How important is valve overlap in a turbo engine?

Overlab is for the most part bad in boosted engines.

 

Sure. The same rules all apply boost or no boost but with boost you tend to lose air and fuel out the exhaust valve during the overlap period. Turbos tolerate more overlap then blowers as trubos cause have higher pressure in the exhaust, but a gerneral rule more mild cams are prefered for boosted engines. Was that your question?

 

Valve overlap on turbo engines may allow a cooling effect on combustion chamber thus reducing risk of high temperature and detonation. In fact you devote a portion of the pressure air generated by the compressor to escape during valve overlap and to "flush" the top of the cylinder , including valve ports. Furthermore on racing applications the fresh fuel-air mixture sweeping through the cylinder may burn in the exhaust duct and expand in the turbine. That effect produces the typical flames out of the pipes and may be coupled occasionally with acceleration of the turbine, which in turn may be welcome after cornering. Valve timing/overlap is accurately designed by engine manufacturers to match the desired characteristic of the engine in terms of torque output, fuel consumption , emissions and durability.

 

Fuel burning in the exhaust is very ineffiecent and will fry the turbine. It's best to burn the fuel in the combustion chamber and let the coolant cool the combustion chamber.

 

The correct answer depends on a lot of little things in the engine.

Valve overlap is the means by which the headers and the intake velocity stacks end up working together. The header resonance needs to be a sub-prime multiple of the intake resonance.

A header is a device with uses the (positive pressure and heat) energy in the spent exahust to create a big broad negative pressure wave back at the intake valve, brining in fresh mixture even before the piston starts downwards.

A turbo (basically) does not have a header. In order for the turbo to get all of the heat energy from the cylinders, the turbo is placed as close as practiable to the heads.

A supercharger is a positive (all the time) intake presurizer.

There are three kinds of pressurizers: Roots which operates as low as idle and have a linear flow with respect to speed, Turbine which have a quadradic flow with respect to speed, and Whipple which have flow like the roots, and efficiency like the turbine. The Roots is fundamentally an air flow generator, not an air pressurizer. Its original use was to create a positive air flow in coal mines so the miners could breath.

Both turbos and supers pressurize the intake system. Turbos extract their pumping energy from the exhaust gasses, supers from the crankshaft.

A: This adds power by carmming more mixture into the cylinder.
B: Part of this air-flow can be used to cool the combustion chamber parts--but this requires that the fuel is NOT injected until the exhaust valve has closed.
C: The pressurization is dependent on the kind of air pump used,
D: The pressurization is dependent on how much energy is extracted to run the pump.

So, depending on how the engine engineer wants the air-flow to produce power and how that engineer cools the combustion chamber parts, determines how much overlap is desired, and what dependencies this places on the other parts of the engine.

Since the turbo uses exhaust pressure to run the pressurizer, you don't want the cool fresh charge to cool the exhaust mixture (or the turbo will become inefficient). Thus, not so much overlap is recommended.

Roots and Whipple chargers can be used with header resonance systems, and are especially effective when the Fuel system can be timed to avoid spraying fuel down the exhaust system. These systems are mostly independent of the overlap, and easily mated to existing engine systems.

 

Normally a turbo system does use a pretty standard header. It's pretty easy to control the heat lossses and having tuned length headers is a pretty big net gain. When you see screwy exhasust set-ups it's normally more about packaging and space than hp.

This part isn't exatly right. Turbo system have pressure in the exhaust system which reduces exhasut flow so normally a trubo engine will want increased exhaust cam duration and lobe centers similar to naturally aspirated engines, leading to slightly more overlap.

A supercharged engine is just the opposite. With the intake pressureized and the exhaust not pressurize there is a huge tendancy to lose intake charge out the exhasut. The standard answer is to increase the lobe separation angle to about 114-115 which is about the highest you can really make work and then decrease the exhaust cam duration a bit.

When you are talking about modest boost (5-8psi) stock cams tend not to be too bad, but the problems increase as the boost goes up. For example on my 308QV with a roots blower at 10 psi I found I was pumping about 450hp worth of air and fuel to make about 360 hp. When I went to a screw type blower and intercooler at 23 psi I was pumping about 850 hp worht of air/fuel to make 550 hp....the rest was going out the exhaust. These engine are extreme examples becasue the stock set-up has pretty low LSA (about 107 degrees) and a head that flows about 20% more exhaust than would be ideal.

 

My 0.02..

Depending on application, less overlap is desirable when using a turbo.
In a turbo application you have higher exhaust port pressure than NA. With that, more overlap can allow backflow near TDC with contamination and heating of the intake charge. Often, lobe centers are moved apart for turbo applications for that very reason.

The problem is much greater in engines where a small turbo is fitted to improve bottom end power and reduce lag.

Trapping efficiency is king…a well designed free flowing intake system reduces the demand on a turbo and improves response… a well designed free flowing exhaust reduces exhaust port pressure… AND properly selected and timed cam(s) will efficiently trap as much uncontaminated charge, and produce the best, and most reliable power.

Basically for turbo, more overlap and narrower lobe centerline angles can only be successful when you free up the exhaust and use mild cams.

Personally, I tend to like higher lift intake, std exhaust profiles, rapid ramps, and wider LCA.

 

How are you measuring that? At a 75 degree lobe center I don't thin the engien would even run......are you talking about degrees of overlap instead of lobe centers maybe?

I think f1 karting is probably right too.....it depends on how much boost and how hot the cams are. With a stockish intake there shouldn't be a problem with flow reversal and I like stockish lobe centers. With big cams increasing the lobe centers to minimize overlap is probably a good idea.


What is it you are building?

 

When I had my 96 Camaro, single turbo, I did some testing with cams. Tried 3 cams on the dyno ranging from standard/reverse/traditional splits and extremely low to moderate overlap.

More overlap made more power every time and this was on a setup that turbine backpressure was double inlet boost pressure. The best turbo cams usually closely resemble a good naturally aspirated cam in my experience.

 

More peak power perhaps, but I would be curious to know more about the bottom end power, lag, driveablilty, and fuel consumption on each of those tests?

'single turbo' leaves a lot of unanswered questions as well..

 

Going from negative 16 deg overlap @.050 to zero deg overlap @.050 resulted in more power across the entire rev range, a greatly extended power range and no loss in transient response or increase in 'lag'.

It was a 357ci LT1 with an 80mm single turbo making about 775rwhp.

In it's final configuration it was 357ci twin 61mm turbos 850+rwhp and got 20mpg.

Many of the turbo cars we build in the 1150rwhp range that are street driven have relatively tight lsa's. Such as 232/232 dur @.050 on a 113lsa. Pretty smooth idle but still rev to 7800rpm.

Obviously like anything else cam specs are combination specific. Best thing to do is start with a goal in terms of power and driveability and spec out all of the components to meet the goal.

Here was my camaro-

http://www.youtube.com/watch?v=9fGQiIlyHTM

http://www.youtube.com/watch?v=Nm_A-Zl5d6k