924Board.org

ideola · Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI

I've been reading Corky Bell lately. I put together a spreadsheet to help me calculate achievable power output numbers, and thought you all might be interested in looking at the results. Here is an image, along with a link to an downloadable, editable spreadsheet that you can all play with.

{EDIT} Now available online as a public Google Docs spreadsheet.

This doesn't take into account every possible variable, but it allows you to plug in the key modifiable parameters of the engine (stroke, bore, volumetric efficiency, rpm and boost) and determine reachable BHP numbers. It also provides flow requirements and pressure ratios, which should be useful in mapping your goals onto a compressor map to see if it will provide the boost and flow required for your application.

Comments and suggestions for improvements to the spreadsheet are welcome. I don't profess to be an engineer, I'm just applying some basic principles and formulas to help estimate (and that's the key word) whether my HP goals are actually achievable.

Enjoy!

{EDIT}
I put the flow requirements in CFM and cm3/sec because some of the maps have flow expressed as CFM and some as cm3/sec ... this should help in the conversion process so you can plot your goals onto a map regardless of the unit of measure used on the map...
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erstwhile owner of just about every 924 variant ever made

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11723 Location: PacNW

bass gt · Posted: Thu Aug 23, 2007 7:43 am Post subject:

SteveVery Interesting.

As a point of reference, my car is astock displacement engine, but with improved intake system, and slightly better turbo. My car made 250 crank BHP at 12 psi of boost, so somewhere in the upper regions of the stock figures.
more boost made more power, but in a diminishing pattern. I think the backpressure of the turbo started to choke it up, and the air temps got very hot
oh, BTW, ou have tooooo much time on your hands

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Front Wheel Drive is the Devil's work.

jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

RC · Joined: 25 Mar 2007 Posts: 2636 Location: Australia

Yes, since you do appear to have a lot of time available how about calculating the piston speed at 7.5K with your stroked crank and comparing it to the stock one at Porsches 6.5K redline.

See you have crower rods but what other mods are you intending on the botton end?

IMHO it would be more effective to do the EFI first but ultimately t is your time and money.

Roger

ideola · Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI

Too much time? Why? I was going thru these numbers anyway to see if my goals were realistic...it was faster to throw together the spreadsheet than to sit down with paper & pencil and write out a thousand different calculations, lose the paper, and have to redo the calcs all over again

Just my analytical mind at work.

ideola · Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI

bass gt · Posted: Thu Aug 23, 2007 6:14 pm Post subject:

i was just ribbing you!! I know how much time it takes to complete these projects!! I have started on next years race config now, even though the season is not starting until April 2008. Time flies ect.
My optimised set up is based upon a good EFI system and ECU to control it. I believe this is the single most significant improvement we can make to these cars. It sems to clean up the running, improve ignition and fuelling, thus increasing the BHP in one move. I would be suprised if the EFI/ECU and Intercooler alone once set up correctly are not worth somewhere in the region of 40-50bhp.
As you say, the book is 10 years old, and although technology has moved on, i think the fundamental principals within hte book are still valid.

Regards,

Steve
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Front Wheel Drive is the Devil's work.

Min · Posted: Fri Aug 24, 2007 1:17 am Post subject:

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11723 Location: PacNW

+1 to that. . .remember how CIS fuel metering works. . .the shape of the cone the metering plate moves up and down in is how the fuel delivery is enginieered into the system for differing RPM and load conditions. One shape=no adjustablility. I think changing basic engine characteristics as you are suggesting (bore, stroke, cam profile) will be very problematic for the CIS system.
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WeiBe (1987 924S 2.5t) - 931 S3

ideola · Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI

Don't disagree. I'M NOT PLANNING TO RUN CIS LONG TERM...JUST LONG ENOUGH TO GET THE MOTOR RUNNING...AND THEN TACKLE EFI.
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erstwhile owner of just about every 924 variant ever made

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11723 Location: PacNW

I understand, but do you want your fresh build leaning out at an inoppurtune time?
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WeiBe (1987 924S 2.5t) - 931 S3

ideola · Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI

OK, I think it's time to start a new thread. I'm open to suggestions on my build, no doubt, but the point of this particular thread was to solicit comments on the spreadsheet as a general tool to plan an engine build.

I'll start another thread to solicit feedback on my specific build.

{EDIT}
I've moved the aforementioned information into a new thread to help keep this one focused on the spreadsheet. For info on my specific build, go to the official Ideola's Ultra Wide Body 931 Build
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erstwhile owner of just about every 924 variant ever made

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11723 Location: PacNW

Ooops, sorry. The chart is quite interesting!!

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WeiBe (1987 924S 2.5t) - 931 S3

leadfoot · Posted: Fri Aug 24, 2007 4:23 pm Post subject:

very nice work, often optimistic to assume %VE efficiencies,
what are the GT cam specs out of interest?

As for your build it sounds excellent,
Leadfoot
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1981 ROW 924 Turbo -
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