K& N Air Filter

The K&N Air Filter is definitely the best bang for the buck. Fourteen horsepower (for real) for $60 US. Not only does it flow more air, it filters better. Every car on earth should have a K&N air filter.

Open Air Box

Like I said above, it all started with the new air filter. When installing the filter I got to looking at the air box. The stock configuration forces the air to go through a couple of 90 degree bends before entering the air filter. 90 degree bends kill airflow by adding a lot of friction. So I got out the tin snips and started hacking on my vette (a moment of temporary insanity). Fortunately it turned out just as planned. The stock filter cover has a small air inlet hole with a plastic cover on top. I permanently removed the plastic air cover and enlarged the small air inlet hole to the size of the entire air filter. I set up the plastic cover so it could be bolted back on in rainstorms. After several severe rainstorms without the plastic cover no water has gotten to my air filter. The project took about 3 hours. If you don't want to snip the stock filter cover you can purchase an open cover from Mid-America for around $50 US.

There are two ram air modifications that can be made. One removes the front license plate cover and ports air directly to the filter. The second turns the air filter around 180 degrees and opens a hole up in the fan shroud. I have not found any documented results from these modifications but they should add power at high speeds. I'm personally reluctant to do a ram air modification for fear of getting rainwater into my engine. Both modifications draw air close to the ground. I raced a car identical to mine except he had the ram air through the license plate. He finished the quarter 3 MPH faster. All our times and speeds were identical throughout the quarter until the finish.

This is more of a warning rather than a modification. The MAF uses hot wires inserted in a known cross sectional area to measure airflow. Higher air velocities will cool the hot wire changing its resistance thereby providing a representative voltage. Many people are suggesting that improved airflow will be gained by cutting the screen out of the MAF. I highly recommend that this NOT be done on F-body vehicles. Most people mistake this screens purpose as preventing debris from hitting the hot wires in the MAF (That is what the air filter is for). Actually the screen is a bank of straightening vanes used to straighten the airflow before passing by the hot wires. Eddies and vortices (turbulence) in the MAF will cause inaccurate readings. Straightening vanes are used in most mass flow measurement situations where 10 diameters of straight upstream conduit is not available. F-bodys intake does not meet this requirement. Straightening vanes are sometimes installed on the downstream side when 5 diameters of straight downstream conduit is not available. An accurate MAF sensor is critical for the engine's fuel/air ratio adjustments.

Quarter mile times on a friends 94 Camaro with the MAF screen removed were two tenths of a second slower. Replacing the screen resulted in an immediate two tenths of a second faster run. The Camaro, Firebird, and TA have a 90 bend right in front of the MAF. Testing has shown that removing the MAF screen on a Corvette helps, but some minor negative spikes in the hp curve will result. My 94 vette gained about 1 tenth in the quarter by removing the MAF screen.

Air Foil

The stock throttle body has two ports with butterfly valves. Between the two ports is an hourglass shaped pocket, which cause air turbulence. This is an obvious source of turbulence on the throttle body. An air foil is an after market part that covers the pocket and smoothes the airflow. Manufactures claim from 11 to 14 hp. The gain is not perceivable mainly because the car still sounds the same.

The down side? $50 to $60 US bucks for something that should cost $2. The hp gain seems exaggerated but when considering the possible hp per dollar, it is still worth the money.

Bypass Throttle Body Heating

Coolant (which is hot) is routed through the throttle body to warm it. This prevents frost from building on the butterflys during extremely cold weather. Unfortunately it hurts the car's performance the rest of the time. Bypassing the coolant flow from the throttle body can be done with a 5/8 hose or you can install a valve to shut it off. Normally the throttle body runs 200F to 230F (the coolant temperature). That heat is dissipated partially by radiating to the engine compartment and partially by convection to the air flowing through it. The heat convected to the intake airflow is lowering the density of the intake air and will lead directly to lower performance. I have seen claims of 2 tenths of a second gain in the quarter mile which would equate to 17 hp. I believe this is a 5 horsepower gain. After bypassing the throttle body I measured temperatures at 100F when the engine was completely warmed up. Temperatures were measured with an infrared thermogun. 100F ought to be sufficient for preventing frost buildup.

52MM Throttle Body

Larger TBs reduce the inlet friction, resulting in higher density air at the combustion chamber. The stock throttle body is 48MM (435 CFM). There are two options available here, 52MM (750 CFM) and 58MM (1000 CFM). However, don't be fooled. Your 350 engine will probably never flow 750 CFM and wont even get close to 1000 CFM unless you're planning on turning 12,145 RPMs, 9,109 RPMs to flow 750 CFM. It's simply a matter of engine displacement x RPM. You have to knock off 23% from the number due to the expansion of air in the intake (vacuum). With larger TBs you will lose throttle resolution of the inlet airflow. On a 58MM 100% of the air will flow by 3/4 throttle. Remember it's a throttle, and you want it to throttle accurately with good resolution. If your not going to modify the intake manifold then I recommend a 52MM throttle body. The stock intake manifold has 52MM ports in it that mate up to the throttle body. Unless you're going to make those ports larger it doesn't seem much good to install a 58MM throttle body. If you're planning serious, bigtime serious internal modifications then a 58MM may be the better choice. I'm running a 383 stroker that turns 7000 RPM and I'm still staying with a 52 MM.  Both sizes usually cost about the same. This option alone will add 10 hp and resulted in a 2 tenths gain in the quarter.  I went with the Edelbrock/BBK throttle body shown below.  The nice feature is the integral air foil between the two butterfly valves.

Music to my ears, with volume control!

There are several Cat-Back exhaust systems on the market such as Borla, Flowmaster, Dynomax, Tri-Flow, Power Effects, etc. I've heard from several sources that the Borla is difficult to bolt on (I do not have first hand experience). In one add Borla claims 80 hp gain which has to be a joke. No way! Running just headers only wouldn't give that much. Reasonably you can expect up to 25 hp gain. After evaluating all of them I selected the Power Effects System. The Power Effects has butterfly valves in the power capsules (mufflers) which can return the car to near stock airflow. Power Effects claims 55% more airflow where the next best is Borla at 40%. This exhaust sounds great. It gets better when the pipes warm up. Temperature must change the natural frequency of the pipes. It's not too loud. It's just right with a lot of base. The full open position gets bothersome after awhile on the interstate. Fortunately, it has the option to turn down the sound. I personally like the aluminum powder coated black pipes better than stainless steel. Stainless steel is begging  to keep it clean, which wouldn't be easy.  

What size exhaust is best? Bigger is not necessarily better. Larger pipes are at the expense of midrange torque and bottom end response. Large pipes are great for high RPMs, if you're there a lot. Here is a table of suggested pipe sizes from Flowmaster.

 Power__ Exhaust Pipe Diameter

HP______ Single____ Dual

200-300 ____3.0"____ 2.25"

300-375 ____3.5"____ 2.5"

375-400 ____4.0" ____3.0"

450-550 ____4.5" ____3.5"

The Power Effects has 2.75" diameter pipes. The car ran the quarter fastest with the butterflys wide open. Altogether, 2 tenths faster than the fully closed position.

Headers

I have 1 3/4" TPIS headers for the L98 installed on my LT1. The work great. Just throw away the Y pipe. There is actually more room for the spark plugs now then with the stock manifolds. The headers are ceramic coated and seem to help cool the engine. That is they pull heat away from the heads.

 Shorty headers are not headers. They are performance manifolds. They are less restrictive but do not take advantage of acoustics (or pressure waves to assist flow). Some dyno testing on the LT1 350 have shown that shorty headers do nothing and long headers gain 20 HP. Don't waist your time with the shorties.

Some say that 1 5/8" headers are better for the 350. To refine that a little better it depends on what you want. These headers are considered "small headers" with tubes 95% to 105% of the exhaust valve diameter. Generally these will make best power right above peak torque on the dyno curve. At higher RPMs they can be restrictive, relative to large headers.

 1 3/4" headers are considered "large" headers and benefit the motor at higher RPMs. I went with these headers because I have a 383 that turns 7000 RPM. That's a lot of CFM.

Starting in 1994 the Corvettes were manufactured with a programmable chip. The engineers at GM set fuel air mixture ratios slightly on the rich side. This is to compensate for lower grades of fuel. However, the Corvette is tuned a little closer than other vehicles. For best performance you want a stoichiometric fuel/air mixture. If there is too much fuel or too much air then the reaction is less efficient, hence less horsepower. This will also get incomplete combustion, which can lead to emissions of CO (carbon monoxide) and other gasses. Hypertech leans the fuel at higher throttles, which provides a better fuel/air ratio, which creates more horsepower. There you have it. More horsepower and more efficient.

Hypertech's 160F thermostat is also a great modification. The 230F temperatures were really worrisome to me. That is just too hot for an engine to be running. The thermo induced stresses are like scratching on a chalkboard. The stock engine is set to run hot for emissions reasons. The same culprit mentioned above, CO. Cooling down the engine does a lot for performance. A cooler engine means less pre-ignition in the combustion chamber. It also means denser air in the intake system. Both will create more horsepower. The Hypertech Power Programmer resets the fans to come on at lower temperatures in conjunction with the 160F thermostat. Now my car runs between 180F and 190F.

While on the subject of cooler engines, one of the greatest features of the LT1 engine is the reverse cooling. The coolant flows in the reverse direction of normal small block Chevys. That means the cooler coolant hits the critical part of the engine first, the combustion chambers. A cooler combustion chamber means less chance of pre-ignition, which means you can have a higher compression ratio. That's how the LT1 can have 10.4 to 1 and the LT4 can have 10.8 to 1(premium only). Everyone else's late model V8 muddles in the 9 to 1 ratio range. Thermodynamically, the efficiency of the internal combustion engine comes down to one thing, THE COMPRESSION RATIO. The higher the ratio, the more energy the engine can get out of the fuel.

The LT1 also has no coolant passages through the intake manifold, which leads to cooler intake air. Cooler air is denser which means a higher mass airflow.

The Power Programmer can also reprogram shift points (automatic trans), set the rev limiter higher, compensate for new rear axle gears or tire sizes, and reset the top end limit of the car. I set my rev limiter from 5800 to 6200 rpm on the old engine with stock rods. 6200 RPM is the limit for stock LT1 Rods.  Trust me, I found out the hard way.  With the new 383 stroker engine I have the rev limitter set at 7000 RPM.  It's designed for it.  Power wise if the engine revs quick to the rev limiter then it's set too low.  Just make sure that mechanically the engine can take it.

The down side? You have to run premium gas. If you own a high performance vette what the heck are you doing running regular gas anyway? Do you want to be fast or not? Running regular in a vette is a sign of opposing priorities.

Don't forget about the tires.  A good brand of tires can increase acceleration, braking, and cornering.  I'm running Firestone Firehawk SZ50. Rear R17/285/40, Front R17/285/40. Same size on all fours. Looks great.

I highly recommend these tires. They completely run circles around the stock Goodyears. I know because I've raced auto-cross on both. They stick better than BF Goodrich Comp T/As and appear to have a good tread life so far. They also stick well on cold pavement. The down side is the last time I checked these tires are not available in R17/315/35. The other down side is these tires grip so well they have eliminated my third gear scratches. (<- Old engine statement, now I can smoke em all the way through third :)

For autocrossing I run Hoosier slicks, 12 inches wide. My racing rims are 12" wide with a 7" backspace. They fit with no problems (for 92 through 96 vettes only).

Adjustable Fuel Pressure Regulator

An adjustable FP has a adjustable spring in it that sets the fuel pressure.. I wouldn't raise it much more than 3 or 4 psi. Fuel pressures can range anywhere from 37 to 47 psi. To high of a fuel pressure setting will prevent the fuel rails from purging vapor lock conditions back to the gas tank. In this case the fuel pressure would have to be set higher than what the fuel pump could generate. If you're going to play with the fuel pressure I would recommend that you do it at a shop where the car can be analyzed. I'm running 46 psig.

 Underdrive Pulley

A smaller pulley on the crankshaft will turn the power steering pump, air conditioner, and alternator slower. A larger pulley on the alternator slows it down even more relative to the power steering pump. This is good for 10 hp. My power steering pump has been susceptible to high rpms (5500 rpm). I've already have replaced the shaft seal once. Turning it slower has more than one benefit, in my case. You'll need to purchase a shorter belt with this modification. This is a good one to do when the engine is already apart for other work.