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Old Mon, May-02-2005, 11:58:06 PM   #11
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Quote:
Originally Posted by m3 hammerdown
Lets assume we are comparing a stock M3 with X brand blower to one with the same blower and mods.

A centrifugal blower will spin only as fast as it is mechanically spun by the belt assembly. For example, lets say the blower pulley (which is attached to the impeller inside) is run directly off of the crank pulley as is most often the case in supercharging (Not turbocharging!! They are driven by exhaust gas). Lets also assume that the final drive ratio results in the impeller (nasty blades inside the supercharger) rotating at a fixed speed of 56000 rpms at 7000 rpms of engine speed. Yes, that fast, sometimes even faster depending on the pulley setup! We now have measurable boost between the supercharger and, say, the throttle body.

Now lets assume your unmodified car, with that blower setup, went from 333 hp to 393 hp (I am picking these numbers out of the air) for a net gain of exactly 60 horsepower. Lets also assume that my car with identical blower setup has a much larger throttle body, more free flowing intake manifold, larger intake valve and head porting, and more aggressive cam lift and duration.

Question 1. Do you think the stock (unmodified) car would make more horsepower than the modified one? VERY unlikely.

Question 2. Which car do you think will make more measurable boost at the same impeller speed? The one with the free flowing intake tract or the one thats like sending a leaf blower through a straw? The stock car makes more boost because it has more backpressure, while the boost in the other car is being used more efficiently for its intended purpose....getting raw material to the cylinder efficiently and making greater horsepower! Do you want more boost on your guage or do you want to stomp the guy with more boost than you in a race???????

Lets also assume that both cars have a compression ratio of 11.5:1 but the modified car has a larger combustion chamber than stock due to replacement pistons or whatever other mods may contribute. Which car is getting a larger amount of air and fuel into the cyinder? Regardless of the ratio to which it is pressurized by the piston (compression ratio), more air and fuel in the cylinder makes more power. You can have 10 pounds of internal pressure in a balloon or ten pounds of internal pressure in a blimp. Which has the capacity to hold more material and produce more kinetic results after combustion?

Therefore the assumptions behind those two myths I had discussed are dangerously inaccurate.

Generally speaking forced induction cars want to open the flow of the intake path as much as possible. The really important and more secretive part of the tuning is in the experimenting with cam grinds, valve overlap, fuel and ignition curves, and head/valve/piston work. The possibilities are endless.

I'll get back to this with some more examples later. My wife needs to finish a paper for her Master's Degree LOL.
Excellent, thanks for the wonderful write up.
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Old Tue, May-03-2005, 12:19:34 AM   #12
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Thank you very much. You have touched up on some topics that i have not fully understood for a while.

I am also scientifically inclined - chem major - so dont be afraid to get somewhat techical with my questions.

Your Subject #6:
I have always thought that if you lower your compression ratio you actually increase the amount of volume of air that you can fit into the combustion chamber. Thus, for simplicity lets say you lowered your compression ratio by exactly 1/2. Thus, in order to achieve the same cylinder pressure as your stock compression ratio you must add exactly twice the amount of air (add boost). So, in this situation you now have two different setups with equal cylinder pressures (Effective Compression Ratio). But one contains twice as many Oxygen molecules and thus a larger amount of energy after combustion.
So, in theory, you can effectively lower your compression ratio, run abit more boost and obtain the same effective compression ratio as before and make more power??
Is this correct?

This can also be achieved by boring out your cylinders to contain more air.
-----
Also, why is high compressin so valuable?
I figure b/c upon compression your adding energy to the molecules (increasing its temperature) and you are returned with same energy + energy from combustion upon the combustion/expansion?
-------
Also, i agree that boost is comonly taken to equate more power. But it is true that a better flowing blower will actually make more power with less boost.

Take a Supra TT stock turbo's at 20psi versus a T-88 at 10psi. The TT @ 10psi will make more power b/c it flows better. Boost just means the amount of air that has yet to reach the combustion chamber, which usually takes part in the power process.

So, i picture boost as a room filled with angry people wanting to get to the other room via doors (valves).

Is this all correct???
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Old Tue, May-03-2005, 12:49:59 AM   #13
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You should ask this question in the thread so other people are informed. Cool down is less important for S/C because it is not turned by exhaust gas and therefore not as superheated during use. It simply runs at the temperature of the engine oil. Good question though. No "turbo cooldown" is necessary.


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Originally Posted by IM3
Hi,
Ive been reading the FI for dummies and I have some questions about having to mantain the FI systems.
I just ordered a ESS S/C system and being shipped atm.

Should I treat the SC system to a Turbo system in mantainance wise?
Do i need to install a timer (cooldown & startup) and Monitoring gauges and water cooling systems? To monitor the whole system?

This is my first time playing with SC.
Billy (IM3)

ANy other inputs are much appreciated as well.
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Old Tue, May-03-2005, 12:51:42 AM   #14
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I have always thought that if you lower your compression ratio you actually increase the amount of volume of air that you can fit into the combustion chamber.
Yes thats true, assuming that you are lowering your compression ratio by making the combustion chamber bigger ( different piston, enlarge chamber with porting, etc...). Then you can add raw material to the cylinder and compress it into the larger chamber, arriving at the same overall compression ratio, but with more air and fuel present. Thatis exactly why racers build "blower motors" with lower compression pistons and larger cc combustion chambers. They are not after "compression ratio" itself as a goal, they want to optimize the air and fuel in the cylinder.


Quote:
Also, i agree that boost is commonly taken to equate more power. But it is true that a better flowing blower will actually make more power with less boost.
Yes! What we are interested in when measuring the efficiency and capacity for work that any given blower provides is not "boost" but cfm. The more volume of air it can move the more efficient it can be at a lower impeller speed. It may also generate less heat at that boost level eliminating the concerns I have mentioned before as a side effect of the inlet air temp rising......detonation, power loss, and potential engine damage.
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Old Tue, May-03-2005, 01:05:14 AM   #15
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Elaborating on IM3's question..............turbochargers function in principle similarly to superchargers in that they compress air or "boost" into the engine. However, where a supercharger is belt driven off of the crank pulley, turbos are driven by channeling exhaust gas from the motor through the housing that drives their impellers.

This results in two fundamental differences.

First, the internal temperature of the turbo can be subjected to thousands of degrees in exhaust gas. The supercharger, by comparison, is usual internally lubricated by the engine oil in most setups (some centrifugal superchargers do not have an external source of lubrication at all) and does not have exhaust gas superheating the unit.

Second, the supercharger doen't suffer from significant "turbo lag" as its impeller speed increases incremently with the increase in crank pulley rpms. Its impeller doesn't build up and lose rpms subject to exhaust flow like a turbo.

BUT (haha you knew it was coming), overall turbos are a much more efficient power adder than superchargers because they don't run as a parasitic drag on the crank. I have seen estimates of the power they consume just "driving themselves" off the engine crank as high as 45%. That means that a supercharger essentially has to make extra power to overcome ITSELF as a burden on the engine. Along with that comes still other concerns such as the stress on crank bearings and crank pulley sideload whcih requires very stout aftermarket crank pulley replacements for high boost applications.

As far as monitoring whats going on with guages (also a good question!).......I would highly suggest instrumentation that can reliably tell you at least these three things: Boost, fuel pressure and oil pressure. It is common, even in some reputable but marginally engineered aftermarket kits for cars to struggle to maintain proper fuel pressure at all rpms. Many kits include a device that ramps up fuel pressure as boost increases by restricting the return line. It works but it doesn't necessarily mean the stock injectors or fuel pump and lines are really up to the task. And guess what happens if you momentarily suffer a severely lean condition.........ugly stuff. When racing or building aggressive street setups we used to put a fuel pressure warning light on the steering column. Any drop in pressure made the light come on and I could get my foot off the gas immediately. Holes in pistons can be expensive!
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Last edited by m3 hammerdown; Tue, May-03-2005 at 01:20:29 AM.
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Old Tue, May-03-2005, 01:15:26 AM   #16
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Quote:
Originally Posted by Grimm333
this should be an FAQ
It will be.
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Old Tue, May-03-2005, 01:37:29 AM   #17
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wow, nice write up, thanks

I offer a couple points for consideration:
in the early 80's BMW extracted >1400hp from an I4, 1.5L, 7:1 Cr...so 1/2 that, with an engine 2x+ larger is entirely plausible...1/4 the efficiency

an engines abs power is ~ to the air pumped thru ~ rpm x displacement
we'll assume a perfect engine 100% volumetric eff, and perfect a/f ratio, and a constant configuration....

cams, porting/polishing, balancing, etc., in an engine as highly developed as a BMW M is a minor factor IMO <10%

at 1 bar (we'll call it 15 psi, actually 14.7) an engine makes X hp
at 2 bar (1 bar boost) it will make 2X hp
at 1.5 bar (1/2 bar boost, 7.5 psi) 1.5X hp

so an oem M3 makes ~280, w/ 7.5 psi ~1.5 x 280 ~ 420, which is within the range we are seeing...actually it will be less, due to increased pressure drop (lost boost) due to the higher flow (Pd ~ cfm^2), but with better intake & exhaust, we'll say these loses are offset...

compression ratio (Cr) has little to do with power in a FI engine...
let's say in a NA you have X cfm and you compress it 10x (10:1)
in a 7.5:1 FI 1.5X cfm (@ 1/2 bar boost) compression ~ 1.5 x 7.5 ~11:1

if this compression factor becomes too high, it ignites, like a diesel...
ie, the lower the Cr, the more boost, that's why it's lowered in a FI
the key is finding the balance, too low no response, too high KABOOM...

same effective compression but 150% more fuel/air...BANG!

now lets put this in perspective...assume oem hp @ 6700 265hp

Scotts car: 6 psi boost ~385hp at 6700 rpm
(1 + 6/14.7) x 265 = 375 hp pretty close to actual, huh

Bogies car: 11/7.5 x 6 times as much boost as Scotts ~ 8.5 psi @ 6700 rpm
(this assumes similar boost curves, not an invalid one)
(1 + 8.7/14.7) x 265 = 420 hp, way off the reported 470+, ~12% more than expected

where could the 50+ hp (12%) come from?
improved flow (cams, porting, etc.)?....some maybe 5%
maybe more boost? 1 psi would yield ~ 12 hp

so it's entirely feasible to get that power...easy in fact....just add boost, until ready to blow, and back off...lol..better head gasket & head bolts will help

btw I calculated the mean piston pressure (mep) for each:
HP = (mep x D x rpm)/833...D=3.2 L

Scotts mep = 149 bar = 2161 lbs
Bogies mep = 184 bar = 2670 lbs

184/149 = 125%, so the gasket & headbolts better be that much stronger
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Old Tue, May-03-2005, 01:46:27 AM   #18
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Compression raio is HUGE when it comes down to the engine's efficiency. Why do you think diesel engines get such good mpg? They have like 20:1 CR.

In the idealized engine cycle (Otto Cycle) the engine's efficiency is proportional to the Compression ratio (ill have to reference my physics book for the exact equation).
That is why i wont go internals unless i have to. It would be ideal if we could run more boost and control detonation.

You could, in theory, achieve more power from a highly compressed engine running the same boost level as a reduced compression engine. Detonation must be controlled.
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Old Tue, May-03-2005, 01:49:42 AM   #19
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Good info Art. We have definitely crossed the technical line that most people will struggle to understand. I'd say the 5% estimate in the search for that extra power may even be a little conservative considering that combination of possibilities we may be dealing with. The cam alone may account for that 5% considering that the M is a fairly optimized NA motor and the demands for a serious forced induction car are quite different indeed. I agree valve and head work may not be a plausible source for significant gains although I must admit a limited familiarity with M specific head design. My mind keeps coming back to the question of piston design. I'd like to have a close look at the top of the pistons in the car in question. Thanks
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Old Tue, May-03-2005, 01:51:31 AM   #20
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damn Art!!! reps if they were still here

this thread is very informative!!!
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