joezero

After reading about the V6 in another post that ran 13.9s, it really got me thinking about doing a rear gear swap to 4.10s. But I'm kinda confused as to how this whole rear gear ratio thing works out. From a previous post I gathered that 4.10s meant that the engine turned a little over 4 times for the wheels to turn once. If this is correct then I don't understand how it really makes the car faster. It seems to me that if you had it the other way around that you'd be really flying. Does the different gear ratio just let the engine rev higher faster? Does anybody know what sort of effect a gear swap like this might have to a 2.8 with a manual trans? Also how much of an effect on fuel economy etc.? Thanks, Joe

AFrikinGoodTime

Just look at it like a ten-speed bicycle. First geat is low speed but lots of torque. tenth gear you have to stand an elephant on the pedals to get moving, but once you are going at speed it will cruise easier at high speeds where as first gear is winding out.

Now w I know you have a gear box that changes gears(we all do) but the rearend gear determines whether you have more torque through the gearbox or whether you have slower starts but better top-end.

There is a fine line between both and they are usually 3.23 to 3.42's with the stock gear boxes based on whether its a stick or auto. once you go up (3.73 or 4.11) you gain torque but will loose top speed and will be winding out more on the freeway, also sucking alot of gas(poor gas mileage)

Most pro drag cars that run gears higher than 4.11's, ( alot higher in some cases) their motors tach to sometimes as high as 10,000+ rpms. This is how they run 200+ in the 1/4 with those gears.

TomP

Actually yeah, it is the other way around. The engine turns once, the rear wheels turn 4.10 times. Combine that with the transmission and the cam, and you could figure out what rear gear you'd need to be in the powerband at the end of a 1/4 mile run.

Right now, I think you have 3.23 rear gears. A 3.73 would probably be the best gear for you; 4.10's seem a little too low. Plus I'm not sure of the gear ratios of the manual trans. The 700r4 has gear ratios of 3.06, 1.63, 1.00, and 0.70. So the first gear of the 700r4, combined with a low rear gear (like a 3.73), makes for a hell of a launch. Problem: Large "gap" between 1st and 2nd gear- the torque convertor helps cushion that gap, but it's still a little too wide. And 4th gear lets you cruise the highway without winding the motor out. If a 700r4 owner puts the B&M 4-3 kickdown sleeve in their trans (or the GM 3-4 WOT kit), you can use 4th gear to accelerate in- it changes 4th from a cruising gear to a useable gear. But the 4th in the 700r4 helps gas mileage. You might want to see what your high gear's ratio is.

As to gas mileage, if your high gear is high enough, you really won't notice it. I didn't notice it when I went from 3.42's to 3.73's, but you'd be making a larger jump (3.23's to 3.73's, or 3.23's to 4.10's).

joezero

How much of a performance difference is there by going to the 3.73s over the 4.10s?

TomP

Not sure; you could probably ask this in the transmission/drivetrain forum... or even do a search for "3.73 4.10" in that forum.

Hmm... just thought of something that I haven't thought of before. Two identical f-bodies. Same transmission, same rear gears, same wheels, same tire size. Difference is, one has a v8, one has a v6. If they go at exactly the same speed, in the same gear- will the engine RPM's be the same? My first thought is "yes"... ? Although, the V6 car might be struggling more, so the driver might be using more gas to go as fast? Hm. Er, well, okay, say both cars are coasting down a hill, no gas pedal, same speed, same trans gear, same wheels & tires.

AFrikinGoodTime

Tom, yes you are correct, they would be identical at same rpm, engine size has nothing to do with it.

Where engine size has a difference is - Larger displacement will generally pull through a gear with a broader powerband but not always. Engine performance is the key.

Lets take the stock 2.8 with a built 700r4 and 3.73. The rpm and shift range(power band) with the pedal floored through the gears would probably be around 2500-4500 rpm's. lets say 2nd gear you would twist those 2000rpm's in 4 seconds and would max 2nd gear speed at 45 mph. (this is a hypathetical sinerio)A slightly built 2.8's power band would shift that same trans in 2nd gear at 5000 rpms instead of 4500 because of higher governer holding pressure or centrifical force.

Now take a built 327 V8 with the same 700r4 and 3.73 rearend.
The rpm range of this motor may sit the 2nd gear range from 3500-8500rpm's. A built 327 could possibly pull those 5000 rpm's in the same 4 secs but the 2nd gear top speed would be more like 80 mph and not 45 like the 2.8 motor.

Basically both cars in second gear at 3000rpms would both be doing the same speed, probably about 25 mph(again just a hypathetical but you should get the point by this example)

Rearend gears have everything to do with an individual engines driving rpm range or in other terms its power band. The wrong governor in the trans can disrupt the proper shift range.

Lesson for the day is- your engine performance and top speed required needs to be married to the trans( gear ratios and governor If automatic, just gear ratio if manual) and rearend ring and pinion gear.

p.s. I could write a real novel on this to really explain things correctly, this is a short version in trying to explain this info correctly. But hopefully it is clear enough

joezero

Again I'm confused. I thought that V-8s were faster because they could accelerate faster (going from say 0-5000 rpms) than a V-6 could because they had more displacement=more power to turn the crankshaft. I remember reading a post on another board (maybe it was power adder, I don't remember) where guys were arguing whether a 305 was making power all the way to 6000 rpms. I understand what you're saying in relation to the powerbands being different, I've experienced it when I've driven those higher revving 4cyl cars. What I don't understand is why a V-6 can't rev higher than a V-8. A 6 has two fewer cylinders it has to fire, so assuming it took 5 milliseconds to fire a cylinder (from TDC to BDC) then it would take 30 milliseconds for a V-6 to fire all cylinders while it would take 40 ms for a V-8. I'm guessing my logic is probably flawed here somewhere, but I'm just thinking back to my ignitions class last semester. We were talking about how the limitations of points ignition was like 5000 rpm because the points couldn't physically open and close faster than that. It seems to me that eventually you would run into a similar problem between trying to advance the ignition so much that you would be firing the spark plugs while still on the intake stroke. Sorry for the novel, I'm just trying to understand why our v-6s can't spin to 8k. (and it they can, somebody please tell me, I love high revving engines )

AFrikinGoodTime

Oh Buddy- you're talkin' my language. There's nothing like the whine and power of high revs!

To your question on V6 vs. V8 revs, it has nothing to do with the amount of cylinders- this is irrealivent. High revs are mainly contributed to light weight bottom ends and not so much stroke of the crank to rod angle as it trust into and outof the cylinders. Also the cam profile has everything to do with the rpm range as well as flow of the intake and heads.

90* motors have a better flow angle by design from intake to exhaust than what a 60* motor has. The intake gases do not have to bend as much.

As for the ignition taking five millisecs to fire so a v6 would fire on rev in 30 millisecs and a v8 in 40, this is not how it works. When lets say cylinder #1 fires, cylinder #2 is already on the 3rd or 4th millisec and #3 is on the 2nd or 3rd millisec and so on. They overlap and they fire with even closer spacing the higher the rpms are. The coil will fire instantaniously, but what takes the time is the distance of the plug wires. In high rev engines (especially Formula One cars at as much as 18,000rpms) they use individual coils per cylinder because there is a point that a single coil can not handle the speed.

pontiacguy1

I think that Afrikkingoodtime was trying to say things the correct way. First of all, what gets your car moving is TORQUE, not horsepower. In fact, horsepower is torque with an RPM factor in it. So the more torque at a given RPM, the more horsepower at that RPM. This gives you two choices to gain HP. You must either increase the torque at the same RPM, or you must make the same amount of torque at a higher RPM. I am going to assume that you are not going to build the engine up right now, so output is constant. I am about to show with a few mathematical calculations why the rear gears will help your takeoff, but won't hurt your fuel mileage as bad as you would think, although it will be affected.

I will calculate engine RPM in overdrive with the two different gear ratios: The overdrive gear ratio in my 700-R4 is .70 to 1. This means that the rear shaft actually spins faster than the engine is turning. I am going to assume you have a 25 inch tire, and are going 60 MPH. The tire's circumfrence is 2*(pi)*radius of the tire, which comes out to 78.5". The car moves 78.5 inches forward for each complete rotation of the wheel. converted to feet, and that is 6.54'. 60 MPH is also 5280 feet/minute. divide 5280 by the 6.54 and you get 807.4 RPM at the axle shaft to go 60 MPH. multily that by the rear gear (3.23) and again by the trans gear (.70) and you get engine RPM (1825.4 RPM with 3.23) Doing this again with the 4.10 gears gives you 2317.2 RPM. This means that in overdrive at 60 MPH, the 4.10 gears would make your engine turn 491 more RPMs. That probably wouldn't hurt your fuel mileage more than a mile or two per gallon.

Say your engine produces 150 lb-ft of torque. The rear gears are 3.23, and your trans (I'll use my automatic as an example)has a first gear of 3.1:1. Multiply the gear reduction of the transmission by the gear reduction of the rear end, and you will get 10.013 to 1 reduction. That means that in First gear, your engine will spin 10.013 times, for every time your rear tire turns. The effect of changing the rear gears to 4.10 would be making the first gear total reduction 12.71 to 1. This means that when your engine produces 150 ft/lbs of torque, the rear axle shafts (not the tire contact surfaces)will see 1,501.5 lb-ft of torque with the 3.23 gears, but will see 1906.5 lb ft with the 4.10 gears! Dividing by the aforementioned 25 inch tire radius (converted to feet), the force on the tires at the ground is 916.6 pounds with the 4.10, versus 721.875 for the 3.23 gears in first gear. That will make your car seem much stronger off of the line. Of course the car will shift sooner, and your top speed will be limited. A gear change will mainly help you in first gear, but it will be quite a noticeable change.

Sorry everyone for such a long post, but I just want to show what difference the gear change will make in actual numbers. You can also go back through all of these calculations and change the gear ratio, tire size, etc... to see what effect it will have on your car. Those are some of the cool things you can do with math!

tamu130

i do believe the rear gears on his car would be 3.42. i also have an 88 2.8L and 3.42s come stock on this model.

just for clarification and mathematical purposes.

jacob

pontiacguy1

Sorry dude. I was using my car as an example, since I don't know the specifics about his car. The 3.42 to 4.10 swap would be less of a change on your cruising RPM, but also a lesser effect on your takeoff. It would still make a difference, though.

tamu130

i was actually replying to a previus post, not yours. i know it wouldn't make much difference. im not retarded. it doesnt take very much math skills to figure out that 3.23 and 3.42 is not much of a difference.

i specifically said it was for classification and mathematical purposes.