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Old 04-24-2007, 07:13 PM   #51
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Would it be better FE wise to have a smaller turbo that came in at a lower rpm say 1500-1800 with a limited boost around 5-6 psi?
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Old 04-24-2007, 07:34 PM   #52
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Most likely.

The thing is, the TurboCoupe came stock with a small turbo and 15psi. (9.5 in low boost) I see full boost (18) at around 2500rpm.

The gains verses what I have right now would likely be minimal. (Heh, If I wanted a 40+mpg car, I wouldn't have a Thunderbird.)

But if you're turbocharging small motor, and just need it for the extra oomf every once in a while, a very small turbo would likely be best. And if you are putting a turbo on a stock N/A motor, for Pete's sake, don't go anywhere over 5 psi! You'll blow up the motor.

So, for FE, a small turbo is best. Low RPM, just enough boost to give you a safe amount of power for those tight situations.

But for me? I'm turbocharging mostly for max power, but still with the option of easy driving and good FE. (Something a v8 can't give you)
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Old 04-24-2007, 08:03 PM   #53
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There's absolutely no reason performance and fuel economy should be exclusive. up to a certain point. Going for 300whp is fine, but there are already production cars that can get 30mpg+ if driven efficiently and make ~400-500whp. The only reason they don't get better mileage is because someone who pays $50k for a car doesn't car much about mileage, but sales would be hurting if the car incurred the gas guzzler tax. So GM put in a solenoid that makes the driver go 1st to 4th at anything less than WOT, and a .5:1 6th gear for highway.

I think you're going after this the wrong way because up to a point, displacement can be replaced w/ gearing. By that I mean, you can have a 300whp Supra that gets ~50mpg cruising at ~65mph, provided it's geared right. There are limits obviously, the corvette is already down to ~1200rpm at 65mph, so it can't go much lower because of what most people consider to be driveability. But if you can put together your own car, you also probably feel that going over 100mph is pointless in a street car because of the risk, tickets and accidents, but having 250-300whp isn't because acceleration is usually fun, and not usually illegal.

What I'm proposing is a 3-4L NA engine that makes ~200-300whp and has the right gearing to average ~50-100mpg@55mph in the hands of the efficient driver. Contrary to popular belief, lost of throttle and low rpm is in fact where the engine is most efficient. We just need to rig the car so that we can use lots of throttle at a low rpm at speeds we usually cruise at. Most people wouldn't like this because the car will feel too slow on the highway, plus, most automatic transmissions have to be calibrated for a certain amount of throttle before they kick down, etc.... But we know that all we need to do to get great acceleration is a downshift.

To illustrate this, here's a *BSFC map of the engine (dotted lines) in my Camry.

It makes a peak of ~185hp, ~66hp at WOT at ~2k rpm, and assuming a stock Camry, I only need ~26hp to cruise at 55mph, with the engine spinning at 2k rpm. If the engine were to instead spin at 1.5k rpm at 55mph, BSFC would improve from ~350g/kwh to ~260g/kwh. The downside being the car would only have a top speed of maybe 110mph instead of 135mph. But, I don't care about breaking the speed limit that much because it'll only mean trouble for me if I get caught, and by dropping engine speed 25%, I can go from ~35mpg to ~(350/280)35mpg=~44mpg. If I go all crazy and cut the rolling resistance and drag coefficient in half, as well as gearing it to still be in that same BSFC region at 55mph. I'll have a car that can get ~80mpg@55mph. Since I cut the power needed to move the car at that speed in half, and made sure the engine was still operating as efficiently.

You can do this to almost any engine, within reason. No need for a turbo, or any other complex stuff. It's all about CdA, Crr, vehicle weight, peak engine power, and most importantly, for the driver that knows what they want, gearing. If I can Drop my car's Cd from .32 to .16, and drop on some LRR tires, I can probably average 60-80mpg w/o changing my driving habits one bit, provided the transmission makes the engine spin at the right speed. Since I'd be going from an auto to a stick, I could probably see an even larger increase. And, if I worked my way around smog, I could run an ITB setup with megasquirt and maybe make ~200-250whp no problem. It's all about how you design your car.

Otoh, if you absolutely have to have really high top speed, it may be hard to do what I'm suggesting. Although, with a rwd car, you could put a gear splitter inline w/ the transmission, so that may help you get around that. In any event. I'd rather have a 3-4L 250-350whp NA car that can get ~60-90mpg cruising at 55mph, while being able to accelerate like a rocket up to ~100mph, then a car with the same power that only gets ~30mpg, or a car that gets the same mileage, but only makes ~100whp.


*I pimp this thing out so much, I bet the other members wanna have me strung up.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 04-24-2007, 08:24 PM   #54
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Quote:
Originally Posted by omgwtfbyobbq View Post
Otoh, if you absolutely have to have really high top speed


*I pimp this thing out so much, I bet the other members wanna have me strung up.
Why have high top speed... I just don't get it... With few exceptions, you're governed by the speed limit (or ticket).... Hell, I know for sure the MKIV+ VWs have tattle codes (they don't show up as a CEL though) when you exceed a preset speed


It's a good graph.... A picture is worth 1000 words - but don't try explaining that to your composition professor
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Old 04-24-2007, 08:32 PM   #55
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Oh my! Alas, all that you speak of is far beyond my budget.

I'll stick with the turbo. I can get to the 300hp range for less than 2k, and it's quite easy to work with.
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Old 04-24-2007, 10:02 PM   #56
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Originally Posted by trebuchet03 View Post
Why have high top speed... I just don't get it... With few exceptions, you're governed by the speed limit (or ticket).... Hell, I know for sure the MKIV+ VWs have tattle codes (they don't show up as a CEL though) when you exceed a preset speed


It's a good graph.... A picture is worth 1000 words - but don't try explaining that to your composition professor
I honestly just want a car that gets 30 mpg at 100 mph, but I'll have to settle with 40 mph for now haha.

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Originally Posted by Biffmeistro View Post

But if you're turbocharging small motor, and just need it for the extra oomf every once in a while, a very small turbo would likely be best. And if you are putting a turbo on a stock N/A motor, for Pete's sake, don't go anywhere over 5 psi! You'll blow up the motor.

So, for FE, a small turbo is best. Low RPM, just enough boost to give you a safe amount of power for those tight situations.

Hmm, I'm thinking the ideal sized turbocharger would give you little if any boost at cruising speeds. On my other car, I actually have it set up to spool the turbo on deceleration haha, so it would stay spooled during sporting driving, but at cruise load/rpm, i was still running plenty of vacuum. The other good thing about the larger turbo is, they dont' over heat the air. A little turbo could probobly stress out a stock coolant system at low speeds.

Just my idas on the subject.
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Old 04-25-2007, 02:19 AM   #57
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Originally Posted by trebuchet03 View Post
Why have high top speed... I just don't get it... With few exceptions, you're governed by the speed limit (or ticket).... Hell, I know for sure the MKIV+ VWs have tattle codes (they don't show up as a CEL though) when you exceed a preset speed


It's a good graph.... A picture is worth 1000 words - but don't try explaining that to your composition professor
Damn, MKIV's have black boxes? I gotta tell my grandpa to stop speeding in his Corolla. I've love these BSFC graphs. I found one for what's essentially my VW diesel, and believe you me, ~100mpg at lower cruising speeds is definitely possible. Maybe even up to 50-60mph w/ the right gearing, overinflated LRR tires, and liberal aeromods (something like Crr=.15 compared to the stock of .45). Course, I'd have to go all Basjoos on it, but maybe that'd get people off my ***? Do you know anything about what manufacturer's considered to be top gear road load in the late 70s early 80s? I found an EPA bit that said road load was only considered to be the impact of aerodynamics as simulated by a dyno, until 85'ish, when they started involving Crr... But I'm really not sure about the older regs/standards.

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Originally Posted by Biffmeistro View Post
Oh my! Alas, all that you speak of is far beyond my budget.

I'll stick with the turbo. I can get to the 300hp range for less than 2k, and it's quite easy to work with.
Whoa. $2k? That's a lot of coin. What I'm talking about could be doable for less than $1k, everything included, if you're thrifty and research the whole deal. Just look for a larger displacement (~2.5-4L) car that'll respond well to aeromods, has lots of gearing options, and isn't too big, preferably for a few hundred. From there, all you have to do is select and install the right gearing, fix it up if it needs it, and drop in megasquirt or similar if you want to run it reliably lean/mess with the FI end.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 04-25-2007, 06:31 AM   #58
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There are limits obviously, the corvette is already down to ~1200rpm at 65mph, so it can't go much lower because of what most people consider to be driveability.
I think that "driveability" is why my Element is geared so low. 3500 rpm at 70 mph. It seems that Honda felt it necessary to make it so that the car can accelerate up incredibly steep hills in top gear. Well, I guess it was actually that customers demand that sort of thing, so Honda built it that way. Anyway, I think I use about 40-45 hp at cruise, and I know I could make that much hp at a much lower rpm. If I had a Honda mechanic as an indentured servant, I'd add 6th gear from the TSX and the taller final drive from the CRV. That would put me at ~2500 rpm at 70 mph. I'd probably drop down to 5th fairly often in hilly places, but I'm a big boy and I know how to shift...

Any idea where I could find those efficiency curves for the 2.4 liter I4 in the Accord, Element, and CRV?
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Old 04-25-2007, 10:23 AM   #59
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Matt -

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Originally Posted by DrivenByNothing View Post
...

It doesn't list how much current it pulls, but the bridge rectifier is rated at 1 amp, so it can't be any more than that.

I know I put a switch on mine. I'm pretty sure the plans call for one. Mine is setup to run the O2 signal straight through with the switch in one position and the signal is modified in the other.

Matt
Thanks, that's what I wanted to know.

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Old 04-25-2007, 10:40 AM   #60
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In the name of FE the turbo wouldn't be worth the effort IMO if it only worked during high range.

The optimal setup would be something like a variable geometry turbo that could dynamically adjust to be effective in a wide range of rpm's. These have been used on diesel motors for years, but have only recently been applied to petrol engines.

This means I have to come up with a way to create a "poor-man's" version of this technology or do some more thinking. I'm not totally bent on doing the turbo, but it's something I've always wanted to toy with.

One of the things I learned via the Eagle-Research documents is that the majority of fuel injected in a petrol engine is used to quench or cool the cylinder. I've read about it elsewhere since, as well. There are other fluids that can take the place of gasoline for that part of the process.

Actual thermal efficiency is much higher when the mixture is lean. If the problems associated with lean burning can be solved, that pushes our fuel economy way up.

Ceramics have been toyed with as a way of keeping the heat local to where it's needed for max efficiency. However, ceramics present their own set of problems (ie keeping them oiled, keeping them from clogging, cracking, etc)

I mentioned that drifting is a previous hobby of mine in the intro part of the forum. Well, one of my good friends built several vehicles for a local drift shop that are still used in the Formula D competitions. The cars are using Nissan SR20DET engines that are putting down over 400hp. I was talking to him this morning and he mentioned that one of the vehicles achieved 38-40mpg over a 750 mile trip as well as on a few other occasions (normally, the cars are transported via trailer, but problems with the trailer forced the cars to be driven on the highway.) They weren't hypermiling and were actually slamming the throttle down quite a bit.

I'll be talking to him about this much more once exams are done.

Matt
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