I have noticed that some engines reach peak rpms sooner than others such as my 351 w spinning up to 5000 rpm sooner than my 4 liter jeep which takes a while but the 4 liter is ultra smooth and my 351 is a little rougher at 5000 rpm but I love the force it puts out. Might a v8 have a liter crank than a inline six? Is it a good idea to lighten the flywheel or crank?

 

The weight of the crank had little or no bearing on how fast a motor revs up under load. The final gearing, weight of the vehicle, and the amount of power the motor makes will determine how fast it revs in any given gear.

 

Rotating mass is part of it. An inline six crank is heavier than a V8, and after that it depends on flywheel weight. Is one an auto vs. the other stick?

But the cam and intake differences between these motors is probably the bigger reason they rev differently.

As for reducing weight, for general use, there is nothing to be gained. If you want to lighten these components, it must be done under an engine teardown condition, with a balance job as part of it. Oh, I suppose you could drop the pan and go at the crank with a hand drill or angle grinder. You'd get some results there!

 

I agree, under load it is not an issue. (and so thus it is of no value for street use to lighten the crank w/o some other significant motor work.) I was talking just revving w/o load, which is what I thought was being referenced initially.

 

Ok well I was just wondering why a 351w in a 1982 f-150 that came with 145 hp stock with only intake and msd as mods reaches 5000 rpm quicker than a 4 liter out of a Jeep that has about 200 hp with headers. The f-150 has a C6 auto and the Jeep has a maual but just reving in nuetral is what I am talking about. I love that 351w as it has been the best and worst experiences for me with cars but I am a novice 19 year old hot rodder. That must be why most manufacturers are running V6s and V8 instead of I6s and V12s. thanks...

I would only machine my internals during a rebuild. Would a 460 or 428 reach 5000 rpm as soon as a 289? It seems that a 429 torino will make the hp at low rpms using higher gears while a 3 or 4 liter would have to rev to 8000 rpm or so with really low gears to be comparable, which seems to be a trend today.

 

351 with intake and igniton mods. Well, it has 4 more valves than the six for one, and it's probably pretty loose..... 460 or 428 with the right cam and intake will rev faster than a stock 289.

As for the 429, well, that's a big motor and makes lots of power at almost all rpm.

As for 3 and 4 liter engines turning 8000 rpm, not stock they don't.

Rules of thumb:

The more you do to an engine to hot rod it, the higher you push hp peak rpm wise. The faster it goes, the more air/fuel it breathes and that's what makes the power.

Now, if you leave everything else the same, the larger you make the engine, the lower the revs for peak power.

An imperfect example would be a 460 vs. a Boss 302. Boss 302 has about the same size valves and carb as the 460. Peak power for the 460 probably 4000, peak for the 302 6000, give or take. 460 is a mild runner, the 302 a near racer. Peak torque for the 460, somewhere around 2200, for the 302, somewhere around 5000. 460 stump puller, 302 needs a push to get off the line.

Don't worry about no load revs. If you keep comparing this facet, you may get carried away and spit a rod out of one of your motors....

 

That makes sense. And I think more valve overlap is needed for 8000 rpm which is hard to get through emissions right?

 

I wouldn't be hard to get a rod to overlap the block @8000rpm.

Lots of lift and duration, solid or roller lifters, a very good crank and rods are what it takes to keep a motor alove @ 8000rpm.

 

I'm guessing the 5.8 makes more torque than the I-6.

If we're talking about automatics, then one way to allow the engine to spool up quicker is with a higher stall converter.

 

Chances are if you build a motor that stock peaks at 5500 into something that will run 8000 you might have emissions trouble. Or not. Lots of aftermarket stuff is EPA/CARB certified. As far as crank, rods, valve springs, valve material and anything else that needs to be beefed to take 8000 rpm, those have no effect on emissions. So if you can flow enough air with a stock intake/exhaust, or with certified parts, and you build the moving parts to take it, you can run 8000 rpm all day. Well, ok, for a few hours.

But then, if you make it run 8000 rpm, how is it going to run a 1800 rpm? Will it still be driveable with the stock gearing?

You see, the thing you have to keep adding to the mix here is money. As long as you have lots of money to stir into the motor, you will be able to take an old 289 and run 10,000 rpm if you like, or push a Chev Ecotec to 10,500. The faster the crank turns, the faster the money burns.

 

Speaking to lightweight vs heavyweight rotating mass, the lightweight crank will accelerate quicker. Taking two crankshafts exhibiting the same static weight, the crankshaft with more weight near its axis of rotation will produce a lower moment of inertia, and thus less resistance to a change in rotational speed. I say this to illustrate the fact that a lightweight crank can possess a higher moment of inertia than a heavier crank, due to placement of weight relative to its axis of rotation. Rods, pistons, pins, etc., must be matched to the lightweight crank to maximize its effectiveness. The less weight in the rotating assembly, the better, assuming equal strength. And minimizing reciprocating rate is more important than minimizing rotating weight. Therefore weight taken off the pistons and rods is more important than weight off the crankshaft, but they actually go hand-in hand. Lightening the rod and piston means removing counter weight from the crank to maintain balance.
http://en.wikipedia.org/wiki/Moment_of_inertia

Rotating inertia is the main power loss during acceleration vs. steadystate running. The quicker the engine accelerates, the higher the losses. So higher gear ratios = quicker engine acceleration = higher rotating inertia losses.


Not JMO.



Pete

 

Just wished to jump in and add my 2 cents,If you go back to the old school racer one of the very first things we would do is respring the distrbutor,those pesky little weights will be held back by to strong of a spring, those of us who could perchase the spring kits,those who could not would bend the pins to let some of the pressure off of them.