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Old 04-11-2012, 02:04 PM   #36
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The original comparison was not a fair comparison...

the money invested to get to the level of the 347 the OP originally stated was a significant amount of machining and aftermarket parts...Comparing a fully modified engine to a factory stock motor you can get for less than $500 complete is noa a comparison...

Equal parts in both motors would be stroker kits, with the same heads, cam, exhaust diameter and so on...351 will be faster if you compare them on a even playing surface.
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Old 04-11-2012, 02:15 PM   #37
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_-Unabashed bench racing crap warning-_

I definitely can't quantify the effect of the bearing size difference, the only "evidence" I have is anecdotal.

5 or 6 years ago, there was an engine builders challenge. Who sponsored/covered it I can't recall, but it included Mopar, Chevy, and Ford based engines. All of the builders turned down the journal sizes as small as they could, specifically to reduce bearing surface area and drag. One of the things each builder measured was how many ft/lbs of torque were required to turn the engine over using a torque wrench on the crank. What stood out to me, is that it wasn't one or two of these builders with a "trick", every single one did this. That tells me they knew something.

The other piece of anecdotal evidence to me, is what I think I know (emphasis on think I know) about friction, being that it is a non-linear function of speed. Meaning that as two objects slide past each other, the friction between them does not increase equally with the speed. So you get more than twice the fiction, at only twice the speed. The way you reduce speed between the bearing and the journal is by reducing the journal's size. So bring these two concepts together and a small decrease in friction measured at low speed (like turning the engine over by hand) will be a large decrease in friction at 6000 rpms.
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Old 04-11-2012, 02:24 PM   #38
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Originally Posted by Wicked03GT View Post
The original comparison was not a fair comparison...

the money invested to get to the level of the 347 the OP originally stated was a significant amount of machining and aftermarket parts...Comparing a fully modified engine to a factory stock motor you can get for less than $500 complete is noa a comparison...

Equal parts in both motors would be stroker kits, with the same heads, cam, exhaust diameter and so on...351 will be faster if you compare them on a even playing surface.
Well I think the idea is to look at it as 2 different ways to get to ~350 cubes, which one will make more power. In that regard, the 302 based stroker will make more power.

But to your point the 347 is built for performance. It's got a lot more $$ in it than the $500 stock 351.

If I can get to 350 cubes any way I wanted, I would use 6 2008 Yamaha R1 engines (1 liter each), with 1 spark plug pulled out of the 6th. My bike dyno'd at 158 rwhp bone stock, so that's 790 rwhp out of 5, plus ~75% of the 6th or ~118.. so 900 rear wheel horse give or take out of 23 cylinders. Go ahead and knock off 30% for my wacky drivetrain I'll still beat up that 347.
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Old 04-11-2012, 02:27 PM   #39
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manual trans will have a given amount of drop based on the gear ratios. in an auto the converter will mask the drop and the extra weight wont have as much negative effect.
Yes: manual trannies.

It will depend on time it takes to shift as well.... time the clutch is depressed.
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Old 04-11-2012, 02:59 PM   #40
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isn't RPM drop strictly a function of gearing? in order to go 60 mph in a certain gear (rear and tranny) you turn a certain RPM (assuming no clutch slip). So with the pedal on the floor if I have heavy flywheel or light and the tires are not spinning after the gear change the rpm drop will be the same. Or with a heavy flywheel the RPM is different going 60 for the same gear (rear and tranny) than a light flywheel? I think the point of the RPM drop was, unless you have a single gear ratio for the entire 1/4 you are going to shift, and the rpm will drop strictly based on your gearing (and yes how fast you shift, depress clutch, etc. but let's keep all that equal). Then the recovery from the rpm drop is what needs to be overcome to accelerate. if your mass is smaller (less friction and inertia to overcome) your engine will accelerate faster. If I can change flywheels and get a different drive ratio for the same gearing then I guess I don't understand what is going on.
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Old 04-11-2012, 03:03 PM   #41
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Originally Posted by andrewtac View Post
isn't RPM drop strictly a function of gearing? in order to go 60 mph in a certain gear (rear and tranny) you turn a certain RPM (assuming no clutch slip). So with the pedal on the floor if I have heavy flywheel or light and the tires are not spinning after the gear change the rpm drop will be the same. Or with a heavy flywheel the RPM is different going 60 for the same gear (rear and tranny) than a light flywheel? I think the point of the RPM drop was, unless you have a single gear ratio for the entire 1/4 you are going to shift, and the rpm will drop strictly based on your gearing (and yes how fast you shift, depress clutch, etc. but let's keep all that equal). Then the recovery from the rpm drop is what needs to be overcome to accelerate. if your mass is smaller (less friction and inertia to overcome) your engine will accelerate faster. If I can change flywheels and get a different drive ratio for the same gearing then I guess I don't understand what is going on.

Inertia.....engines are not exempt from it.
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Old 04-11-2012, 03:14 PM   #42
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Originally Posted by andrewtac View Post
isn't RPM drop strictly a function of gearing? in order to go 60 mph in a certain gear (rear and tranny) you turn a certain RPM (assuming no clutch slip). So with the pedal on the floor if I have heavy flywheel or light and the tires are not spinning after the gear change the rpm drop will be the same. Or with a heavy flywheel the RPM is different going 60 for the same gear (rear and tranny) than a light flywheel? I think the point of the RPM drop was, unless you have a single gear ratio for the entire 1/4 you are going to shift, and the rpm will drop strictly based on your gearing (and yes how fast you shift, depress clutch, etc. but let's keep all that equal). Then the recovery from the rpm drop is what needs to be overcome to accelerate. if your mass is smaller (less friction and inertia to overcome) your engine will accelerate faster. If I can change flywheels and get a different drive ratio for the same gearing then I guess I don't understand what is going on.
Drop isn't a great word for this topic.

A lighter flywheel permits an engine to change RPM up or down, more quickly.

This hurts you on the launch, but this helps you accelerating through a gear.

This can hurt you between gears if you allow the RPMS to get under the correct matched RPM for the next gear. If you can shift fast enough this isn't an issue, or simply don't completely reduce power from the engine when you shift ala "flat shift".

To your question, the flywheel weight has nothing to do with your RPM @ 60mph in a manual transmission.
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Old 04-11-2012, 04:10 PM   #43
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Runaway train lol. Just ask yourself why guys pay 1000's upon 1000's for lighter cranks, rods, pistons, flywheels, driveshafts etc and you will find the answer.
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Old 04-11-2012, 08:11 PM   #44
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Guys it's really simple, a 347 with AFR 205 heads, super victor, 600lift solid roller 200 shot of spray etc vs a 351 will all the same parts.. the 351W will blow it's doors off.. I mean how fast can you really go with a cracked block?
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Old 04-11-2012, 09:43 PM   #45
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Guys it's really simple, a 347 with AFR 205 heads, super victor, 600lift solid roller 200 shot of spray etc vs a 351 will all the same parts.. the 351W will blow it's doors off.. I mean how fast can you really go with a cracked block?
because stock windsor blocks dont crack

Must Read....PSA for Windsor Roller block guys
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Old 04-11-2012, 10:48 PM   #46
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Inertia.....engines are not exempt from it.
roger, neither are they exempt from physics. The inertia the engine must overcome to accelerate back up should be easier for lighter components with less friction. My point is if I am shift from 1st at 7000 and when I let go of the clutch my engine is now at 6200 rpm, would this not be a function of only gear ratios. As a heavier or lighter flywheel won't change gear ratio. I understand you are saying the inertia of the heavier flywheel will try to keep the motor up high, but if the tires hook there is only one rpm possible for given gear ratio. If I don't hook up then sure the heavier flywheel will keep he tires spinning more than a lighter one will and the car might still accelerate. The drop or decrease of rpm from one gear to next is due to the gear ratio. And finally Woody's brought up the obvious capitalist answer, why else would people strive for lighter components. Not because they cost more. Why does any race team spend money, to go faster.
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Old 04-12-2012, 09:16 AM   #47
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because stock windsor blocks dont crack

Must Read....PSA for Windsor Roller block guys
No they can but a stock 351W is good to 700+ hp the stock 302 cant come close to that. It all comes down to the fact that this comparison is maxing out the 347 and restricting the 351W.

To put it another way what would be faster, a mustang with a t-5 or a tko600? The t-5 will rob less hp to the wheels then the tko600 so it's faster right? If you restrict the power the motor makes then yes the t-5 would be faster. But once you add in the fact that the tko600 can take about double the power of the t-5, and you then add that power into the equation the tko600 would be much faster (broken trans vs still racing). Same thing applies with the 347 vs 351 apply more power and we have the same thing (broken block vs still racing)

The only way the 347 vs 351 comparison works n favor of the 347 is if you max out what the 347 can safely due and restrict the 351W.
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Old 04-12-2012, 09:34 AM   #48
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roger, neither are they exempt from physics. The inertia the engine must overcome to accelerate back up should be easier for lighter components with less friction. My point is if I am shift from 1st at 7000 and when I let go of the clutch my engine is now at 6200 rpm, would this not be a function of only gear ratios. As a heavier or lighter flywheel won't change gear ratio. I understand you are saying the inertia of the heavier flywheel will try to keep the motor up high, but if the tires hook there is only one rpm possible for given gear ratio. If I don't hook up then sure the heavier flywheel will keep he tires spinning more than a lighter one will and the car might still accelerate. The drop or decrease of rpm from one gear to next is due to the gear ratio. And finally Woody's brought up the obvious capitalist answer, why else would people strive for lighter components. Not because they cost more. Why does any race team spend money, to go faster.
LIGHTER PARTS ALLOW FASTER CHANGE OF DIRECTION AND THEREFORE ACCELERATION.........THEY DO NOT HOWEVER = MORE POWER.
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Old 04-12-2012, 09:38 AM   #49
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LIGHTER PARTS ALLOW FASTER CHANGE OF DIRECTION AND THEREFORE ACCELERATION.........THEY DO NOT HOWEVER = MORE POWER.
This is true when speaking about gross power.

But, if we're talking about net power at the wheels, lighter parts does = more power.
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Old 04-12-2012, 10:27 AM   #50
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No they can but a stock 351W is good to 700+ hp the stock 302 cant come close to that. It all comes down to the fact that this comparison is maxing out the 347 and restricting the 351W.

To put it another way what would be faster, a mustang with a t-5 or a tko600? The t-5 will rob less hp to the wheels then the tko600 so it's faster right? If you restrict the power the motor makes then yes the t-5 would be faster. But once you add in the fact that the tko600 can take about double the power of the t-5, and you then add that power into the equation the tko600 would be much faster (broken trans vs still racing). Same thing applies with the 347 vs 351 apply more power and we have the same thing (broken block vs still racing)

The only way the 347 vs 351 comparison works n favor of the 347 is if you max out what the 347 can safely due and restrict the 351W.
did you even read the link? windsor blocks are cracking in the #2 and #4 cam bearing journals. what does HP have to do with it cracking cam bearing journals? 302s arent doing that. the point is, since windsors are also cracking take the block out of the equation. 351w vs 347 everything else the same, the 347 accellerates faster.

Last edited by cab0154; 04-12-2012 at 10:31 AM.
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Old 04-12-2012, 10:29 AM   #51
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This is true when speaking about gross power.

But, if we're talking about net power at the wheels, lighter parts does = more power.
the heavier assembly will put more power down on the dyno, the lighter assembly will accellerate faster.
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Old 04-12-2012, 10:37 AM   #52
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the heavier assembly will put more power down on the dyno, the lighter assembly will accellerate faster.
Absolutely false.

All things being equal, the energy in the combustion will be the same between the two (gross power). However the heavier assembly will steal more of that energy than the lighter one.

The rate of acceleration is strictly a function of available power. The lighter assembly accelerates faster because more power is available.

It's like your gross income before taxes, and your net paycheck after. Assembly weight is power tax. The heavier the assembly, the greater the power tax.
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Old 04-12-2012, 10:37 AM   #53
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LIGHTER PARTS ALLOW FASTER CHANGE OF DIRECTION AND THEREFORE ACCELERATION.........THEY DO NOT HOWEVER = MORE POWER.
right. but extra weight does not minimize rpm drop, that is a function of the gearing assuming the clutch doesnt slip and the tires hold.
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Old 04-12-2012, 10:42 AM   #54
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Absolutely false.

All things being equal, the energy in the combustion will be the same between the two (gross power). However the heavier assembly will steal more of that energy than the lighter one.

The rate of acceleration is strictly a function of available power. The lighter assembly accelerates faster because more power is available.

It's like your gross income before taxes, and your net paycheck after. Assembly weight is power tax. The heavier the assembly, the greater the power tax.
my 302 lost 6hp peak as well as everywhere along the curve (using the same dyno in the same conditions) with my fidanza aluminum flywheel (13lbs vs. my 24lb Ram billet steel unit) but the car was consistently .2 quicker in the 1/8 mile only picking up .3 mph.

the dyno measures torque, not hp. it calculates the hp from torque. the heavier assemblies make more torque at every point on the curve, thus make more hp. but the lighter assemblies do accellerate faster for the reasons you state. the dyno lies. what you are saying is correct, but the dynojet doesnt work that way.

Last edited by cab0154; 04-12-2012 at 10:47 AM.
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Old 04-12-2012, 10:46 AM   #55
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right. but extra weight does not minimize rpm drop, that is a function of the gearing assuming the clutch doesnt slip and the tires hold.
YOU ARE INCORRECT, sorry, but fact is fact.
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Old 04-12-2012, 10:48 AM   #56
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my 302 lost 6hp peak as well as everywhere along the curve (using the same dyno in the same conditions) with my fidanza aluminum flywheel (13lbs vs. my 24lb Ram billet steel unit) but the car was consistently .2 quicker in the 1/8 mile only picking up .3 mph.

the dyno measures torque, not hp. it calculates the hp from torque. the heavier assemblies make more torque at every point on the curve, thus make more hp. but the lighter assemblies do accellerate faster for the reasons you state. the dyno lies. what you are saying is correct, but the dynojet doesnt work that way.
This is EXACTLY what you would expect.....ALL the result of something called INERTIA.
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Old 04-12-2012, 10:56 AM   #57
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YOU ARE INCORRECT, sorry, but fact is fact.
if this is the case, and the tires are hooked and the clutch does not slip, please explain to me how with the same gear ratios in a manual trans and everything else being equal other than rotating assembly weight, how the rpm will not drop a given amount?

i can see how the rpm would stay up if the clutch slipped, but if that doesnt happen in either case they should recover at the same rpm for a given shift point because of the mechanical connection of the gears, everything else being equal other than weight.
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Old 04-12-2012, 11:11 AM   #58
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if this is the case, and the tires are hooked and the clutch does not slip, please explain to me how with the same gear ratios in a manual trans and everything else being equal other than rotating assembly weight, how the rpm will not drop a given amount?

i can see how the rpm would stay up if the clutch slipped, but if that doesnt happen in either case they should recover at the same rpm for a given shift point because of the mechanical connection of the gears, everything else being equal other than weight.
Dood....you ain't quite gettin it. Got nothing to do with clutch slip or hook.
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Old 04-12-2012, 11:15 AM   #59
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Dood....you ain't quite gettin it. Got nothing to do with clutch slip or hook.
well that sure explains it.
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Old 04-12-2012, 12:37 PM   #60
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well that sure explains it.
I thought it would.

To see it better, you can look up CVT vs Manual shifting....years ago I came across charts on it ( I was tuning snowmobile clutches back in the day).....

Essentially it looks like a sawtooth pattern when you look at what the rpm does on a manual car....you overshoot the target rpm then it falls off as clutch is depressed and again builds back up.

The amount the rpm falls off is related to the inertia of the spinning assembly as well as other factors obviously such as friction from components and oil viscosity, etc....
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Old 04-12-2012, 12:38 PM   #61
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Runaway train lol. Just ask yourself why guys pay 1000's upon 1000's for lighter cranks, rods, pistons, flywheels, driveshafts etc and you will find the answer.
Dammit Woody, stop making sense.

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my 302 lost 6hp peak as well as everywhere along the curve (using the same dyno in the same conditions) with my fidanza aluminum flywheel (13lbs vs. my 24lb Ram billet steel unit) but the car was consistently .2 quicker in the 1/8 mile only picking up .3 mph.
What are the "same" conditions? Was the room temperature the same? Was the humidity the same? Was it on the same tank of gas? Was the engine at the same temp? Etc, etc. 6 HP is easy to gain or lose through even small variations.

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the dyno measures torque, not hp. it calculates the hp from torque. the heavier assemblies make more torque at every point on the curve, thus make more hp. but the lighter assemblies do accellerate faster for the reasons you state. the dyno lies. what you are saying is correct, but the dynojet doesnt work that way.
Not all dynos measure torque, there is also what is known as an "inertia dyno", in which a cylinder of a known mass is accelerated by the engine/vehicle, and rate of acceleration is measured, thus determining horsepower. I believe most chassis dynos use this system. It is the same concept of estimating HP by trap speed of a car of a known race weight. Also same concept used by Dynolicious. By knowing mass and the rate of acceleration, one can determine power.

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if this is the case, and the tires are hooked and the clutch does not slip, please explain to me how with the same gear ratios in a manual trans and everything else being equal other than rotating assembly weight, how the rpm will not drop a given amount?

i can see how the rpm would stay up if the clutch slipped, but if that doesnt happen in either case they should recover at the same rpm for a given shift point because of the mechanical connection of the gears, everything else being equal other than weight.
Think of it this way: If your car was sitting still and you were to rev your engine to 6K rpm, and simultaneously drop the clutch and kill the ignition, the car would move forward and experience some amount of acceleration, solely from the inertia of the moving engine parts (remember, the engine is off). I think he is trying to argue that a car in a drag race would experience the same phenomenon of acceleration due to moving engine inertia during an upshift.

... And it probably does, to some extent. However, is it enough to justify the losses of accelerating the heavier parts in the first place?

My engineering experience tells me that no mechanical system is perfect, and you never get 100% of the energy that you put into getting a mass moving back when you try to stop the mass. That's why there is no such thing as a perpetual motion machine. So, I'm thinking that the power wasted to accelerate the heavier engine parts is probably NOT recovered during the upshift, when the mass of the car is working to decelerate the engine.

So... I still think lighter parts win.
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Old 04-12-2012, 01:03 PM   #62
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So... I still think lighter parts win.

Are you saying, with your engineering experience that lighter parts make more power?

Because that was the original question.
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Old 04-12-2012, 01:11 PM   #63
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Are you saying, with your engineering experience that lighter parts make more power?

Because that was the original question.
NOONE cares about power as long as they go faster. A 347 will walk away from a stock stroke windsor with the same parts, regardless of power, could care less about a dyno sheet, its meaningless.
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Old 04-12-2012, 01:28 PM   #64
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NOONE cares about power as long as they go faster. A 347 will walk away from a stock stroke windsor with the same parts, regardless of power, could care less about a dyno sheet, its meaningless.
Not trying to piss on anyones corn flakes, it was the question asked, and not by me.
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Old 04-12-2012, 02:15 PM   #65
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my 302 lost 6hp peak as well as everywhere along the curve (using the same dyno in the same conditions) with my fidanza aluminum flywheel (13lbs vs. my 24lb Ram billet steel unit) but the car was consistently .2 quicker in the 1/8 mile only picking up .3 mph.
A dyno is not a precision instrument. It's a tool that can help, and for the same car on the same day, results between pulls can provide useful information. However, that data is only relevant between those pulls.

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the dyno measures torque, not hp. it calculates the hp from torque. the heavier assemblies make more torque at every point on the curve, thus make more hp. but the lighter assemblies do accellerate faster for the reasons you state. the dyno lies. what you are saying is correct, but the dynojet doesnt work that way.
Correct. Dynos measure Torque, because engines produce Torque. They do not produce Horsepower. We as people choose to calculate horsepower because it tells us what we want to know, namely how *fast* can Work be done.

Torque is a Force, specifically a twisting force, and we typically use ft/lbs as the unit of measurement. Work means moving something. To move something (do Work) you must apply Force.

The Rate (speed) at which you do Work is what we call Power.

Moving 3000lbs down the 1/4 mile is Work. It is the same amount of Work regardless of how long it takes you to do it. If the distance is less, it is less total Work. Or, if the weight is less, it is less total Work.

The faster you do Work, the greater your Power.

Therefore the claim that people are making in this thread that the lighter parts accelerate faster, but don't have as much Power, is fundamentally wrong.

By definition Power is married to rate. Faster = more Power.


TL;DR version. More Power = More speed/acceleration. To say something has more acceleration but less or equal power is an oxymoron. It's just plain wrong.
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Old 04-12-2012, 03:58 PM   #66
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A dyno is not a precision instrument. It's a tool that can help, and for the same car on the same day, results between pulls can provide useful information. However, that data is only relevant between those pulls.



Correct. Dynos measure Torque, because engines produce Torque. They do not produce Horsepower. We as people choose to calculate horsepower because it tells us what we want to know, namely how *fast* can Work be done.

Torque is a Force, specifically a twisting force, and we typically use ft/lbs as the unit of measurement. Work means moving something. To move something (do Work) you must apply Force.

The Rate (speed) at which you do Work is what we call Power.

Moving 3000lbs down the 1/4 mile is Work. It is the same amount of Work regardless of how long it takes you to do it. If the distance is less, it is less total Work. Or, if the weight is less, it is less total Work.

The faster you do Work, the greater your Power.

Therefore the claim that people are making in this thread that the lighter parts accelerate faster, but don't have as much Power, is fundamentally wrong.

By definition Power is married to rate. Faster = more Power.


TL;DR version. More Power = More speed/acceleration. To say something has more acceleration but less or equal power is an oxymoron. It's just plain wrong.
Typically it is ASSUMED that power from an engine is the TORQUE it produces.
HP is just calculated as you say.
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Old 04-12-2012, 05:02 PM   #67
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did you even read the link? windsor blocks are cracking in the #2 and #4 cam bearing journals. what does HP have to do with it cracking cam bearing journals? 302s arent doing that. the point is, since windsors are also cracking take the block out of the equation. 351w vs 347 everything else the same, the 347 accellerates faster.
I did read it and that really doesn’t have anything to do with my point. The 351W have been in production sense the 60s, and the only ones with the cracking cam bearings is the roller blocks that where made for a small window of time compared to the whole production period, and of those only some have been cracked. And according to our resident stroker expert it seems that the cracks are due to heat and if it isn't cracked it's good to go for a hi end stroker motor. Now with that being said how many stock 302W blocks have held up to 700hp? Now lets compare that to how many 351W have. My point still stands..

But to play along with the hypothetical.. If we take out the strength difference between both motors and just compare a 347 vs a 351 with the same parts (or as close as possible) the 347 has after market crank rods and pistons, so why wouldn't the 351W also have aftermarket crank rods and pistons? So again to play along with the rules we keep the stock 3.5” stroke to keep the CI as close as possible. So if your not going to stroke it you might as well make the rotating assembly as light as possible. So now who is faster?

The downside to the 351W is the 3” mains will cause more friction..

But the 351 (356 with a 30 over) will have more CI
it also has a better rod to stroke ratio
larger stroke = more torque
Comparing the same intake manifold, the 351W will flow more cfm compared to the 302 version.
Due to the 351W block being much stronger it's safer to turn a much higher rpm.

Not really as cut and dry as people are saying.. And this is all with imposing limitations that you don't have to stick to when building a 351W..
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Old 04-12-2012, 05:17 PM   #68
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Original question..........


347 stroker or a stock shortblock 351w


The 347 wins. You guys went way off track with all the "what ifs"
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Old 04-12-2012, 05:47 PM   #69
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Like I think most would agree,

the pretty safe limit of a 347 (regardless of rotating assembly) is around 450rwhp

what would the "safe" limit be on a STOCK (rods/crank/piston) 351 shortblock? about the same?
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Old 04-12-2012, 06:24 PM   #70
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Oh yeah, what about rod ratio???














Sorry Jim, have to get the train all the way off the tracks!!
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