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morghen
Joined: 21 Jan 2005
Posts: 9095
Location: Romania
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 Posted: Fri Aug 24, 2007 7:19 pm Post subject: |
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hmm...i knew the 924 and the 924 carrera gt cam are the same....
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Supercharger and EFI kits
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Fri Aug 24, 2007 11:19 pm Post subject: |
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| Rasta Monsta wrote: | | Ooops, sorry. The chart is quite interesting!! |
Thanks! 
By the way, if anyone downloads the spreadsheet, I should point out that all you have to do is edit the variables in the yellow cells. That is, boost, bore, stroke, rpm, and VE (you can also edit the two constants, atmospheric pressure [if you want to see how poorly your car will perform at 6500 feet, for example] and the # of cylinders [in case you want to consider the ol' Grand National engine swap  ]). Everything else is calculated. The goal was to produce an idea of a range potential BHP output (in the red cells), but also to produce the required flow and pressure ratio numbers, which are required in order to plot your power goals onto a compressor map. On most compressor maps that I've seen, the pressure ratio is on the Y-axis (vertical) and the flow is on the X-axis. Some of the maps represent flow in CFM and some in cm3/sec, which is why I calculated both numbers.
So for example, with the spreadsheet, I was able to conclusively determine that the K26 cold side is NOT sufficient for my targeted goals. Hence, I am pursuing a hybrid turbo with a K27 cold side, which according to all of the maps I've seen should easily accommodate my goals. The nice thing about the K27 is that it will be much more efficient at producing the range of CFM I will require, which in turn will reduce the charge temp, which will make more power.
That was the idea behind the spreadsheet.
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Fri Aug 24, 2007 11:24 pm Post subject: |
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| leadfoot wrote: | | ...very nice work...As for your build it sounds excellent |
Thanks!!!
| leadfoot wrote: | | what are the GT cam specs out of interest? |
| morghen wrote: | | i knew the 924 and the 924 carrera gt cam are the same |
According to Jorge at Eurorace, the Carrera GT cam has a different profile, because they list it separately. When I first inquired, he didn't have the numbers handy, so I don't know precisely what they are. But I do believe they are different. It is an assumption right now on my part, and I have yet to confirm precisely what I'll do with the cam. When it comes time to send the head out for rebuild, I will confirm everything. In the meantime, if anyone has authoritative info on the Carrera GT profile, it would be great to see it.
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Fri Aug 24, 2007 11:42 pm Post subject: |
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Carrera GT cam specs:
Data: B=87,4 S=84,4 V=2025cc Valves I=42,4mm E=38,4mm CAM = "DRAKE 476" lift 0,476" or 12mm Compression ratio = 6,7:1
More info here.
And from our tech section:
| Quote: | Camshaft
Schrick in Germany had a few good grinds 15 years ago; but I do not know who is dealing for them here in the US. JPasha
Webcam is another cam grinder, favored by a lot of PCA Club Racers in the US
David Ewing, "I was chatting with Mr. Dimitri Elgin at Elgin Cams about the 931 camshaft profile. He related the camshaft used in the 924 and 931 are
identical. Porsche engineering relied on the boost to
increase the charge volume, no provisions to optimize the breathing of the head with the turbo.
The part number is located on the casting near the distributor gear."
Intake Duration@.050" Lift Exhaust Duration@.050" Lift
222 .470" 222 .465" |
So it appears the Carrera GT profile is a bit different...
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Min
Joined: 04 Nov 2002
Posts: 2368
Location: Vernon, British Columbia, Canada
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| Posted: Sat Aug 25, 2007 1:20 am Post subject: |
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| ideola wrote: | | 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 feel the need to point out that your last few posts have very little to do with your spreadsheet.
And of course, ignore that your the one who posted all that information on your build in this thread.
Nice spreadsheet.
Min
p.s. talking about your build, is actually on topic since its directly related to the spreadsheet and why you made it. You just told us to stop talking about it becuase you didn't want to hear the things we were continueing to say. At least thats the way it appears from my perception.
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1980 n/a with EDIS and Megasquirt II Injection. 7 different colors and counting. |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Sat Aug 25, 2007 1:30 am Post subject: |
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| Min wrote: | | your last few posts have very little to do with your spreadsheet |
Agreed...I was as guilty as the rest at allowing the topic to drift off-topic. My point is that I was hoping to solicit feedback on the spreadsheet as a useful tool. I did not intend for this thread to be about my specific build. I will start another thread on that topic and you can all have at it. When I start the new thread, I will edit my posts here and cross-link to the thread on my build.
In the meantime, if there are suggestions for how to make the spreadsheet more valuable, useful, etc., I would greatly appreciate it.
| Min wrote: | | You just told us to stop talking about [your build] because you didn't want to hear the things we were continuing to say. At least thats the way it appears from my perception. |
Well, there you are wrong. First, I never said stop talking, and I explicitly stated that I wanted people's input on my build. So how you can say I didn't want to hear what you all were saying is beyond me. I had / have a difference of opinion regarding the priority of adding EFI. We'll take that offline into another thread. But just because I have a difference of opinion doesn't mean I dont' want to hear what people think. Sheesh! Lighten up a bit, man.
I do value people's input. I want suggestions for my build. But I'm reiterating, I did not intend for this particular post to be about my build. Yes, I built the spreadsheet initially to help with my specific build. However, I posted it here in hopes that it would be a useful tool for others who would presumably have to go thru the same thought process when considering a modified engine.
{EDIT}
I've moved my specific build information into a new thread to help keep this one focused on the spreadsheet. For info on my specific build, go to the official Turkey Shoot Open Invitation: Dan's Ultimate 931 Build
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erstwhile owner of just about every 924 variant ever made
Last edited by ideola on Sat Aug 25, 2007 3:41 am; edited 1 time in total |
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Min
Joined: 04 Nov 2002
Posts: 2368
Location: Vernon, British Columbia, Canada
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| Posted: Sat Aug 25, 2007 2:38 am Post subject: |
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| ideola wrote: | | In the meantime, if there are suggestions for how to make the spreadsheet more valuable, useful, etc., I would greatly appreciate it. |
Nope, no suggestions, looks good to me. I was planning on doing something similiar eventually after I read maximum boost, just never got around to it.
Min
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Sat Aug 25, 2007 3:43 am Post subject: |
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Incidentally, correct me if I'm wrong, but all of the stuff in the spreadsheet would also apply to a supercharger setup. Of course, it doesn't calculate for the parasitic power loss of a SC, but it should still be helpful in terms of determining target BHP as well as determining SC flow requirements...
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Min
Joined: 04 Nov 2002
Posts: 2368
Location: Vernon, British Columbia, Canada
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| Posted: Sat Aug 25, 2007 4:54 am Post subject: |
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| ideola wrote: | | Incidentally, correct me if I'm wrong, but all of the stuff in the spreadsheet would also apply to a supercharger setup. Of course, it doesn't calculate for the parasitic power loss of a SC, but it should still be helpful in terms of determining target BHP as well as determining SC flow requirements... |
I believe the basics would apply, someone out there with corky bell's supercharged book should pipe up though.
Min
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RC
Joined: 25 Mar 2007
Posts: 2637
Location: Australia
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| Posted: Sat Aug 25, 2007 8:53 am Post subject: |
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Although this basic spreadsheet appears to give a similar power output in the baseline column to a stock 931 it does not take into account ALL the VARIABLES that will affect your planned rebuild.
You may get a vague indication, which may be sufficient for your purpose. Professional engineers in F1 and similar use very complex modeling that won`t be available as a free download.
We do not know accurate VE, headflow data, CR, rise in turbo and exhaust back pressure, rise in IAT, cam and flow data at lifts, etc.
The factor that stood out like a neon sign to me was the 7500 RPM with your stroked crank. Longer stroke will improve torque at the expense of higher RPM, due to increased rod angles and piston speeds. While it may be possible to get this engine to do 7.5K by forced induction it will be your gamble to do so. A stock stroke engine at 6.5K has a max piston speed of 60 feet/second or 3600ft/min which according to my information (that is out of date, but more modern than our motors and CIS) is at the upper limits for a stock engine. Your intended stroke and 7.5K corresponds to a piston speed of nearly 4500ft/min, a 25% increase, which places it in the extreme racing category.
FYI google piston speed calculate for heaps of online calculators, and more maths and information than you will want to read. Do your own research and decide how fast you want to spin with a welded crank and stock main bolts.
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Sat Aug 25, 2007 10:38 am Post subject: |
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Good stuff, RC. Couple of comments:
1. The spreadsheet is obviously NOT a replacement for high-end modeling. It was intended solely as a sanity check...All I really wanted to do was validate whether or not it might be feasible to hit the 300-350 BHP...and I believe, for that purpose, the spreadsheet is useful. I also want to generate an idea of the flow and pressure ratio requirements so I could do an adequate job of finding a suitable turbo setup. Again, for that purpose, I believe the spreadsheet is useful.
2. Let me reiterate what I've said at least a couple of times already: my build is NOT based on Scenario 3 in chart. It is based on Scenario 2. In that scenario, I simply plugged in 7000 RPM, being ignorant of the necessary formulas to figure out achievable RPM. It was a seat-of-the-pants guess. Ironically, if you look at numbers for the GT, GTS, and GTR, they had RPM limits of 6800-7000 RPM, so in retrospect, I think it was a pretty darn good guess on my part. Notwithstanding, the stroked crank might adversely impact what is achievable, meaning I'll have to adjust my expectations to some extent.
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Sat Aug 25, 2007 11:10 pm Post subject: |
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I've updated the downloadable spreadsheet to now include a calculation for flow requirements in lbs/min. I also tweaked my target build just a bit (reduced the RPM range to match what we know works on the GT series of 924s). Here is a jpg of my updated numbers, with my target boost level highlighted. The prediction ends up being between 296-366 BHP, which is what I've been targeting all along. So far, so good.
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Tigger937
Joined: 11 Apr 2004
Posts: 919
Location: PCA Milwaukee Region
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| Posted: Sun Aug 26, 2007 5:16 am Post subject: |
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| ideola wrote: | Carrera GT cam specs:
Data: B=87,4 S=84,4 V=2025cc Valves I=42,4mm E=38,4mm CAM = "DRAKE 476" lift 0,476" or 12mm Compression ratio = 6,7:1
More info here.
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I wasn't aware the GT had a dry sump???
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jrcook320
Joined: 02 Nov 2005
Posts: 67
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| Posted: Tue Oct 28, 2008 2:38 pm Post subject: |
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Nice spread sheet. Ideola, you might be interested in the sheet I put together for my research.
I'm in a similar boat with no head flow data, but I used desk top dyno to try to make a decent model of my engine prior to the turbo, then use those VE% numbers to calculate lbs/min while taking into account as many factors as I reasonable could. You could easily download desktop dyno from your favorite torrent website, model your motor, punch in your VE #'s, bore and stroke and your turbo's efficiency, ambient temp, etc.
I can email it to you if you're interested, or you can download it here:
http://sites.google.com/a/e21legion.net/jrcook320/Home/turbo-project/turbo-calculations
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Tue Oct 28, 2008 10:52 pm Post subject: |
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JR, I've download the goodies at your link, thanks for the reference! Also, I've been searching high and low for desktop dyno, have not been able to find it...
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