4l60e guys, questions on shift points
09-14-2011, 09:57 AM
#1
4l60e guys, questions on shift points What: My Chevelle with the ls1, 230/224 cam, FAST 90/90, longtubes, SS3600 stall, and 3.42 12 bolt on 325/50r15 MT ET radials.
Cliffs: Do you have shifts the same on the 1-2 as the 2-3? Has anyone had better luck short shifting 3rd a tad if your cam falls off peak quickly?
Why I ask:
My peak power is at 6000rpm and begins to fall off pretty quickly past that point (not a downward spike, but immediate arch).
I used to race mostly 1/8th mile so I was barely getting into 3rd if at all at the stripe where the car would trap 90mph in 1840+ DA with a weight of approx 3700lbs. This is with the shifts occurring right at the rev limit of 6600rpm. Basically I kept bumping the 1-2 and it kept picking up small amounts of MPH. I'm thinking this was due to the huge 1-2 gear split and my stall/gear size.
At the 1/4 mile track, I am only trapping 110-111 in the 1/4 in the same DA. I think the car should gain a little more on the backhalf than that. Not much...but a little. (Or maybe it's just the aero taking effect vs a similar fbody car). The 2-3 is occurring right on the 6600rpm limiter and I'm thinking of trying a lower shift on the 2-3. I'm thinking I might fall into a better part of the power band VS the power past peak on that shift since the gear split isn't so great.
Or am I thinking about this backwards? Has anyone tried this with a cam that falls off quickly past peak rpm? Either way I need to adjust it a touch because the car will barely kiss the rev limit on the 2-3 at the track (doesn't do this on my street tires which are a tad shorter). Just wondering if anyone has experience with losing mph from getting too aggressive on the RPMs in 2nd.
Cliffs: Do you have shifts the same on the 1-2 as the 2-3? Has anyone had better luck short shifting 3rd a tad if your cam falls off peak quickly?
Why I ask:
My peak power is at 6000rpm and begins to fall off pretty quickly past that point (not a downward spike, but immediate arch).
I used to race mostly 1/8th mile so I was barely getting into 3rd if at all at the stripe where the car would trap 90mph in 1840+ DA with a weight of approx 3700lbs. This is with the shifts occurring right at the rev limit of 6600rpm. Basically I kept bumping the 1-2 and it kept picking up small amounts of MPH. I'm thinking this was due to the huge 1-2 gear split and my stall/gear size.
At the 1/4 mile track, I am only trapping 110-111 in the 1/4 in the same DA. I think the car should gain a little more on the backhalf than that. Not much...but a little. (Or maybe it's just the aero taking effect vs a similar fbody car). The 2-3 is occurring right on the 6600rpm limiter and I'm thinking of trying a lower shift on the 2-3. I'm thinking I might fall into a better part of the power band VS the power past peak on that shift since the gear split isn't so great.
Or am I thinking about this backwards? Has anyone tried this with a cam that falls off quickly past peak rpm? Either way I need to adjust it a touch because the car will barely kiss the rev limit on the 2-3 at the track (doesn't do this on my street tires which are a tad shorter). Just wondering if anyone has experience with losing mph from getting too aggressive on the RPMs in 2nd.
09-14-2011, 02:36 PM
#2
Guess the bigger question is what RPM are you seeing in 3rd in a 1/4 mi track. It has always been my understanding is that you want to be as close to peak power or useable power @ the 1/4 mi mark. Having a car set up for the 1/8th vs the 1/4 are generally 2 different setups and no one would be optimal for both. So I would not expect a huge return on a 1/8th mi car running the 1/4.
Depending on how far back the RPM drop from 2-3 and how much you get back up is a big question and remember, in a 1/4mi run you start dealing with things such asdrag coefficent and its not like this is a Fbody with smooth curves.
What is exactly the power drop from 6,000 to the limiter??
I could be wrong, but these are the things that stick out to me.
Depending on how far back the RPM drop from 2-3 and how much you get back up is a big question and remember, in a 1/4mi run you start dealing with things such asdrag coefficent and its not like this is a Fbody with smooth curves.
What is exactly the power drop from 6,000 to the limiter??
I could be wrong, but these are the things that stick out to me.
09-14-2011, 03:11 PM
#3
I will see if I can find my dyno sheet. Appears I never scanned it. Wasn't exactly proud of it.
Not sure about RPM crossing the stripe, but a gear change is out of the question anyway. I will be going back to a shorter tire once these burn up, but that's the best I can do there.
Not sure about RPM crossing the stripe, but a gear change is out of the question anyway. I will be going back to a shorter tire once these burn up, but that's the best I can do there.
09-15-2011, 11:04 PM
#5
that looks to be a heavy valvetrain to drop off like that ^^^
some instability...
but to the question at hand I short shift my 2-3 about 100 rpm due to the shift extension being smaller on the 2-3
some instability...
but to the question at hand I short shift my 2-3 about 100 rpm due to the shift extension being smaller on the 2-3
09-16-2011, 09:44 AM
#6
I was expecting it to carry more as well. But the valvetrain is all stock type things (stock rockers/valves, pac spings, comp pushrods, etc).
I had put the combo together shooting for more. But I can't seem to find anything wrong so I'm not sure what could cause the car to underperform but still run so well for so long.
That graph is over a year old and the car was on the road for 1.5 years before that as a daily driver. Perhaps by then the springs were just too weak? I didn't expect it to be an issue with such low lift on the cam.
I had put the combo together shooting for more. But I can't seem to find anything wrong so I'm not sure what could cause the car to underperform but still run so well for so long.
That graph is over a year old and the car was on the road for 1.5 years before that as a daily driver. Perhaps by then the springs were just too weak? I didn't expect it to be an issue with such low lift on the cam.
