[pastry_chef]

Originally Posted by PontGuy

You mean the same HP, right Adam? Engine torque is force as measured at the crank. But torque at the wheels varies by the gear ratio. So HP better represents what is getting to the ground through the gears. (Yes, another engineer here.)

Exactly correct.

Acceleration now in G = horsepower now * 375 / (Mass lbs x MPH now)

The video is totally correct.
He states the wider the trans gear split, the more engine torque must dive to justify shifting - otherwise engine redline will be the best shift point.

[pastry_chef]

Another excellent video.

https://www.youtube.com/watch?v=4BOryrhs9iA

With a 3 foot bar I can generate 600 ft.lbs of torque by hand.
That is supposedly a substantial value.

Even with four buddies no way we will generate a lowly 200 horsepower.
Just too much energy!

Last week I was playing with converting engine cylinder pressure to car acceleration. Without using torque numbers or gear ratios.

[AdamIsAdam]

I have to watch that long video, but the Engineering Explained video says to use torque and not HP, then he goes on to calculate how torque is affected by different gears, and therefore, when you should change those gears. So when I do the math based on my own torque curve (which is very flat) I found that the torque in 1st at 5300 RPM is 704 and the torque in 2nd at 3950 RPM is 705, making that the ideal RPM to shift. I'll repeat this exercise for the other two shifts as well. I found it interesting that the answer was 5300 RPM since that's about the number most experienced Pontiac'ers tend to use.

If you are saying to use HP instead of torque, then aren't you challenging the validity of his video entirely?

[pastry_chef]

Originally Posted by AdamIsAdam

I found that the torque in 1st at 5300 RPM is 704 and the torque in 2nd at 3950 RPM is 705, making that the ideal RPM to shift.

Where is 704 and 705 coming from?
What are the gear ratios?

What hits the rear wheels is the important thing.
If you provide your specifics I can show you both methods that end up with the same final result.

[PontGuy]

If you are referring to calculated torque at the wheels being the same then you are right. I always think of it in terms of engine HP at resulting engine RPMs though. Mathematically it all works out either way. And we don't much talk about wheel torque when discussing cars.


Sent from my iPhone using Tapatalk

[AdamIsAdam]

Originally Posted by pastry_chef

Where is 704 and 705 coming from?
What are the gear ratios?

What hits the rear wheels is the important thing.
If you provide your specifics I can show you both methods that end up with the same final result.

Using the formula from EE's video and my own torque curve from when I first dyno'd the car on the chassis dyno 3 years ago, I got the following math:

2.2 (1st gear) x 320 lb.ft = 704lb.ft @ 5300 RPM
1.64 (2nd gear) x 430 lb.ft = 705 lb.ft @ 3950 RPM

and to get the RPM drop of 3950 it was:

2.2 5300
---- * ------ = 3950
1.64 x


At a 5300 shift point, the torque was the same before and after the shift.

[pastry_chef]

Originally Posted by AdamIsAdam

2.2 (1st gear) x 320 lb.ft = 704lb.ft @ 5300 RPM
1.64 (2nd gear) x 430 lb.ft = 705 lb.ft @ 3950 RPM

From that you have more horsepower at the lower RPM

320 lb.ft @ 5300 RPM = 322.9 horsepower

430 lb.ft @ 3950 RPM = 323.4 horsepower

Engine torque is dropping like crazy up top. Agree you would not shift higher.

[AdamIsAdam]

Yes. (remember that this is in 1st and 2nd gears, not 4th gear). But yes, that's my point.

If you do this math at 5400 RPM shift the resulting numbers are 660lb-ft in 1st @ 5400 and 705lb-ft in 2nd @ ~ 4025 RPM.

If shifting at 5200 RPM the final torque numbers are 748 in 1st and 705 in 2nd. Oh wait, does that mean I should shift at 5200 and not 5300 (theoretically of course, I know that 100 RPMs difference in 1st gear would be hard to vary at this level of performance)?

[JSPONT]

Adam track rental is friday, we can get it sorted out, I will be willing to test your car if you want!!

[AdamIsAdam]

Originally Posted by JSPONT

Adam track rental is friday, we can get it sorted out, I will be willing to test your car if you want!!

I'm finally having my new 1/2" fuel system put in this week (from pickup to carb). I don't think it'll be done for Friday which is why I didn't sign up for track day. But I do want to go this year for sure!

And thanks for the offer to drive my car. I've got it covered

[PontGuy]

Yup, slightly higher torque at the wheels and corresponding slightly higher HP after the shift according to the math.

Glad your car is a stick, 'cause otherwise we would have to get into the effects of a torque converter!!

[AdamIsAdam]

I have to go out now but I'll run these numbers down the RPM range more because my peak RPM is 450 at 3000 RPM (well, it was before I changed my intake and carb).

So if I look to be at 3000 after I shift, that means I'm shifting at 4024 RPM. Ironically, one of my best ever 0-60 times was when I shifted at 4000 because I was losing traction in 1st gear.

NOW I really need some track time!

[johnta1]

That dyno sheet your using is from a car dyno?

I would think the torque readings are what is after the gear ratios?

[johnta1]

Try the dyno chart that was posted for Cliff's engine?

See what it shows?

[AdamIsAdam]

Originally Posted by AdamIsAdam

THAT'S NO FUN!

I guess I was hoping for a theoretical ideal as a starting point. I live too far from the closest track to test regularly. I'm planning to hit the track this year, maybe later in the season in case I break something I don't lose the entire summer. So I was looking for starting points.

I'll be taking the car to the dyno in the next few weeks once my new fuel system is in. I have a benchmark dyno report when I first bought the car, but i later discovered that I wasn't getting WOT, so that's not much of a benchmark. I've since changed carb, intake and did a valve job with new springs (the old ones were VERY weak as per the machine shop who measured them).

I know nothing beats opinions faster than a time slip, but indulge me.

PS Hey, 2,000th post!


(I've attached that old dyno run from 3 years ago as a reference point.)

this is the chart I'm using

Chassis dyno

[pastry_chef]

Originally Posted by AdamIsAdam

down the RPM range more because my peak RPM is 450 at 3000 RPM (well, it was before I changed my intake and carb).

So if I look to be at 3000 after I shift, that means I'm shifting at 4024 RPM.

Shifting near 4000 RPM would kill your power average down the track.

http://performancetrends.com/blog/?p=7

From your info, from around 4000 through to 5300 your engine is making near 320 horsepower or more.

450 torque @ 3000 is only 257 horsepower.

Acceleration now in G = horsepower now * 375 / (Mass lbs x MPH now)

So imagine "now" you are at 3950 RPM making 323.4 horsepower, then you shift causing RPM to drop to 3000, now your putting 257 horsepower to the ground.
Better off instead to make about 320 HP at 4100, 4200 etc and continue on.

[PontGuy]

I think you are getting confused Adam, its torque at the wheels that matters. Holding first to get more torque to the wheels is more important than the engine running at peak torque after the shift. You got it right the first time.

One of the reasons I prefer to think of it in terms of HP. Less confusing and more intuitive.

[AdamIsAdam]

Originally Posted by pastry_chef

Shifting near 4000 RPM would kill your power average down the track.

http://performancetrends.com/blog/?p=7

From your info, from around 4000 through to 5300 your engine is making near 320 horsepower or more.

450 torque @ 3000 is only 257 horsepower.

Acceleration now in G = horsepower now * 375 / (Mass lbs x MPH now)

So imagine "now" you are at 3950 RPM making 323.4 horsepower, then you shift causing RPM to drop to 3000, now your putting 257 horsepower to the ground.
Better off instead to make about 320 HP at 4100, 4200 etc and continue on.

That link was an interesting read. So I laid all this math out in a spreadsheet. Based on the original video's theory of shifting for peak torque, I'd be shifting around 5200-5300, but when I added a conversion to HP for each of those figures, it raises the shift points a few hundred RPM, especially in 1st gear.

Take a look at this spreadsheet:

https://www.dropbox.com/s/xxeg08ql8x...COPY.xlsx?dl=0

[PontGuy]

Remember HP is not a function of gear ratio so calculating it like you are doing is not correct. Hence the 700+ HP numbers which are way more than your engine makes. Just read the HP right off the dyno chart. So for 1-2 your car will be making 320 HP at a 5300 RPM shift which matches nicely with the 320 HP at 3950 after the shift. And so on for the other shift points. Doing it this way is a lot easier than calculating wheel torque and you end up with the same results.

The curves in your dyno chart are very flat which would mean that the shift points should not be that critical at the track. But then again with carb/fuel flow issues resolved the curves may look a lot different. And real world never works out quite like theoretical due to other factors in play.

[AdamIsAdam]

Sounds good. Thanks for all the chatter. As always, very informative and fun.

I hope to have the new fuel delivery system completed this week. I'll drive it a bit, then off to the dyno. THEN, I'll plan a trip to the track for some "put up or shut up" real testing! My goals at the track are to:
1) NOT break anything
2) not break anything too expensive
3) find a consistent launch pattern (traction is not easy with this car)
4) run 12.50's or better (car weights 3700lbs with me in it)
5) have fun! (which means goal #1 & 2 must also be met)

I figure between the new carb, intake, valve job and new springs, the timing/tuning improvements and the fact that now I have WOT (which helps!) I'd LOVE to see 500 crank HP but will settle for 450. (So 400+ at the rear wheels I guess using 15% drivetrain loss factor for manual.)