How much mpg do you loose adding horsepower?

Question

I have a 1987 Chevy 1/2 ton pickup with a 350 throttle body, 4 speed auto (700r4), 3.64 gears, 37 in tires, stock motor besides headers.

I get a little under 10 mpg now. I plan on rebuilding another 350 with throttle body to replace this one that will be built for mild power. I want to cam it, redo the top end, put forged parts, double chain, bigger throttle body rebuild, intake, ported, typical power adders in a 350....and I was considering boring it over like .10 or .20

My question is....what kind of loss of mpg will I be looking at with those type of power adders or possibly with a bigger bore, or possibly hookin it up to a turbo 400 (of course i will loose highway mpg without fourth gear, but I figure Ide just throw that idea out there). So if you have experience with this kinda thing or have a car you repowered or built up...maybe you could tell me what mpg loss you took.

Any advice appreciated.

chevyraceman_383 · Accepted Answer

If done right it's the other way around.

Stock engines like that has poor Volumetric Efficiency (VE) at around 85% or less. 

If you keep build mild, matched and better flowing heads, compression, intake, exhaust, etc your VE will come up to around 90-95% and you will gain fuel mileage 

Keys to this is keep parts light, and keep stuff matched together for the RPM you will be driving in. 

Keep your 700r4, no need in the monster th-400 unless your pushing over 500 HP. The overdrive is worth about 30% fuel mileage. 

No need in forged parts for a mild engine. Even at 400 HP or so and 6000 RPM shifts a new scat 9000 cast crank, hyper pistons, stock rods and ARP rod bolts will be just fine. 

Heads is the major thing here. You need vortecs or better aftermarket heads like brodix IK 180's or AFR's. The gain the better heads will give you is more than worth the price they will cost you.

? · Answer

If an engine gains torque at the RPM you cruise at, then you gain mileage, not lose. TH400 transmissions will cost you a significant rear wheel HP loss compared to your 700 overdrive, btw.
At and below 5,200 RPM, a gain in torque will also be a gain in HP at that same point. 

For what you are doing, perhaps consider a Big Block.In regards to your 1500-2500 RPM cruise range which is primarily where you are going to spend 99% of your time driving/cruising, a Big Block is much more suited to that area than a Small Block because of where it makes the numbers. Just a thought. 

Intake port air velocity spells torque. Therefore, the shape of the intake port of your cylinder head is far more critical than how big it is. The bigger it gets, the faster the engine will need to spin to keep the air speed in the port up(especially in ports with tight turns) unless you are dealing with a raised runner head which typically has very gentle turns inside the ports, and that keeps air speed up. For your 350 and your specific goals with it, nothing will touch a factory Vortec head for the price. Not even close.

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How much mpg do you loose adding horsepower?

2 Answers