle and at -10" at 2200 rpm @ 65 mph in overdrive. If you hog it the gauge goes to the plus or pressure side and as rpm's increase the pressure increases. If I remember right the crank pulley is 6-5/8" diameter and the blower pulley is a nominal 3-3/4" diameter. According to Joe the max boost with this pulley ratio is around 6#. (PSIG) Does anyone on here know the cuin/revoloution on a 4-71? You may be able to work the numbers backwards if you can find out that information.
Joe Abbin recomends a 471 be underdriven on a flat... How much would depend on the motor. From what I've heard about your motor, I would think it would have to be pretty underdriven. Joe will take the general specs of your motor and suggest a good ratio for the blower. I think he charges a little for the service but it wasn't that much when I talked to him. He has some programs he runs to figure it out along with his experience. It might be worth it to give him a call. http://www.flatheaders.com/blown.html
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