# Advanced Engineering Tech - Displacement formuals

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Cop Car
11-10-2005, 09:15 PM
I know there is no replacement for displacement... except boost. Is there a mathmatical formula out there to calculate the "displacement" of a boosted motor related to its displacement (cubes or CCs) and the #s of boost?

Ben R
11-10-2005, 09:45 PM

11-10-2005, 10:56 PM
Try this:

http://www.motorsportsdigest.com/tech/forced2.htm

http://www.turbofast.com.au/tfcalc.html

I guess it's not a one step calculation. You'd have to figure out what the
engine is ingesting at a certain elevation, and what the VE of the engine is
naturally aspirated. Maybe some sort of correction factor as well.

From that data, every atmoshpere theoretically doubles the effective displacement.

Just keep in mind that math is as good as the data you start with, as it's only
going to get you in the ball park.

Ben R
11-10-2005, 11:05 PM
I can assure you those calculators are NOT accurate.

11-10-2005, 11:06 PM
Never said they were (as per post). But please go ahead and assure me.

I'd like to know how far off they actually are?

11-11-2005, 01:14 AM
Here's another good article from Hot Rod Magazine. A few references to
math in this write up as well.

http://www.hotrod.com/techarticles/engine/113_0312_turbo/index2.html

Damn, that turbo calculator was only off by EIGHT HP when mathing up the
variables on the compressor charts.

Richiec77
11-11-2005, 01:54 AM
I don't have anything in front of me but you can do rough calculations.

BUT, that is it. Just rough, ballpark figures. You have to account for a bunch of variables that can change too easily due to atmospheric conditions to accuratly predict it. Just like in an NA application. NA is easier due to less complexity, but even those charts are plain rough also.

I'm basically explaining What Ben meant by No.

11-11-2005, 02:09 PM
I think all formulas are meant to be used as a guideline.

Does that mean we need to get rid of all formulas because they are not exact?

Even calculating DCR is subjective. As long nobody is pointing a dyno at
select parts, and setup budgets for your engine builds.

I don't think anyone can dispute that boosting a 350 cube motor by 15 PSI
will get you an effective displacement of about 700 cubes. That's basing the math
on 100 efficiency.

Garbage in = Garbage out

DanO
11-12-2005, 12:56 PM
It depends where you measure boost at. Every component in the intake track has a pressure drop.

For example(theoretical):

Engine #1
Stock 346, ls1 intake, etc...
Place a FI unit and measure boost. You get 9lbs

Engine #2
Place SAME FI unit and measure boost. You get 6lbs

What happened? Engine 2 is making more power on 6lbs?!!?!? Its all volumetric efficiency (VE) as stated above. The FI unit does not have to compress the air as much, less heat generated, FI unit in more efficent range, more flow = more power!

So in essence, measure the mass airflow not the #. In all reality PSI doesnt mean crap. The only thing PSI matters for is the efficiency range the FI unit is operating in. Mass flow is what you are concerned with for power

Cop Car
11-12-2005, 02:13 PM
yeah im familar with 9 psi on x motor not being the same as 9psi on y motor, its just a measure of the manifold pressure. just wondering if i could get an idea what the theoretical displacement on a motor is. 4 cyl guys brag about how much more power they make on less cubes, but becuase of FI they have the same efficency as a larger N/A motor

Moldmaker
11-12-2005, 11:11 PM
If you are after a barometer of engine efficiency then study up on BMEP (brake mean effective pressure).

You can find more with a search than I could possibly post here.

Good luck.

DanO
11-13-2005, 09:26 PM
If you are after a barometer of engine efficiency then study up on BMEP (brake mean effective pressure).

You can find more with a search than I could possibly post here.

Good luck.

I belive there are a number of parameters to measure for a complete comparison, however, i belive the term you were looking for is BSFC for engine efficiency (Brake specific fuel consumption)

Generally, an optimized small FI engine will have a much greater BSFC than a larger displacement NA engine.

For example Eatons SuperTurbo program (Now called VW Twincharger) has much greater BSFC than a larger NA engine of equal power.

Moldmaker
11-15-2005, 10:52 AM
No, I was actually looking for BMEP. IMO, BMEP is the better yardstick for comparing efficiency -an engine's ability to turn an a/f mixture into hp- than comparing fuel consumption per bhp. I'd compare based on peak output, tq/cid at peak power if you will.

Trying to determine some theoretical displacement for a turbo/supercharged engine IMO isn't worth the hassle. Not with so many variables in the mix. I'd rather calculate the means to the end, ie. manifold absolute pressure required to meet X horsepower target.

DanO
11-15-2005, 01:41 PM
No, I was actually looking for BMEP. IMO, BMEP is the better yardstick for comparing efficiency -an engine's ability to turn an a/f mixture into hp- than comparing fuel consumption per bhp. I'd compare based on peak output, tq/cid at peak power if you will.

Trying to determine some theoretical displacement for a turbo/supercharged engine IMO isn't worth the hassle. Not with so many variables in the mix. I'd rather calculate the means to the end, ie. manifold absolute pressure required to meet X horsepower target.

I'll agree to your statement interms of comparing engines, however, in terms of turning fuel into hp, BSFC is the direct relation... however, as you stated, there are far to many variables to determine "comparitive displacements"

Old SStroker
11-15-2005, 02:39 PM
No, I was actually looking for BMEP. IMO, BMEP is the better yardstick for comparing efficiency -an engine's ability to turn an a/f mixture into hp- than comparing fuel consumption per bhp. I'd compare based on peak output, tq/cid at peak power if you will.

Boy are we on the same page! BMEP (tq/cid) at peak power has been my mantra for a while now. In the current RaceTech rag there is a good article on Everham's Cup engine development. Best current numbers are power @9100, up from 8900 earlier. Max rpm is 10K, but gear rules are limiting that to 95-9700 with as much as 850 hp on a longer track open engine. That's about 490 lb-ft @9100 or 1.37lb-ft/in^3 @ power peak. That's about 205 PSI (14 Bar) if I remember my formulas. Torque peak was at about 7200 rpm.

2003 BMW F1 engine numbers from BMW indicated max rpm of 19200 with an extimated power peak of 18000 and 900+ hp. Using 900, and 18000 I got 1.44 lb-ft/in^3 or about 5% better than a Cup engine. It amazes me that the Cup guys can get that kind of efficiency from 2 valve flat tappet pushrod gasoline endurance engines. If you don't make the power to get to the front, BSFC isn't going to win you many races.

Vents
11-15-2005, 07:21 PM
:edit

missed the point. dont mind me..

Moldmaker
11-17-2005, 10:42 PM
Thanks Old SStroker,

I'm glad I'm not the only one who looks at BMEP as a barometer of engine development. In all honesty, I never thought much of the matter until reading a paper by Yashio Nakamura -Honda R&D- some years ago on the subject of high performance engine development. I find myself calculating BMEP often now. :)

Ric
11-19-2005, 01:51 PM
I know there is no replacement for displacement... except boost. Is there a mathmatical formula out there to calculate the "displacement" of a boosted motor related to its displacement (cubes or CCs) and the #s of boost?

There's really too many variables to come up with a feasible, relatively easy formula. You'd have to consider things like ACTUAL head flow, measured boost pressure, volumetric efficiency, blah-blah blahh.

Let's talk big Mopar motors, for example. Stock 440 Magnum pushed 375hp. I've got a Mopar 440 pushing mid-500s hp. A friend of mine also has a Mopar 440, same displacement(including overbore, stroke, etc.), pushing over 1500 hp in a Pro Street drag car. We haven't even talked about a boosted 440 yet, either. My guess is it'd be scary fun, though. :)

DanO
11-19-2005, 07:21 PM
BSFC isn't going to win you many races.

I dont think anyone said it was....

Old SStroker
11-19-2005, 09:34 PM
...BSFC isn't going to win you many races..

I dont think anyone said it was....

..in terms of turning fuel into hp, BSFC is the direct relation....

HP wins a lot of races. Evidently I misinterpreted you, Dan.

DanO
11-22-2005, 07:48 AM
If you are after a barometer of engine efficiency then study up on BMEP (brake mean effective pressure).
.
HP wins a lot of races. Evidently I misinterpreted you, Dan.

Not knowing if that was extreme sarcasm or not...

BSFC = Fuel to HP (efficiency, like i stated)
BMEP = Displacement to Work output (independent of fuel input..i.e. cannot gauge effieciency)

When efficiency was mentioned, i said BSFC should be used. Am i wrong?

When comparing and engines power output capability per displacement you should use BMEP.(i.e. great for comparing engines...)