Troque and Gear Ratio
12-08-2014, 11:13 PM
#1
Instructor
Thread Starter
Troque and Gear Ratio
I have been looking on the internet and this forum for the last several hours and cannot find the answers I am looking for, and that could be because I do not know where to look esp. on this forum. This is a good forum and I like it. Anyway...
Questions:
Does anyone know the RPM points of the 300hp and 340lb-ft of the stock 94 C4 LT1?
Does anyone know the torque at 1200 to 2500rpm of the same engine? Most dyno charts I have looked at start at 2500rpm and go up. I have one article that states the 325lb-ft of a 96 Camaro LT1 at 2000rpm. Accurate?
In May this year I bought a 94 Vette with about 75K on the clock - LT1 and 4L60-E. The powertrain is solid and smooth. I have fixed several minor problems and have a few more yet, but what I am really interested in is changing the rear gear ratio from 2.59 to 3.45. I have found that I can get better mpg in 3rd gear at 55/65mph than in 4th! I am sure this is because I am so low in the torque curve. The 3.45 would put me higher in the torque curve, but still below peak. Acceleration would be a lot quicker too!
Empirical Data:
I ran a trip at 55/65 (mostly at 55) all in third gear and with a lot of stops/slower speeds and got 22mpg. The best to to date! Earlier on a 5.0 hour round trip, in 4th gear, at 55/65 mostly cruising I got only 20mpg. Big difference! Soon, I am hoping to take a 4 or 5 hour trip cruising in 3rd gear and check the mileage and noise level of the Flow Master exhaust system at those rpms.
My data:
2.59 ratio, 3rd gear: 1841rpm @ 55, 2175rpm @ 65
2.59 ratio, 4th gear: 1281rpm @ 55, 1514rpm @ 65
3.45 ratio, 4th gear: 1706rpm @ 55, 2017rpm @ 65 (slightly below the rpm of the 2.59 and 3rd gear, but could be the sweet spot)
Questions:
Does anyone know the RPM points of the 300hp and 340lb-ft of the stock 94 C4 LT1?
Does anyone know the torque at 1200 to 2500rpm of the same engine? Most dyno charts I have looked at start at 2500rpm and go up. I have one article that states the 325lb-ft of a 96 Camaro LT1 at 2000rpm. Accurate?
In May this year I bought a 94 Vette with about 75K on the clock - LT1 and 4L60-E. The powertrain is solid and smooth. I have fixed several minor problems and have a few more yet, but what I am really interested in is changing the rear gear ratio from 2.59 to 3.45. I have found that I can get better mpg in 3rd gear at 55/65mph than in 4th! I am sure this is because I am so low in the torque curve. The 3.45 would put me higher in the torque curve, but still below peak. Acceleration would be a lot quicker too!
Empirical Data:
I ran a trip at 55/65 (mostly at 55) all in third gear and with a lot of stops/slower speeds and got 22mpg. The best to to date! Earlier on a 5.0 hour round trip, in 4th gear, at 55/65 mostly cruising I got only 20mpg. Big difference! Soon, I am hoping to take a 4 or 5 hour trip cruising in 3rd gear and check the mileage and noise level of the Flow Master exhaust system at those rpms.
My data:
2.59 ratio, 3rd gear: 1841rpm @ 55, 2175rpm @ 65
2.59 ratio, 4th gear: 1281rpm @ 55, 1514rpm @ 65
3.45 ratio, 4th gear: 1706rpm @ 55, 2017rpm @ 65 (slightly below the rpm of the 2.59 and 3rd gear, but could be the sweet spot)
12-09-2014, 12:11 AM
#2
Team Owner
Pro Mechanic
About 300-320
I have found that I can get better mpg in 3rd gear at 55/65mph than in 4th! I am sure this is because I am so low in the torque curve. The 3.45 would put me higher in the torque curve, but still below peak. Acceleration would be a lot quicker too!
Empirical Data:
I ran a trip at 55/65 (mostly at 55) all in third gear and with a lot of stops/slower speeds and got 22mpg. The best to to date! Earlier on a 5.0 hour round trip, in 4th gear, at 55/65 mostly cruising I got only 20mpg. Big difference!
Empirical Data:
I ran a trip at 55/65 (mostly at 55) all in third gear and with a lot of stops/slower speeds and got 22mpg. The best to to date! Earlier on a 5.0 hour round trip, in 4th gear, at 55/65 mostly cruising I got only 20mpg. Big difference!
(sample size of one, each way) you will get better mileage the lower you spin your motor, in almost all cases.
Last edited by Tom400CFI; 12-09-2014 at 12:38 AM.
12-09-2014, 01:43 AM
#3
Race Director
Change to the 3.45 and don't worry so much about the numbers. You'll be really happy.
12-09-2014, 11:50 AM
#4
Instructor
Thread Starter
Thanks guys, big help! 383vett, if I change to 3.45 will I lift my front tires like you!?
This morning I thought to look in my 94 owner's manual (last place to look). I did not have it last night.
300hp @ 5000rpm
340lb-ft torque @ 4000rpm
I think my problem is more related to hp than torque:
Rear End: 2.59
TransRatio MPH RPM HP Torque
.696 55 1281 95 280
.696 65 1514 105 295
Rear End: 3.45
TransRatio MPH RPM HP Torque
.696 55 1706 125 300
.696 65 2017 160 305
I understand and agree: lower rpm = better mileage - to a point. If you are too low in the power band the engine will have to struggle to climb a hill or resist air drag. This has been proven on dynos: fuel consumption vs rpm and load. I will run a few more trips before I change the gear ratio and will try to report here.
Thanks again guys!
This morning I thought to look in my 94 owner's manual (last place to look). I did not have it last night.
300hp @ 5000rpm
340lb-ft torque @ 4000rpm
I think my problem is more related to hp than torque:
Rear End: 2.59
TransRatio MPH RPM HP Torque
.696 55 1281 95 280
.696 65 1514 105 295
Rear End: 3.45
TransRatio MPH RPM HP Torque
.696 55 1706 125 300
.696 65 2017 160 305
I understand and agree: lower rpm = better mileage - to a point. If you are too low in the power band the engine will have to struggle to climb a hill or resist air drag. This has been proven on dynos: fuel consumption vs rpm and load. I will run a few more trips before I change the gear ratio and will try to report here.
Thanks again guys!
12-09-2014, 12:45 PM
#5
Team Owner
Pro Mechanic
Can you post a link to test that proved that? I'd be interested in reading that one.
12-09-2014, 01:52 PM
#6
Race Director
12-09-2014, 04:03 PM
#7
Safety Car
I understand and agree: lower rpm = better mileage - to a point. If you are too low in the power band the engine will have to struggle to climb a hill or resist air drag. This has been proven on dynos: fuel consumption vs rpm and load. I will run a few more trips before I change the gear ratio and will try to report here.
Thanks again guys!
Thanks again guys!
12-09-2014, 07:06 PM
#8
Team Owner
Pro Mechanic
Well said.
12-09-2014, 07:59 PM
#9
Burning Brakes
There may be some truth to this statement with some combination of gears, car, and engine design, but I doubt it is true for a 1994 corvette that is all stock as designed and built by GM. GM ran 2.59 gears and an overdrive transmission to get the better gas mileage out of the car then the 2.73 or 3.07 optional gears. If the 2.59 gears could not get better mileage then the optional gears GM would not have built the car that way. Running in third gear is essentially running the car with a 3.70 rear gear as far as engine RPM is concerned and 3.70 rear gears do not help mileage. (2.59/.7) = 3.70
12-09-2014, 08:34 PM
#10
Team Owner
Pro Mechanic
Your understanding is correct.
Overdrive is a numerically lower gear ratio, or, a "higher" gear, allowing the motor to turn lower RPM at a given speed.
Overdrive is a numerically lower gear ratio, or, a "higher" gear, allowing the motor to turn lower RPM at a given speed.
12-09-2014, 10:33 PM
#11
Instructor
Thread Starter
Bsfc
BSFC - Brake Specific Fuel Comsumption
Look it up, study it. Here is one link I went to: http://www.hotrod.com/how-to/engine/...l-consumption/
In the chart and in paragraph 12:
"... an engine tends to be the most combustion efficient at or near peak torque, ..."
Also:
"Ideally, you'd like to create a "flat" BSFC curve, but that's not entirely possible. What you can do is work toward establishing that condition, using the value of BSFC at peak torque as your target. However, a couple of conditions play into any attempt at "flattening" a BSFC curve. Among them is the fact that at engine speeds below peak torque, there's an increased amount of time for heat losses to the cooling system and related parts and passages. And especially if the engine is using a carburetor, air/fuel charge quality tends suffer from less efficient atomization (mixing) at these engine speeds than higher rpm. "
"Beyond peak torque, there is the issue of mechanical separation of air and fuel as signified by a corresponding rise in BSFC values. In other words, combustion efficiency tends to deteriorate, accordingly. Plus, in the higher rpm ranges, while there is an increase in the amount of otherwise useable heat from combustion, high rpm shortens the time available to deliver that heat, resulting in a loss in power."
I am not sure why GM put a 2.59 rear end into the autos. Did they want to go 236mph @ 5500rpm? Was it to save the trans, rear end, or the safety of the driver? The stock 94 LT1 puts out 340lb-ft torque at 4000rpm. The 4l60E is rated for 360lb-ft max. My stock 94 has a trans shift kit and I have barked the stock tires when grabbing second at less than full throttle. Would the tires break lose with a 3.45 rear end at full throttle and, possibly, cause me to lose control? Would 3.45s shorten the life of the trans or rear end? Boys will be boys! If it is there, we will use it.
Look it up, study it. Here is one link I went to: http://www.hotrod.com/how-to/engine/...l-consumption/
In the chart and in paragraph 12:
"... an engine tends to be the most combustion efficient at or near peak torque, ..."
Also:
"Ideally, you'd like to create a "flat" BSFC curve, but that's not entirely possible. What you can do is work toward establishing that condition, using the value of BSFC at peak torque as your target. However, a couple of conditions play into any attempt at "flattening" a BSFC curve. Among them is the fact that at engine speeds below peak torque, there's an increased amount of time for heat losses to the cooling system and related parts and passages. And especially if the engine is using a carburetor, air/fuel charge quality tends suffer from less efficient atomization (mixing) at these engine speeds than higher rpm. "
"Beyond peak torque, there is the issue of mechanical separation of air and fuel as signified by a corresponding rise in BSFC values. In other words, combustion efficiency tends to deteriorate, accordingly. Plus, in the higher rpm ranges, while there is an increase in the amount of otherwise useable heat from combustion, high rpm shortens the time available to deliver that heat, resulting in a loss in power."
I am not sure why GM put a 2.59 rear end into the autos. Did they want to go 236mph @ 5500rpm? Was it to save the trans, rear end, or the safety of the driver? The stock 94 LT1 puts out 340lb-ft torque at 4000rpm. The 4l60E is rated for 360lb-ft max. My stock 94 has a trans shift kit and I have barked the stock tires when grabbing second at less than full throttle. Would the tires break lose with a 3.45 rear end at full throttle and, possibly, cause me to lose control? Would 3.45s shorten the life of the trans or rear end? Boys will be boys! If it is there, we will use it.
12-10-2014, 10:42 AM
#12
Team Owner
Pro Mechanic
Yes, I know what BSFC is. Realize that the "peak tq" RPM changes with throttle opening (size). IOW, We aren't operating our engine at WOT driving down the road, in most of the time. So, peak BSFC occurs at a different (lower) RPM at part throttle.
I think gears are a fantastic mod....but they won't get you better mpg.
I think gears are a fantastic mod....but they won't get you better mpg.
12-10-2014, 10:50 AM
#13
Safety Car
BSFC - Brake Specific Fuel Comsumption
Look it up, study it. Here is one link I went to: http://www.hotrod.com/how-to/engine/...l-consumption/
In the chart and in paragraph 12:
"... an engine tends to be the most combustion efficient at or near peak torque, ..."
Also:
"Ideally, you'd like to create a "flat" BSFC curve, but that's not entirely possible. What you can do is work toward establishing that condition, using the value of BSFC at peak torque as your target. However, a couple of conditions play into any attempt at "flattening" a BSFC curve. Among them is the fact that at engine speeds below peak torque, there's an increased amount of time for heat losses to the cooling system and related parts and passages. And especially if the engine is using a carburetor, air/fuel charge quality tends suffer from less efficient atomization (mixing) at these engine speeds than higher rpm. "
"Beyond peak torque, there is the issue of mechanical separation of air and fuel as signified by a corresponding rise in BSFC values. In other words, combustion efficiency tends to deteriorate, accordingly. Plus, in the higher rpm ranges, while there is an increase in the amount of otherwise useable heat from combustion, high rpm shortens the time available to deliver that heat, resulting in a loss in power."
I am not sure why GM put a 2.59 rear end into the autos. Did they want to go 236mph @ 5500rpm? Was it to save the trans, rear end, or the safety of the driver? The stock 94 LT1 puts out 340lb-ft torque at 4000rpm. The 4l60E is rated for 360lb-ft max. My stock 94 has a trans shift kit and I have barked the stock tires when grabbing second at less than full throttle. Would the tires break lose with a 3.45 rear end at full throttle and, possibly, cause me to lose control? Would 3.45s shorten the life of the trans or rear end? Boys will be boys! If it is there, we will use it.
Look it up, study it. Here is one link I went to: http://www.hotrod.com/how-to/engine/...l-consumption/
In the chart and in paragraph 12:
"... an engine tends to be the most combustion efficient at or near peak torque, ..."
Also:
"Ideally, you'd like to create a "flat" BSFC curve, but that's not entirely possible. What you can do is work toward establishing that condition, using the value of BSFC at peak torque as your target. However, a couple of conditions play into any attempt at "flattening" a BSFC curve. Among them is the fact that at engine speeds below peak torque, there's an increased amount of time for heat losses to the cooling system and related parts and passages. And especially if the engine is using a carburetor, air/fuel charge quality tends suffer from less efficient atomization (mixing) at these engine speeds than higher rpm. "
"Beyond peak torque, there is the issue of mechanical separation of air and fuel as signified by a corresponding rise in BSFC values. In other words, combustion efficiency tends to deteriorate, accordingly. Plus, in the higher rpm ranges, while there is an increase in the amount of otherwise useable heat from combustion, high rpm shortens the time available to deliver that heat, resulting in a loss in power."
I am not sure why GM put a 2.59 rear end into the autos. Did they want to go 236mph @ 5500rpm? Was it to save the trans, rear end, or the safety of the driver? The stock 94 LT1 puts out 340lb-ft torque at 4000rpm. The 4l60E is rated for 360lb-ft max. My stock 94 has a trans shift kit and I have barked the stock tires when grabbing second at less than full throttle. Would the tires break lose with a 3.45 rear end at full throttle and, possibly, cause me to lose control? Would 3.45s shorten the life of the trans or rear end? Boys will be boys! If it is there, we will use it.
Just think about this, at idle the throttle is closed the engine is idling at 550 RPM there is not any extra torque being produced at that throttle opening to move the car and all since all of the power the engine producing is being used to keep the engine idling. All the engine power is being used up on frictional losses, and the BSFC would be terrible since no power is being produced to do any actual work. (The engine cannot rev any higher then 550 RPM so assuming you need to operate at 4000 RPM to get the best BSFC is incorrect) It must always be referenced to torque output at a given throttle opening. Also assuming that you must operate at the highest torque output per throttle opening is nice but not always attainable due to limitations of gear ratio's available.
12-10-2014, 10:54 AM
#14
Team Owner
Pro Mechanic
The stock 94 LT1 puts out 340lb-ft torque at 4000rpm. The 4l60E is rated for 360lb-ft max. My stock 94 has a trans shift kit and I have barked the stock tires when grabbing second at less than full throttle. Would the tires break lose with a 3.45 rear end at full throttle and, possibly, cause me to lose control? Would 3.45s shorten the life of the trans or rear end?
2. A numerically higher rear end ratio lowers the burden on the transmission.
Also realize; the 3.45/m6 combo has about the same total gearing in high gear as you 2.59/a4.
.696*2.59=1.80
.5*3.45=1.725
12-10-2014, 01:57 PM
#15
Race Director
Afgunn, as I mentioned before, you are overthinking this. I'd hate to see you grocery shopping with a list of items. Many on here have run those gears in an auto transmission equipped Vette with great results. You are not reinventing the wheel. You will be nothing but happy after you do the swap.
12-10-2014, 10:52 PM
#16
Instructor
Thread Starter
383vett
Actually, I was not over thinking this. I collected some data in 3rd gear and did some calculating to determine what my rpms would be with a rear end at that rpm. I shared what I had collected to see what data others had. I was told my data was wrong, then I was challenged as to where I got such ideas. I posted that and was told that was wrong also. And no one else has shared any data yet - only comments. So, I will collect some more data and determine what to do after that. It will probably be Christmas before I can drive that 5 hour round trip again - in 3rd this time.
It was a good point that a 3.70 rear end would be closer to what I have experienced in 3rd gear, so I might should go with that ratio. I know the car accelerates and drives a lot better in 3rd than 4th.
Actually, I was not over thinking this. I collected some data in 3rd gear and did some calculating to determine what my rpms would be with a rear end at that rpm. I shared what I had collected to see what data others had. I was told my data was wrong, then I was challenged as to where I got such ideas. I posted that and was told that was wrong also. And no one else has shared any data yet - only comments. So, I will collect some more data and determine what to do after that. It will probably be Christmas before I can drive that 5 hour round trip again - in 3rd this time.
It was a good point that a 3.70 rear end would be closer to what I have experienced in 3rd gear, so I might should go with that ratio. I know the car accelerates and drives a lot better in 3rd than 4th.
12-10-2014, 11:30 PM
#17
Team Owner
Pro Mechanic
I don't recall you asking for others' data but my '92 m6/3.45 gets it's best mileage in 6th gear and that is over 30 mpg hwy. My AVERAGE mpg for many tanks in all driving conditions is about 24 mpg.
VETTE Data
You can't run through two trips and make any accurate conclusions. Too many uncontrolled variables.
VETTE Data
You can't run through two trips and make any accurate conclusions. Too many uncontrolled variables.
Last edited by Tom400CFI; 12-10-2014 at 11:33 PM.
12-11-2014, 01:28 AM
#19
Race Director
Actually, I ran a 3.73 gear with a 700r4 for many years. It was perfect for my car which saw both street and strip use.
12-11-2014, 08:42 AM
#20
Burning Brakes
3rd gear should be 1:1 for the ratio, and this means that in 3rd gear you experienced a 2.59 rear axle ratio not a 3.70. I think that would be closer to being accurate if your in 2nd gear.