Next, if you really want to get down to it, if you replace the cab on your '77 with the '73 cab and leave the complete running gear from the '77, it'd be considered an engine swap into the '73 vehicle. The engine has the VIN stamped in the deck from the '77? If this is the case, a smog tech could require you to have all the emission control systems in place and functioning for the '77 powertrain. This is a stretch, but that is the law.

As we said in the beginning, the answers we got were probably just as gray as the answers you got from the CHP officer. You should have enough information to make a decision which way you are going to go. Good luck with your project.

Cubic Inch And Power Range
Q: My question is about operating range and cubic inches. I've read that for every 50ci displacement increase, an engine's operating range will drop 500 rpm. If I have a 350 with a Comp Cams 256, can I go to a 400 and run a Comp Cams 268 basically in the same operating range? Thanks for all the great info you give all of us.
Paul Oterski
Antioch, IL

A: This is a pretty good swap if you're not going to change the cylinder heads, intake manifold, or exhaust header sizes. If the components are sized correctly for the increased displacement, the power peaks should fall very close to the same. Of course, you would see the attending power increase. Going from 350 to 400 cid is a 14 percent increase in cubic inches. If you have stock cylinder heads on your engine, we'd say that a 400- to 500-rpm delta in power peak could occur. If your cylinder heads are not a limiting factor, for instance, if you have aftermarket aluminum cylinder heads that are more sized for your 400, you wouldn't see as large of shift in rpm range.

Back to your camshaft change to compensate for the displacement change, the smaller camshaft in the 350 produces great slow-speed torque. The 400 with the larger camshaft raises the torque peak rpm and hurts the slow-speed torque by increasing the duration and overlap area. The 400 will give you a boost in torque in all ranges because of the displacement increase. This would mask most of the torque loss from the larger camshaft.

In the end, it's a pretty good deal what you've come up with for shifting the powerband. We agree with most of the logic for engines equipped with smaller camshafts. When you have cams over the 230-plus range at 0.050-inch tappet lift, your rpm shift is out the window by then.

It's A Truck Kinda Month
Q: I have been getting your magazine for some time now and have used some of the things you've had stories about on my pickup: an '04 1500 Silverado four-wheel-drive with 67,000 miles. It's a 5.3L with a TH-4L60E tranny. I have put 0-ohm plug wires on, an aftermarket mass airflow sensor, a 1-inch spacer plate, a cold-air kit, and a K&N air filter, and I've removed the Onstar fuse. Wow, talk about waking up a truck. It really will run hard if you want to, and it's not hard to hit 5,000 rpm or more. In addition, the mileage went up to 22-23 mpg, which I like too. One problem I have is that the check engine light will come on with 87 gas, so I burn 91. It will still come on in trailer mode. I have read the code and it says the left or right bank is too lean. The light will clear itself, though.