How far to rev this 273 commando build

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stroker402

1968 dart GTS convertible
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Just wondering ...........This is the T-3 custom grind isky cam specs I installed in my 273 commando . what is meant by the 2* advance , does it mean it is at 106* or 110* ? Rpm range is 2000 - 6400. does this cam need to be revved to 6400 to make good power or what? I've taken it up to 5900 rpms so far and it's still pulling hard . is there any need to go to 6400 rpms? 65 barracuda with 4 speed , 3.55's gears , hooker headers , pertronix ignition , cleaned up 920 heads . real stock bore 273 commando engine with dome pistons and .028 head gaskets.
also......... what about the 44* overlap concerns ?what does that mean? this cam has 15"vac at 850 idle .

here are the specs of the cam installed .

my-coammdo-cam-specs-ii-jpg.1715285896


my-commando-cam-specs-jpg.1715285897
 
I like to rev them until 1) they stop pulling hard, 2) the valves start to float, or 3) a connecting rod comes loose. In that order. You should let off or shift before 2 or 3 happens, LOL. I wouldn't rev past the cam makers red line.
 
Just wondering ...........This is the T-3 custom grind isky cam specs I installed in my 273 commando . what is meant by the 2* advance , does it mean it is at 106* or 110* ? Rpm range is 2000 - 6400. does this cam need to be revved to 6400 to make good power or what? I've taken it up to 5900 rpms so far and it's still pulling hard . is there any need to go to 6400 rpms? 65 barracuda with 4 speed , 3.55's gears , hooker headers , pertronix ignition , cleaned up 920 heads . real stock bore 273 commando engine with dome pistons and .028 head gaskets.
also......... what about the 44* overlap concerns ?what does that mean? this cam has 15"vac at 850 idle .

here are the specs of the cam installed .

my-coammdo-cam-specs-ii-jpg.1715285896


my-commando-cam-specs-jpg.1715285897
How's the bottom end? Say from 1000-3000? No need to rev it to the moon if you don't need to.
 
I shift when she stops pulling, typically never at the cam listed max rpm. 65'
 
How's the bottom end? Say from 1000-3000? No need to rev it to the moon if you don't need to.
Hi tmm ! fresh commando rebuild from last summer Mike . about 2000 miles on it already. time to let her stretch her legs some. lol I only took it up to 5900 rpms so far. I did call isky today , Ron said if you have a good bottom end .. take it up gradually in increments to 6400 rpms where the cam is in it's glory . said the cam is just coming alive in the upper rpm band. I could be giving up considerable hp stopping at 5900.
 
The Commando even with the factory cam was a revver. Not a lot of grunt down low (torque peak at 4K!), but it loves to wind. Look at this chart from the factory brochure.
Screen Shot 2019-02-06 at 5.03.08 PM.png


Don't be afraid to see how it likes it at 6K -- it's overbuilt as hell for the displacement.
 
I like to rev them until 1) they stop pulling hard, 2) the valves start to float, or 3) a connecting rod comes loose. In that order. You should let off or shift before 2 or 3 happens, LOL. I wouldn't rev past the cam makers red line.
solid cam ....no valves to float .. still pulling hard at 5900.
 
solid cam ....no valves to float .. still pulling hard at 5900.

Never really got into high RPM HP but from this comment my assumption of valve float is coming into question.

I always had the understanding that valve float was caused by the RPM high that the valves could not close fast enough due to relatively low spring pressure for the RPM. Now it sounds like it is hydraulic lifter pump up that causes the issue at high RPM.

Would someone set me straight. I-I-I'm Sooo Confuuuused!
 
Great graph!! Thanks for posting.

My little Commando comes alive about 3500 and really starts pulling, but I can see in the graph just how far I need to push it.
 
Never really got into high RPM HP but from this comment my assumption of valve float is coming into question.

I always had the understanding that valve float was caused by the RPM high that the valves could not close fast enough due to relatively low spring pressure for the RPM. Now it sounds like it is hydraulic lifter pump up that causes the issue at high RPM.

Would someone set me straight. I-I-I'm Sooo Confuuuused!
could be either or . imo
 
In this day and age it's easy to figure out shift points 'n such. There are several vehicle "performance gauges" I have a used G-tech for example. Anything that measures torqu or acceleration will do ya. You generate a curve for RPM in a known gear, and then plot that curve corrected for the other gear ratios. "Where they cross" is where you shift

Similar to this.......One axis is (here thrust) can be torque, acceleration or HP, the other axis speed or RPM.

Figure_10-1024x624.png
 
That graph in post #4 is incorrect. Torque and HP always cross at 5250 RPM.
 
solid cam ....no valves to float .. still pulling hard at 5900.
Huh ?
You can float them with a roller cam and a rev kit if you have no regard for parts or the laws of physics..
With a solid cam , the springs will let you know when your right foot needs to go on a diet.
The only valves that cannot float to my limited knowledge is the Ducati motorcycles with their desmodromic valve actuation.
 
For a drag race, that graph in post #6 tells you to shift just past the hp peak ~5500 - 5700 rpm and operate around peak hp.
It does no good to go too far past peak hp @5200 before shifting. You lose hp and risk hurting the engine. For top speed in 4th, the Commando engine goes easily to 6000 rpm.
 
Rob can you explain? I just used Google and looked at approx 10 graphs. Only 1 where they intersected close to 5250.
I read wiki and got confused quick lol.
I'm not saying above graph is correct by the way.
That graph in post #4 is incorrect. Torque and HP always cross at 5250 RPM.
 
Rob can you explain? I just used Google and looked at approx 10 graphs. Only 1 where they intersected close to 5250.
I read wiki and got confused quick lol.
I'm not saying above graph is correct by the way.

It's over my head, Steve, but it has something to do with physics. Every time on every dyno run torque and HP always intersect at 5250. I don't know why.

Steve, I found this. I got lost pretty quickly, but maybe it will help. lol

Why does hp and tq crossover at the same point always? - Power and Drivetrain
 
This thread is a loaded question. We can't tell the op how high rpm he can take his 273. Each engine is different. I hope 7000 rpm doesn't come up and the op scatters it all over the neighborhood.
 
It's over my head, Steve, but it has something to do with physics. Every time on every dyno run torque and HP always intersect at 5250. I don't know why.

Steve, I found this. I got lost pretty quickly, but maybe it will help. lol

Why does hp and tq crossover at the same point always? - Power and Drivetrain

It's simple Rob. A dyno DOES NOT measure HP. It measures torque. HP is a mathematical calculation from torque, look at the formula. "There is no such thing" as HP in effect. It is in fact a product of mathematics

Horsepower = torque X rpm / 5252

EDIT read your link LOL, explains it better.............

Torque Vs. Horsepower



Have you ever wondered why “torque vs. horsepower” has always drawn so much debate?

I think it’s because since the definitions involve trigonometry and physics most people don’t want to go through the hassle of deciphering it all.

Let me try to simplify it a bit.



Here’s the short of it: Horsepower (HP) doesn’t really exist, just torque!

You don’t believe me?

Here’s definition: Horsepower = (Torque x Engine Speed)/5252.

So, HP is just a function of Torque.



But first we need to understand a few concepts:



Force is the pressure of one mass against another. In the metric system force is calculated in “Newtons”. Gravity is an easy example of a natural force and in the English system its unit is pounds. So we also use pounds as a basic unit of force.



Work is defined as force over distance and is calculated as Work = Force x Distance.

Work is achieved when a force causes an object to move. The force placed on the object and the distance it moves is the work done.



Power is the amount of work that can be done in a certain amount of time and is calculated as Power = Work / Time. Power was originally defined by James Watt (inventor of the steam engine) who noted that a horse could lift at a rate of about 550 lb-ft per second in an 8 hour shift.

Rotation.jpg



Torque is the tendency of a force to rotate an object about an axis and is defined as the force at any one point on the edge of a circle in the exact direction of rotation, multiplied by the radius (distance from the center).

As we explained above, in the metric system, force is calculated in Newtons, and distance is in meters, so the standard torque unit is Newton-Meter (N-M). In the Standard/English system, force is calculated in pounds and distance in feet. So the torque unit is lb-ft, usually called “foot-pounds” and sometimes also written as “ft/lb”.



Torque is the measure of the turning force of an object, such as a flywheel or a bolt. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.



Horsepower is a unit of power. It can be defined in multiple ways, but in its basic sense , it is defined as work done in a straight line as described above under “Power”. When the work is not done in a straight line (as in an automobile engine), it must be defined in a different way: Torque.

Horsepower = (Torque x RPM) / 5252.

It’s worthwhile noting that horsepower is not recognized in the International System of Units.



The reason Horsepower started being used was to compare the output of horses (which everyone understood) with that of the (then new - 1702) steam engines that could replace them. In a study done in 1993 to try to prove (or disprove) the theory it was found that one horse could produce a peak power over a few seconds as high as 15 hp. However it was observed that for sustained activity a work rate of about 1 hp per horse is consistent with what was determined in 1702.

Going back to the formula: HP = (Torque x RPM) / 5252 we can see that at an engine speed of 5,252 RPM Torque and HP are equal. That’s why when we see a graph of Torque / HP we see the lines always cross exactly at 5252 RPM.

Curve.jpg

But where does the 5252 come from?



Watt’s definition for HP says: 1 HP = 550 lb-ft / sec, or 1 HP = 550 lb x 1 ft / 1 sec



If we convert straight-line work to its rotational equivalent the formula becomes:



1 HP = 550 lb-ft x 1 rad / 1 sec (a rad is a unit-less measure of a circular distance)



When we convert radians (rad) to revolutions and seconds to minutes the formula again evolves. Note: 6.28 rad per revolution (2 π) and 60 seconds in 1 minute



1 HP = (550 lb-ft x 1 rad / 1 sec) x (1 rev / 2 π rad) x 60 sec / 1 min)



Multiplying and canceling, the formula simplifies to:



1 HP = 5252 lb-ft x 1 rev / 1 min which is the same as: 1 HP = 5252 lb-ft x 1 RPM



So, Horsepower = Torque x RPM / 5252.



If you’ve rear this far I applaud you, since you now know where 5252 comes from and why the HP and Torque curves cross each other.
 
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Del thanks. I had seen you say this before and knew it was right but I cannot get an understanding on it. You made it better.....with the link I posted. LMAO
 
I dont fully understand it, but for example you have a "normal" diesel genset. Any that I've seen rev nowhere near 5200 rpm, but there is hp and torque? So that would mean for this example the 5200 is theoretical?
 
Download a G-meter app onto your phone.
Get one that works off the accelerometer chip in the phone, not GPS.
Tape or secure the phone near the tach, where they are visible.
Boot it in each gear, - note when the Gs drop, that was your shift point.
Don't be surprised if it's different in each gear.

Hydraulic lifters pump up from lack of valve spring pressure.
 
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I dont fully understand it, but for example you have a "normal" diesel genset. Any that I've seen rev nowhere near 5200 rpm, but there is hp and torque? So that would mean for this example the 5200 is theoretical?

Pretty sure it's factual, Steve. I don't understand all of it myself at all. I will say this......don't tell anybody the new generation Ford Powerstrokes won't rev to 5200. Mine sure did. LOL

Did you read that post I linked to that Del posted the text of? I did and I'm still lost. Although a little less lost. A little.
 
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