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 Post subject: shift points
 Post Posted: Sat Nov 17, 2012 10:44 am 
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Joined: Wed Apr 19, 2006 5:39 am
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Location: Stranded in Iowa
What method should I use to pick starting shift points on my new combination? I can use the SWAG method but wondered how you guys pick a starting point. I realize it will take some testing to come up with the numbers the car likes best.
Thanks

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 Post subject: Re: shift points
 Post Posted: Sat Nov 17, 2012 10:59 am 
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I took advise from my converter guy and that seemed to work just fine...Leave at 5000rpm shift at 7000rpm...It copies time slips =; ..

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 Post subject: Re: shift points
 Post Posted: Sat Nov 17, 2012 12:21 pm 
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What I did with my 18* SBC was start out shifting at 8600 and each pass I dropped that by 200 RPM until it slowed down, then went back up to that previous best, which for my combo wound up being 7600.

I guess you could do the opposite though and start lower to see where it drops off going up? Same thing only different!

FWIW, some guys shift by x.xxx time after the hit.

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 Post subject: Re: shift points
 Post Posted: Sun Nov 18, 2012 5:06 am 
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Joined: Sun Apr 23, 2006 5:41 am
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Location: Newville PA
If your engine was on the dyno, shift at peak HP. then move shift point down 100 revs each time until it slows down. Bob J


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 Post subject: Re: shift points
 Post Posted: Sun Nov 18, 2012 11:37 am 
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I usually have a good idea where it should make power from the Engine Pro software I use based on engine and port size, cam, carb and headers. If you dyno or use the software expected peak hp rpm you can do this ... Start by shifting a little above peak hp and gradually move up until it doesn't go quicker. Sometimes you need a couple passes at each rpm to make sure. You need to keep good record of weather. This will get you close. In general engines with bigger/heavier rotating assy, cars w/more gearing and lighter cars will need shifted closer to peak hp and the opposite will need shifted farther above peak hp.

After I have a few races behind me I might try raising or lowering shift point by 100 or 200 rpm and leave it there for that race. This will get more passes in to see a small difference.

Rick


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 Post subject: Re: shift points
 Post Posted: Mon Nov 19, 2012 6:23 am 
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Joined: Wed Apr 19, 2006 5:39 am
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Location: Stranded in Iowa
I dont have a reliable dyno sheet on this engine. It has probably 100 passes on it. Its going in the car as is. I will dyno it at freshen up next year. I think I can zero in on it now with testing but thought I would see how everyone else might do it. My favorite track does almost no test and tune days. The ones they do are early in the year and if the track is cold and not well prepped my car is a handfull. I can just test on race days but its hard to get in very many passes. May have to visit a couple of other tracks this next year.
Thanks for the help. I dont uaually say it but I really do appreciate it.

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 Post subject: Re: shift points
 Post Posted: Mon Nov 19, 2012 7:26 am 
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Location: Adelaide Australia
Just to throw a spanner in the works , check this out. One of the drag cars I do we did a test on it one day, took off in top gear (glide)and it ran exactly 0.2 seconds slower over the quarter. Just using the one gear. And when we did normal runs and shifted so that the engine rpm dropped mathematically way way back into the convertor and we let the convertor do some work it ran the same times as revving the ring out of it. We have developed that convertor for many many years and its a gem of a design. Its got a 3speed behind it now and we have now changed the engine so its max power is at 7800 rpm. (412 SBC, old slayed valve heads) The engine's pretty stout making 650hp at 5500 rpm so it can pull some convertor multiplication, it makes 882hp at 7800. Different diff gears and its running the same times it used to revving the thing to 9300(as in previous setup), so it heaps cheaper to run now. Its pretty obvious that all this stuff about shift points sometimes means nothing to some particular setups. I think spending money like dyno to find just that information is a waste of money, I prefer to just figure it out on the track. the convertor design is a big part of what latitude or range you have to work with, then you fiddle with tuning issues and re-fiddle with shift points. It a blend of factors, not a rule type of thing to me.


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