Any way to tell what rpm range your engine is most efficient in?(mpg talk)
04-07-2007, 06:56 PM
#1
Any way to tell what rpm range your engine is most efficient in?(mpg talk)
As many know you will get better mileage at 55mph then 80mph, the difference obviously being less aerodynamic drag and lower cruise rpm. The more teh tq you make in the given "cruise" rpm the better gas mileage you will get(to make things simple). Im trying to figure out where my new motor is going to be most efficient at to gear my truck accordingly. Once my truck is up and running it will see 80% highway driving and I usually cruise at 75+/- 5mph. With 4.56s and 37" tires Im looking at ~2200rpm, with 4.10s and 35s Im looking at ~2050. I wan to target my tq "bubble" for as close as possible to that rpm range as possible since Im also going to be running a stock 80e stall(~2600 with my setup, up to 3K brake stalling). Im not worried about off-the-line performance since I can brake boost it and bring the power on up, but Im wondering how to figure efficiency at a given rpm. Changing gears all the time is not an inviting prospect, not to mention expensive with a 14 bolt rear. With the stock 5.3 I got great mileage at ~70mph which was a tad under 2K rpm. That doesnt help me anymore since obviously Im not running a stocker motor or even close. My cam specs are going to be in the low 220s to give me awesome tq out of boost(aka mileage). Wondering how to "adjusts" the cam specs to give me a good balance between mileage and power, with the scale tilted more towards mileage. I dont think Ill have a problem in the power area with the 418, turbo and nitrous if need be. So cam/engine theory experts, lets hear your ideas. I have my own thoughts as to what I should run but Im by far no expert, so lets hear your ideas.
Full specs on the truck will be
418 with 9.5:1CR, WCCH Stage II 317s, Vic Jr with 4Barrel TB, Cam(to be decided obviously), 80e with stock 80e stall(should brake boost to a tad over 3K & flash to 2600), STS with gt-67 and FMIC @ 12#s, 35s or 37s(depends on what I can fit with the lift), Im going to put either 4.10s or 4.56s in it to get the right cruise rpm since again Im not worried about the power just getting the right rpm for good mileage. Remember the truck will be close to 6K with replacement front and rear bumpers(tax return time
)
What this whole post boils down to is I dont relish doing a gear swap front and rear to find the right "sweet spot" for mileage not to mention the cost. I realize Ill probably just have to end up doing a trial and error thing but figured I would ask before going to PITA/expensive route.
Full specs on the truck will be
418 with 9.5:1CR, WCCH Stage II 317s, Vic Jr with 4Barrel TB, Cam(to be decided obviously), 80e with stock 80e stall(should brake boost to a tad over 3K & flash to 2600), STS with gt-67 and FMIC @ 12#s, 35s or 37s(depends on what I can fit with the lift), Im going to put either 4.10s or 4.56s in it to get the right cruise rpm since again Im not worried about the power just getting the right rpm for good mileage. Remember the truck will be close to 6K with replacement front and rear bumpers(tax return time
)What this whole post boils down to is I dont relish doing a gear swap front and rear to find the right "sweet spot" for mileage not to mention the cost. I realize Ill probably just have to end up doing a trial and error thing but figured I would ask before going to PITA/expensive route.
04-07-2007, 07:18 PM
#3
Nobody wanted to give me what I feel the sts kit is worth so Im going to throw it on there. I already have the vic jr and my nitrous pro-flow plate system so I might as well use it
Already have the rest of the stuff to get it back on the road just need time now, 80 hours of work a week plus school take alot of time away from wrenching
Already have the rest of the stuff to get it back on the road just need time now, 80 hours of work a week plus school take alot of time away from wrenching
04-07-2007, 07:33 PM
#5
Originally Posted by silver-mod-o
Cody changed his underware today.... thus changing his mind LOL.... would you pick something and build it already!!! 
Its too late to change my mind now though, I already ordered injectors for for FI so atleast I have to stick to that and I already have the sts kit so...
80e should be here monday, SE input gear conversion next week, injectors, fuel pump, 80e stall already here, got the block back yesterday so the shortblock will be done next week, WCCH heads will be here next week, everything is coming next week, but I go out of town next weekend
04-08-2007, 07:48 PM
#7
not sure if it will help or not but my ls1 with the lpe cam and 33's with 3.73's like 80mph better than 70mph. at 80 it was turning 2200 rpm or so and i got 21 mpg at that speed. below that speed and rpm the truck just didn't pull itself as easily and took more pedal to get over hills and what not but at 80-85 it would cruise right along with no hesitation. with that tall of tire i would go with 4.56's just so your in a good torque range. but of course i'm sure you'll have plenty of cruiseing torque with a 418 and all. so 4.10 might just work also. lol i would shoot for a gear that allows the truck to pull itself the esiest at the lowest rpm. like mine at 2000 rpm was a slug and got like 17-18mpg but at 2200-2300 rpm it would pull itself with ease and got 20-21 mpg.
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04-08-2007, 07:52 PM
#8
Thats what Im looking for sort of. Right now Im going to have to wait and see what it dynos at and do some cruising and find the sweet spot and find out how the taller tires are going to affect it when I throw the lift on. I believe TurboGibbs is going through the same thing that you are tlaking about with his 6 speed. I need to play with the dyno sim software some more.
04-08-2007, 08:03 PM
#9
TECH Veteran
After all kinds of research into this issue the consensus was that RPM is much more important than speed for engine efficiency (and therefore MPG).
For the LS-series of engines, optimum efficiency seems to lay between 1800-2000 RPM, so whatever gearing puts you in this range at cruising speed is what you should be aiming for.
For the LS-series of engines, optimum efficiency seems to lay between 1800-2000 RPM, so whatever gearing puts you in this range at cruising speed is what you should be aiming for.
04-08-2007, 08:11 PM
#10
Originally Posted by TBSS
After all kinds of research into this issue the consensus was that RPM is much more important than speed for engine efficiency (and therefore MPG).
For the LS-series of engines, optimum efficiency seems to lay between 1800-2000 RPM, so whatever gearing puts you in this range at cruising speed is what you should be aiming for.
For the LS-series of engines, optimum efficiency seems to lay between 1800-2000 RPM, so whatever gearing puts you in this range at cruising speed is what you should be aiming for.
