Re: Best Street Gear Ratio?? -HELP
[Re: DaytonaTurbo]
#60670
05/21/08 11:02 PM
05/21/08 11:02 PM
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cause i'm not in a hurry to go anywhere. if i really, really have to go on a superspeed long trip.. 20mins and the 2.76's are in.
Kinda defeats the purpose if you have to change every time you want to go on a longer highway trip tho. I know some guys do the swap quick but I'm not that good.
I've got 3.55's with a 27" tire and it's okay, but still revving high for me at 60mph. Doesn't really matter that much for me because I don't take my car out on the highway much anyway. 3.55's are a compromise gear. They'll cruise on the highway and they'll race down the strip, but they won't do either one that well. However I still think they're a better middle ground than 3.23's. But if you've got 3.23's already, is it worth the $ to change to 3.55's? IMO, no. Play with the speed/rpm calculator linked to a few posts up and get a feel for what you can handle. I think 3.73's would give me a little more punch and not give up much on the highway, but again, not worth the $ for that little of a change.
IF i had too. but i have no problem going 70mph either with 4.30's on the hiway. if your engine can't handle that for a couple hundred miles.. build a better engine, or get EAR plugs.
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Re: Best Street Gear Ratio?? -HELP
#60676
05/22/08 12:39 PM
05/22/08 12:39 PM
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Joined: Jan 2003
Posts: 27,421 Balt. Md
383man
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amen to that Ron.
You got it as I love the 4.30's in my 63. I was going to use 3.91's but I wanted a taller tire in my B-body wheelwell as we use 3.91's in my sons Dart with 26" tires and they look fine in his smaller A-body wheelwell. With the 29" tire I run the 4.30's work perfect for a good launch and I can still drive the freeway. And the 29" tire looks good in my stock wheelwell. I agree....Amen to that ! Ron
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Re: Best Street Gear Ratio?? -HELP
[Re: chagjr]
#60677
05/22/08 12:42 PM
05/22/08 12:42 PM
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Joined: Jan 2003
Posts: 27,421 Balt. Md
383man
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I currently run 3.91. I also have a 4.30 pumpkin. If your big block is putting out good tourqe you do not need a lot of gear. With a good tourqe converter 2800 or so, 2.94 and or 3.23 are great gears. I ran a 14.65 @ 96mph with 2.94, 11 inch converter. The 3.91 gears just get me down the track quicker 14.018 @ 95mph.
You are giving up 6 tenths in the 1/4 mile so I would say you do need the gear for drag racing in that combo. If you dont care about your 1/4 mile et then yes the gear dont matter. I do agree some bigblocks make enough torque that you wont need as much gear as a combo with less torque. Ron
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Re: Best Street Gear Ratio?? -HELP
[Re: A4SixtyNineRR]
#60678
05/23/08 06:23 AM
05/23/08 06:23 AM
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Posts: 8,162 USA
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(here's an attempt to figure out gear ratio for highway cruise in a scientific way, based on piston speed and intake manifold vacuum. comments welcome) ------------ Now lets discuss gearing and fuel economy.
In city driving gearing doesn't matter nearly as much as not having a 'heavy foot' or hauling a lot of weight in the bed of the truck.
A vehicle that is either overgeared or undergeared will lose MPG at highway cruise.
An engine is numerically overgeared if it is not cruising at about 60-75% open throttle.
An engine is undergeared and "too weak for the job" if it has to operate with its pistons going faster than an average of about 1200 feet per minute when at 75% throttle.
Note that you could reduce engine weakness by turbocharging as well as the much more common thought to just make the engine larger in cubic inches. This is the reason nearly all diesels have become turbocharged in the last 20 years.
How do you calculate this average piston speed?
Multiply the stroke of the engine in inches by two (because the piston goes both up and down on one rpm) and then divide by 12 to convert inches to feet. Multiply this by the rpm.
So where does 1200 feet per minute end up for various engines? Here some examples:
For the 3.58 stroke of 3.7, 5.7, 5.9 Dodge engines: 2011 rpm = 1200 ft/min /( 2 x 3.58 inches/12 inches per ft)
For the 3.405 stroke of the Dodge 4.7V8 2114 rpm = 1200/(2x3.405/12)
For the 3.00 stroke of the Ford 5.0L V8 2400 rpm = 1200/(2x3.00/12)
For the 4.72 stroke of the Cummins Inline 6 diesel 1525 rpm = 1200/(2x4.72/12)
Is it possible for an engine to be made to get its best fuel economy at a piston speed above the typical 1200 feet per minute? Yes. Very thin piston rings, using less than the normal 3 rings, slippery coatings on piston skirts, extremely hard but slick coatings on bore walls like "NikoSil" and keeping the bore walls very hot so that the oil there will be thin and less viscous can all allow the 'best economy' piston speed to be raised but even 'state of the art' giant marine diesel where nearly every trick is used seldom exceed 1500 feet per minute.
Honda has announced that over the next 5 year period that the major part of their engine research $ will be spent on ways to reduce internal engine friction, and this is from a company that NASCAR engine builders already admit has the best rod and crankshaft bearing material for sale.
Which is more important: having the rpms near this 1200 foot per minute speed for the pistons, or having the throttle in the 65-75% open range? Answer: it is more important to be in the 70% throttle range and you should slow down the rpms to get there. There is not much change in efficiency when piston speed drops from 1200 down to 800 ft/minute but there is a big change in efficiency when the throttle goes from 70% open to 40% open.
Why?
Imagine that you have a disassembled engine in front of you. Put a loose piston in a bore and pull it down against the friction of the rings. It won't be very hard.
Now imagine that same piston in the bore has the top of the bore sealed off and has a vacuum sucked of 12 inches of Mercury. 12 inches of vacuum is about 6 psi of negative pressure. If the piston is 4.00 inches in diameter it has an area of about 12.5 square inches (remember pi R squared but most cherry pies are round?) 6 pounds per square inch times 12.5 square inches = 75 pounds would be required to pull the piston down - you probably couldn't do it with the grip of just a couple fingers! An engine running with a high vacuum in the intake manifold has to do just this, and for eight cylinders!
Engineers refer to this as one of the two parts of "Pumping Losses". The other part of Pumping Losses is due to pushing the exhaust out the tailpipe. Most engine owners easily imagine that exhaust loss, accept it, and spend a lot of time and money on mufflers, headers, Y pipes etc. These same engine owners would do well to remember the "Pumping Loss" on the intake side and try to keep it low during highway cruise conditions.
{Note to engineers: Yea, I know, 'negative pressure' does not really exist and what actually happened is 14.7 psi of air pressure is pushing the underside of the piston and there was just less pressure on the top of the piston to create a delta P - but the general public thinks in 'vacuum' terms. And it is not just the general public - talk to the biologists who think -100 psi or more of 'negative' pressure pulls water up capillaries to tree tops}
Setting Gear Ratios to suit the Customer's Expectations
Most auto manufacturers intentionally overgear their vehicles because they know from experience that customers will bitterly complain about a vehicle that downshifts to climb a hill.
Engineers know that to give their customers the best possible highway MPG the transmission would downshift on EVERY hill, but the sales department reminds them that that every employee at the automaker (including engineers) will lose their jobs if customers {even ignorant ones} get mad and buy someone else's vehicle that meets their expectation.
So vehicles are overgeared to 'feel strong' during the 20% of the time they are climbing hills in Overdrive, even if it means this hurts MPG a bit during the 80% of the time the vehicle is on level roads or going downhill.
An engine is numerically undergeared if it is trying to cruise in the 'Power Enrichment' zone above about 80% throtttle opening where the PCM computer greatly enriches the air to fuel ratio from 14.7 to 11. On most vehicles with automatic transmissions it is not possible to cruise in OD on level highways in the power enrichment zone because the transmission will downshift to a lower gear long before the accelerator is pressed that far.
For the same reason anyone who presses the accelerator down all the way near the floor whether they are driving in the city, highway, climbing a hill, or pulling a trailer is going to get about 25% worse MPG than someone who presses right up to about 70% throttle but never goes over it.
This is why the old advice to install a vacuum gauge can certainly help MPG if the driver watches it out of the corner of his eye.
When the vacuum gauge reads about 1 to 6 inches of mercury you are in the fuel guzzling 'Power Enrichment' zone that you want to avoid except when you want the highest horsepower and accept the higher fuel usage.
When the vacuum gauge is showing about 7 to 9 inches of mercury vacuum your engine is converting fuel into horsepower at its most efficient level.
Each engine goes into Power Enrichment at a slightly different MAP sensor level, but if you listen closely to the engine sound you can hear it happen as the engine gets a bit smoother with the very rich mixture.
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