1996 Camaro Z28 Engine Modifications

One of the first things I did was to junk the stock exhaust. The Borla cat-back has a larger than stock intermediate pipe (3" vs 2 3/4")  and a free flowing muffler. The system is adjustable by means of by-pass baffles. There are 5 different baffle plates that let you select how much of the exhaust is going through the muffler and how much exits through the unmuffled passenger side exhaust pipe. As for power, it adds about 7-12hp. That might not be a whole lot of gain, but the gorgeous sound makes it worth it.

The stock airbox is restrictive on purpose. It's the main reason the LT1 in the Camaro/Firebird was rated at 285hp vs the 300hp in the Corvette. Several companies offer better airboxes. I chose the Moroso version. It also features a conical K&N filter that sucks air from the wheelwell. The airbox adds about 15hp.

The above 2 changes improved my 1/4 performance from a 14.0 @99.4mph to a 13.8 @101mph.

Next I got more serious. I took the cylinder heads off and gave them a moderate port-job. I did the porting myself. I highly recommend goggles and earprotection. Trust me on this one. As for what I did, here's a breakdown:

Thorough pocket port.

Improving  the short turn radius (intake port only).

Reshaping the valveguide to a teardrop shape.

Widening of the intake port where it's narrowed down due to the pushrod.

Gasket matching of the intake port. The intake manifold was left untouched to decrease reversion.

Removing sharp edges in the combustion chamber. This was done to prevent detonation with the 11:1 compression ratio.

Headgaskets from a Impala SS were used as they are thinner than the F-car gaskets and raise compression half a ratio. The stock 10.5:1 compression was thus bumped to 11:1. It runs happily on 93 octane (but I won't try anything less). I kept the stock valves. No machining was (such as milling) was performed as the heads were still straight.

A stock LT1 head measures 157cc intake and 63cc exhaust. I ported mine to 175cc intake, 68cc exhaust. This isn't a very big increase. The reason for this is that I feel there is nothing to be gained by hogging the LT1 heads out unless you're going for really serious performance (12 flat and faster). Most 190cc+ heads just decrease low end torque without giving any horsepower gain until you're over 5500 rpm. As always, your mileage may vary, but this is what I've found.

A set of Competition Cams 1.6:1 Pro-Magnum roller rackers replaced the stock 1.5:1 stamped steel units. As the pro-magnums are non-selfaligning, guideplates were needed. I used a set of Manley stepped plates, but I'd recommend using the flat-style instead. This is because the stepped style requires much more modification of the internal re-inforcement braces inside the valvecovers. I also installed a set of 7/16" ARP studs. Beginning with the '96 model year, GM started using non-hardened pushrods, so I also had to get a set of hardened pushrods.

With the 1.6 rockers, the stock springs get alittle too close to coil bind for comfort, so I opted for a set of Crane #99846 springs. Some people that have used these springs have had problems with the damper binding to the valveguide and actaully had the damper break, so I don't recommend these springs. On my car they've worked great, though. Sofar I have 15k miles on them with no problems.

I figured better breathing was needed so I got a 52mm throttle body. Mine is a bored-out stock unit rather than the BBK one pictured. I don't feel the throttle body made any real improvement.

An Airfoil... What can I say? It did make a small difference and it wasn't very expensive... Check out the dyno page for a dynotest I did.

I installed a MSD6A and an Accel coil. The stock ignition is very good, so the upgraded ignition isn't really needed, but it did manage a small power increase as shown on the dynotest page.

After I ported the heads, my peak power was at 5600 rpm, only 200 rpm before the factory rev-limiter. That and the 4.10 rear gears I installed made me get a Hypertech Programmer. It let me set the revlimiter to 6400 rpm and corrected my speedo for the 4.10 gears. It also made a small power increase as seen on the dynotest page.

With the 6400 rpm revlimiter, the power steering pump gets to be a problem. I've heard of alot of PS pumps biting the dust with increased rev-limiters. The solution is to install a underdrive crank pulley. I guess there might be a small power increase as the accessories are underdriven, but since the waterpump is gear driven and thus unaffected by the pulley I suspect the power gain is modest at best.

I also did some small mods such as bypassing the throttle body coolant passage. The throttle body has coolant routed thru it to prevent icing in really cold climates. Well, Texas isn't one of those really cold climates, so the coolant is not needed. By bypassing the coolant, the throttle body gets somewhat cooler and might decrease intake air temperature.

Finally, I relocated the IAT (Intake Air Temperature) sensor. The stock location is in the intake elbow where it picks up alot of underhood heat giving a falsely high reading. By relocating it to the K&N filter element it will read correctly. I did some dyno experiments and noticed less of a power drop as the engine heated up.

Here is a table comparing a stock LT1 with the engine after the modifications. I'm using a 17% drivetrain loss to estimate flywheel power.

The stock numbers are from several stock LT1s I've seen on the dyno. It seems GM is slightly underrating the LT1 by 10hp and 20ft-lbs.

It should also be noted that although the peak torque is at 4200rpm on the modified engine, the torque curve is very flat and is at 330 ft-lbs at the rear wheel at 2400 rpm, only 7 ft-lbs less than at the 4200 rpm peak.