This is topic is the corral blowing smoke up my ass (as usual?) in forum Tech Talk at Northern California Ford Owners  .


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Posted by shade-tree (Member # 298) on :
 
I posted a question asking stock optimal shift points for a '93-'95 cobra motor (GT-40 iron heads) and they responded with:

1-2 6000
2-3 5800
3-4 5700

when I ran the time in my sig I was shifting 5200-5300 (on an accurate monster tach w/shift light)

is the corral blowing smoke up my ass?
 

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Posted by cobraman1994 (Member # 467) on :
 
thats wher i shifted in my 94 cobra. but i had a freakish pull up top around 5500-6000.
 

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Posted by shade-tree (Member # 298) on :
 
turbo? come on, GT-40 heads don't pull past 5200.... or do they?
 

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Posted by sic91sleeper (Member # 779) on :
 
Take it up till it stops pullin....
 

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Posted by 88gt (Member # 579) on :
 

quote:

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Originally posted by sic91sleeper:
Take it up till it stops pullin....

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Ya Shade tree, looks like your but dyno needs re-tuning. You mean you can't just feel it when it stops pulling??
 

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Posted by cobraman1994 (Member # 467) on :
 
thats what i thought too. but right after it would fall on its face(around 5300 or so) it would get this crazy pull all the way up to 6. then i would shift cause thats pretty high on a stocl 5oh. lol
and the car was stock all the way down to the paper filter. except for the monster tach
but after my first couple of mods, it went away and never came back

[ November 05, 2002, 01:40 PM: Message edited by: cobraman1994 ]
 

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Posted by shade-tree (Member # 298) on :
 
well, I well track test it Sat. if it doesn't rain and I'll make a post about it. I have my reservations, but I'm not afraid to spin the motor to 5k. Besides, I'm good at replacing oil pumps now, lol.

5k, lol freudian slip, I meant 6k.

[ November 05, 2002, 01:42 PM: Message edited by: shade-tree ]
 

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Posted by AaronC (Member # 86) on :
 
I believe in staggered shift points with a T-5/Tremec (like the ones suggested). The gear spacing will require more RPM's in 1st/2nd gear. I shifted the stock motor at 5500 in 1st/2nd and 5300 on the 3-4. Usually they say a good guesstimate is 500 RPM past peak HP. Could be more, could be less though.
 

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Posted by shade-tree (Member # 298) on :
 
thanks Aaron. I'm just surprised to learn that GT-40 iron production heads rev that high. I mean I thought they had the same springs as on the E7TE which we all know have issues beyond about 5200 (actual) speaking for myself, my factory tach underestimates about 200-300 rpms under 3k, and overestimates about 200-300 rpms over 5k, lol POS.


 

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Posted by onesicklx (Member # 285) on :
 
i rev my gt40 irons to 5,800-6,000 im not really sure if they are making power...
 

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Posted by AaronC (Member # 86) on :
 
Try the shift point calculator on www.prestage.com. If you have your dyno chart you can plot torque per RPM and your gear ratios and it'll calculate it for you. Not sure how accurate things are but it's worth a shot at trying.
 

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Posted by st5150 (Member # 51) on :
 
I've been considering writting an online calculator for proper shift points based on a dyno graph. Its a simple optimization problem...

Calculate your RPM drop between shifts, we'll call these X1 and X2. For example shifting at 5400 RPM from 2nd to 3rd gear drops you to 4000 RPM for our make believe tranny here.

X1 = 4000 X2 = 5400

The difference between the X1 and X2 is 1400 RPM

No take several data points from the engines dyno graph and graph it in EXCEL.
Use the curve fit function to derive a quadratic equation that represense your dyno graph's HP curve in your powerband.

Magazines and cyber racers always talk about "area under the curve". I'm sure you've heard it on many corral infomercials... but here is where "area under the curve" fits into the whole scheme of thing.... INTERGRALS!

Take the intergral of the function that represents your HP curve (from EXCEL) between the RPM points X1 and X2. The number you get will represent the total HP being made. Now try bumping up your shift point to 5600 RPM (X1) so your RPM after the shift point will be 4200 RPM. Run the intergral again and see if this total HP number is larger than your previous shift point. Once you got it narrowed down, you'll know the best shift point for second gear.... now you can calculate the RPM drop between 3rd and 4th gear and do the math all over.

Does this make sense? Not easy to explain with out pen and paper, but it is the *TRUE* way to accurately calculate shift points. Its also something they don't tell you in those corral infomercials. This concept is very powerfull in calculating flow of heads at given valve lifts to compare two different cylinder heads with the same cam as well as many other things automotive and not automotive related.

With that said.... guess&check is a time honor method as well

[ November 05, 2002, 07:57 PM: Message edited by: st5150 ]
 

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Posted by st5150 (Member # 51) on :
 
No comments, input or questions???
 

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Posted by AaronC (Member # 86) on :
 
Sawson-

I like your method. Makes perfect sense to me.
 

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Posted by FasterDamnit (Member # 442) on :
 

quote:

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Originally posted by st5150:
No comments, input or questions???

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integral.
Good luck explaining area under the curve and approaching a limit to this crowd. Maybe if you post the proof for calculus...


 

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Posted by shade-tree (Member # 298) on :
 
actually, I don't see the increased accuracy of a 'least fit squares' or even a newtonian approximation to a dyno's ouput of data. I mean, we have precise 100 rpm increments. But that's just me.
I got my car dyno'd and the stupid @@$%^&! shop didn't even give me a printout. shweet.
I'm going to have to stick with the tried and trued "guess and check method". I figure anything beyond 5500 rpms on these heads and I'm deluding myself.
 

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Posted by S281 01-570 (Member # 1451) on :
 
A simpler idea. I used www.302cobra.com/stuff/topspeed.html you enter your gearing, wheel size, tranny etc. It tells you what rpm you are turning in each gear. Next you use your dyno sheet and figure out where you start coming into your hp and when you start dropping off. Shift so that you keep in your power band. I have 3.55s but discovered that 4.10s would be better and keep me in my power band. I have to run 1st to almost 6250 rpm to shift and not be too low in second.
 

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