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Engine Technical Questions in Plain Language

Q: How Do I Check Piston to Valve Clearance?

Recommended procedure to check piston to valve clearance

1.     Assemble the crank, 1 rod and piston into cylinder number 1

2.     Install one intake valve spring and one exhaust valve spring (with an OEM spring if available).

3.     Place the head gasket you plan to use on the block, place the cylinder head on the gasket.

4.     Torque down the cylinder head (since this is only a test fit, you may wish to use a spare set of OEM bolts)

5.     Set piston #1 to TDC.

6.     Depress the intake valve down until it lightly but definitively contacts the piston and record this distance.

7.     Now do the same for the exhaust valve until it touches the piston and record this distance.

8.     Subtract your valve lift at TDC from the piston contact distance.  This will give you your piston to valve clearance.

For example; if the intake valve has 10mm of travel until it touches the piston and the lift at TDC is 4mm then you will have 6mm of piston to valve clearance, this is acceptable. Typically, we like to see 2.5mm Piston-to-valve clearance or more but tolerances can be tighter for race engines.

Q: Why has VAC chosen CP Pistons as its piston of choice?
As one of the biggest suppliers of high performance BMW engine and racing parts in the world, we could have chosen any company to provide forged pistons for our engines, but "good enough" is not enough for us as a company.

CP Pistons has shown us time and time again that their talented staff of engineers and technicians produce top quality products the first time, every time.
Their products have worked flawlessly on countless race engines, rally engines, turbocharged and supercharged engines, as well as NA, and daily drivers. From mild to wild CP has never let us down and delivers the goods.
We work closely with them to make sure our product delivers exactly what we say it will for each application and at a competitive price.
If we don't make a product in house we search until we find the best; with CP Pistons, we look no further.

Also, CP includes rings, pins, and locks in all there pistons sets. We probably don't stress enough the quality of the included accessories, however, they are top notch and we can always trust there product. Again, CP Pistons kits and accessories always provide excellent results, we need change nothing.

We won't name names but we can assure you, we have played the field of the piston world so you don't have to! Quality product is more than just words.  The bottom line is: not all forged pistons are created equal!
Q: Why Chose to Fit VAC Motorsports Valve Springs?

Beware when you are shopping for Valve Springs: Many brands are out there... how do you know what you need?

First, a little simplified tech background. There are three common types of Valve Springs in the BMW market with one design we feel being superior in high performance use to the others...

First is single spring. Single spring valve springs are inexpensive to make, engineer, and offer large margins for the manufactures. The get the job done for most engine in OE form. Enough said.

When it comes to Dual Spring Valve Springs, the first design consists of a Large Outer Spring and a Smaller Inner Spring that DO NOT come into contact with each other. The second design consists of a Large Outer Spring and a Smaller Inner Spring that DO touch each other, also commonly known as an Interference Type Valve Spring. Both will work but the Interference Type Dual Valve Springs have clear advantages over the former in high performance use...

While they both have the advantage over the single-spring-per-valve configuration (that if one spring should fail, the other spring can help keep the valve from dropping) the Interference Type Dual Spring configuration is designed so the two springs will dampen each other, that is preventing the springs from vibrating in their own resonant frequencies which often lead to valve float and severe valve train failure, piston to valve contact, etc.
The dangers these frequencies pose on a hydraulic lifter engine are especially serious; the hydraulic lifters can tend to pump up during such a situation and over extend the valve which will almost always lead to Piston-to-Valve contact.

Please note that VAC sells only Interference Type Dual Valve Springs unless otherwise requested by the customer. Our design was co-created with a Formula 1 valve train expert, and all VAC Valve Springs are specially designed and manufactured to VAC specifications. All batches go though thorough inspections and shot peening process to insure extremely low failure rate, minimum fatigue, and high consistency, thus leading to the highest reliability and performance possible.


Beyond these details, uprated valve springs ensure your valves are as controlled as possible during high RPM use. If you have fitted aftermarket camshafts, we highly recommend upgrading to aftermarket valve springs. This will ensure that in the event of an over-rev or mis-shift, you can avoid failures that are more common than they should be, simply because the camshaft manufacture states that the cam is OK for use with the OE valve springs.

Q: What camshaft/s are best for my goals?

If you have a street car, a ''mild'' cam or cams tend to offer excellent gains across the powerband vs. stock, and tend to play well with the factory ECU.
In the same token, in a track or race vehicle relatively mild cams are also more forgiving of the driver as they can seem to make power ''everywhere'' Vs. only a t peak RPMs.

For the experenced driver or extreme street car longer duration, high lift cams move the torque (power) band into the higher RPM range. These cams have a narrower RPM ''sweet spot'' where they are effective, but produce a lot of power in that range. These would be great on the track, but note that many aggresive cams can feel a little flat on the street and may make the vehcle idle less smooth or even ''bumpy''.