April 2012 Chevy High Performance Q&A

Kevin McClelland Feb 22, 2012 0 Comment(s)

Fuel Economy Testing

My ’69 Chevelle currently has a 454 bored to 4.280 inches, a four-bolt main, Eagle SIR rods, and Federal-Mogul pistons, L-2349F 030 with 0.210-inch dome. The heads are GM 781 castings, ported and polished, with 112cc chambers, and 2.19/1.88-inch valves. The compression with this combination is 10.2:1. I’m using a COMP Cams XE 294H Xtreme Energy hydraulic flat-tappet that specs out at 250/256 duration at 0.050-inch lift. It has a max lift of 0.588/0.593 inch and is ground on 110 centers. The intake is Edelbrock Pro-Flo injection. The engine builder estimated horsepower about 550. Would that be close?

This is a 100 percent street car. Isn’t the cam a little steep for street use? I’m currently running a vacuum pump for the four-wheel power disc brakes. Also, the engine shop told me to use only Pennzoil 25W-50 oil. Why that particular grade? I have to order it as no one stocks it. Can I use another grade?

What size exhaust should I be running on this car? I currently have 2-inch Flowmaster dual exhaust on it. When I removed the tailpipes I saw a big difference in power.

Also, gas is coming out of the gas tank vent lines—it doesn’t start right away and after driving it 10-20 minutes the leaking starts. It is a stock-style tank with sender mods for the return line for the EFI. I thought at first it was due to the O-ring seal leaking at the sending unit. It has slowly gotten worse in the last year, and now I am losing about a half cup fuel per minute out the vent lines! I contacted Edelbrock, and they had no real answers to my problem. I switched to a vented cap and it barely helped, and I even ran without the cap. I removed the tailpipes, thinking that maybe it was super-heating the tank somehow. That made the biggest difference, but I still needed the vented cap. When there is more than a half tank of gas I would still leak, but nowhere as much. I installed new vent lines and believe that they are 5/16 and 1/2 inch. I extended the lines further up in the tunnel hoping that would help. I have the purge valve but just found out that the flapper is missing. Could that be the whole reason behind the leaking? I checked the fuel pressure and return line pressure, and they both checked out, even though Edelbrock doesn’t have specs on return line pressure, mine is 3-4 psi. I only checked the pressure while idling. I would have to get a fuel isolator to check while driving. I went to different forums and no luck. I have been fighting this for four years now and not really getting anywhere. Any help would be greatly appreciated.

Michael Labus
Menomonie, WI

Your nasty Rat can’t be getting great gas mileage to start with. Then dumping a half-cup of fuel on the ground every minute must really shorten your driving range! You’re lucky you haven’t caught the thing on fire if you’ve been dealing with this problem for four years. Hopefully we can give you a few tips to remedy your problem.

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