HP with fuel efficiency

ok ok ok I know you have horses, you need to feed them I understand that. HOWEVER...

when building an engine such as a nice 10.5 -1 roller cam aluminum head 482 type of build.... is there any specific "thing" that will really help out fuel efficiency? OR is it a matter of having the overall build as efficient as it can get, such as cam, torq converter, heads, rear gears all working well with each other? EFI makes a difference. Maybe not any single thing will make a change but an entire package deal. also sure it depends on the drivers driving style.

can a 500 HP or so car/engine/however you want to look at it get "decent" fuel mileage ? or are you looking at 6...4... MPG? again Im not loking to make 25 MPG.... Im thinking somewhere around 10.


I guess mostly this is about simply asking the question. If I build this engine and its getting say... 4 mpg highway/freeway with turbo 400 and 350 gears... for example. and I mention it and im told well if you would have done XYZ, or changed THIS on your cam to THAT or whatever... your MPG would have improved.

Again not asking or looking for a miracle, just curious if there are things that can be done with this kind of a build.

 

You are on the right track.
Efficiency and HP can be on the same plane. That is how newer engines can produce greater HP per CI and still get greater mileage than earlier engines.

But, a lot of that has to do with much improved induction, variable valve timing and overall engineering advancements. You can get a great deal of efficiency and HP from older engine designs by taking advantage of newer engineering, and a lot of "aftermarket components" as well as learned engine mods applied, however, there are limits as to what you can get from older engine designs.

Fuel injection is a biggie.

Correct gearing for specific driving conditions is another factor.

Along with building the most efficient engine from available parts without getting crazy chasing after really small gains, you can have a reliable powerful engine that gets good mileage.

Design on paper, and get as much information on achieving the realistic goals before spending a nickel.

 

HP and MPG do not have to be mutually exclusive. It absolutely has to do with the combination of parts and the efficiency of the engine. More compression means more torque, but can also increase MPG through more efficient burning. My '78 Cutlass Supreme got about 9 mpg when it had a 260 2 barrel in it, when I switched to a 350 Olds with literally over twice the power and no other changes (same engine design), it got 16-18 MPG easy. My beater truck used to get about 8 MPG when the engine was totally stock and rated at 165 HP. Now I've stroked it, put on aluminum heads, EFI, custom cam, etc. in it, and it makes about 400 HP and 500 ft.-lbs. and now it gets 14 MPG all day long with no changes to trans or rear gearing. Going from carb to EFI on that truck got me another 2 MPG. Efficiency.

 

We have a customer that we built a 502 for. It's in a 1990 as regular cap pickup truck....motor is about 9.5:1, AFR 305 heads, headers, roller cam, still runs power brakes so it's not a crazy cam, 1000 cfm 4150 Holley, I would guess its somewhere near 550-575hp.

Trans is a th375.......so same internals as th400, just a smaller output, 3.73 gears, has gear vendors overdrive, regularly gets 13 mpg on the highway, and is plenty fast when you want to light the candle.

Yes even on 1000cfm carb it getting 13 an all highway trips. Combo tanks get 10ish, around town only 6-7.

I think alot had to do with "how" you drive it.

But the more power a motor makes in theory the easier it is to do the job at hand......

I've done boat motors where an extra 150ho betted them twice the fuel mileage as b4, when driven the same spee . .....but they always seem to run it harder and are driving faster now that they can and see 0 change

 

The Dodge 707 hp Hellcat is rated at 22 mpg EPA highway.

 

Generally speaking stay conservative or middle of the road on everything and you'll get your best mileage. Go more rowdy if you want best ET at the track, you have to decide. Around town forget about mileage but 15 mpg on the highway shouldn't be a problem going middle of the road with 3.55's and a mild converter.

Higher the stall converter the lower mileage compared to a lower stall of the same manufacturer. Brand to brand may have efficiency differences between them.

4.10's worse than 3.55's, 2.73's better but acceleration worse. Overdrive would be great with 3.42/3.55's especially if it had a lockup converter. IMO 3.23's is about the lowest you can go with an OD and still keep the rpm at a high enough level to keep the engine happy. 3.23's is also about the lowest I'd go if I wanted decent mileage and snappy acceleration.

Once the cam starts to get lumpy at idle the mileage will start dropping. Maybe not much but the bigger the cam MPG will go down. 220-230 intake about the max there. Still plenty of power to be had at 230* intake in a roller. A 482 would easily make 500 HP with 230* intake, if not more.

Smaller the engine the better but that has left the station with a 482.

I would think that a carb to FI would make a big jump and be even more noticeable around town. I don't care who builds the carb it just cannot be better in start and stop driving. A Q-Jet or even better a Thermoquad will be better than a Holley double pumper because every time you crack the secondaries a bit the secondary pump is trying to dump fuel. The vacuum secondaries draw fuel when they open up a bit but don't dump. On the highway it is different when pretty much holding at a smooth steady throttle setting the carbs and FI differences usually get smaller.

Another factor which has nothing to do with the car is how fast do you want to drive on the highway? Stay at 55-60 and mileage will be better than at 65-70. I would think RPM between for 2000 and 2500 would be a good place for best mileage with a hot rodded engine. Cruising at an engine rpm lower than the cam(lumpy) likes definitely doesn't help or at least does very little for MPG.

So, to sum up stay middle of the road with cam, converter, and gears and you'll be fine out on the highway if you keep the speed below 70.

Now if we could just get GM to put all their resources behind a completely redesigned 700 HP 455 they could probably get the same MPG. I often said that about the 3800 Series I/II when they came out with it, too bad they didn't pour some R&D behind a 350 BUICK or even better a 455.

I should mention that your setup with the middle of the road gearing/converter/cam should easily be an 11 second combo.

 

Just re-read the original post: If you're looking for one engine design parameter that will help with MPG, as much compression as possible is the thing.

 

This is all great info. Makes me feel better about the parts I’m putting together.

Larry, my rear gears are 350 with a shorter tire (I keep forgetting to look and I can’t remember the exact size ) I’m also running the TSP 9.5” converter. Our combos seem to becoming very similar. Makes me happy you are getting the mileage you are.

 

I agree. As a pilot who flew an airplane with what we call a constant-speed propeller, I was very interested in what manifold pressure/RPM combination was best for economical cruise. After getting into engineering data (some of which used math above my head) I found that the higher the manifold pressure combined with relatively low RPM would yield the greatest fuel economy. What you wanted was a high Brake Mean Effective Pressure (BMEP) in the cylinder, but not high enough to cause detonation. The late Max Conrad set many distance records with his Piper Comanche using this method in the early '60s; at least a couple of which still stand - Casablanca Morocco to LA is one of them. FWIW his original destination was New Orleans, but when he arrived over that city he had so much fuel in his tanks that he went on to LA. He landed in LA with enough fuel to have made it to San Francisco, but aviation distance records are base on straight-line distance from the airport of departure to the destination airport. If you look at a globe, SF is about 10 miles further from Casablanca then is LA, so the 500 miles to SF wouldn't have gained Conrad much of anything.

 

There are a lot of paths you can take.

1. Sit back, decide how much fuel you can live with burning annually (for me that is never a factor). A car seeing less than 10K miles a year is not worth spending $2000-$3000 to gain 5 mpg.
2. Do your homework. Talk to people who have gone down the path you decide on.
3. Do a balance sheet on cost to obtain desired fuel mileage and hit the horsepower levels you desire so you know going in the costs and do not have to back up and redo things or park it because it it too expensive.
4. Tech today can be worth the price of admission on DD's.

Mikey

 

The engine has to run efficiently at your cruise rpm. Lugging a motor with a monster cam at 1800 rpm at 70 will get you terrible mileage. An old timer engine builder told me that the best cruise rpm for any engine is when the engine reaches maximum vacuum. That is its most efficient speed and will get the best mileage.