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Old 03-31-2008, 07:22 PM   #1 (permalink)
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Accelerating and shift points

The way I think about it is, you can accelerate slowly and have to be in the throttle for a longer period of time, or you can accelerate quicker and get to cruising speed much sooner. Ive also got two different cars to consider, and the only thing they have in common is they're turbo 4 cyls. One is a 5 speed and one is automatic, so for the 5 speed another thing to consider is shift points (and you can also consider that on the auto to an extent).

The shadow has a stock vac/boost gauge whereas the tbird only reads boost, Ive been wanting to pick up an aftermarket vac/boost gauge for it. Ive always assumed that accelerating with the most vacuum is best, and try to keep it between 7-10in of vac when accelerating (cruises at 14-15). However, Ive noticed that acceleration doesn't really improve much with more pedal until you get to around 2 to 0 inches (remember its a turbo car so 0 vac doesnt mean wide open throttle. At that point, close to 0 inches the car seems to accelerate a good bit better, even though you arent giving it hardly much more pedal. Between 0 vac/boost and 2psi of boost is where I would consider "normal" non-hypermiler acceleration.

Basically I just wonder if it would be better to accelerate slowly (7 or more inches of vac for me) and have to stay accelerating for a much longer time, or to accelerate at a more "normal" pace (2in vac to 2psi boost) and get up to speed much sooner.

Its a pretty different story for my Thunderbird. Since its a 5 speed theres no torque converter to slip around and keep the engine from lugging. Its happy accelerating from at least 2000 rpms, and anything under 1500 is a no-no, except in 1st or 2nd gear. In the upper 3 gears its like a semi truck jake braking under 1500, sure to rattle the bearings and such pieces apart in time. So its not only a matter of how far to open the throttle, but what rpm range to use as well. Should I accelerate in the range that seems "easiest" on the engine or try to keep the rpms as low as possible without rattling the car apart. Theres different combinations of rpms and throttle to try too, low rpms and low throttle, low rpms and more throttle, high rpms and low throttle, etc.

So what are you peoples opinions? Accelerate slow or quickly? Keep rpms as low as possible or use more of the engines efficiency range?

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Old 03-31-2008, 07:33 PM   #2 (permalink)
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Quote:
Originally Posted by bhazard View Post
The way I think about it is, you can accelerate slowly and have to be in the throttle for a longer period of time, or you can accelerate quicker and get to cruising speed much sooner.
Careful - correct conclusion for the wrong reasons.... In the miles per gallon equation, there is no "time" variable. mpg is independent of time. If your engine efficiency stays the same all the time - it would take the same amount of fuel regardless of how fast you accelerate... But, efficiency changes with load

As for your question - I'll let someone else answer for a turbo My explanation won't be very eloquent
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Old 03-31-2008, 07:37 PM   #3 (permalink)
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Feel free to speak your mind. Im open to opinions.
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Old 04-01-2008, 08:39 AM   #4 (permalink)
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Quicker acceleration is better as the engine will operate where it is most efficient. Driving for FE can also be fun sometimes. The goal is to get to your cruising speed in the shortest reasonable distance without completely putting the pedal to the metal.

Don't be affraid to use boost when accelerating, it just makes your engine more efficient under load.
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Old 04-01-2008, 09:28 AM   #5 (permalink)
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Quote:
Originally Posted by tasdrouille View Post
Don't be affraid to use boost when accelerating, it just makes your engine more efficient under load.
This is true as long as it isn't kicking into open loop.
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Old 04-01-2008, 12:53 PM   #6 (permalink)
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I saw somewhere else on the forum a discussion of specific fuel consumption and an "island of efficiency" related to torque loading of the engine - the load and speed where it is making the best power for the fuel it burns. Find a BSFC map for your engine (good luck - I can't find anything like that for mine), then do the math for your drivetrain to determine what that equates to for your gears. I'm thinking that if you do all your accelerating in that sweet spot, you'll be hitting the best possible load/fuel burn.

Just guessing.
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Old 04-01-2008, 03:22 PM   #7 (permalink)
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I know that the ECU in the tbird goes to open loop at 3000 rpms, so I would have to keep the rpms under that, but Im not sure at what point on the throttle side of things that it goes to open loop, obviously at wide open throttle, but Im not sure if theres a certain amount of throttle that causes open loop as well.

I dont really know anything about the Shadow as far as open loop goes, not sure if theres a certain rpm or throttle opening.
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Old 04-01-2008, 05:44 PM   #8 (permalink)
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So could you say as long as I'm staying in closed loop its the same or even better to accelerate at normal brisk pace from a stoplight instead of an agonizing crawl? That would sure make alot of folks behind me less annoyed.
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Old 04-01-2008, 06:29 PM   #9 (permalink)
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Thats my guess.

Could try say, 3 tanks of gas accelerating slowly, and then 3 tanks accelerating more normally and compare.
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Old 04-01-2008, 06:32 PM   #10 (permalink)
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Quote:
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So could you say as long as I'm staying in closed loop its the same or even better to accelerate at normal brisk pace from a stoplight instead of an agonizing crawl? That would sure make alot of folks behind me less annoyed.
It can't be worse for sure. But give it a try, it's really easy to test. It can be as short as a 1 mile test. Start at a stop, pick a target cruising speed at which you'll stop accelerating, reset your trip on the SG and do runs accelerating like a granny and runs giving it a lot of pedal.

I usually set my target at 40 mph in the city. I go 1-2-3-5 shifting a bit over 3k rpm.

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