Leverage vs Engine volume

I know that going from a V6 to a V8 is about a 33% increase in engine displacement, and was looking for a way to calculate the gearing for the rig based on wheel and tire diameter.

If I divide 6 (cylinders) into 8 - I get a factor of 1.33, which is what I expected.

Since displacement pretty much follows available power, and work is expressed as leverage times the power applied, if the power is increased by a third, it stands to reason that the leverage it acts through can be changed in order to balance it out.

NOW, with the same gearing in the truck that I had before the engine swap, if I go to a larger tire size these are the numbers-

If the stock rim/tire combo is 28 inches tall, and I go to a 36 inch, then 36 divided by 28 gives me 1.28.

37 inches divided by 28 = 1.32, which is right about on the money.

Another way to figure it is in forward motion:

A 28' tire has an effective forward roll for 1 revolution of (28 x 3.14) 87.92 inches

36' = 113.04"
37" = 116"

-These numbers can be factored together...

From what I can see, a 37" tire will equal the original power to ground figures pretty closely, and a 36" would provide a small amount of advantage over the original. This would mean the engine RPM's would be approximately the same as stock, at a higher speed.

Do these numbers add up/make sense?

Is there a way to figure ideal matches between drivetrains and power plants?

Anyone? Jump right in here....

 

uuhhhh..... 6?

;-)

 

Greywolf, what the hell do you smoke, man??:7

Don't take it wrong, now.

Raul

 

[updated:LAST EDITED ON 29-Nov-01 AT 04:18 PM (EST)]
What do I smoke?

~TIRES, usually....

*snikkerezz*

 

Another thought -

For a V6 to move a truck around (even a half ton) the gearing had to have been set up so that it would rev higher, in order to make more power available. So the gearing is on the "Torky" side anyhow...

I bet it launches like a scalded rabbit!

 

How do you launch a scalded rabbit?

A modified potato cannon?


Roger Lane

 

Err, a bucket of boiling water usually does fine...

*He mumblezz*

 

Instead of using the number of cylinders, it'd be more accurate to use the relative displacement of the engines (in cubic inches, or liters, or whatever). All V8s are not created equal.

I think the best bet is to come up with a combination that turns the engine at approximately the proper revolutions on the highway. IIRC, they go something like this:

big-block V8: 2000 rpm
small-block V8: 2250 rpm
I6: 2000-2200 rpm
V6: 2500 rpm

Now these are just ballpark guesstimates, but I'm sure you can find the real figures someplace online - or from one of the more knowledgeable people on the board. If you're going from a V6 to a big-block V8, you might be okay increasing the tire size from 28" to 36" at the same time - but if you're going to a small-block, I think you might be disappointed. Also, the newer V8s seem to like higher RPMs than the older ones...I don't think any of them would be happy turning as slowly as the 300 I6 or the older big-blocks.

LK

 

You bring a valid point.

302/250=1.208

About a 20% increase in available power, I think.

34/28=1.214....

So 34" is closer to the ballpark. But that is still at 2500 RPM.

On the other hand, a larger tire will have greater rotating mass, and therefore inertia. So the losses to turn it at high speeds would be greater.

I like the look of 35 to 36 inch tires, and I supose some gearing tech will be needed here.

I planned on a nine inch rear end for it though, so the option is open.

What combinations do other people have that come close to the best freeway efficiency?

(THIS time expressed as "Real World" MPG)
(Umm, for a 302...)

 

Seems to me that a good ballpark rpm of highway cruising is the rpm of the torque peak, that is the bottom of the power band which puts you in good shape for sudden need to accelerate. Cruising at the torque peak puts my 300 I6 at 2000 RPM (which is what it does at about 67 mph), but my wife's aerostar would have to be at 3600 rpm, though I think it cruises closer to 2800, as evidenced by the fact that even moderate acceleration from highway cruise speed invokes a downshift, whereas my torquey 300 I6 will pull strong from highway cruise speed in OD.

 

 

Yep, that's along the lines of what I was thinking.

But what are your tire sizes and other essential data?

 

>But what are your tire sizes and other essential data?

Whole thing is bone stock except for K&N filter, modified intake duct, and modified muffler. Tires are 235, 75 (or 70?), 15. Rear ratio: 3.08. Tranny: Mazda, OD = 0.80.

 

[updated:LAST EDITED ON 04-Dec-01 AT 11:44 AM (EST)]Using the torque peak to determine a good cruising RPM seems to work better for the older engines than it does for the new ones. For example, the torque peak for the 4.2L V6 or 4.6L V8 is in the neighborhood of 3400-3500 RPM - which is faster than you'd really want to be turning them on the highway. I think the key is that the torque curves are pretty flat, and if you have 90% of your peak torque at 2500 RPM than it is perhaps better to use that rate instead of the higher number. Maybe a good rule of thumb is that your highway RPMs should be at the point where the engine first produces 90% of its peak torque...though you need to have good graphs to figure out where that point is. There might be dyno graphs on-line for all the various Ford engines, I dunno...

Of course, all I'm doing is making things even more confusing.

Greywolf, from your post I gather that you're thinking about swapping in a 302? If that's the case, I've found that the 302 seems to prefer higher RPMs, at least in stock form...and depending on your stock gearing, I wouldn't go over 32s or 33s. My '91 F150, which had approx. 3.55 gearing and an auto tranny, was tolerable with 31s...but with 33s it could no longer pull hills without coming out of O/D (at 65-ish mph). If you turned off O/D, it wasn't *too* bad for towing...but it wasn't that good, either. Personally, I wouldn't consider 33" tires or larger unless you have 4.11 axle gears (or lower). You can get by with something like 3.55s on the highway, but you won't have much extra power for carrying a load or pulling a trailer.

Of course, this also depends on the year of engine, if it's modified, and things like that. With enough engine you can make up for a bad gear ratio.

As far as mileage, hmmm...with an extended-cab 4x4 F150, it was around 12mpg with the 31s (highway driving), and maybe a half-mpg or 1 mpg less with the 33s. The oversize wheels/tires seem to really cut into mileage because of the larger rolling resistance...though maybe if you went with something narrower (like a 33x9.50) it wouldn't be as bad. Oh, and this truck had a 500 pound brushguard on the front (and no, I'm not exaggerating), which I'm sure cut into the mileage a fair amount. I might have gotten 13-14mpg without it, but it protected the truck from deer damage and I seem to hit a lot of deer.

LK