GoRideSno

So I have taken the car around the block a few times now. I have never driven a car with a roots blower. I had imagined that there would be a steady sharp gain in boost pressure off idle to WOT. I had expected to have boost during any acceleration.
Mine seems more like a turbo though with far more boost during hard acceleration. I can slowly rev it to 3500 in neutral and see only 1 or 2psi or vacuum. I can accelerate hard in 1st and see the neddle jump to 10PSI. I have not experimented much yet because I have no where to do so (crowded streets near LA) and the 10PSI number scares me.
Has anyone driven in a vehicle w/ a roots blower? Maybe someone has a neighbor w/ a Lightning or Cobra and can ask them if this seems right.
BTW the car feels strong.
<img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />
Andy K

ErnestSw

Sounds like your plumbing may be wrong for the low end.
Does it have an adjustable pop off valve?

Jim Nowak

Andy,

That sounds like your bypass valve is working. When you mash the throttle the valve should shut immediately and you should have instant boost. When you are operating under high vacuum the valve should remain open so your engine is not consuming power spinning the blower and producing boost when it is not needed. My supercharger system does the same thing as yours due to the bypass valve.

GoRideSno

Thanks for the replies.

Ernst,
There is really no plumbing between the blower and cylinders, just a manifold.
I am using the Eaton throttle style by-pass valve.

Jim,
Would you say there is more boost when the engine is under heavy load?

I drove the car again after the post above. With half throttle it left well over 20' of rubber from a 5mph roll. This thing is going to be strong when I get it running right.
<img border="0" alt="[burnout]" title="" src="graemlins/burnout.gif" />
Andy K

ErnestSw

I could be completely wrong, but wouldn't the geometry of the manifold affect the boost curve?

Normy

My understanding has always been that manifold pressure depends on two things: 1. How much air gets shoved into the manifold, and 2. How much air the engine sucks out.

A roots blower spins at a rate that is mechanically fixed, and its intake and output is constant per revolution. This is not true with a turbocharger or centrifugal-type blower, where the unit actually is far more efficient at packing the intake manifold with air at higher speeds than at low speeds.

Unless I am mistaken, you should have nearly full boost, even at just off idle, immediately upon opening the throttle. In fact, I think it is far more likely for just the opposite of your experience to be normal- engines with wild camshafts have the ability to take far more air from the intake manifold at higher speeds than low speeds, and in these engines, Roots-developed boost would tend to drop off with higher RPM as a result.

Remember, centrifugal compressors are more efficient at higher speeds due to aerodynamic factors. This is not true with Roots blowers.

Normy!
'85 S2 5 Speed

ErnestSw

How does the vacuum number relate to boost if the boost is constant? I guess I don't understand the concept.

ViribusUnits

From the looks of your picture, I'm willing to bet that the throtal body is the silver thing on the passangers side of the car, upstream from the blower?

OK, my understanding of the eaton by pass is like this. When the throtal body is closed, it makes a good bit of vacume compared to atmospheric, between the throtal body and the blower. This vacume causes the by pass valve to open, and allow air from the high pressure side of the blower to flow to the low pressure side.

If you don't open the throtal up, the vacume on the upstream side of the blower never gets below a threashold that closes the by pass. With the by-pass open, there is no way you can develop any boost. All of the air the blower just pumped into the manafold flows right back around to the low pressure side of the blower. Like when you reved the engine in neutral, I'll be you never opened the throtal wide enough for the vacume to drop enough to close the by-pass valve.

When you go to WOT, or nearly so, the vacume is basicly nill. This lack of vacume causes the by-pass to close, and you get boost. If the by-pass is progressive, then you'd get a steadly increaseing boost as the vacume on the upstream side of the blower is decreased. If not, it all should hit you at once, with a bit of control as you decrease the vacume further on the upstream side.

The fact that you have too much boost eighter says to me that you've got a bad guage, or maybe the wrong pully? I'm unclear if the by pass is suppost to hanndle over boost too.

I haven't honestly played around with this myself. This is what I get from seeing other cars, and from the Eaton stuff.

Jim Nowak

Andy,

When you rev your engine at idle you will probably not see much boost but if your engine is under load then you should see boost. On my supercharged car, I have a boost gauge before the blow-off valve and an additional vacuum/boost gauge that takes its reading a few inches away from the intake valve. I can rev the engine all day long in park and only make one or two psi of boost near the intake but the boost gauge before the blow-off valve is reading every pound of boost that blower is producing.

GoRideSno

Thanks Jim,
I think that answers my question. That is pretty much what happens. I measure boost/vac only in the intake manifold.

Normy,
The roots does have a greater volumeteric efficency at higher rpms. An example is below.

GoRideSno

<img src="http://boards.rennlist.com/upload/Mp112Graph.gif" alt=" - " />

rjtw

Congratulations Andy -- super achievement. Sounds like power will NOT be a problem! ;-)

At the risk of asking about something you may have posted on already, what in the end did you do with fuel enrichment... RRFPR, injectors, ? Or are you sticking with stock so far?

Can you give me a general overview of the design parameters on your manifold? It looks beautiful, by the way. Did you basically just fab something that fit the intakes on one hand, and the SC on the other with proper clearance, and move the MAS and throttle to the side? Any fancy volume or airflow considerations? Did you have to move around any components like the filler neck? What did you do for injector placement? What materials did you use? I'm wondering how you solved (one of) the hard parts, namely mating up with the intake ports and matching their shape (perhaps clever reuse of an old manifold?)

Finally, pulleys... I have yet to figure out what the exact deal is on pulley sizes on the various kits out there. Ideally, all pulley sizes (on the crank) would remain exactly as stock, then you either drive the SC off an existing belt location or put on an additional SC-dedicated belt (obviously preferable). What did you do, and was it hard to get a custom crank pulley manufactured? (Can you share references?)

All just in case one day I decide to go the custom route too, on my '83. I've been following your project with GREAT interest!

Way to go dude!
Rick

rjtw

Hmmm, now that I go back and look at your picture in the last post, I see the big red RRFPR at the back! duh..
Rick

Normy

Go-

Volumetric efficiency refers to the engine's ability to fill its cylinders, not the superchargers' output. VE depends more on camshaft timing and port design than blower output, which is constant.

This is proven with the inlet flow graph, which shows a linear relationship between flow and RPM.

N-

John.

Better watch that 10 psi number, you are on cast pistons.