Fuel economy vs gas prices

[The Dark Side of Will]

A few months ago I did some botecs...

@$$uming that the difference in gas mileage between two engines with different compression ratio, but otherwise identical is simply the difference in ideal thermal efficiency of the Otto cycle for those compression ratios...

And making soem @$$umptions about how much compression an engine can run on 87 octane vs 91 or 93...

When regular gas hits ~$4.00 and @$$uming premium is only .30 more or so...

Then the cost per mile of a high compression engine that requires premium becomes lower than the cost per mile of a low compression engine that can run on 87.

I don't think I'll ever worry about building an engine to run on regular gas again.

Thoughts?

[befarrer]

My 98 Neon with the DOHC motor says in the manual to run 89 octane (midgrade) gas. At the time I was running regular, and getting high 20's for mileage. I filled up with 91 octane since midgrade is only available at 1 station in town, and my mileage went to 35 on that tank, and stays around there while running midgrade. That more than pays for itself to cover the extra .10/L it costs.

Alot of cars now adays do require premium, it is probably easier to get fuel mileage with premium if the engine was designed for it. I guess since you can run higher compression and effectively get more HP out of the same size motor, so you can put smaller motors in larger cars I guess.

[The Dark Side of Will]

It's not just more HP out of the same engine... it's more HP out of the same FUEL (better thermal efficiency), especially at part throttle.

[whipped]

Or..... run mexican 85 octane on a 12:1 engine with water injection. It'll be cheaper assuming the water doesn't steal any of it's thermal efficiency (and it will.. question is how much)

[Atilla the Fun]

I wouldn't build an engine for premium, for a couple of reasons. One, you have to keep the chambers carbon-deposit-free, and 2, you hafta change something to get your compression back down if you decide to add boost.
Around here, we have 85 to 91 octane, we're at 4300 feet, and for a 350 chev, iron heads, 9.25:1 is about it. With most aluminum-headed engines, 10:1 is really pushing it around here, and going to 9.75:1 isn't gonna cost you 1 mpg, nor 10 hp, but will let you run 89.
So, try more if you like, but remember a thicker gasket to reduce compression will kill quench, and then you will have detonation and worse gas mileage.
I am a professional automotive machinist, I do engines for a living. Play it safe, you'll have no cause for regret, and no lost sleep.

[lucky]

Ahh the thermal evil that is ethanol. My car only really likes to run on premium. Methinks my stock fuel system doesn't like my performance ignition. O well. I've been stretching my gas dollars by running 89 and letting every other tank run real low. On the low tank I've added 3 or so ounces of lucas system safe fic. Quart bottle is a deal, and it boosts your octane by a couple points.

[The Dark Side of Will]

I wouldn't build an engine for premium, for a couple of reasons. One, you have to keep the chambers carbon-deposit-free, and 2, you hafta change something to get your compression back down if you decide to add boost.

That assumes that I would be so wishy washy as to build an engine for all motor performance and then want to boost it later.
Why would a high compression engine accumulate more carbon than a low compression engine?

[p8ntman442]

My 98 Neon with the DOHC motor says in the manual to run 89 octane (midgrade) gas. At the time I was running regular, and getting high 20's for mileage. I filled up with 91 octane since midgrade is only available at 1 station in town, and my mileage went to 35 on that tank, and stays around there while running midgrade. That more than pays for itself to cover the extra .10/L it costs.
.

Your ECM is probably running a retarded ignition map because of knock. Therefore your terrible gas mileage with low grade.

[p8ntman442]

A few months ago I did some botecs...

@$$uming that the difference in gas mileage between two engines with different compression ratio, but otherwise identical is simply the difference in ideal thermal efficiency of the Otto cycle for those compression ratios...
And making soem @$$umptions about how much compression an engine can run on 87 octane vs 91 or 93...
When regular gas hits ~$4.00 and @$$uming premium is only .30 more or so...
Then the cost per mile of a high compression engine that requires premium becomes lower than the cost per mile of a low compression engine that can run on 87.
I don't think I'll ever worry about building an engine to run on regular gas again.
Thoughts?

I think building a motor for fuel efficiency is silly. How much will you spend on the rebuild? $2k minimum. OK, now how much gas mileage do you need to make up to make that back in less than 2 years.

[The Dark Side of Will]

Obviously the engine isn't going to pay for itself in fuel savings.

But in situations in which one is building an engine ANYWAY, then it doesn't make economic sense to build it to run on regular.

[Atilla the Fun]

I'm not saying high compression accumulates either more or faster or both. I am saying it's more critical that it doesn't. Carbon deposits, platinum plugs, overheated exhaust valves, and sharp edges all can cause detonation. The higher your dynamic cylinder pressure climbs, before the burn begins, the more likely it is that a hot spot will cause detonation, or preignition, or both. In every engine, one cylinder will have a tendency to be the one that is most prone to detonation. Causes like the engine's inherent hot spots due to the water circulation you can't do much about. But this contributes to where the carbon deposits accumulate. You can address the symptom. And if you want to try 11:1 on 85 octane, you hafta address everything possible. Ceramic-coat your exhaust valves. De-burr everything, including your spark-plugs, to the extent possible. Now someone is gonna read that sentence wrong. But engine internals I have 10 years of hands-on experience with, from sixties V8s to VTECs at 19 pounds of boost. I've yet to have a comeback, knock on wood

[The Dark Side of Will]

http://airflowresearch.com/articles/art ... /A3-P1.htm

[Blue Shift]

I made the choice to run 8.5:1 in my 383 Chevy in the boat on the assumption that 87 octane is more economical at ~2 MPG. From a thermodynamic efficiency point, I wonder how much more efficiency one would need to squeeze from a premium only 383 to break even in running costs, and if you're only averaging ~2MPG, if that'd be any wider or narrower a margin than with a car. My assessment in this unique case has been that it's not worth it for my application.

However, some other factors that apply to boat use include continual severe duty operation, and unavailability of 91 (93 for non CA people) at some marinas in the event we need to get her home.

Provided my engine isn't fucked up to begin with (or maybe even more so if I need to replace the heads), I have my eye on a set of Vortec style heads - compression goes up to ~9.5:1, but due to more efficient, faster burning combustion chambers, the likelihood of detonation is said to be no more than old style heads at 8.5:1 - which is why they put them on HD trucks. I'd gain power and thermodyamic efficiency while still using 87. Oh, and they're iron which takes a little better to raw water cooling, too.

Sorry if this isn't relevant to cars /hijack

[Aaron]

Will, you'd also have savings in the brake pad department as well. And there is one thing I hate more than buying new brake pads--installing them.

Chris, Vortec heads FTW! Do it!

[Blue Shift]

http://airflowresearch.com/articles/art ... /A3-P1.htm

Holy balls, I did it backwards. I should have built a 6.2" rod 377 Chevy instead!

[The Dark Side of Will]

The 4.125 x 3.250 engine is 347 cid.
You can bolt it together with OTS parts...
Pistons for a 400 w/ 6 inch rods
6.250 rods (Eagle)
3.250 crank (283, 327 or Eagle)
All production 400 blocks and all 3.250 cranks are two piece rear main.

You could build a 377 the same way by using a (aftermarket) 3.500 crank with 6.125 rods. It would be a smidge bigger than 377, of course, but that won't hurt.

[Atilla the Fun]

The vortec heads are my specialty. I've ported and flowed them literally hundreds of times. Part of why they resist detonation is the smoother combustion chamber. Part is their high swirl. (and I don't mean the pre-96 swirl-ports) If you're gonna go vortecs, please find out your cam specs and post them. Absolutely don't try Any porting on the intake side. If you want to improve the intake side, get the best 1.94" intake valves you can find, with undercut stems, and have a good machine shop put a small 30 degree back cut on them. You're done. On the exhaust side, they're easy to screw up. Do taper the guide boss. It won't show any improvement number-wise, but it changes the sound on the flowbench in the way that indicates smoother flow. I recommend a 5-angle valve job. 15-30-45-60-75, with a 1.60" valve, back cut and under cut, 75 it until the 60 is approx .050" wide. Lightly port the short-side to a smooth radius, remove the ridge in the floor, and polish it.
As it's a boat, and as you hafta buy an intake manifold anyway, go to the Edelbrock RPM AirGap.

[Atilla the Fun]

As for that article at AFR, realize that as the annual EMC proves, engines are far less octane tolerant in a car than they are on a dyno. 87 octane on the dyno requires at least 89 in a real car, more so if it's a manual trans.

[The Dark Side of Will]

I was thinking of building a 347 as described above w/ Keith Black hypers having 3.25 cc net dome. With Vortec heads, assuming 65 cc chambers (I think you said somewhere else that this is not a good assumption?), and .035 quench the compression comes to just under 11.4:1. What's your assessment of this engine's streetability on 93 octane?

I know I haven't listed cam specs. I haven't picked a cam yet, because I needed more input on how much I'd have to tweak the IVC event to run 11.4 compression. I absolutely want a roller cam.

I'm planning TPI for the 347 because I think it will give me the right powerband to move the car on the street. Obviously I'll need the Vortec TPI base. I'll also be going with upgraded aftermarket runners, but probably a stock plenum. It's got 2.5" Ram's Horn exhaust manifolds right now. I'd love to build custom headers for it, but I think the Ram's Horns are a far more likely choice for quite a while.

The application is for my dad's Jaguar. It's currently running one of the last 400 crate motors w/ TPI, towing cam & 350 heads at mid 9's compression. This engine requires premium. It's got a built 4L60E, LT1 ECM and 3.31 axle. It's a street car through and through.

My plan was to build this 347 and swap to a 2.88 axle. I'd like to see what mileage I can get out of it, but I also think a built 347 with good heads and a good roller cam will outperform the current flat tappet 400 in both power and torque. With the better engine, the change in rear ratio should result in no net loss of vehicle acceleration, while improving gas mileage, especially on the highway.

Also, the Summit listed prices for conversion roller lifters are incredibly high... like $500 for the kit. What's up with that? The AFR article budgeted $200, IIRC for the conversion lifters.

[The Dark Side of Will]

I think building a motor for fuel efficiency is silly. How much will you spend on the rebuild? $2k minimum. OK, now how much gas mileage do you need to make up to make that back in less than 2 years.

Push the repayment window out to 100K miles...
At 12K/yr that's 8.3 years. $2000 over 8.3 years is $240/yr
At 20 mpg and $4/gal, fuel cost per year is $2400. Saving $240/yr is only 10%... From 20 mpg to 22 mpg. That's available through compression.

Considering that the short block that's in the car now has ~300K miles on it and the top end has over 250K, 100K is a conservative estimate of repayment period.