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Magazine Feature

Gm throttle body injection article from Trucks magazine


GM TBI - Camshafts and Cylinder Heads

OK, now we will talk about camshafts and cylinder heads!

NEW!!! This is unbelievable!!!

We have tbi performance kits for GM's 330hp Vortec Crate Motor*!

 Rated at 330hp at 5,000 rpm and 380ft/lbs torque at 3800rpm** , this engine will make you smile from ear to ear!!!

This engine features - Vortec cylinder heads, 9.1: 1 compression, 4 bolt block, +  a 212/222 at .050" hyd. cam, .435/.460 lift, and 112LSA..

Available for all GM tbi powered vehicles equipped with the above engine, these kits include:

  1. High flow ported and blueprinted 5.7 GM Throttle Body - $229 + core return
  2. High flow and matched pair of injectors - $250 outright
  3. Custom computer program for most GM Tbi Full Size Trucks - $250

Please call for your application.

*Engine is available thru all GM Performance Parts Dealers.
**Tested by GM with a  4bbl carb and intake.

Okay, we just had to mention that. Now on to the info.

If more modifications are performed ie: cylinder heads and/or slightly larger camshaft computer re-programming will be necessary. Why? The engines breathing characteristics will be much different than stock and the engine's vacuum levels may be changed from stock.

 The GM Tbi uses a MAP sensor and programmed tables in the computer that correlates the engine's vacuum levels to a specified amount of air that should be entering the engine. If the vacuum levels are lower than stock, the computer believes that the engine is flowing more air and is under more load. It will then reduce timing and add more fuel. This is why an incorrect camshaft can confuse the computer and cause many drivability problems. Now, say the stock cam is used and  better flowing cylinder heads are used, the vacuum level will increase.  The computer will then lessen the fuel added because it believes it is under less of a load and less air is entering the engine.  This is exactly opposite of what you want to happen! More air is entering the engine while the computer is lowering the amount of fuel added! A double whammy! Not good.

Any change in camshaft, cylinder heads, and/or cid will almost always dictate the need for re-programming and in many instances require a larger injector. Note: not all engine modifications can be programmed for. There is only so much you can do with the GM computers programming and only a certain selection of injector sizes available for the tbi unit.  Because of this, it is imperative that you speak to the chip programmer you are going to use (Hopefully us!!!) before you change the engine, camshaft, cylinder heads, and/or the cid.  It would be horrible to spend all that time and $$ to find out that a program cannot be made for it. It happens!

Note: a higher compression engine can tolerate a larger camshaft than a lower compression engine can. Why? A larger camshaft bleeds off cylinder pressure - a smaller camshaft raises cylinder pressure. Higher compression adds more cylinder pressure - lower compression lowers cylinder pressure.  Keeping cylinder pressure within programming limits can be a fine balancing act. 

Note: a camshaft that has the same specifications as another camshaft are not equal. Again, they are not equal! Sure, the spec's for duration at  .050" , lift, and the Lobe Separation Angle might be the same but what about the duration at .004", .006", .010", .020" and etc, etc? What about the velocity rates lifting and closing the valves? This effects vacuum and performance!

Confusing? Maybe - but it really isn't. Basically, don't try to re-invent the wheel here.  Before you build an engine, replace the cylinder heads, and/or replace the camshaft call your chip programmer(s) of choice , discuss your needs or wants, and find out what they have successfully written programs for.  A certain "package" that has been successfully used will usually be a much better choice than one that has not been used and/or proven to work.

 

sales@cfm-tech.com

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