Re: 79 DOHC 750 Turbo project

Posted by turbocity tom on Nov 17, 2014; 5:10pm
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/79-DOHC-750-Turbo-project-tp4054442p4054478.html

Here is  a quick little video- nothing crazy or wild- just to show how the turbo works.

http://www.youtube.com/watch?v=aqzPuDY5s0k

I really like how it works- the bike was built to ride and not a racer.

Turbo 101

For those of you that are not familiar with how a turbo works:

The point at where a turbo “makes” boost is dependent on engine RPM, throttle position, and to a lesser extent, engine load.

There are certain design points around how the system operates also (size of wheels- A/R ratios of the housings) – size of the exhaust- size of the carburetor.

There is a lower RPM boost threshold. It is the point at which the engine does not produce enough exhaust volume to drive the compressor to produce more air than the engine is consuming.  (My system is approx 3700 rpm)

The position of the throttle (carburetor opening) will also limit the amount of air drawn into the turbo- allowing modulation of the boost at any RPM over the lower boost threshold.

The system will run both non boosted and under boost. The boost gauge normally swings to both sides (in a draw thru system).  When it is on the left side of the gauge, the engine is operating as a naturally aspirated engine- power levels will be very close to stock levels. As the gauge nears the 0 point (center) the turbo has begun to supply extra air to the engine. The power level is increased to around the max potential of the naturally aspirated configuration. When the gauge is to the right, which is where the turbo really begins to function. Typically, the power increase is approx 7% per pound of boost (may system is waste gate limited to 7 psi or approx 50% more power than stock.

At low rpm/ full throttle, the gauge will be at 0 (centered) and as the rpm increases over the lower rpm boost threshold, the boost will increase up to the controlled limit. (7 psi)

Under cruising conditions, above the lower boost limit threshold, the system will still operate under vacuum (left side of gauge). Now the boost level becomes a function of throttle position more than RPM. In the video, you can see the tach (rpm) change very little, but the boost will swing up dependent on throttle position. This is “power on demand” and is why turbos work so well for driven type applications.


Thanks for looking    Tom