Thread: Gen I 350 - Performance & Gas Mileage?

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In the realm of the possible? An impossible dream? Pure lunacy?

I know that the what manifold / cam question gets raised frequently. Sorry to do it again. I have a moderate amount of knowledge on the SBC but I just can't seem to make up my mind on cam/manifold combo. Perhaps you can help me.

I'm going to build a 350 for a 1970 Nova with stock rear gears, turbo 350 w/ stock converter, 600 holley vac secondaries, ~9.5 CR, HEI, headers and duals. Won't be revving much above 5500 RPM.

The engine will have to deal with a stock bottom end and the stock heads with new springs and a inexpensive clean up and perhaps some light porting for the time being (will pick up a set of brodix IK 180s in the following summer season).

I've got the cam selection down to Comp Cam XE262H or XE268H and manifolds down to the Performer EPS or the Performer RPM (likely not the air variety of each).

I hope to use the car regularly during spring, summer, and fall with only the odd trip to the track so I don't want to totally kill my gas mileage. I had a 396 in a 1970 Monte Carlo and just couldn't deal with the gas consumption.

It seems obvious to me that the XE262H would be the best for around town driving between the two but, as silly as it seems, I want a rough idle because the sound of these classic cars really adds to the whole experience of driving one.

Seems like I could go with either the EPS or RPM with the 262. The 268 looks to be better matched with the RPM (or would it not be so bad to use an EPS).

Last, would there be much of a difference mileage with between the two manifolds and between the two cam choices?

Any other tips, outside of the obvious, for achieving better mileage?

I really appreciate you feedback.

Cheers,

J

Just a suggestion, but if you're after moderate performance and still get some decent fuel mileage, how about using a junkyard takeout 5.3 or 6.0 LS engine and trans??? Around 300 horse, and great mileage.... With a mild cam and freeflowing exhaust the performance and mileage improve....

The LS engine would be great for my purpose and it is certainly something that I would work towards. For the moment though, it is not in the budget.

In sticking with the Gen I 350, what advice (manifold, cam, etc.) would you have regarding my original post?

Ultimately, I'll try to keep things light with aluminum manifold and H2O pump (eventually heads) etc. and keep my gearing a bit taller. 700R4 for the overdrive would be nice too.

Thanks again guys,

J

If you don't have any engine parts yet---think about getting a 350 out of a 96 or later pickup/tahoe/suburban---they made excellent power, decent milage---carb manifolds are available


But---lopey idle sound does not equal milage

Thanks for the advice guys. Seems like the LS is well recommended around here. I would want to stay carbureted if I went with the LS motor. What makes the LS so much more efficient?

I'll have to live with the gen I for a couple years though, so I suppose it's best to avoid spending too much on it and simply install a cam and manifold and clean up the heads a bit. I was hoping to get some advice within these constraints.

Your best mileage gain will be a overdrive trans. Also, if economy is important, dont use too big of a carb, it's just dumping unburned gas out the tailpipe. Use the carb size formula: The math formula is: CARB CFM = Cubic Inches x Max RPM's / 3456 x VE%

Max RPM's for a good street small block is probably around 6000. VE is Volumetric Efficency. A basic stock 350 is about 75% efficent while a well built small block can achieve 100%. Work it both ways and it'll give you idea of the range of sizes you're looking at. The goal is to completely burn the fuel as much as possible.

That would put me in the range of a 587 cfm (100% VE) to 783 cfm (75% VE) carb.

Was planning on going with a 600 CFM vac. secondaries.

In a former combo in my 69 nova I got close to 20 mpg with a 2.56 gear and th350. The motor had maybe 9 to 1 compression 76cc smog heads with cheap flat tops. It also had a 268 high energy comp cam 218@.050 454 lift for both intake and exhaust. on top was a Performer Rpm with a 625 Carter Afb. It ran 15 flat @ something like 88mph(this was going through traps in 2nd gear) It was by no means quick but the mileage was good for long road trips. This was on a 350 bored .030. hope this helps

That does help. If I can get ~20MPG and be in the 14's that would be fine by me.

Cheers,

J