100 octane, do you live next to a race track? I cant even find anything over 92 within 20 miles of me.

 

A followup question for you blower guys.
Have you seen more failures from the 250 blowers with sealed rear bearings?

 

That's a good point. If all you can find driving across country on even in your home town is 93 or 94 absolute maximum what good does it do to build and tune a steet engine for a non existing gas?
This is a street engine. One required to have good maners. You might even be forced running across country to run on regular 80 something. Will your race motor tolerate it?? My street engine will.

 

IM quoting the article in the previous post they used 100 octane,

 

I think it will fit with a thinner filter.....the plate system adds 0.5 inches I think.....it's an NOS system. If you look at the pic I posted in reply to you up top, you can see the lip of the air cleaner at the scoop opening pictured in the inset. Drop an inch or two with the filter and ditch the plate and you should be there. As I mentioned, pics and measurements can be taken for you if you need more precise info.

Jim

 

I've seen some comparisons here, that essentially seem to be that a Roots blower and Centrifugal (Paxton, ATI, etc.) are the same...

They are the same, in that they force air into the engine. BUT, there is something very important to consider...

A Roots blower takes a certain volume of air, and mechanically forces it into a space. Double the speed of that Roots blower, and it doubles the output, in a (nearly) linear fashion.

However, a centrifugal compressor takes the fluid (air, water, whatever) from the center, and flings it to the outside of the housing, causing it to have higher energy.... The key here, is that at low RPMs, the blower doesn't add anything significant to the air velocity... But as you double the RPM, you square the output...

So, let's use an example: Suppose you have your 350 SBC, and you're comparing boost levels of two superchargers, each one designed to make 10 pounds of boost at 6000 RPM... Seems pretty simple, yes? Ok, we work back from 6000 RPM. At 3000 RPM, the boost level of the Roots blower should be about half of that, or 5 pounds. The centrifugal supercharger, however, makes the square-root of 10 (pounds of boost), or 3.16. At 1500 RPM, the roots? 2.5 pounds of boost, the centrifugal blower comes in at 1.79 pounds...

(Before you experts ALL start ******* on me about throttle position, and load, these are variables that cannot be factored in, therefore, ignored for this simplified discussion...)

The POINT of this comparison, you ask??? Simple: A centrifugal supercharger works well across all loads, if it's driven by a prime mover that is NOT the engine crankshaft- ergo, a turbo. But when the centrifugal supercharger is crank-driven, you must be extremely careful to not overspin the engine, or the boost levels will increase so quickly that it'll blow your motor. Also, since the boost of a centrifugal supercharger is a logarithmic scale versus a linear scale (as on a Roots blower), the boost will not come into play until you're in the upper third of your RPM band... and that power band will be very narrow, indeed...

I like the Roots superchargers, simply because you can have significant boost at lower levels... and (depending on how your drive is configured) you can have it be VERY snappy even at low RPMs, as Norval has stated...

As to compression ratios, JMHO, but it seems that the dynamic compression ratio should be at a level where you can fill the tank anywhere, for a street car... Whether with slightly higher static C.R and lower boost, or vice-versa, is largely a matter of preference... And frankly, tuning abilities. As you make larger amounts of boost with a lower C.R., it seems that the wider the difference (C.R. versus boost to attain that max. dynamic C.R.), the more problematic it is to dial in carbs to span such a wide range of conditions...

For me, to solve this dilemna, I have opted to build a custom EFI system... A la Force Fuel Injection, but for much less cash outlay...

I hope this clarifies the issue, and I've not embarrassed myself TOO much (it's only been 20 years since I studied centrifugal pump-theory in any real depth)...

Norval, would my observations seem fair (boost vs. C.R.)?

 

They are more, but I don't know about double. I thought you were more worried about fitting everything under the hood...

 

I don't follow this?
A roots blower puts out full boost at ALL rpm. Yes as low as 1500 rpm full boost can be acheived. IT has nothing to do with rpm. It is strickly throttle position. The blower wants to produce maximum boost all the time, forget rpm. but the throttles are closed so the blower can't get the gulp of air it wants so it doen't produce the boost. Open the throttle and the blower instantly grabs a big gulp of air and rams it into the motor.
Boost level is flat throughout the rpm. From idle to redline it is flat.

You are right about the supercharger. It is totall rpm dependent. I know a few guys with them and under 3000 they get nothing even with the throttle floored. It requires rpm.
Around town NOTHING can match a roots for down low grunt. Crack the throttle and boost is right there.
My wife has a roots blower on her SSEI supercharged bonneville and she gets 7 pounds just merging on the expressway.
The supercharger needs rpm to build boost the roots does not.

 

Norval full boost in theory yes in reality no,It takes time to get maximum boost which is ramped up with rpm,It all happens quickly but at 1500 rpm and you go to full throttle it will develope max boost as rpm goes up and it is filling more than the engine can consume,assuming the blower and ratio are right.This has to relate to the ve of the engine too.At higher rpm the engine can use more but the blower is building up boost equally,As an example my engine is not even showing boost until 3000 rpm at full throttle than ramps up from 6lbs to over 18-22 lbs, Part of this boost is the heated inlet charge which is being compressed. This all takes place in milliseconds but it still is not the same at every rpm. Now if you are putting in way more than can be used you will see boost earlier but it will still read higher at higher rpms,If the supercharger is tight clearanced and capable of this.

 

I'm worried about hood clearance but I'm also on a budget. I'm a college student who works part time at the school food court. I'm making $7 an hour .

I already have the block, heads, and a new cam. But I would still have to buy the pistons, pay for the machine work, buy the blower, and possibly a blower carb and blower cam.

I want to plan out a nice blower motor combo on paper now. Then I'll see how much money I have when next spring comes around.

 

Correct. The only way to make it an apples-to-apples comparison is to assume an RPM increase of say 500 RPM/sec, for a roots-supercharged engine versus a centrifugal-supercharged engine. In this way, the variables of boost "stack" in the intake manifold is eliminated, and we are left with RPM versus manifold pressure, since all the other variables cancel out (which is why I didn't want to get into the area of throttle-position introducing any confusion into such a comparison).

Strictly speaking, from this perspective, a roots setup will come in much earlier, and in a linear fashion, while a centrifugal setup will be a dog for the first 1/3 of the RPM band, start to come alive in the middle third, and fly through the last third of the RPM band... In other words, it looks cool, but it's a silly thing to use except for pure racing applications (where the RPM band is quite narrow).

A positive-displacement supercharger, or a turbo-charger, will always be a better deal for all-around performance, then a crank-driven centrifugal supercharger..... IMO.

 

The roots blower was originally ment for trucks. Trucks that probably had 3000 as a red line.
The term 8-71 means 8 cylinders displacing 71 cubic inches per cylinder or the 8-71 was meant for a 568 cubic inch motor.
It is an air pump, a positive displacement air pump that is designed to positively move so much air regardless of rpm.
If the truck motor is spinning at 1200 rpm with open throttle the blower fills the motor to a certain level at 1200 rpm. If the motor is reved to 2000 the positive driven blower responds by again filling the motor to a set level at 2000.
Both my 8-71 and my wifes whatever size roots blower create maximum boost right off idle. I have driven this combination for 5 years and yes I do pull almost ALMOST full boost off idle.
By almost I qualify this by saying I don't go to absolutely full throttle off idle without violently spinning the tires and at this point you are not looking at gages but watching the road, can't look at a tac either. I have never seen my maximum boost with the throttle floored but off a light under a brisk throttle bordering on lightly hazing the tires I see 9 or 10 pounds all the time.
Of coarse I watch my wifes boost gage and it seems to be hooked directly to the throttle. Step on the throttle and the boost heads over to the 7 pound maximum.
Even going up a hill you see the boost gage climb, just openning the throttle a tiny bit causes the motor to see more boost.

I am cruising at 100, about 2600 rpm and I lightly accelerate out of corner, nothing hard, just smooth flow out of a corner and you glance at the boost gage and it moves up from vacuum into the positive range.
I run my blower 15% overdrive to get instant throttle response and good boost levels down low where the motor spends 99% of it's life.
Why have something that needs spooling up for the few times to go there and the rest of the time it is just a drag???

Mike our motors are not anything alike. Yours is pure brute power. Mine is a street motor, nothing more.

 

a blower does nto create pressure or boost as you call it, it's a result of trying to cram air into an engine, so flow through a restriction. The roots blower is just an air pump, a non adiabatic device. The centrifugal compressor/supercharger and turbocharger are adiabatic devises. As the air is pulled inot the impeller and forced outside into the scroll it merely changes direction, the scroll is sort of a collector that channels the air to the outlet. The scroll gets bigger so the charge velocity goes down and we all know that slower moving air has a higher pressure than fast moving air (air plane wing, it develops lift because the air over the top has to take a longer path, and thus move faster to get at the end of the wing at the same time as the air moving under it, the wing gets sucked up due to a low pressure area on top). The roots blower only compresses over the rotors but once the air reaches the outlet side the're's no pressure increase anymore. Furthermore all non spent air is recirculated and this causes turbulence and a lot of heat.

 

a blower does not create pressure or boost as you call it, it's a result of trying to cram air into an engine, so flow through a restriction. The roots blower is just an air pump, a non adiabatic device. The centrifugal compressor/supercharger and turbocharger are adiabatic devises. As the air is pulled inot the impeller and forced outside into the scroll it merely changes direction, the scroll is sort of a collector that channels the air to the outlet. The scroll gets bigger so the charge velocity goes down and we all know that slower moving air has a higher pressure than fast moving air (air plane wing, it develops lift because the air over the top has to take a longer path, and thus move faster to get at the end of the wing at the same time as the air moving under it, the wing gets sucked up due to a low pressure area on top). The roots blower only compresses over the rotors but once the air reaches the discharge side there's no pressure increase anymore. Furthermore all non spent air is recirculated and this causes turbulence and a lot of heat. Simply put, the roots blower is merely an air pump, the compressors are just that..compressors. :o

The engine's air "consumption" is also not directly linear w/ the rpm, intake & exhaust efficiency and cam events make that the volumetric efficiency is not the same all over, just look at the torque curve for an idea of VE w/ max VE being @ max TQ. So, if you extrapolate the torque curve over the rpm range (you have to know at least the VE at one rpm to properly do this, as a reference) then you have an idea of the true demand vs. rpm.

 

EXACTLY

 

Kid -

Have you checked out this site?? There are some good tech articles also...

You might also look into some used hardware - it's often a good bargain.

http://www.superchargersonline.com/content.asp?ID=76
http://www.superchargersonline.com

 

I took this directly/copied it from the first link.

Notwithstanding minor amounts of air-leak at low rpms, the roots supercharger cannot flow backwards like a centrifugal supercharger, and is thus nearly as efficient in its ability to pump air at low rpms as it is at high rpms. What this means is that roots superchargers are very capable of making large amounts of boost even when engine rpms are very low. This makes for great low-end and midrange power, and also makes them great for trucks and towing vehicles. The roots is also self lubricated, and is the simplest of the supercharger designs, meaning it is reasonably priced and very reliable. This is why roots superchargers have been the choice of GM, Ford, Mercedes, and Toyota for OE applications.

 

I agree