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1slw4dr 12-19-2013 06:32 PM

Speed Racer trying to become Fuel Saver . . .
Alright so I currently drive a '94 Honda Prelude Si. I don't know how well people on this page understand automobiles on a mechanical stand point but I am a mechanic and a performance enthusiasts. I've always worried about how to go faster and blah blah but now I drive 100+ miles a day and would like to extend my fuel range. Currently I get 350-380 to a tank of gas. Mostly just 350 though. I don't understand what happens during the times I manage 380 to make that possible. Well for those interested here are my automotive specs currently.

1994 Honda Prelude Si

Stock motor has been changed out for S model Prelude motor F22a1. Allows me to run 87 octane fuel instead of 93 and is the same motor as found in 90-93 DX Accords so is known for be fairly fuel efficient. Problem is the shorter gear trans the car comes with keeps me from peaking the usual 30-35 mpg FE that I seen some Accord guys say they have gotten.

Wheels have been changed from 15 to 16 inch but I plan to go back to 15 inch to save weight and change the tire size to a 195/60r15. Factory size is a 205/55r15 but I'm hoping the taller tire with less width would help with acceleration and in turn reduce fuel consumption.

Well for now that's all I got so here I am and there is some starter info on my background. Hope to learn a lot here. Good day!

jeff88 12-19-2013 08:54 PM

Welcome to the board! Two things you'll find is there are a lot of mechanics on here (professional and DIY, old and new) and that we aren't afraid of racers! In fact we have a lot of members in your situation, where they want the FE for everyday and the speed and acceleration on race day. We actually have members that have proven that they can do both.

The nice thing is that most mods you can do help with both, especially aerodynamics.Grill block, air dam, belly pan, rear diffuser, kammback, boat tail, mirror delete, etc. If you need to search for those key terms and you'll find loafs of info on here.

Since you're pre-OBDII, you'll want to get an MPGuino. It will help determine best driving practices, both for fuel economy and performance.

I can't speak for the brand personally, but Volk makes some really light wheels you might consider... spendy though.

I don't know if this is possible as I'm not familiar with your brand of vehicle, nor engine, but maybe consider a different first gear and final drive. It could give you better FE, but also keep a little zip for you. I'm assuming you are trying to stay away from higher octane fuels, but what about milling the head? Have you considered a small turbo (just big enough for low end torque, not for the upper end of the RPM range)?

Again, welcome to the board and good luck with your endeavors! :thumbup:

wmjinman 12-19-2013 10:04 PM

Grille block should help quite a bit. Pumping the tires up to max pressure should too.

But the biggest thing will probably be driving style. If you're like most "speed racer" types, you're in the habit of stepping on the gas, out-accelerating the rest of the traffic to try to get a couple extra car lengths ahead at each light, wait until the last moment to brake, etc. All these things are huge wasters of gas. Look ahead to the next light, and if you're going to have to stop at it anyway, start coasting as soon as you figure that out. Let all the other speed racers get the couple car lengths ahead of you. Who cares, right? In fact, there's an actual hypermiling technique known as "rabbit timing", where you let the Ricky Racers all get way ahead of you for the next red light so THEY can trip the signal and get it to change. If you time it right and it works out, you can just cruise on through the light that has just recently turned green, thanks to them and their hot rod driving.

Of course, I don't know you, or whether you actually drive like that or not, but if you happen to, changing that can increase your gas mileage a LOT!

The concept has to do with the brakes. Whenever you brake, you're throwing away momentum that was hard won by burning gas. So try to USE that momentum instead of wasting it - coast up to stops, letting your momentum fight wind and rolling resistance instead of more gas to maintain speed, and then throw it away by using the brakes.

The other related concept is conservation of momentum. I'm not too good at this myself because I hate to step on the brakes, and delay braking too long, but in some cases, braking EARLY can pay off so it takes you longer to get to the red light - - - long enough that it's not red anymore! See, if you can avoid coming to a complete stop, that saves a lot of gas, too. So, slowing down a few mpg early, so you can roll through the light in second gear is better than cruising right up to the intersection, stopping, then having to take off in FIRST gear when it turns green.

But if you can get to treating fuel saving like a game, it can make it fun. Whenever I go out driving, I'm looking for a new "high score" - in mpg. It's a lot like racing, just with a differnt goal.... instead of trying to get there the fastest (the least amount of time spent), you're trying to get there with the least amount of gas spent. :thumbup:

1slw4dr 12-19-2013 10:41 PM

Well damn that was way more response then expected lol Thanks for the tips guys I'll have a look into it. I've thought of changing out the transmission for that of an older Accord because the gearing would allow for better fuel mileage but with how the car is currently setup I worry about loosing a lot of the fun factor I have of driving the car. Also with the tire pressure mod it can be quite tiring because it changes the handling characteristics of the vehicle and with me occasionally taking a back country road to and from certain places it can make the car a little sketchy in the sharper turns where I tend to not slow down. Another way I look into saving gas. Lowered car with better handling I don't have to slow down for curves and turns therefore no acceleration required or waste of momentum. i learned this during the first couple years i had m license growing up in a country like area.

wmjinman 12-20-2013 04:08 AM

You're on the right track keeping the speed up through turns. My girlfriend frequently screams at me for scaring her when I do this.

Lowering the car is good for aerodynamics as well as allowing you to corner faster (which means, with less energy used in braking, then re-accelerating). Just be sure not to wreck the alignment. You say you're a mechanic, so I'm sure you're on top of that. The alignment should be as "straight ahead" as you can make it. As far as tire pressure vs. cornering speed, that sounds like a place where a compromise will need to be made. I pump my tires up to 60 psi, and don't really have any complaints about my car's cornering ability. I'm sure it suffers some, but it isn't usually an issue.

It was once, though. I was entering a "roundabout" and I had gone about 640 miles on that tank already, and was REALLY nervous about running out of gas. I was headed to a gas station, and trying to maximize everything. So I didn't brake for the roundabout, and ended up skidding into the curb HARD - bent my rim and my lower control arm. OOPS! I sure felt stupid after that. But I think that incident was more a factor of going too fast than having my tires aired-up too much - - - in retrospect, I doubt I would have made that turn even with optimum tire pressure. :o

deathtrain 12-20-2013 08:12 AM

If your worried about a few PSI in the tires doing that much change to your handling in the corner.... then you might be going to fast.

1slw4dr 12-20-2013 08:49 AM


Originally Posted by deathtrain (Post 403656)
If your worried about a few PSI in the tires doing that much change to your handling in the corner.... then you might be going to fast.

Ya sometimes I get over excited. Right song on the iPod starts playing and the blood gets pumping and I'm in it to win it. I put a lot of time and money into my cars so every once in a while I like to make sure the money was well placed. For now though I'm testing out the whole higher PSI trick to see how it works so my driving is a lot more reserved. Handling aside at work I've seen what too much PSI can do to tread wear and that also worries me. I'm going to test it out but if the center starts to wear out faster than the outer edges I'll have to drop the PSI back down. I see it happen all the time with cars and trucks people over inflate the tires mistaking tires max allowed pressure for what the cars pressure should actually be. I usually make my own educated choice on tire pressure but I drive 100+ miles a day with no power steering so I tend to feel it more then your average person especially considering the performance aspect of its importance to me. Less pressure up front with slightly higher in the rear to promote oversteer since in a front wheel drive car I am plagued with understeer issues.

Fat Charlie 12-20-2013 09:59 AM


Originally Posted by 1slw4dr (Post 403590)
I've always worried about how to go faster and blah blah but now I drive 100+ miles a day and would like to extend my fuel range. Currently I get 350-380 to a tank of gas. Mostly just 350 though. I don't understand what happens during the times I manage 380 to make that possible.

Welcome- and you can have both worlds. Just remember that hypermiling is high performance driving, performance that can actually be measured. You aren't slacking off, just focusing on a different area.

You're also setting your sights too low. In my Subaru I went from 350-380 mile tanks to well over 500. Enjoy watching your numbers go up!

BLSTIC 12-20-2013 11:00 AM

Yeah hypermiling and performance aren't completely unrelated...

But to further your own knowledge on driving style v efficiency, read this article NOW. Not later.

Autospeed Brake Specific Fuel Consumption

After realising this I typically accelerate at high loads and use the gears to vary my acceleration. This puts the engine load (and efficiency) measurably higher than varying throttle to change acceleration.

Another thing that you will want to learn about is injector cut-off. It's when the fuel is shut off completely on zero throttle and taking full advantage of it can alter fuel economy markedly when coasting. It also promotes coasting before lights as you automatically think "injector cut = infinite mileage while slowing, so I'll back of earlier and get a free 50 yards at every single light"

If you don't have any fuel consumption display you can still discover it. Start at about 3000rpm and back off the gas pedal completely. When you feel the engine braking drop suddenly it means the injectors are opening again and you are using fuel. Above that point, zero throttle means zero fuel. This point can vary with coolant temperature and air conditioner usage, but is typically 1500rpm (my '13 Suzuki Swift is 1100rpm with a/c off, 1500rpm on).

Just those two techniques (which don't actually impede progress at all) are the difference between me and my stepfather driving in traffic. He got 7l/100km indicated driving my car, I got 5.5 on the same loop

BLSTIC 12-20-2013 11:28 AM

Incidentally what to modify on your car comes down to three questions:

On a scale of traffic jam to highway, where do you drive?

Traffic jam: Mods are geared to weight reduction and engine being off whenever not required. Extend your coasting time as much as possible. Kill switches, lightweight flywheel, wheels, empty the trunk, underdrive pulleys (always a good idea for efficiency, only real concern is alternator not producing enough power at idle. But a real hypermiler never idles anyway), carbon fibre everything etc.

Highway: Aero, gearing, and lean burn. Undertray, lose the factory rear spoiler, ditch a wing mirror, wheel covers/skirts (the general shape of a prelude is pretty good actually, it's the details that kill cars of the 90's), get your computer tuned to make lean burn a reality. Tallest gearing you can get.

How much money have you got?

Ever seen a coroplast undertray that can take a scrape? Nope, but ABS plastic can. Genuine moon hubcaps v pizza pans. Get an aftermarket computer (or re-tune) to take care of injector cut-off and trans control for you while us plebs deal with toggle switches and buttons on our gearsticks. Carbon fibre doors.

How crazy are you about fuel consumption?


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