Is a 300hp 20mpg Gen-I sbc possible?

[evan]

I’ve got a first generation 5.7L motor that I’m planning on swapping into my Malibu. About 18 months ago, with the help of a friend, I rebuilt it before putting it into my truck.
-stock crank (polished)
-.050 duration the cam numbers are 204/214
-rebuilt stock iron heads
-flat top pistons
-Edelbrock Performer intake
-Edelbrock Performer 600cfm carb
-Dynomax 1-1/5" primary headers
-Dual 2.5" exhaust

This engine in my 1/2 ton truck driving around town averaged 13mpg.


Is there a combination of parts that can produce 300hp and still maintain good fuel economy of at least 20 mpg? I’d like to leave the bottom end stock so I’m thinking specifically of
-cam
-heads
-intake
-carb/fuel injection

[68Formula]

It would be difficult (but not impossible) without overdrive. You also didn't mention your rear gearing.

Start with Vortec-style heads if you don"t have them already. Either GM Iron or aftermarket (EQ has some good heads, Edelbrock too). They are much more efficient than older heads (although you didn't mention the engine year so maybe you alerady have them).

FI or Carb is up to you, but it can still be done with a carb you just need to know how to tune it. If you go carb, probably the Edel Airgap Intake would be best.

Best carb is the one you know how to tune, because if you can't figure out how to tune it, it's never going to be optimal. And I mean tune all the circuits (idle, acceleration enrichment {pump shot}, power enrichment {power valve on a Holley}, main circuit, secondary circuit). If you only adjust the idle and main circuits for best efficiency without touching the others, you may get driveability issues or power issues when the other circuits become active. Spreadbores usually have smaller primaries which helps when you're foot is not into it. I'd probably start with the carb you have if you're already familiar with it. Another thing to watch for is make sure you're secondaries aren't active during normal highway cruise speeds.

Don't ignore tuning the distributor. You need to optimize not only the mechanical curve (rate and max), but the vacuum advance (rate and max) as well. This is where a lot of people miss out, and there is easily a couple mpg here on a modified engine.

For tuning at a minimum you should have a dial-back timing light (or timing tape w/standard light), vacuum gage (with enough hose to run it inside the car while you're driving), and a tach. A wide-band O2 is also helpful (don't bother with a kit that uses a narrow band, despite what the say they are not accurate once rich or lean from stoich), but usually cost around $300. Maybe you can split the tab with some friends.

A friend of mine had a 69 with a 383, Stock Vortec GM heads, and Airgap with 750cfm carb (don't know the cam) and put out about 310hp at the rear wheels. Very torquey and with the 700R4 on street radials he put down a 12.85 at about 107mph (shocked the heck out of us).

Good luck, let us know how you fare.

[JMarsa]

If you are wanting 20 MPG in a truck I don't think it's going to happen based on that motor. I have a Envoy that has a 300HP 5.3L (Gen 3) it's all aluminum and has fuel injection and an overdrive. I can bare pull 20-21 MPG on 4 hours trips at an average of 65-70 MPH. You have 2 strikes against you...the size, shape and weight of the truck and a carb.

--JMarsa

[Goatman]

Yep, big brick. Don't know of any truck that would get close to that. The car won't be much better. Even the LSx motors aren't pulling down 30mpg (in reality) and those cars are pretty sleek..

[evan]

I'm planning on swapping it into my Malibu.

Thanks for the info amcmike. I am using non-vortec heads, so I'll look into the vortec heads for the rebuild. I'm going to have to do some more reading about carb tuning to completely understand your post but it's all appreciated.

[TPI Monte SS]

Well, FWIW, when I had my old '87 Monte SS, that car would turn about 21mpg on the highway; that had a 180hp 305, with a 200-4R and 3.73s. I'm sure you could get the same mileage with a 300hp 350, as long as you kept your foot out of it. As long as the motor is efficient and you don't have too big of a carb dumping fuel into it, she should do OK in a Malibu.

[chicane67]

A 430hp, 355cid on 87 octane with a crab will get 20+...

Its all in the DCR, quench and cylinderhead.

[evan]

A 430hp, 355cid on 87 octane with a crab will get 20+...

Its all in the DCR, quench and cylinderhead.
That sounds good. Can you elaborate on it a bit?

[chicane67]

A few years ago... there was a peticular build up that everyone was really into pulling off. A 400+ hp small block that ran on 87 octane... and for obvious reasons.

Even the most basic build (that was a big part of a few of the magizine builds) ran a GM Goodwrench 250 crate engine. Its available for around $1150-1300 from places like Scoggins Dickey. Drop on a mildly ported set of TFS or AFR 195cc aluminum cylinder heads with a certain Fel-Pro head gasket, Edelbrock Performer RPM Air Gap, a relatively small hydraulic camshaft from Comp Cams...... and viola !!

425hp on 87 octane.

I did the same thing back in 97-98, before the magazines jumped in and with a few different components... I was able to make just under 450hp with the only change being the cam and valvetrain. I opted for a small soild cam (kinda near the same specs as the hyd) and Jesel shaft rockers.

Your MPG is going to have a lot to do with the weight of your car, the transmission and rear end gearing... and obviously... the way you drive. But, 20+ MPG is a very easy goal to obtain... if you get the rest of the drivetrain right.

[500hpc4]

I have a 512 hp 383 that puts down 23 average in town and 27 on the highway. I did better before my non-lockup torque converter. I use a Lingenfelter SuperRam with Edelbrock heads, a GM 849 cam and a built 700r4.