It occurred to me while looking at my dyno that maybe shift patterns might generate differences in 1/4 mile times. for those of you who are chipped and have run the 1/4, where does your truck shift? Did you specify to your tuner or did you take it as you got it? It seems to me that the higher you shift, the more HP you will be making in the next gear... Before you say, "yeah, but my truck runs out of steam at the top and I want to get into the next gear"... look at your dyno. All the way to 5600, you never drop down lower than where you will be once you shift into second.

My math here is as follows: MPH=(rpm*tire size in inches)/(3.73*336). result divided by gear ratio the tranny is in...

someone check my math and make sure the speeds at these rpms look right... I didn't factor in a torque converter slip percentage, though I probably should have...

What I did was look at what speed the truck would be running at when the shift occurred and look at that speed in the next column. I took the closest number and used it for where we would be in the RPM band. The resulting HP is in the column.

Note that my HP readings are from my dyno chart and while fairly accuracte, they are still estimates because I do not have a table provided by the dyno computer. Also note that the MPH matchup is not exact so there are a few +- horsepower there also.

I would think that shifting with an extra 10-20 ponies would make a diff going down the 1/4... any thoughts?

 

Checks out with my program. Goof thinking Ken.

Mike

 

My first superchip for my harley shifted around 5400 or 5500. A couple of different burns on my diablo have shifted at 5600. My thinking is that now that I really know where my boost levels are and my blower rpms, I am not as high as I once thought. Given that, I might consider having my chip shift at 5500 rather than 5250... And leave my boost level where it is...

Ken

 

I wanted to shift higher to but mine is at 5200-5250 per JLP.

Maybe they set them higher now but I think the weak rods had something to do with it.

Chip is old very old but I like the numbers I get.

One of the first few JLP burned.

 

GT, I agree, the RPMs scare me abit, but if you go back to the charts.... lol... the 8lb guys are turning 16,154 RPMS at a WOT RPM of 5250 (assuming it's that and not 5200). Shifting at 5500 with my 6lbr, I peak at 15,231.

The problem isn't so much the blower rpm, but can the motor handle running up there without tossing a rod through the side from the rate of spinning?

I'm going to post over on the L forum and see what they are doing over there... I think I'll do NLOC, though, cause I never seem to get any response from the L community here.

Ken

 

I am totally prepared to be absolutely wrong on this one but wouldn't you get better acceleration shifting at high torque rather than high HP? From looking at most dyno graphs, max TQ is higher lower in rpms than HP. Would it make sense to graph TQ rather than HP and make decisions about shift points from there? Forgive me if the TQ thing is just an urban myth.

 

Would the extra 20 ponies from the higher RPM's cause excessive wheel-spin upon shift I wonder? But Ken has those nice Hoosiers now. Hrrrm. Guess and check I suppose.

Mike

 

same post on nloc...

The transmission itself is acting as a torque multiplier, is it not?

In other words, let's say the engine produces 500ft lbs at a 3400RPMs. So using the logic from above, a 10 speed gearbox that keeps rpms around that 3400 level would be ideal for maximum accleration.

First gear on our tranny is 2.7:1. If you are putting 500ft lbs at 3400 rpm, does this not create an effective torque rating of (500*2.7) or 1350ft lbs at 3400 in first gear? If this is the case then it would stand to reason that staying in 1st the longest, regardless of where you fall back to on the shift into second would net the lowest possible ET on a 1/4 mile strip.

Isn't this where the small engines get their acceleration ability from? There are some very small displacement engines with low torque numbers that put out decent HP, but it always comes way up high in the rpm range... like 10,000 or more... So they look for higher ratios in 1st and 2nd so that they can multiply their limited torque for maximum acceleration.

Horsepower is measured as HP=Torque*rpm/5252

So, Hight torque and low RPM nets similar to low torque and high RPM, does it not? Two engines: 500ft lbs and 250 ft lbs. Plug in 400HP as your desired result and solve for X and these two engines produce that HP rating at:

The 500ft lb engine will produce 400HP at around 4200RPMs. The 250ft lb engine will have to spin to 8400RPMs to achieve the same.

So, in a perfect world, RPMS AND TORQUE would be nice to have...

Correct me if I am wrong in my assumptions please...

I'm not necessarily advocating shifting at higher RPMs, or that the engine can take it. I am just trying to make sure I understand completely the benefits of when/where I shift...

thanks

ken

 

Ken You are right but I dont think it would be wise to do.

The more HP under the curve you use during a run the better.

 

Read the NLOC post so I think TQ is a good way to look at this. That said, from your HP dyno and the TQ formula, I computed a TQ curve. Seems that the TQ curve has a nice fat hump from 3200 to 3700. Working backwards from the speed computations you posted, I tried to find the shift point that fit best via speed matching. I threw out the 1-2 shift because it seems to be a safety/longevity issue associated with 5250 (and because it would not work with the rest of my speculation). I think that you are correct though, in theory, a shift at 5600 or better (6000 appears to speed match to 3400) should produce a better run by optimizing the engine output at the shift.

For the 2-3 and 3-4 shift, you get the value closest to the 3400 rpm TQ max by shifting at 5100. The 2-3 shift speed match comes in at 3350ish. The 3-4 shift comes in 3600.

Dropping to a 5100 rpm shift point has the additional benefit of allowing the use of the L 6 lb lower without significantly overspinning the SC - I continue to stick with 14000 as max safe SC rpm.

 

Here is an approximation of the torque curve in a spreadsheet... I highlighted in blue the optimal shift area, but there are far too many estimates in sheets at this point. I need a legitmate dyno computer provided table to work the numbers.

mmeyek... you hit the nail on the head. the shift points need to be different for each gear such that that you don't go too far in one gear and have tq drop below what it would be in the next.

1st gear needs to run as long as possible... 2nd and third need to shift sooner...

I would say that pushing the shift point up to 5450 (up a couple hundred from stock) might be an ok compromise in terms of not tearing the engine apart and still getting a little benefit from moving up the TQ/HP curve...

Also, doesn't the stock rev limiter kick in at 5600? If that is the case, I would think that one could make the assumption that (in stock form) 5600 rpms is considered the very upper edge of safe...

It's a fun discussion, but I don't think I am going to mess with my shift points...

Ken

 

Hey Ken, Whassup

I've had numerous tuners and numerous diff chips over the last 4 years. Ken Payne (Ford-Trucks.Com) James Parker and Ivan (Runnin with the Devil), Johnny Lightning (JLP), and now Sal Mennella (PSP)
I got to say I prob get more involved in my tunes than anyone else I know, and I have questioned each of those tuners and more on the shift points.
I can only tell you what they ALWAYS tell me. It's MUCH MORE important to be at a certain RPM AFTER the shift, than it is before.

Although I have had shift points as high as 6000 RPM and 5700 RPM, there is NO DOUBT the spot they all seem to feel is optimal for the truck to actually
shift the next gear is @ 5500 RPM. BUT some still set it at 5250-5350 because as you get up in the power you'll find the trans wont always shift EXACTLY where you set it especially at WOT, and even more so with the n20. For instance if you set mine at 5500, it WILL actually shift as high as 5700 - 6000 RPM on the spray, and thats a bit too high.
For this reason they usually set my n20 program shift points @5000 RPM, and my non n20 program @ factory shift points of 5250. With those setting on and off spray I seem to shift just about on the money at 5500 RPM.

BTW,
I also usually have a little tweaking on the duration of the shift "MS",
I believe it stands for mili-second delay between shifting and we usually set mine @200 MS. I also have had Torque reduction turned off since my first tune a few weeks after I bought this L back in early 02. I always have my Rev Limiter/Fuel Cut Off moved waaaaaaaaaaaaay up, usually we set it for 6900 RPM or turn it off all together.

You are right on the money with your thoughts but you need to understand it will shift at diff points under diff situations. For instance do a down shift from 2 to 1 and stay WOT and look where it shifts. Then launch from a dead stop and stay WOT through a few gears and watch again, I bet it shifts at a diff spot, then put slicks on it and do it again.
The quest is to find the happy spot between them all, for this reason I do believe actually shifting the geat AT 5500 RPM will be that spot, but again to do that the shift points may need to be set lower