Rear end ratio / power transfer

[xtreme80]

I'll keep this simple and to the point. If you look at my signature, you can see what I have done to my motor. This was a 290 horse crate, and I added the intake manifold, carb, exhaust, etc.

My question is this: Even with a 2.73 rear end gear ratio, should I, theoretically, still be able to spun tires if I punch it from a stop? They are brand new tires, so they are sticky, but still... I would think with a 290 horse crate with my add ons, I should be able to spin these things.

Something just doesn't feel right in my truck and I wanna make sure it's not just my imagination.

[71chevylowrider]

2.73? Maybe not, but 3.73 definitely. What tranny? torque converter?

[benoit454]

Man I know EXACTLY how you feel.

For a truck with 2.73's yes you should, I base that statement on this...

Our stock 84 with 305 h.o. and 2.73 rearend with a th350, everything stock as far as I know and if non of it has changed, you can break them loose god with it.


Now I went through the same thing with my monte carlo. The motor in it was a stock 350 4 bolt, 2.02 iron gm replacement heads, weiand intake dual plane, edelbrock 600 cfm carb, comp 268 cam and lifters, hei dist., flowtech ful length headers. it had a stock th350 and stock convertor, also had stock non ss monte rearend with a 2.29 gear ratio.

I took that motor from my old 76 truck, back then it had a bigger cam, old holley dual plane and had a holley 600cfm vac. sec. mech. choke carb, same as my edelbrock, but it was backed by a stock 3 speed standard shift, and what we think was 4.11's and I know for sure it was posi.

The truck would absolutely smoke tires, you could barely get a chirp, they's just start spinning. We ran 31x10.50's on it and it still smoke the mud tires off. it was freakin awesome, mean as hell, very, VERy quick, but on the highway about 60-70 you let off and it was like gearing down, almost.

Same motor just with the stuf mentioned above in my monte, with the 2.29's, no way I could smoke em. I could barely, BARELY break them loose. I changed the gears to a set of 3.42's added a shift kit and a 2000 rpm stall to it. it will break them loose from a dead stop about half down on the pedal.

I haven't done much else, but with the shift kit when I hit 2nd it'll chirp the tires really good. As for as smoke tires I haven't tried it much. I don't feel like it do it that good, like it not do it as good as the 84 with the 2.73's, but I'm not sure since I haven't tried. I was just stabbing it down halfway in the carport playing around with it. I also have not really dropped it at the stall speed, but from what I've done playing a stall is nice, great launches out of it.

Everyone told me it was my rear gears, but I'm not sure, I think like you, maybe something isn't quite right in the motor, it did run great, but it sat for awhile and only started every couple weeks, and wasn't driven for about 5 years.

Our 86 k10 4x4 had 3.08's factory or so it said, and I know for sure somebody changed the rear because it was a 12 bolt, and the only 12 bolts wer earlier years, but still, it would break them loose good too about half pedal or so, and we ran 33's. I found a gear ratio/tire size calculator online and 3.08's with 33's compared to 235/75's factory put the final ratio at about 2.72.

SO I'm in the same boat as you, everyone says get a stall and gears to keep your motor in it's powerband, like say it tops out making power at 6k, you don't want something that makes the motor run past 6k alot or any at all.

I'm just as lost on it as you are though, maybe now with warm weather I'll take my monte out more and play with it to see how big of an improvment the new gears are compared to before.

[b454rat]

I have a couple things for this one. My old man had an 84 1/2ton 2wd, 305/350C, 2.73 open rear, with 235s on it. Motor ran like crap, so dumped that and got a 350 out of another truck. Have no clue what the motor was, but it was a GM Goodwrench motor. Anyways, if you hit the gas too much, it would spin the tire till you let off. It would just keep going, would rev to the moon. Swapped in 3.73s with a Govlock, and would spin them effortlessly, but not as much as with the 2.73s. Miss that truck. Other one was my old car. 200k 400, 400 trans, 2.73 open 10 bolt. That would spin the tire aboot 50 yards or so. Put in a 468, manual v/b 400, 9" rear with 4.10s, so tire spin was pretty easy after that.

So go by what you did to your truck, you should be able to get some nice pinstripes on the road.

[xtreme80]

Man something can't be right here! My buddy has an 07 sierra classic style with a 5.3 and 3.42 rears. Stock tires. We raced from about 55 to about 80, and he pulled on me the whole way. Left me like I was standing still. I don't see why I shouldn't have kept up with him, seeing as his truck only has like 280hp stock with 305 ft/lbs of torque. I'm roughly 315hp/330ft/lbs. And at that speed, I should have hung with him better.

Im so stumped on why this wont run right

[71chevylowrider]

Hey Extreme just a guess/theory here. How tall are your tires? That could effect your ratio, if you have say 28-29" instead of 26"-27" it could make a difference in your launch capability. In other words if you have shorter tires, that would effectively give you lower rear end gear ratio to make the truck launch faster, if you had taller tires say 30" it could make it more sluggish off of the line.

[xtreme80]

I've got 225/70/15's on it right now, so it's not a big tire at ALL. And with my old engine, I could smoke the 30's that were on it when I got it. They were by no means bald either...

[71chevylowrider]

weird, is your kickdown cable working right? Is there a chance your not starting out in first gear for some weird reason?

[piecesparts]

2.73 and tire spin are not a common mix. If you want to get something that is more apt to do what you want go for a 3.42 or 3.73 geared rear end. What you have for a tranny is also involved in this mix.

The horsepower don't mean much if it is being lost on the way to the ground. Your buddy is kicking your butt, because he has a 5.3 that is a quicker response style motor with fuel injection, with a lower geared rear end and a better power range capability. Have you considered how you have your engine set up? What carb, what is your timing set at, and how is your air cleaner and exhaust set up? As you can see, just adding an engine that is supposed to be 290 HP doesn't make it so, the other items make it do what it is supposed to do. The tire size is another factor, the 225/70/15 is a 27.40" diameter tire, that is a fair size but with a 2.73 geared rear end that is pretty high and is robbing your acceleration. Do you run a tach? What is your RPMs at 70 MPH---my guess is that they are realitvely low.

[xtreme80]

Thanks for the responses guys. piecesparts, Im currently using a TH350 tranny. I plan on doing 3.73 gears in the rear, but it's so expencive right now!

As for how it's set up, the carb is an Edelbrock 1400, 600cfm elec. choke. Should be more than enough for this engine. The exhaust is pretty much open. Stock manifolds and stock cats, but they are straight pipes with no mufflers. I'm not sure what you mean about the air cleaner. It's a chrome 14" air cleaner.

Also, I was reading up on some things today, and I think my engine is running a little too rich. I'll get some black smoke out of the exhaust when I'm cruising, and at idle, it smells very rich. Since I have no way of checking the actual AF ratio, I'm just going to buy a carb calibration kit from our speed shop here and start leaning it out with new jets or metering rods. Hopefully this can help me out, because after messing with the idle adjustment for about an hour today, I actually have it idling REALLY good finally.

Any thoughts?

[benoit454]

Man you don't need rods and jets, the mixture screws are right on the front of your carb, they'll lean it out or rich it up whichever you need.

Also you maybe getting black smoke from the cats if you have no mufflers, and they might even be clogged up, if you don't have to pass a sniffer test I poke them out, just gut the cats out and then you'll have no worrys of them ever plugging up. However it'll be ALOT louder with gutted cats.

We drove our 86 down to get gutted cats and nw pipes with just headers, and cats, no mufflers and it was quieter then my monte carlo which has gutted cats and glasspacks.

As for expensive rear gears your should find some in a junkyard or on here who is parting one out, then swap the whole thing or swap the gears and carrier out. I did that on my car, first one I ever did, bought the guts online shipped for $75.

[piecesparts]

Originally Posted by benoit454

Man you don't need rods and jets, the mixture screws are right on the front of your carb, they'll lean it out or rich it up whichever you need.

Also you maybe getting black smoke from the cats if you have no mufflers, and they might even be clogged up, if you don't have to pass a sniffer test I poke them out, just gut the cats out and then you'll have no worrys of them ever plugging up. However it'll be ALOT louder with gutted cats.

We drove our 86 down to get gutted cats and nw pipes with just headers, and cats, no mufflers and it was quieter then my monte carlo which has gutted cats and glasspacks.

As for expensive rear gears your should find some in a junkyard or on here who is parting one out, then swap the whole thing or swap the gears and carrier out. I did that on my car, first one I ever did, bought the guts online shipped for $75.

First off; the air mixture screws are for the Idle circuit only. It has nothing to do with the fuel metering circuit after the transition from the idle slots to the power circuit. That is where the rods and jets come into function. HOWEVER, be careful that you do not take the engine too lean and burn a valve. You will need to spend time looking at your exhaust pipe color and your spark plugs to see if you are going to far. I skipped the purchase of a tuning kit and just bought a collection of rods and jets that went two to three steps on both sides of my factory installed parts. I now have a full set of rods and jets from over the years, but I can figure a fuel adjustment to a gnat's tail, if I want to. Do you have the original carb install book from Edelbrock? It gives the rods and jets that were in it for assembly and it gives you directions that you can go to improve on that. If not then borrow one or go to a store and copy one from a box on the floor.

Setting your idle mixture is definitely best done with a vacuum gauge connected to the intake manifold instead of by ear. Take one of the idle mixture screws in to a point that the idle drops off to a stutter and then start opening up until the idle stops increasing. Now turn the screw back inward an 1/8th of a turn. Do the opposite screw the same way. repeat this a couple times to ensure that the settings are at their best point.

Take a look at your timing and see where it is set at. If you advance initial timing some it will pick up your acceleration and take away some of the doggyness that you hate. Listen closely for detonation sounds on acceleration and also watch for "dieseling" when you shut the engine off. I timed my truck and my Son's truck with a timing light that has the advance feature on it and set them up for a TOTAL advance setting around 36 degrees. Not all engines can be set there, but you can get close.


Believe it or not a motor works a little better if there is some slight back pressure. I never like a straight pipe, but many do. So that is your choice, but the cats may be plugged and causing you some grief on your acceleration. Dispose or replace as necessary. Headers are a must for horsepower building. I figure your system is just enough to make it work well, but you have a ways to go, just my .02 worth.

The air cleaner is important, the more air in, the more power to work with. What height of filter element, preferrably a 4" tall one. The little short filters and a chrome cleaner assembly may be causing a turbulance issue above your carb and making you loose Horsepower. Look into a quality filter and set it up to work for you.

The 3.73 gears is a hard step for a TH 350 tranny, that will kill any highway mileage that you want. I would consider a set of 3.42 gears at the lowest and may a set of 3.08 for every day would be better. When we took my son's 86 SWB truck to 3.73 gears, we had already changed it out to a 700R4 tranny and that works well with his combination. I have a complete 10 bolt 3.08 rear end sitting by my garage, with the drums. brakes, cables, hoses, housing, and fluids still in it.

This can be done with time and a little creativity.

[benoit454]

hmm on my edelbrock it ran rich, after driving, I pulled the plugs and they were black, I tuned the mixture screws as stated in the book and then they burned clean.

They are IMS though, Idle mixture screws, I forgot that.

My edelbrock book tells you what to do and to test for a lean or rich driving mix and what to do about it.

Like one was be up to speed and stomp it and if it done this then it's way one way or the other and this was needed to correct it, and so on.

[xtreme80]

Setting your idle mixture is definitely best done with a vacuum gauge connected to the intake manifold instead of by ear. Take one of the idle mixture screws in to a point that the idle drops off to a stutter and then start opening up until the idle stops increasing. Now turn the screw back inward an 1/8th of a turn. Do the opposite screw the same way. repeat this a couple times to ensure that the settings are at their best point.

I see what you mean, but what I don't understand is where the vacuum gauge comes into play? What would I look for on the gauge to help me with this?

As for the exhaust, I'm sick of it. It's way too loud and raspy, and the shop that did it did a horrible job on it. I've got a set of headers sitting in the garage, but they aren't expencive ones.

As for rear ends, I don't really do much highway driving at all, and I'm not worried about the gas mileage, yet. (wait till prices come up more)

I've got a 12 bolt that I'm gonna go look at this weekend. It's a 4.11 ratio in the rear. It's a little higher than I wanted to go, but the price is so dang cheap that I'm going to just give it a try and see where I can go with it. If it's too much, I'll just resell it and find something geared higher in a junk yard.

[benoit454]

Other then the high rpm's while driving around town and highway I bet you'll love the power transfer and tires smoke you get from the 4.11

You'll probably never keep tires on it with a 4.11

When he says vacuum guage he means when you have it hooked up and are tweaking the idle mixtues screws you set them to get the HIGHEST reading/vacuum possible. That's the optimum air/fuel idle mixture setting, can't get much better then that.

[piecesparts]

The vacuum gauge is a telltale meter for reading your vacuum as you adjust the idle mixture. Most vacuum gauges measures vacuum in inches/mercury (0" to 30" with 0" being atmospheric pressure and 30" being a near perfect vacuum) and the higher numbers are readings of a better vacuum. Once you turn one of the mixture screws, on the carb, inward to get the intial setting the truck engine will stumble and run poorly, also at that time the vacuum reading will be really low (bad vacuum). As you turn the screw outward, the idle will improve and the vacuum reading will get higher. The idea is to turn the screw outward until the vacuum gauge numbers quit climbing and stay at one point on the dial. (do this slowly so that you do not overshioot) That is the point that you turn the mixture screw back in 1/8th of a turn from. That will be the OPTIMUM vacuum/fuel mixture setting for your carb. The vacuum reading is actually a readout of your fuel metering and the engine's response to the mixture. This should be done on both fuel mixture screws to get them set exactly the same.

Many carb specialists have been using vacuum gauges for setting idle mixtures and balancing multiple carb setups for years.

You will not get the setting any closer by using your hearing. Try it and you will see what I mean. Vacuum gauges are not that expensive at many parts stores and they are useful.

For your exhaust, may I suggest a set of dual exhaust pipes from the headers, with an "H" pipe or "X pipe crossover at the tail end of your tranny's output shaft. Then run 2 1/2" exhaust pipes to the outlet points through your mufflers. That would about the most optimium exhaust pipe size for your engine's size.

The 4.11 rear end is pretty steep for any daily use and will make your truck a City block SPECIAL. After a city block the other guy will kick your proverbial butt, because your engine will be topped out.

[xtreme80]

Originally Posted by piecesparts

The vacuum gauge is a telltale meter for reading your vacuum as you adjust the idle mixture. Most vacuum gauges measures vacuum in inches/mercury (0" to 30" with 0" being atmospheric pressure and 30" being a near perfect vacuum) and the higher numbers are readings of a better vacuum. Once you turn one of the mixture screws, on the carb, inward to get the intial setting the truck engine will stumble and run poorly, also at that time the vacuum reading will be really low (bad vacuum). As you turn the screw outward, the idle will improve and the vacuum reading will get higher. The idea is to turn the screw outward until the vacuum gauge numbers quit climbing and stay at one point on the dial. (do this slowly so that you do not overshioot) That is the point that you turn the mixture screw back in 1/8th of a turn from. That will be the OPTIMUM vacuum/fuel mixture setting for your carb. The vacuum reading is actually a readout of your fuel metering and the engine's response to the mixture. This should be done on both fuel mixture screws to get them set exactly the same.

Many carb specialists have been using vacuum gauges for setting idle mixtures and balancing multiple carb setups for years.

You will not get the setting any closer by using your hearing. Try it and you will see what I mean. Vacuum gauges are not that expensive at many parts stores and they are useful.

For your exhaust, may I suggest a set of dual exhaust pipes from the headers, with an "H" pipe or "X pipe crossover at the tail end of your tranny's output shaft. Then run 2 1/2" exhaust pipes to the outlet points through your mufflers. That would about the most optimium exhaust pipe size for your engine's size.

The 4.11 rear end is pretty steep for any daily use and will make your truck a City block SPECIAL. After a city block the other guy will kick your proverbial butt, because your engine will be topped out.

Thanks pieces and benoit454 for the information. I already have a vacuum guage, so I'll give that trick a try tomorrow and see what I can come up with.

As for the exhaust, it'll be a while. The first thing I have to do is pass emissions in June. What is the purpose of an X or an H pipe? Does that just balance both sides?

The rear end is a little steep for my tastes, and after reading what you're tellin me, I think I may hit a junk yard and try to find one before I go with this one.

Thanks again for the info!

[boatpuller]

i'm thinking about that same thing with my truck.i did some research online and with my tall rear tires and stock 3.08 rear end.i'm down in roughly 2.73.as 71lowrider said tire size make's a hell of a difference.i could feel it when i put new ones on.also i used to tow a 23 ft. boat with an 02 dodge diesel i had it came with stock with 265(32or33)tires.i put 35's on it and i really felt it.escpecially in the hills we have up here in washington and oregon.i figure a 3.73 ratio with my tires will put me in the 3.50 area.which ought to be about ideal.

[xtreme80]

Originally Posted by boatpuller

also i used to tow a 23 ft. boat with an 02 dodge diesel i had

Is that why they call you boatpuller?

No seriously though, I believe I have smaller than stock tires, and when I went from the 30's it had to these 27 inch tall tires, I noticed a hell of a difference. Performance AND looks!

Anyone know of a good final drive ratio calculator that will include the tire size as well?

[Zoomad75]

I've got some experience with 2.73's in my 69 Nova. When I first got it, the engine was a wheezy 307 2bbl with a TH350, 2.73 gears and puny 195/70 14's. Even with the teeny tires that wheezy 307 took a lot of effort to spin the right rear. Pumping the brake, turning right to unload the right rear and such. Swapping in a 4bbl helped but I had bumped up the tires to 205/60 14's. Once I put the 350 in smoke shows came a little easier. Changing to 2 1/2" duals and then an edlebrock intake/holley carb woke it up. Thats where its at now. Even with the 2.73's it can do a wicked peg leg burnout.

It all boils down to the tuning like others have said. Each time I changed something, I had to go back and re-adjust the timing, carb or both. It can be done with 2.73's but you got to dial your combination in better.

Here's a little example of what my Nova does with 2.73's....

[piecesparts]

I have the calculations for a ratio and it includes tire sizes. I also have a tire size website that take the numbers of tires and gives you the tire sizes. Let me do a little research and I will get them.

[69BBsuburban]

This may have already been addressed, but your old engine probably made more low end torque, as opposed to the new engine with the bigger cam, which doesn't make power until higher RPMs.....a slightly higher stall on the converter would help, but probably won't be needed once ya swap in the 3.73's......

My truck would BOIL the tires with 2.76's.....but of course a built 454 helps!

[boatpuller]

Originally Posted by xtreme80

Is that why they call you boatpuller?

No seriously though, I believe I have smaller than stock tires, and when I went from the 30's it had to these 27 inch tall tires, I noticed a hell of a difference. Performance AND looks!

Anyone know of a good final drive ratio calculator that will include the tire size as well?

yup,bought boats,sold boats......... therefore,...pulled alot of boats.

[piecesparts]

Try the following formula for calculating gear ratio and speed to different tire sizes. It works real well, If you have a overdrive tranny, the number for that will not be (1), it will be more like 0.68.


NEVER MIND, this dang website won't let a photo into it that is of any size. So here goes :

MPH=
RPM X Tire Diameter
Rear Axle Ratio X Trans output X 336

RPM =
Axle ratio X MPH X Trans output X 336
Tire diameter

Tire diameter=
Axle ratio X MPH X trans output X 336
RPM

Trans output =
RPM X Tire diameter
Axle ratio X MPH X 336



Also attached is a webssite that has a tire size calculator. It can take any tire size and give you dimensions or even comapare to other tire sizes.


http://www.discounttire.com/dtcs/infoTireMath.dos

[benoit454]

Well with my 3.42's I can definately turn the tires over alot easier then before.

The 2.73's in the 84 truck sure will smoke them off too, so did the 86 4x4 with 3.08's but 33's.