[ReverendDexter]

Okay, so I know the rule of thumb than a short stroke motor should be able to see higher rpm than a long stroke motor, but I'm assuming there's a lot more to it than that.

Rather than just continuing to spout the same canned answer, can someone educate me in a little more detail what exactly the correlation is between bore:stroke ratio and how much rpm you'll get out of a motor?

I mean, you're obviously fighting intertia when you're trying to change the direction of a piston, and a longer stroke motor will have to accelerate that piston more, and have it going at a faster speed than it would be with a shorter stroke, and thus have more inertia to "fight" at every direction change.

So, how much more rpm could you get out of, say, a 4.6, if you were to take some stroke OUT of the motor? I'm not sure what the actual crank throw is, but say you had a crank with a 1" shorter throw, and put longer rods in to make up the difference, would you see an increase in how "happy" the motor was to spin? Would you have increased the overall rpm capacity? By how much, if any? Or would you just have cost yourself displacement and torque?

[4.6LMadness]

well if you reduce the stroke you are obviously reducing displacement but im sure you could get that back if you bore it over. As for rpm's, it dosnt take much to get the motor to rev higher.

Remove some accessories and shortin the stroke and you"ve got yourself a v8 honda motor capable of 8k or more. Might float a couple of valves in the process but im sure it would sound wicked.

[SWEET2KSTANG]

you are right about inertia of the stroke, and the last part you are talking about is destroking. all that plays a factor, but you have to think about piston/rod ratio, dwell time, motor set up/use, etc. depending on motor, you can change certain aspects of the set up and be able to get a little more revability and efficientcy out of it. natuarally a squared motor will give best all around performance. another factor in reving a motor to think of is in the heads and valvetrain. you can do all you want in the rotating assembly, but all the power is in the heads.

[ReverendDexter]

Where this thought process started was in my thinking about how much power an N/A 4v makes relative to an N/A 3v. Near as I can tell, with the architecture of the 4.6, the 3v heads flow well enough for any amount of rpm that the motor can turn. It seems to me that the 4v heads are superfluous, as the 4.6 just cant' turn enough rpm to justify the extra flow.

Seems like the power potential of the 4v is only realized when you put boost to 'em, and artifically create the airflow that the extra rpm would generate.

[SWEET2KSTANG]

the 4.6 can turn the rpm's you want, just got to make the rest of the set up go with it. depending on set up of each motor, the 3v heads can be more than suitable for street power n/a. the 4v's can make the same power and have room to go if you wanted to push a little more from the combination. if you set a motor combo up to turn the rpm's and make the power, the 4v's will make more power than the 3v.

[ModMustang97GT]

Well, here is the real answer that i think you are looking for. we all know that torgue is the force of the motor spinning, and horse power is a function of the rpm's times the torque. This is why a engine that makes 200 lbs ft of torque can make 600 hp. It revs to 11k. The power will make it rev. The reason your parts fail is friction and balance. The two add to eachother. if the engine is out of balance, which it always will be, there is no such thing as a perfect balance. And, like anything in the air, there is terminal velocity. There is a theoretical limit to the speed your motor can rev. however, cars are no where near that limit, so its irrelivant. The real issue is, with racing oil and 4" bore. A big block 511 can rev to 8k rpm and hold it for half an hour. I crew for a Hydroplane and that is the name of the game. The real issue, is the balance. If you can balance your motor really well, when you rev up really high, the motor internals won't be wobbling due to the inconsistencies in the parts. Destroking and boring simply make it easier to turn those numbers, because with a large stroke you have to worry about the rods clearing the block. There is alot of machine work that goes into the block of a largely stroked race engine. That is why a 1000hp n/a big block costs 100k and a supercharges 1000hp big block costs 25k. The machine work, and the balancing. You dont' care about the balance at 5k rpm, because its only 5k rpm. But at 9k rpm, there is a exponential amount of force on the rotating parts. making that little wobble at 5k rpm, a catastrophic failure at 8k rpm. Now, if you could have a block that was extremely strong, a super well balanced motor, with a good set of bump sticks and some monster springs to go on them. You could reach the flow limit of the 4v without too much trouble. They only flow like 600-700 cfm all together. With a 4.6l turning 9k rpm and 14:1 compression. not an insane set up for a race motor. You could be easily into the power range where your 4v heads would be maxing out. The problem is, the parts to build such a 4.6 do not exist. No one makes a high quality enough crank and rods to make those rpm's and the heads aren't designed to hold the 500lb/inch springs you would need for the cams.

Ultimately it comes down to the whole cost factor. The best N/A set up I have heard of, was the guys from Logan motorsports using their stage 3R ported heads making 400 hp. I'm sure the TF heads could do a little better, and with a stroker kit, you could hit it up to the 450 range. However, even the big boys don't see anyone making a 500 hp n/a 4.6 any time soon. The aftermarket that makes those type of parts just aren't into our cars. There are not enough corner carvers out there for the big circle track guys to make the parts. a Mustang unfortunately, is deemed a drag car. So you will most likely be limited to f/i or giggle juice to get to the ground pounding numbers you seek. Unless you only want a mild 400-450 hp. then you can still go n/a.

[SWEET2KSTANG]

the stock cobra crank or a kellog crank can turn those rpm's with no issue as long as rotating assembly is balanced. there are billet rods out there to spin that rpm and hold it as well. there is currently no 2v head that will flow enough to turn those rpm's or make power at those rpm's efficiently. you can do 500hp from a 4.6 4v if you put the $$$ in the right parts and combination. 450 can be done based around pump gas street DD car, turn up th ecombination and 500 can be done.

[bitemark46]

Scott Cushman destroked a 5.4 down to a 322(ish) and achieved 10k rpms. Made around 700-750 at the crank. So a destroked 4.6 I'm sure can hit the same rpm level but probably not the same hp level. Off the shelf aftermarket parts nowadays can hold 8k-8.5k. Stock cranks (forged) too. Definately nowhere near a streetable setup but it can be done. At that rpm you have to worry about the inertia on the downstroke. So much stress on the rod bolt and the piston (around the wrist pin) that quality parts are a requirement and with quality parts come $$$.

-Mark

[ModMustang97GT]

Ok, wasn't aware the parts were quite available. Although you are right, you need to have some monster rpm to make power. And with the low amount of displacement, its just hard to make that much power n/a. It can be done, its just really expensive, which is why everyone loves FI.

[banned]

ok some of this is just too dumb so I have to come in here. His question is completely dependent on the bore/stroke ratio. Thats why people destroke 5.4's when they want to turn 10k+ rpms. 5.4's have a 3.55 x 4.16, 4.6's are a "square motor" which is essentially perfect 3.6 x 3.6. A 4.6 has a 1.67:1 rod to stroke, this ratio is good to go for just about as many rpms as the quality of the materials in the rotating assembly and valve train will allow. The 5.4 has a 1.60:1 ratio, which is still "good" for any oem application, but in a performance standpoint isnt ideal for going over 7k rpm. (again provided the materials are up to the task). So there would be ZERO sense in de-stroking a 4.6 and pretty much no sense in destroking a 5.4 STREET car, you will never need to spin more than 7k rpm.

Now, the big block rpm comment... a N/A big block can make 1000 hp for a hell of alot less than 100k bucks, more like 15-20k, and FI big block can make 1000hp for 5-10k. But again there is no need to de-stroke anything, in a street car you want your power as soon as you can make it, esp for drag racing. The purpose spinning retarded rpm is use the cylinder vacuum and valve timing to create non mechanical boost = more power.

In general you want more bore, less stroke or equal amounts, but anything with a rod/stroke ratio of 1.6 or better is good to go.

[ModMustang97GT]

My reference to cost is on blown 454's and stroked 454's (511's) for hydroplane racing. Water racing requires a lot higher tollerances as they run at red line for 30 minutes at a time, with their prop coming out of the water, making there be no drag on the prop, and since they are direct drive that means random 13k rpm excursions for the 511 designed to be running at 8k rpm. In a car, these motors would be closer to NASCAR engines. Drag cars don't bother with the workmanship of these engines because of the small amount of time they spend at full throttle by comparison.

[banned]

^ say no to drugs...

[NeverEnuff]

Originally Posted by ModMustang97GT Ok, wasn't aware the parts were quite available. Although you are right, you need to have some monster rpm to make power. And with the low amount of displacement, its just hard to make that much power n/a. It can be done, its just really expensive, which is why everyone loves FI.

"expensive" is a realative term. To one person 100 bucks might be a lot...to another 1000.

IDK if EVERYONE loves FI.

I for one don't love it, while it does make power...it also comes with other hassles.

You don't need ANY special parts to turn 7-7200k rpms on a mod motor. Hell I did on my Aviator motor with just bolt ons, cams and springs. Hell Boss330 did it on Motorcycle valve springs. Nothing special and 9000+rpm.

Like Mark said....Scott Cushman did his 324ci to 10+ and 700+hp...

Nazman has done over 450rwhp on his BB setup no stroke, and over 500rwhp on big bore and stroker 4.6. He ran 10s on full weight...I think it works.

Also another big factor is piston speed...the longer the stroke the more pistons speed it has...combine a long stroke with a lot of RPMs and it can spell disaster fast. Hence why Scott choose to destroke his AND do a big bore, bringing the motor back to 'square'...

[ModMustang97GT]

Well said, my only point is that the work necessary to make good N/A power on our cars is less common knowledge. You are likely to have to pay a machine shop to help you out with things that alot of people are not as familiar with. The 4-5 grand for short block work and 2k for a good intake is alot more than the $2600 and 3k for the super charger kit that will make 600 hp. Most mustang people will recommend going FI and forget about it. I like N/A personally, but its definitely more work than going FI.