Intake Restriction
 

The last thread about intake restriction left out the throttle body. Before I got my 914 2.0 I was driving an 1.8l 8v Scirocco. The water cooled VW's have the dual butterflys with a progressive linkage opening the small one first.

What seems strange is that the larger butterfly is apx the same size as the 914 TB and its only supplying a 1.8l engine. The 2.0 VW's use a TB with a much bigger main butterfly, which is also used as an upgrade for the 1.8 claiming about 5-8hp.

Going on this, I have been considering a TB swap on the 914. I believe one of the best options I have come up with to keep the TPS is to get a second TB (off a VW Bus maybe) of the same size and build a split plenum to fit on the existing intake runners.

Does this sould like a viable option? Would it yield a big enough increase in power to make it worth the trouble? And how would the MAP sensor react?

BTW My engine is a 2.0 with stock FI, mild cam, larger valves and lightened flywheel.

--Alex

Check this link: http://members.rennlist.org/chuxter/#ThrottleBody

Similar idea--larger throttle area. Also scroll up a bit, you'll see a note about a modified stock throttle body.

I know of someone who is in the planning and material acquisition stage of a 914 CIS setup and will probably use a Rabbit throttle body. Similar idea...

--DD

If I was going to do it over I would look at this: http://www.millerfi.com/


TMK
73 914S 2.1L

Thanks for the sites, sounds like the idea is worth looking into. Now if I go ahead and build a new split plenum for dual throttle bodies, which two cylinders should have the same plenum?

How 'bout throttle bodies? Are any of the VW Bus TB's matches for the 914 one?

Opening up the intake sounds like a good idea, but keep in mind that if you change the runner volume or the plenum volume, you can significantly change the shape of the torque curve. This is due to a resonance that occurs when the valve opens. There is pressure wave that bounces to the plenum and then reflects back to the valve. At a certain rpm, the reflected pressure wave helps push more air into the cylinder, increasing power. Changing the intake runners or plenum will change the rpm where this occurs, changing the shape of the power curve. Porsche took advantage of this in later cars by having a valve change the size of the plenum, essentially having 2 rpm's where you get the boost.

I was able to gain 15 hp at a particular rpm on a motorcycle engine by tuning this. Got another 10 by tuning the exhaust the same way.

Well, I'm going to keep the stock intake runners for simplicity sake, so its really just a matter of plenum volume. I'm looking for low end power, like 2500-3000 or so. I'm debating just using the stock volume and just splitting it into two chambers, again, to keep things simple.

Now if I were to change the plenum volume, would I want to go larger or smaller? And would this actually give me more power, or just move it around in the rev range?

The plennum and runners are designed to provide enough volume for the engine to breathe at higher rpm, but still be small enough to provide the necessary velocity of air at lower rpm. Too big and the engine will "bog" at lower rpms, resulting in a loss of low end torque. Too small and the engine will be choked at higher rpm. It all depends on what you will use the engine for, and how it is built. I know everyone has heard this before, but it is important enough to repeat: all engine components must be matched to work together properly! Decide on what you want the engine to do, and what you are willing to give up. If you want a high revving, race motor, then increase the size of the plennum and runners, or shorten the runners. If you want an engine that is smooth enough to run on the street, leave it alone. Again, a larger intake will not help (it will hurt) unless the other components (cam, valve train, heads, exhaust) are properly matched to flow the correct amout of air. Eliminate the bottlenecks so that everything works together.