Increasing compression on 350 chevy?

Fuel has to be vaporized before it gets into the cylinder, for it to be of any use.

The Quadrajet is a superior fuel atomizer. What kills 72 Quadrajets is that they were a second year emissions calibration. You should have no problem getting 17 MPG out of that truck with 308 gears when things are set up right.

Compression will increase efficiency.

Tightening the piston to head squish clearance will raise compression but lower octane and spark advance needs. It's combustion efficiency. Another very smart move for you is a pair of stock Vortec heads with your flat top piston change. Deck your block and use a fel pro 1094 head gasket. It will run with 28-30 degrees total timing like that.

Until you change the IFR's, PVCR's and put about .026 air bleeds in that Holley, it'll never get good milage. IF it does not have annular boosters in it, then it will never vaporize/atomize well, and so I would not use it.

Several other adjustments to gain power and milage for you, but this will get you started.

Get rid of the SMOG cam, as well. It's killing cylinder pressure, which in essence is destroying the dynamic (effective) compression.

camshaftshaun@gmail.com

Chevy 350 Compression

It sounds like you know a bit, so here is something: Carb size isn't going to change efficiency. Not sure how Holley managed to scam the public with that one, but if you run an engine lean (which is all you can do with just a carb change) you will destroy it. If nothing else the difference in tune is what made MPG better. Quadrajet gets excellent MPG when tuned correctly. The Quadrajet's only came in two sizes, and the only difference was in the primary side...250 CFM vs 300CFM. Again, the engine has to suck in as much fuel as it needs, if you aren't into the throttle heavy, the 500CFM secondaries are meaningless in regards to mileage.

Where are you measuring those MPG's? If that is combined city/freeway, or city, you aren't going to improve on that. Sorry, you won't, at least not in a way that makes $ sense. It's a truck, starting and stopping uses massive amounts of fuel because of it's mass.

At ~8:1 compression you shouldn't be using mid-grade, you should be using the low grade. Resistance to detonation doesn't help things, in fact it can hurt since ignition is harder.

GM reduced compression because high combustion temps increases NOx. Once EGR was introduced, and electronic means to control timing better, compression started to increase again.

Those cylinder heads are GARBAGE. A set of Vortec heads would immediately increase compression with no additional mods. They also flow far better, so better power.

Dome pistons are also garbage, and should only be used as a last resort if there is no other way to increase compression. They interfere with flame propagation and quench, both of which when optimized allow higher compression without increasing octane requirements.

Here's real-world examples: Stock 305 (9:1 compression was stock), carbed with a quadrajet, OD trans, 3.08 gears. Stock 350 (~8:1 compression), carbed with quadrajet, no OD manual trans, 3.42 gears and 33" tires. 350 with Vortec heads (9.6:1 flat top pistons), injection off a 1989 Camaro, roller camshaft, headers, no OD manual trans, 3.42 gears, 33" tires. Me driving every single one, the exact same chassis with each drivetrain change.

Best economy ever is 18MPG, freeway only. NONE of the modifications from the last one made any noticeable difference in economy. Power is increased, driveability is increased, but economy didn't change, because trucks are heavy vehicles with bad drag numbers. And that was carbed or injected...same economy freeway, MAYBE a hair better in town, but not enough to be worth the cost.

You could spend a boat-load of cash and increase efficiency a dozen different ways and I doubt you will get an additional 2-3 mpg. Most attempt efficiency gains then lose it with a heavy right foot. Save the big plans for later. Keep that truck and never let it go. Don't go crazy with modifications because so many will look and say; "I'll bet it was nice before he started messing with it". A Plain Jane gas guzzler is way cool over a toyed up truck. Put some Headers and FlowMaster 40's, maybe some better heads and a intake manifold.

Suppose you put $500 into it and raise the mpg 3 or 4 pts, still sounds terrible. I'd get a used Honda car that was engineered from start to be efficient.

Think headers and free flowing exhaust. That WILL improve the mileage.

what really kills it is 72cc heads