On Boost or not?

[datman]

After sorting a few problems I have my car running really well. I do have a question about boost/timing though.

How do you know when you are actually pushing air into the engine rather than it sucking it? I assumed that anything over 100kpa or 0 on my bost gauge was boost but this is not the case, If an engine has poor flow and restrictive manifolds it be pushing air before reaching 0, if this is the cse do I start retarding timing the crossover point?

Is there an easy way to measure this?

 

Thanks

 

Oh, obviously I am talking about Turbo engines

[gjc5500]

anything above idle is technically boost. as soon as the needle starts to move, you are pushing some

[datman]

Not according to Mr Bell it's not

[datman]

"because many stock cars have restrictive intake systems, it is not at all unusual to record a manifold vacuum of 4 - 5hg (more than 2psi) at wide open throttle. Therefore, when we add some form of supercharging to a car like this, anymanifold pressure higher than -4hg will in effect be boost pressure, and fuel mixture will begin to become lean long before manifold pressure climbs above 0psi.

 

I guess I could just unhook the intake from the IC log a drive at WOT and see what kind of map reading I have.

 

Or...is this not necessary and I'm missunderstanding the whole thing? (probably )

[WizardBlack]

It's really a matter of semantics or how you look at it, to be honest. Most people refer to boost as positive pressure (with respect to ambient). In reality, the car will flow more air even at vacuum levels close to zero since the turbo is 'helping it along' compared to normally aspirated.

I think you are wondering about this from a timing/tuning standpoint. Generally timing starts to taper down after zero psi (relative) although some engines may like a degree or two less right at zero. Most of the time, once you reach the minimum rpm (~2500rpm or so), the car uses about 40 degrees timing at all vacuum point until you get to the limits at almost full vacuum or right at zero; both where it may use 37~39 degrees.

Ultimately, you can search the megasquirt section; there are timing maps posted.

[Tony D]

Boost is a manifestation of restriction to flow.

If you have 100KPa in the manifold, you are WOT, and running an efficient air pump.

Generally, because of the way carburettors work, there has to be at least 4-5" Hg Vacuum in there at WOT, at maximum RPM...but BEFORE that point you will register 0"...it's not until the engine starts flowing more than the manifold supports will you notice the restriction...Many times the engines are flowed to be at peak torque. WOT max RPM is not really given that much consideration in most passenger vehicle manifold designs. Peak Torque is where it's optimized.

Like WizB said, it's semantics. I run 35kpa at idle, and don't start pulling timing until I'm ABOVE 100.

 

Usually 1 degree per psi from a total of 38 when at high cruise. Say 3500rpm and 30-40 map. Were I at WOT at that same rpm, and running 10psi, my timing would be around 28 degrees, and closer to 18-20 at 20psi.

 

Don't worry too much about what Mr. Bell has to say, there is some dispute to some of his contentions, and his book is using dated technology. He is not, as contended by some, Gawd...

[datman]

Thanks Wizzard and Tony. I'll modify my timing now I have that info. My map tells me I have 30kpa at idle.

[Tony D]

Yes, with a stock cam and setup that is about right, depending on altitude. I don't figure much elevation in the UK... I am at about 700-1000 ft above sea level to get the 35 kpa at idle.

Our Bonneville Car, on the other hand, idles around 55-60 kpa...a bit more cam there! LOL That one uses an alpha-n map 'blend' software mix to get it off-idle smoothly as with MAP based only, and all the throttle area we have, a simpl cracking of the throttle will flood the engine as it immediately goes to 100!

 

Timing is usually 1 psi per pound of boost on the gauge. Some places more, some places less. All depends on what you find when tuning, and hte octane available where you live/drive. For reference the old Corvairs ran 24 total advance at 5-7 psi, and my old N/A distributor was set to run around that same point at 10psi. So it is possible you can run a lot more advance below 10psi than you think, and then start retarding the spark above that point. All depends on what you find. The Megasquirt Stickie on the board has a bunch of maps on it that you can take a look at to get an idea what other people are doing---that will reveal many similarities especially if you lay it all out on graph paper and write the bins in to correspond to your kpa readings...you will see the scaling makes more of a difference than each individual engine does. For a stock setup they are all pretty much about the same.

[datman]

Thanks Tony, I'm 30mtrs above see level here (pretty low) The air is nice and cool and my MAT hardly goes above 22deg C even under 10psi boost. The fuel I use is unleaded 99 ron...100 ron is available at some petrol stations but 99 is everywhere, expensive in comparison with your Gas..right now its around 1 pound (GBP) a litre. 4.55 Ltr to the Gallon (UK) so thats about 10$ a gallon.

[Tony D]

When I was in Liverpool last year, it was 99p/litre for diesel.

Gave my 'driver' 60 L for his Mitsu SUV for taking me on a pilgrimage to Wrexham... Couldn't be in the area without visiting 'The Demon'! LOL

[datman]

The Demon as in "tweeks" ?

[Tony D]

Is there any other?