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I found this on LindseyRacing.com...it's a good starting point...hope this helps...Royce~

15 Top Upgrades for Porsche 951
This list includes most of the available performance upgrades and their order of importance. It concentrates on the less expensive items first.

Before doing any upgrades, we highly recommend some basic maintenance first such as replacing the timing belt, boost pop off valve, and check the intake system for vacuum leaks. Inspect your belts and hoses for possible problems.

1.) Conical Air filter: cost $155.00
The first thing to consider is a good conical air filter. This is one part that you need to decide just how far you intend to take your mods, some other performance upgrades come with a conical filter, such as a MAF. If one of these is in your immediate future, save a few bucks and apply it to your MAF.

2.) Lindsey Boost Enhancer: cost $89.95
This product is bar none the best bang for the buck available for the 944 Turbo. It makes a significant improvement in the turbo lag present in all 944 Turbo’s. Again, this product needs to be considered in an overall look at what your intentions are for the future. The turbo lag is caused by design characteristics in your factory wastegate. Replacing the wastegate eliminates the need for the boost enhancer, see part 4 of this list for more explanation.

3.) Chip Upgrade: chipsets range in cost from $299 to $349.
Chipsets do several things to improve performance, they make changes to the fuel curve, increase the engines timing and raises the engines rev limit. Depending on other upgrades, they can increase the boost level as well.

4.) Wastegate with manual boost controller: cost $399 + $65
This is probably the best money you could spend in improving the overall performance of your car. It does two very important things, it greatly improves the turbo lag and helps to control the boost drop off that occurs with sustained high boost conditions. If this upgrade is within your budget, the Boost Enhancer will not be necessary. Look for a direct bolt-in unit, requiring no exhaust modifications. With this configuration, you will have the ability to adjust the boost level from as low as 8 pounds to as high as 23 pounds.

5.) Adjustable Fuel Pressure Regulator: cost $159.95
If you follow this sequence of upgrades, once you have the ability to turn up the boost on your motor, you need to know what the fuel mixture at all times. An adjustable fuel regulator in conjunction with our Air fuel monitor will help you maintain the correct fuel mixture.

6.) Instrumentation:
There are several items that are included in this category. This is one area that many people tend to ignore. Without proper gauges, there is no way to accurately access the changes you are making and to know whether you might be doing damage to the engine.
6a.) Boost Gauge: Cost $50.00.
The factory boost gauge does not have small enough graduations to give you a good feel of the boost pressure, especially above 15 pounds.
6b.) Air Fuel/Injector Cycling Monitor: Cost $99.95.
This product is more of a safety issue, as you make performance upgrades to your car, the need for instrumentation to monitor the engines settings becomes very critical.
6c.) Fuel Pressure Gauge kit: Cost $59.95.
This gauge combined with an air fuel ratio gauge will allow you to properly tune your fuel and help you to know when it is time to move up to larger injectors.
6d.) Wide Band O2 (oxygen sensor) Cost $450.
This is the Eugo 1000, one of the best and best values in high speed tuning. This unit has a digital readout that is easy to read and when combined with our Mafterburner digital fuel controller, you can read the A/F ratio live in the tuning software.

7.) Intercooler Upgrade: cost $375 to $575
This upgrade reduces restrictions in you engines air intake system and allows the turbo to spool up much more quickly as well as lowering the air temperature of the charge air. We recommend our Stage I upgrade for 2.5-liter streetcars with a stock or one up Turbo.
The Stage II is for cars with 2.8 liter or more displacement with big turbo’s. If you buy a Stage I you can trade it in on a Stage II.

8.) Exhaust system. Cost $469 to over $1500
This is a critical part of your performance upgrade, there are a large variety of exhaust systems for both the 944 NA as well as the 944 Turbo. Even with a 3” exhaust system, with a large enough turbo and a free flowing enough intake system, you can still have dangerously high backpressures. We saw backpressures as much as double the boost pressures and more.

9.) Mass Air Flow: Cost $1199
There are a variety of Mass Air Flow units available. These units cost in a $699 price range and can be tailored to the size of the engine and the intended HP output. The key to a successful MAF upgrade is the use of a piggyback computer to allow the driver to fine-tune the fuel curve (A/F ratio) of your particular components and local atmospheric conditions. Data logging that includes Wideband O2 output is critical to tuning properly.

10.) Turbocharger upgrade: From $899 to $1175
There are a variety of turbocharger upgrades. From rebuilding the stock uint up to turbos big enough to hold 23 psi of boost. The important thing in picking a turbo is sizing it for the peak boost you want to maintain to readline. The bigger the turbo the more boost you can maintain, but also the later in the rpm band the turbo will start making power.

11.) Fuel Injectors: cost $400
There are several standard injector sizes, 42# 48# 55# 72# 83# and 93#. The stock injectors are 36#. The two larger inejctors are typicall used in engines with engine managements systems with sequenial rather than batch fireing. When to do this upgrade will depend on what combination of parts and the desired boost level being run. Don't get fooled into going with too big of an injector with the notion of being able to upgrade into it later down the road, that rarely works. An Air Fuel Monitor will help you in deciding when this becomes necessary.

12.) 3” down Pipe: $399
The 3” down pipe is not necessary on all turbo upgrades, there comes a point in the sizing of the turbine wheel and hot housing where there are benefits from this upgrade.

13.) Cylinder Head, O-ring and Port and Polish: cost $499 to $2300 +
Cylinder heads are offered in various stages. A Level I head gives you a complete rebuild including new guides and seals. The head is solvent and sonic cleaned, inspected for dimension, flatness and ceramic port damage and measure valve springs. The deck surface is milled for flatness and the finishing touch, a precision cut 5 angle valve job. This option typically costs $499 plus valve springs if necessary to replace.

A Level II head included everything described in the Level one but includes porting the intake port, which takes the intake port flow from 185 CFM at the Level I to the 235 CFM. There are other options such as O-ringing the deck surface for added cylinder sealing for higher boost applications. Another very successful upgrade is our Hi-velocity exhaust which increases the speed of the exhaust leaving the head from under 400 feet per second to the 550 fps range, that is a 40% gain! The higher velocity exhaust helps the turbo spool faster and improves cylinder filling.

A Level III head takes it one step further and increases the size of the intake valve. This brings the flow numbers of the head up to the 250 CFM range.
You can also get new factory valve springs with Titanium retainers to fit them.

14.) Solid Lifters: Cost $917
This is a recent product released by Lindsey Racing and is an exclusive product to Lindsey Racing. These light weight, strong and durable lifters replace the heavy factory hydraulic lifters. They machine inspection ports in the cam box to make it possible to measure the valve lash without removing the cam box. They also adjust the height of the valve tip on the cylinder head, measure and calculate the exact lash cap necessary to run your particular camshaft. The kit includes 8 solid lifters, 8 lash caps, 8 inspection port plugs and plug seals, and all measurements and machining. All of this for $917. This must be done in conjunction with the cylinder head off the car to allow for tipping the valve length.

15.) Camshaft: Cost $450
There are several companies offereing camshaft upgrades. Look for one which gives good power throughout the RPM range with a nice aggressive idle. The cam is not a real benefit unless you have done a good head upgrade.

Lindsey Racubg is working with Web CAm to offer special Solid lifter cam upgrades to work in conjunction with their solid lifter package. They have cams for both 2.5 and 3.0 Liter applications, and offer a cam for more midrange torque, one for a good over power band, and one with a power band that comes on after 3500 rpm, but will pull to well over 7500 rpm.

14.) Intake Manifold: Cost $1300
Lindsey Racing offers a totally new intake manifold. This unit is cast aluminum with 4 matching intake runners, a plenum just large enough to supply equal airflow to all cylinders but not so large as to cause any lag. This manifold flows more than 25% more air than the stock manifold and flows equal to all cylinders. The factory manifold has a huge variation between cylinders although this does not become a big concern unless you have installed a high flow ported head. This manifold is not for everyone, it is intended for high horsepower application and is capable of supporting flow to over 8000 rpm.

15.) Stage V Intercooler: Cost $1199
This intercooler flows more air and cools better than the stock unit by far. It produces an additional 20 degree drop in inlet air temperatures as well as a drop in flow restriction. This translates to quicker spool up and more top end horsepower.