Heres a question for all you theoritical types:

with respect to "STOCK" cars/trucks, if you do things to improve H.P. of a given motor should not the fuel economy go up also ???

so in theory, if you remove the stock restrictions of factory performance limiters such as exhaust manifolds,intakes,carbs,exhaust systems and replace them with high performance parts you would be allowing the engine to breath better and make more power using the same amount of fuel, thus, requiring less fuel to maintain a desired speed than was required prior to the modifications.

this would apply to all cars/trucks providing you didn't get to the point where you started using more fuel.


discussion ???????

 

Maybe only if the modifications made increase horsepower in the normal operating or cruising range.

 

How is youre enging running right now in terms of its air to fuel ratio? Because for however much air that is the air/fuel ratio should be quite good. If you start allowing for things that will have more air coming in youd have to use more fuel. On the other side of the car it might make sense. Exhaust wise id see it being a good thing getting the excess stuff out of there asap because youd end up with a better burn....probably worse for emissions but better for fuel economy.

 

Actually, the stock manifolds do help gas mileage a little bit. Just the way they are designed, some of the fuel that ends up going right through to the exhaust without being burned can get forced back in by the backpressure (i think) that all crappy manifolds have. Headers don't do this, they will just suck it right out.

Most of those parts that you listed actually make more power by being able to feed the engine more fuel and air without drowning it. That is why when you go to a bigger carb or a set of headers, you gas mileage will go down. Headers prevent unburned fuel from being forced back into the cylinder by the exhaust backpressure. A larger carb will allow the engine to suck in more air and, to keep the A/F ratio correct, more fuel as well.

The theory is good, and it makes some sense, but its just not quite that simple

 

An engine that is modified to be more efficient will also provide more power at a given intake flow. However, increasing the flow through an engine to make more peak power doesn't necessarily make it more efficient. Efficiency is the amount of useful power that can be extracted from a given fuel charge. By simply increasing the amount of fuel/air charge than an engine will move, the peak power should theoretically increase, but efficiency could remain the same or decrease.

There are means to make an engine more efficient and powerful, such as:

• Increasing the dynamic compression;
• Improving the combustion duration and mechanical transmission of power to the crankshaft by incresing the connecting rod length;
• Reducing internal friction;
• Reducing the mass of the rotating and reciprocating parts;
• Retaining heat in the engine so that combustion produces more gas pressure;
• Managing intake and exhaust flow through changes in valve size rather than valve lift;
• Other engineering steps.

Many things that race engines use to produce maximum power will also provide improved efficiency. At a given rate of fuel burn, those modifications can provide more power at the crankshaft and drive wheels.

 

Could you elaborate on this? I am looking at a cam for my SBC that has more lift than is actually needed. The engine is a torque motor and fuel economy is a big concern. Valve sizes are 1.94/1.50

 

What type of cam is it? Most aftermarket camshafts that are designed for low end do increase power and tend to make major improvments as far as fuel economy. The best cam Ive found is a Melling MTC-1 RV cam, a few others market this cam as thier own also.

I noticed with the 70's LT1 cam grind I'm running that is supposed to make peak tq @3000 RPM if I drive @3K my highway milage jumps to 21+ MPG even though vacuum leves are still lower then I'd like.

 

It is a mild Comp # 08-500-8 hydraulic roller with an advertised duration 258/264 and duration @ .050 206/212 with a 112º LSA lift is .480/.488 with 1.5 rockers heads are L98 iron versions with mild porting(cleanup/blending), a nice valve job and a 30º backcut on the valves. The induction is TBI with an Edelbrock performer TBI intake. I dont feel the induction will support .480 lift

If you read anything by David Vizard his theory is "the more lift the better" and " a short duration high lift design makes more torque"

If one is running a cam with more lift than the heads/induction will flow, will fuel economy suffer? Is too much lift a bad thing?

Thanks

 

Compared to the stock, straight stem 1.94/1.50 valves operated to 0.500/0.500" lifts, 2.055/1.6 valves will give you more free area and flow at only 0.450"/0.450" lifts. The 2.055/1.60" valves will flow 2.66% better on the intake side, and 5.33% better on the exhaust side at 0.450 than the 1.94/1.50 valves at 0.500". The real beauty is that at all of those lift points below the peak (where the valve spends most of its time) there is even more of an advantage for the larger valves. At equal lifts, 2.055/1.60 valves will flow 14.1% and 17.0% better, respectively. A 2.02 valve will flow 10.1% better than a 1.94.

Restated, the total area under the curve plotted by the cam lobe will strongly favor the larger diameter valves, and the increase in flow (and power) will be measurable. Valves spend very little time at peak lift, but a lot of time getting there and back to the seat. That's where the bigger valves will really outperform the smaller ones.

Since the engine is going to expend less energy opening the valves to a lower peak, you not only get better flow but more free power to the crank instead of it being eaten by the valve train. You can even use a slightly lighter rate spring to control the valves, freeing more wasted power.

If you are anxious about going that large on the intake valve, even a 2.02" intake valve at a 0.450" lift will flow nearly the same (99.04%) as a 1.94" valve at 0.500" lift.

Given your stock valve sizes and your provided cam specs (0.480/0.488), the flow past 2.02/1.60 valves will equal or exceed the available peak flow of the stock valves at only 0.437" intake and 0.417" exhaust lifts. Install a moderate (about 0.450/.0460) cam and you'll flow a lot more with less wear, less energy waste, and less valve float.

In a torque engine, the shorterer duration will maintain good lower RPM cylinder pressure and filling, and the engine will be able to flow more at higher RPM due to the increase in free area alone - short duration or not.

It's a very old concept that far too many people have forgotten or choose to ignore. Everyone used to install larger valves instead of stupid-tall cam lobes, probably because roller cams were so expensive and rare. I've even fit non-Chevy valves in Chevy heads to make more flow. Pontiac and Ford Windsor valves fit well, and keep the geometry pretty close to optimal. They're available in larger sizes than the most common SBC replacement valves.

Clear on that now?

 

"Install a moderate (about 0.450/.0460) cam and you'll flow a lot more with less wear, less energy waste, and less valve float"


What you say here is exactly what i have been thinking all along.

Thanks for the informative reply, it helps alot