3000 @70 in a 59 el camino.with a.................. wait for it..........................305 runs all day long.right in the middle of cam range.but I do wish it was lower.
Wow. Alot to learn from all you guys. Once again. Thank You for all the info. I think I'm gonna keep it where it's at and be happy. I love this forum. Lots if knowledge and help. Once again, Thank You everyone.
my pickup which weighs 7000 pounds runs at 2100 at 60 mph and starting to come onto the torque curve ( O/t 454) 2000 at 70 is at the bottom of the curve but should still have plenty of torque .and is a excellent cruise speed .
No body has mentioned converter stall speed. What ever RPM you gear for at 70mph, just make sure the converter is locked up at that RPM. If the converter isn't fully locked up during extended highway speeds it will build heat and fail fairly soon. My RPU is running a TH 350 with 2:91 gears in the QC and 31" tires. At 65/70 it's turning around 1900 to 2000 RPM's. So, I run an 1800 stall converter. Yeah, I know 2:91 is pretty tall, but the blower makes butt loads of torque, and in a car that weighs 2700lbs, it will still blow the tires off when it's hammered. With two 600 Holley double pumpers it's not about the mileage. It's about long life for the motor. If I want mileage, I'll drive the wife's Accord.
According to my calculations, I'll (eventually) be doing 2200 rpm @ 70 mph, using .64 overdrive and 3.70 gears. Yours are acceptable, maybe just a little on the low side.
Good rpms for all around driving. I'm doing 70 @ 2400 rpm with a C4 and 273 gears and a mildly souped 302.
It all makes me think of the 70's when the Feds instituted the National 55 mph speed limit in an effort to (in their infinite wisdom) curtail national fuel consumption. My daily driver at the time was a low compression 350 Corvette with I believe had a 3:08 rear gear and a Turbo 350. My mileage was always better between 65 and 70 than at 55 because it was the "sweet spot" in the rpm range for optimum fuel efficiency.
Ok. Thank you. I feel better about my RPM's after this thread. Once again, Thank You everyone. I learned a lot.
That is what my 27 is turning..........3500 @70. (deep gears). 2100 is terrific, my 23 was something like that and I regularly got 25 mpg and sometimes even 30 mpg. Don
I've driven a few rental cars with intantaneous mpg displays and computer controlled automatics. For steady state cruising they will generally get into a gear that gets the engine into the 1200-1500 rpm range. Even the little four cylinders. As soon as I step even a little on the throttle they will down shift to get some throttle response, as there would be none in such a high gear. Nobody would choose to drive that way with a standard shift. Many modern engines have such broad, flat torque curves that the torque at "peak torque " rpm may be just a slight bump in the curve. If the engine is making good torque it implies the cam timing, manifold, air fuel ratio, and ignition timing are all doing a good job (at full throttle), the cylinder filling is good, and the rpms aren;t so high the friction is lowh. here is a Brake specific fuel consumption map for a Ford Zetec engine. http://ecomodder.com/wiki/images/e/ee/Ford_2.0l_zetec_bsfc.JPG Note the torque is good from about 1000 rpm to 6000 rpm. The "peak" torque is at 3500, or is it 5500 rpm? But the best fuel economy (lowest BSFC. lbs of fuel per HP) is at nearly full throttle, around 2200 rpm, where max torque is down about 45 Nm (about 33 lb-ft). For several years Corvettes have been rated and achieve over 20 mpg highway. http://www.roadandtrack.com/cm/roadandtrack/data/vettecomp.pdf highway cruising rpm is about half the "max torque rpm." In the 2014 Corvette they are even using cylinder deactivation, which has the effect of moving upward on the BSFC curve. Chevy claims its worth 10 - 20% better fuel economy. http://www.motortrend.com/roadtests...let_corvette_stingray_first_look/viewall.html
Let's try to understand a couple of things by looking at what is going on at different RPM's with your 454. As you can see the Horsepower is fixed, so the slower you turn the engine, the more torque you need to make 55 HP. Load is Wheel Torque/ Max wheel torque (425). Also look at the FHP, at 3000 RPM the engine is burning fuel to make 102 HP, 55 to the wheels and 47 to friction. At 2100 it is 81 HP worth of fuel 55 to the road, 26 to friction. So in general the lower the RPM the better the fuel economy. What about the Vette story above, either selective memory which we all have or a problem in the fuel calibration. If the sweet spot was that far off something bad was happening, which is certainly possible. The only danger in going to a taller gear is opening the power valve or other enrichment circuit. In the example above if our Power Circuit came in at 9 inches then 1800 would get worse mileage than 2100 because of the extra fuel flow, about 20 Pct. So the more load the more efficient, and more load comes from gearing (most cars don't enrich until about 65 - 70 pct). Here is a sample of V8 cars and how their fuel efficiency looks by load.
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