Different theory on turbo cams

[1fastII]

This was posted on another board I'm on, but I thought I would post it here to see what you guys think.

In a general sense, camshafts for nitrous and forced induction applications are typically ground on a 114 or wider centerline to reduce overlap and retain as much cylinder presssure as possible. A different theory was presented to me the other day that made incredible sense to me, but I had never thought about before. A supercharger is driven mechanically by the motor itself, so If you have a lot of overlap, your cylinder pressure is wasted out the exhaust, and you lose some efficiency. On a TURBO application, you're deling with a closed system. Meaning that a camshaft on a 110 or so allows some of the cylinder pressure through the motor, out the exhaust, and back through the turbo allowing for faster spool time and GREATER efficiency. In other words, cylinder pressure that escapes the cylinder because of overlap is not wasted out the tailpipe, but recirculated back through the turbo. This theory is being utilized VERY succesfully by the gentleman that presented it to me.

[DAVEZ84SVO]

Intresting, very intresting.. I'd love to see a few dyno runs to have that proven.. 110* is getting fairly ratical for a street cam.. It should have a noticably different idle, but it will be on the edge of running any vacuum things like power brakes.. My buddy has a 110* lob seperation cam in his 70 Torino with BB & has had some issues with the power brakes getting really stiff in stop & go traffic..

Dave

[Alex L]

exaust manifold pressures are normally 2x+ the intake pressure. (unless you're running a VTAN or ginourmous turbo) Charge air isnt going out the exaust during overlap, its getting blown up the intake.

see 'miller cycle' engine if you want to improve eff % based on cyn pressure.

[NavySVO]

This is totally new to me. The "energy" that goes out the tailpipe from valve overlap is not "recirculated", it is power lost. It is like having an exhaust valve that won't close all the way and gets pushed out of the compression stroke.
It sounds like he is trying to convince himself (and others) that it somehow excessive valve overlap somehow enhances the spool up of the turbo CREATING more energy?

I recommend you read a book by Corky Bell called "Maximum Boost". You can get it almost anywhere online.

He is the "Godfather" of turbocharging, and talks about this very subject, as well as numerous others. Minimum valve overlap is extremely critical.

[Alex L]

I second the recommendation of 'Maximum Boost'. It is a well written book, documenting all parts of a turbocharged system, explaining the theory and application.

Amazon

[NavySVO]

Amazon

If you are an eBay-er, www.half.com is one of their cronies. Your eBay account will work there, and you don't bid, you just buy.
People go there to sell books, CDs, DVDs, etc. Things there are rated from NEW-all the way to used condition. I got a new copy of this there -shipped- for around $15.00.
Shipping is around $3.00.

[SVOeric]

the thought is that the air going past the valves makes the turbo spin - but the velocity of this air would be nil compared to the velosity of the air coming out after combustion
-- if anything, it would slow the turbo down, becasue the air is moving so slow.
I'm not buying that one.

[65ShelbyClone]

Well, it seems to me that high overlap cams have been used with turbos before and more power was to be had with less overlap.

[Ted B]

Meaning that a camshaft on a 110 or so allows some of the cylinder pressure through the motor, out the exhaust, and back through the turbo allowing for faster spool time and GREATER efficiency. In other words, cylinder pressure that escapes the cylinder because of overlap is not wasted out the tailpipe, but recirculated back through the turbo.

First of all, a 110 deg LSA is only moderately tight for a turbo motor that makes power up to maybe 7000rpm. Tightening the LSA to 108 or even 106, with proper configuration and support, does indeed improve spool characteristics and midrange torque.

Obviously, this is difficult to ascertain with a single-cam engine like a turboford. However, with the EVO, I've demonstrated this time and time again. By tightening the LSA from 110 to 106, the midrange torque curve is bigger, and the turbo spools noticeably quicker.

This is due to the fact that mechanical efficiency (VE) is improved in the midrange with a tight LSA. A wider LSA tends to flatten the torque curve and broaden the powerband. Obviously, there are many factors that influence the results, but you'll get the general idea.

[under pressure]

Lets see your before & after dyno sheets. Turbos don't like overlap. In a normally aspirated engine the overlap helps with savaging, turbos and superchargers don't need it. Rick

[Ted B]

Turbos don't like overlap. In a normally aspirated engine the overlap helps with savaging, turbos and superchargers don't need it. Rick

Very well. But before I post the sheets, I am curious to see an explanation of your theory as to why turbos 'don't like overlap' and also why turbo and supercharged apps 'don't need it'.

[under pressure]

A normally aspirated engine needs scavaging to draw the air/fuel mixture thru the intake manifold to the combustion chamber. That's were the overlap comes into play. A turbocharged or supercharged engine has the intake manifold pressurized so it doesn't need the scavaging to get the mixture to the combustion chamber. Adding overlap in your cam in a turbocharged engine is just going to blow your mixture thru the combustion chamber and burn less of the mixture. Supercharged & turbocharged engines have been used for over 90 years. Look at the spec. sheet for cam lobe seperation on any turbocharged engine and compare it to a normally aspirated engine. But if it works for you go with it. Rick

[85ttops]

so does this mean i can put a dual plug head from a non turbo 2.3 on my svo and it will run better , spoll faster and go faster

[under pressure]

No to all of your questions.

[Ted B]

A turbocharged or supercharged engine has the intake manifold pressurized so it doesn't need the scavaging to get the mixture to the combustion chamber.

A very common misconception is that a turbocharger or supercharger is 'force feeding' the engine. This is false.

In reality, the engine isn't being force fed anything. The only thing a turbo (or SC) is doing is increasing the density of the intake air. It is an increase in air density, not volume, that provides the increase in power. Equally importantly, that increase in intake air density creates a simultaneous increase in exhaust air density, which increases exhaust pressure proportionally. The 'force feeding' theory would be true if the exhaust pressure was lower than the intake pressure, but this is false. The exhaust pressure is *always* equal to or greater than the intake pressure, and that is why the 'force feeding' theory is a misconception.

Adding overlap in your cam in a turbocharged engine is just going to blow your mixture thru the combustion chamber and burn less of the mixture.

This is false.

Excessive overlap becomes a problem in a turbocharged engine not because the air/fuel charge is being 'thrown out the window'. This could be true if the intake pressure was greater than the exhaust pressure, but that notion is fantasy. In reality, the potential drawback is exactly the opposite problem (reversion). In other words, there becomes a point whereby the exhaust pressure is great enough to force a portion of the exhaust charge back into the intake during overlap. Where this point occurs determines where overlap becomes a problem, and fortunately, is easily manipulated.