[Stan Weiss]

If you run an engine on an inertia dyno. Where you have a constant resistance and a variable acceleration rate. You will see the fastest RPM rate of acceleration right around peak torque.

Lets look at what happens as you go down the 1/4 mile. The important factor in wheel torque. I will stay in first gear until the wheel torque will be less than it will be in second gear after I shift. As you can see by this graph that will be well after peak torque in second gear. etc.

Stan

[steve25]

Why are you talking about a form of performance/ racing that is so darn far removed from what we are discussing in this thread?

The OP of this thread probably does not want to run better then mid 11s and your talking about 1300 hp pro stock motors!

[slowbird]

Originally Posted by steve25

Why are you talking about a form of performance/ racing that is so darn far removed from what we are discussing in this thread?

The OP of this thread probably does not want to run better then mid 11s and your talking about 1300 hp pro stock motors!

Because only on a dyno is your statement true. In a car the gearing greatly efforts your statement. Both of our 11sec street car also never see peak tq again after launch. So are those examples better?
If you're racing (don't care if it's an 11 sec car or 6sec car) and you're pulling the engine down to 400rpm below peak tq on gear changes odds are you're losing et (i can think a few cases where your not but it's not the norm).

[i82much]

Originally Posted by Stan Weiss

If you run an engine on an inertia dyno. Where you have a constant resistance and a variable acceleration rate. You will see the fastest RPM rate of acceleration right around peak torque.

Lets look at what happens as you go down the 1/4 mile. The important factor in wheel torque. I will stay in first gear until the wheel torque will be less than it will be in second gear after I shift. As you can see by this graph that will be well after peak torque in second gear. etc.

Stan

if you had an infinitely variable cvt and perfect traction, you would always accelerate most quickly by running the engine at peak HP rpm - true?

[Stan Weiss]

Originally Posted by i82much

if you had an infinitely variable cvt and perfect traction, you would always accelerate most quickly by running the engine at peak HP rpm - true?

YES

Stan

PS - T400 first run - CVT Second run Locked at Peak HP RPM- CVT Third run Locked at Peak Torque RPM

60 Foot ET = 1.1675
330 Foot ET = 3.4159
1/8 Mile ET = 5.3630
1/8 Mile MPH = 126.1423
1000 Foot ET = 7.0458
1/4 Mile ET = 8.4689
1/4 Mile MPH = 158.6642

__________________

60 Foot ET = 1.1569
330 Foot ET = 3.3514
1/8 Mile ET = 5.2532
1/8 Mile MPH = 128.7825
1000 Foot ET = 6.9099
1/4 Mile ET = 8.3167
1/4 Mile MPH = 160.7694

Number of Gears Used 1/4 = 172750
Last Gear Ratio Used 1/4 = 3.9947

__________________

60 Foot ET = 1.1620
330 Foot ET = 3.3880
1/8 Mile ET = 5.3191
1/8 Mile MPH = 126.8088
1000 Foot ET = 7.0018
1/4 Mile ET = 8.4308
1/4 Mile MPH = 158.2600

Number of Gears Used 1/4 = 178374
Last Gear Ratio Used 1/4 = 3.4708

[ta man]

Originally Posted by i82much

if you had an infinitely variable cvt and perfect traction, you would always accelerate most quickly by running the engine at peak HP rpm - true?

Exactly like snowmobile clutching.

[ta man]

Originally Posted by slowbird

All fast na cars from Pro Stock to Super Stock, none will see peak tq after launch.

I'd say this is true for almost any vehicle going down the dragstrip...the only ones that I can think of are Big Rigs.

[i82much]

Originally Posted by Stan Weiss

YES

Stan

PS - T400 first run - CVT Second run Locked at Peak HP RPM- CVT Third run Locked at Peak Torque RPM

60 Foot ET = 1.1675
330 Foot ET = 3.4159
1/8 Mile ET = 5.3630
1/8 Mile MPH = 126.1423
1000 Foot ET = 7.0458
1/4 Mile ET = 8.4689
1/4 Mile MPH = 158.6642

__________________

60 Foot ET = 1.1569
330 Foot ET = 3.3514
1/8 Mile ET = 5.2532
1/8 Mile MPH = 128.7825
1000 Foot ET = 6.9099
1/4 Mile ET = 8.3167
1/4 Mile MPH = 160.7694

Number of Gears Used 1/4 = 172750
Last Gear Ratio Used 1/4 = 3.9947

__________________

60 Foot ET = 1.1620
330 Foot ET = 3.3880
1/8 Mile ET = 5.3191
1/8 Mile MPH = 126.8088
1000 Foot ET = 7.0018
1/4 Mile ET = 8.4308
1/4 Mile MPH = 158.2600

Number of Gears Used 1/4 = 178374
Last Gear Ratio Used 1/4 = 3.4708

very cool, thanks.

[i82much]

Originally Posted by ta man

Exactly like snowmobile clutching.

i can’t even drive my muncie, let alone a snowmobile

[Steve C.]

And keep in mind many have NO CLUE at what RPM their specific engine makes its peak torque at. Only a guesstimate or a number based on a VERY SIMILAR combination that has dyno numbers to support it.


.

[Dragncar]

Originally Posted by Stan Weiss

What is the diameter and depth of that dish (32 cc's)?

Stan

Spec sheet says .200

[shermanator2]

Originally Posted by Steve C.

And keep in mind many have NO CLUE at what RPM their specific engine makes its peak torque at. Only a guesstimate or a number based on a VERY SIMILAR combination that has dyno numbers to support it.
.

Just thinking out loud here, but in modern times with a Vbox or other logger, isn't it trivial to know this? Just accelerate in a fixed gear from low RPM to red line and with the weight of the vehicle, from speed and acceleration you get torque and horsepower to the rear wheels. I have never used a Vbox, but it seems that they would just give you this. True it is not engine torque and power, but what gets to the wheels is what counts anyway.

Peak acceleration will be at peak torque, and peak horsepower is where the product of speed and acceleration peaks.

[ponjohn]

What cc would be considered a "large" dish?

[Dragncar]

Originally Posted by firechicken

Race weight?

I weighed it only once and it has had changes since then. it was not quite 3400lbs empty and then I added a rollbar. I would bet I am around 3600 race weight.
I will try and take it to the scales before I yank the old engine.

[Ponchy]

Originally Posted by Speargun

Which would give better performance when dialing in a CR, a head with a larger chamber, i.e. 87cc, and a smaller dish in the piston or a smaller chamber, i.e. 72cc, and a larger dish?
Or does it really matter?

Back in the 60's, engine builder Smokey Yunick surmised that the best piston design was a dished piston.
This was because with a pop-up piston, the a/f charge reacts against it kind of like the air does when a bullet flies through it.
It is deflected off to the side in a smooth and cordial fashion.
With a flat top piston, you don't get that reaction, but as the pressure of the burning charge hits the piston, it slides across the top. With nothing to stop it, it ends up, once again, ending at the cylinder wall.
Better, but not quite perfect.
With a dished piston, the charge reacts like it would with a flat top, but because of the depression in the top of the piston, the burning a/f charge is encapsulated on top of the piston.
If it deflects anywhere, it would be sent back upwards to react against the combustion chamber, adding thrust to the charge already reacting against the piston.
Of all the piston shapes available, the dished piston is the only one that actually attempts to keep the burning a/f charge away from the cylinder wall, thus less erosion of the cylinder.
Ever since I found out about that, I prefer a dished piston with a small, but open, combustion chamber to gain power.

[PontiacJim1959]

"Back in the 60's, engine builder Smokey Yunick surmised that the best piston design was a dished piston. This was because with a pop-up piston, the a/f charge reacts against it kind of like the air does when a bullet flies through it.
It is deflected off to the side in a smooth and cordial fashion."

Tell that to Bill Jenkins who used domed pistons in his small block drag cars pushing 700 HP - it is all he used.

Personally, I don't think it matters. In theory you may look at the differences and assume a flat top with big chamber heads or dished piston with small chamber heads have an advantage over the other, but there are far too many other variables to consider - bore size, rod ratio, stroke, piston weight, piston skirts/rings, piston speeds, chamber design, spark plug angle/location, quench distances, valve type/size, cam lift/speed, port velocities, A/F separation & tumble/turbulence, exhaust flow, etc., etc., etc..

Bottom line is to simply achieve the desired street compression you want/need and match piston CC with head CC.

[pastry_chef]

Originally Posted by PontiacJim1959

Tell that to Bill Jenkins who used domed pistons in his small block drag cars pushing 700 HP - it is all he used.

He needed domes to obtain the compression. In his 1976 book he has 17 pages dedicated to piston and ring prep. He says they would cut away as much dome as possible that would still obtain the target CR.

[Tom Vaught]

I have Bill's Book and remember that information in 1976.
You will notice that he ran two of the Holley Furl Pressure Regulators because the
tiny control orifice in the regulator would not pass the flow for any real power.
Holley later designed the 7/16" control Ball regulator for the engine systems that worked well on higher HP engines. I threw the Red and Blue small Holley regulators in the trash years ago. If you are trying to make power, small or large combustion chamber, it means nothing if the fuel is not available at the intake manifold.

Tom V.

[Stan Weiss]

Car Craft '75

Stan

[steve25]

Not much to be gained up there especially once the dome starts to slow down flame travel and your not even burning some 20% of the fuel and air charge on the lifter valley side of the chamber.

Prostock motors however fight for ever tied bit of compression because of the Cams they run and other factors!