I suspect that the scavenging effect on a boosted application is far less than on an NA engine. On an NA engine with a larger cam, scavenging can be up to 75% of the engine's breathing ability (the exhaust sucks much harder on the intake port than does the descending piston). But a turbocharged engine with the restriction in the exhaust might be different to some degree.

Moreover, scavenging is generally a higher RPM phenomenon (or in a CTD case, high boost). Since many CTDs are running higher TIP than Boost pressure at WOT, the scavenging effect is minimal or at worst negative.

 

Scavenging is non issue. It's allowing all the exhaust to exit instead of forcing some of that to be recycled in the cylinder by way of after though EGR.

Cam designs are based on meeting emission requirements first and performance second.

The new the truck, the better a cam performs. As emission standards tighten, so do the requirements for reducing pollutants.

By adding lift and duration to the intake event, we can help fill the cylinder more completely. When that valve closes has a direct impact on cylinder pressures. Overlap is commonly a gasser way of looking at determining dynamic compression and thus, fueling and airflow requirements. However, most aftermarket gasser cams are pretty "small" when used in turbo charged and or supercharged applications.

The rules change a little in charge air motors. While improving scavenging has a benefit at true zero boost, anything after that is getting force fed. After all, we are putting a minimum of 2 atmospheres in the cylinder and modded trucks a few more. So, we really can do more with less. By adjusting the cam exhaust cycle, we can still get great off idle performance when we are running on the vacuum side of the motor and still let the cam get out of the way for boosted applications.

A cam is one of the best ways to spend your money. It helps the engine breath easier at all throttle settings. Not to mention the effects of better milage and lower EGTs due to introducing a clean, cool charge of air into the cylinder. In other words, a cam is like a bigger turbo in many cases. By taking good quality charge air from a compressor operating in it's efficiency range, we put more of that volum into the cylinder than without the cam.

Since these motors cannot get too much air, we have more oxygen content to burn when the fuel is introduced.

There are cam designs that are counter productive, but they are usually designed for the application they are used in. Be sure to match the components to your needs. This avoids disappointment down the road.

Dave

 

I'm not trying to argue, but, what if the stock cam provides enough lift=open time to allow the cylinder to completely fill with fresh air? If that is true, you will gain nothing with a cam. I don't have a cam like you so I can't compare the two, I'm just raising questions that need to be answered to determine the net effect of a cam change.

 

Great post, Dave

... and Blue - if the stock bumpstick allowed the cylinders to completely fill with air, there'd be no need for aftermarket cams like the Helix, which always have a net benefit in terms of spool-up, EGT reduction and power under the curve.

 

Completely? That assumes there's some kind of ceiling of filling that cannot be exceeded.

This isn't a black/white thing where you either get all the air or you don't.

Rather, it's a continuum where more is always better.

OEMS actually discourage maximum air ingestion, because the leftover exhaust gases in the cylinder slow the rate of combustion and reduce NOx emissions. The whole idea is to limit efficiency, in a sense, because the same conditions that produce optimum efficiency (advanced timing, lots of clean air) also have the highest emissions, and thus emissions considerations kill cam performance, as Dave said.

First, the cams in the factory engines often have improper event timing-- for example, intake valve closure is intentionally delayed too far in hopes of reducing the amount of fresh air trapped-- more fresh air speeds combustion and increases emissions as it lowers EGT.

The intake valve also tends to open too early, letting exhaust get pushed back into the intake tract, which dilutes the incoming air charge.

Note that both of these are a form of "EGR" in that exhaust gases are retained in the cylinder for the purposes of reducing emissions.

It's really no different than swapping an OEM cam for a performance cam on a gasser. Even with a mild cam optimized for the same RPM range, EVERYTHING gets better: torque, mpg, response, etc because the aftermarket is only constrained by emissions considerations on those parts for which they are seeking a CARB E.O. or similar certification.

Justin

 

Good conversation guys.. I am using "gas" technology backround, all be it natural gas as I work mainly with stationary NG engines.. So the turbo charging vs cam I am aware of. Even with different models of say the Waukesha engines from their standard VHP Turbo engine to the Lean-burn version, the cam profile is mostly affected in the exhaust duration and overlap of the intake. And like Justin brought to mind, there are cam "tricks" used to try to beat the emissions and still provide adequate output power.

Another example engine is some old Superior engines that also were upgraded from old rich burn technology to lean burn. Just by changing the cam profile, the boost air pressure can be doubled on the same displacement engine and EGTS's drop dramatically (NG engines behave like diesel, lean=cool EGT). The overall flow through the engine was greatly affected, just by the cam.

I am still a believer in the aftermarket cam, just on the points of efficiency alone.

 

Let's say your example is correct. And BTW, it's not far off based on the timing parameters of the intake lobe. Since these are charge air motors, the VE is way up no matter the intake cycle. But, the timing does play a role here.

More importantly is that exhaust valve timing event. As Justing pointed out, that's where most of the issue lies. If you cannot get all the spent gases out of the cylinder, you cannot fill it completely with fresh gases.

Take a 1 gallon container. Fill with a fluid or gas. Dump out .8 gallons. put 1 gallon of fresh fluid or gas into that container.

Basically, that's what is happening in the cylinder. Yes it is to reduce emissions. It also serves as a RPM buffer within the engine. So, it's more than getting good air into the motor. We have to get as much air in as possible.

This is why some cam profiles with more overlap can better fill those areas with oxygen at a lower boost threshold. The down side is lower combustion efficiency at lower RPMs depending on the intake vale timing and the lobe centers.

The heart of the matter is that most aftermarket cams make the engine better able to make power with less incoming charge. Many times, a cam swap will yield lower boost numbers, lower EGTs, better mileage and faster spool. It's all about where that energy goes and how fast it gets there.

Dave

 

Nice thread guys, thanks that's some good stuff.

- Craig

 

No but I had electric fans so it was hot either way

 

so the best mod for lowering E.G.T. is an upgraded cam shaft? where do I buy one? this driving around at half throttle for fear of high temp is for the birds. I've got the snap-on timing set on the way.

 

Somis, I think there's more to it in your case than just a cam swap.

First, is the combination of auto trans, 3.54 gears, and 315 tires. That's all but a guarantee of EGT problems even with a stock truck. The larger turbo isn't helping if it's laggier (most are).

A Cam will help, imo-- but if you're hoping for a miracle, don't be too surprised if this isn't the Immaculate Reception.

 

This got turned into a cam thread, I have one and I would say F2D4U is more right. It has been awhile since my cam went in, so my memory is not fresh, easily 150-200F drop in WOT EGT as well as around 4 psi drop in boost. Boost cruising is zero.

Jim

 

Are you trying to say, IC inlet temperature vs IC outlet temperature?

Jim

 

Bump your timing before you do anything else. I may be an ignorant cuss but I predict that bumping your timing will be the most cost effective approach to lower egt's. You should also notice a little bit more fuel economy.

 

I put a water to air intercooler on my 91 D250,before the intercooler the charge air temp was almost 600* at 35psi(max boost),after the intercooler the charge air temp was 120* with a max boost of 27 psi.The charge air temp dropped 480* and my EGT did not change.but I went from a 15.08 to a 14.49 in the 1/4