Let's say you have a cam for:

• 5psi boost
• 10psi boost
• 15psi boost

How does increasing boost affect cam choice? Does LSA have to be increased? Does the exhaust side need to mimic a bigger motor?

I'm hoping someone can explain how boost affects cams.

 

I start by thinking of boost this way.

Atmospheric pressure is about 14.7 psi, no pressure is vacuum or 0 psi.

So if you start with a vacuum in your intake manifold, you have no pressure (0 approx.) when idling, as you start off you open the throttle and let air in and your engine is producing more and more power as you open the throttle and increase the air pressure until it's wide open and the pressure is 14.7 (or approx. 15) psi (ideally).

Now let's add some kind of compressor (air) and with the throttle wide add 5 more psi, lets call it now 20 psi instead of the 14.7 (15.0 approx.).

So we have gone from 0 psi to 15 psi to 20 psi in one smooth sweep and the engine obviously is making more power. So we add some more air pressure (boost) until we've added about another 15 psi to the approx. 15 psi that we started with for a total of 30 psi or twice as much as we started with before adding the compressor.

The engine adds fuel in proportion to the air flowing through so now we are burning twice as much air + fuel as without the compressor and getting about twice as much power.

So what do we have to do with the camshaft?

Basically nothing! Everything is working fine!

But now we think that maybe the valves are getting in the way of the air/fuel at the higher flow density (pressure), so we make the cam lobes wider to hold the valves open longer and we flow more air/fuel by this enlargement of the flow path and we make 20% more power.

However, because the intake and exhaust cams are close together there is some overlap which confuses the flow into momentarily going backwards, then forwards again causing dillution of exhaust and intake charge which degrades combustion.

So we move the cam lobes apart maybe 5 to 10 degrees (Lobe Separation Angle, LSA) and that seems to calm things down at the lower speeds and the engine is runs reasonably well at all speeds.

Now we think that if a little is good, more is better, so we make the lobes really wider and bigger (lift) and that gives us another 20% more power-WOW, but the damn thing won't run under 1500-1800 rpm and makes it hard to drive slow.

So we give it even more lobe separation angle and that just screws it up this time-less power and rougher running.

So we go back to 5 degrees of LSA and get a lot of power at high speeds, but proportionally less at low speeds and very rough running. So we reduce the duration of the cams back a little and we're running good.

Now we get the idea that we are forcing a lot of air in with high pressure but just letting it fart out with no special assistance on the exhaust side. So let's try a bigger exhaust cam than the intake-WOW that works but now it's running great at high speed and even worse at low speed again. But we decide to accept that situation as the sacrifice we must make to get what we want.

Now we decide that we are going to get a free ride by using exhaust and a tubine to drive the air compressor, but after the change it barely runs 'cause the exhaust builds up pressure turning the turbine and forces itself way back into the intake manifold really messing up the air/fuel ratio. So lets make the exhaust cam smaller and sure enough with a 230 degree intake cam and 220 degee exhaust cam the thing runs great and puts out almost 2 1/2 times more power than we started with.

However, the engine is only running well at very high speeds but we're tired of the frenetic pace of the engine so we go the other way and reduce the duration of the cam lobes and it is nice and smooth running and puts out a lot of power, but what is that tinkly noise at high speeds and full loads? Egad! It's detonation!

At the compression ratio of the engine with the boost now available, the dynamic compression ratio of the engine is too high and it's knocking. So widen up on the lobes and add a little more overlap and hey, we're back to good running/power on the top end and mediocre/acceptable performance on the bottm end.

So what is this shaggy dog story leading to?

Well most of the changes we made to the cam were made for other conditions other than directly from boost considerations, however all these parameters are interrelated and one thing changed typically results in other things changing too, maybe not the way we wanted.

If you think of absolute pressure of 20 psi instead of 15 psi atmospheric + 5 psi "boost", the relationships seem easier to understand. I didn't really answer your question and I don't think I can but I put out a lot of considerations that others can chime in on and help us all understand Boost vs. Cams

To sum up my feelings, if atmospheric pressure on earth was 30 psi absolute instead of 15 psi absolute, I don't think cams would look much different.

Roy

 

good information. I hate to ask ... do you know how to build a watch, just kidding

 

I think the short answer is NO.

7.0sc I'm kidding as well...good info.

 

We have made over 700rwhp with a stock cam. Could we of made more with a more agressive cam, sure, but the point being that you do not have to change the cam if you do not want to.

 

Great write up Roy!
In fact this was exactly what went through my mind when I decided to build my F/I vette.
You either want a H/C car or F/I but not both, now that being said, I will say a little increase in lift won't hurt if it has negligable affect on duration or LSA. (like 1.85 ratio rockers)
Another area that likes a little attention is the valve dia. and head flow characteristic's.
But again a lot is not better then a little in this case, too big an intake valve and your low end tq. suffers.
And any change in valve dia and head flow should be kept proportional.
IMHO

 

Simple.

The best place to build one is across the street from the girls dorm.
You just find a stable place on the sidewalk, open your 10 ft ladder, climb up noting the warning not to stand on the highest rung, get out your 6x binoculars and watch.

If you,re across the street, the police are less likely to note your presence.

Thats about the best watch I ever made and one of the easiest to construct! Hope that helps you.

Roy, U of Ill., '61

 

I remember reading in the magazines a lot in the late '80's about lots of 5.0 Mustang guys running 10 to 20 psi boost on stock cams and winning at the drags!

Roy

 

Sounds like experience talking! I have a 114LSA cam with a mild lope. Never have been able to get good boost numbers, and I've suspected the cam for a while. The previous owner couldn't make numbers with the same cam and a D1SC, so me figures 'try it again for different results'.

If I'm not mistaken, it sounds like more boost means milder cam. That would mean the stock LS1 cam is even better than an '02 LS6 cam once the boost gets high enough.

 

So what's the key to building high output blower motors? Strong components, high boost, and big exhaust? If a big cam is bad for FI, then what about porting heads or porting for FI?

I tossed my stock cam and was considering an 02 LS6 cam, but for high boost is a LS1 cam better? I'm running 12-14psi with stock manifolds, and have WCC/Melrose 22" 1-3/4" headers and 2-1/2" midpipe waiting for install.

 

The inlet tract, air cleaner, ducts, IC, MAF, throttle body, manifold, head ports, valves (in/out), exhaust system are resistance to flow.

A mechanical blower (belt driven centrifugal) is a dumb thing and just cranks air through mechnically dependent on engine speed.

You can think of the inlet tract of having resistance to this air flow.
The resistance is what builds up the air pressure (boost)!
Just like a very small copper wire builds more electrical resistance than a larger dia. wire.

The object of the game is not to build boost but to flow the maximum amount of air though the engine.

However, this is a compromise 'cause you need good flow velocities to properly mix the air/fuel, to properly fill the combustion chamber, to have a homogeneous mixture (swirl) in the combustion chamber, and to scavenge the chamber. So increasingly larger has diminishing returns.

You think of more boost as more flow, but as soon as you start to change the size of the inlet/exhaust tract items, that isn't necessarily true or even proportional.

In general, an overly large inlet tract will give lots of flow, but lower boost (gauge) and poorer mixing characteristics, and an overly small inlet tract, better mixting and lower flow/power, but higher and better use of the air/fuel (efficiency).

All the above is also a function of the required engine speed range, at 4000-7000 rpm, the big open flow tract may be the most powerful and useful. At lower speeds, 1000-4000 rpm, something much smaller may be the most efficient and usrful.

It's hard to have it both ways and the guys who do this compromise the best go the fastest!

But again, boost is not the most accurate measure of the power, boost is only proportional if you don't change anything on your engine. As soon as you change a valve size, port size, cam duration/lift, the relative boost changes, but the air flow is whats important and is not in the same proportion as before the change relative to boost.

So you strive to flow the most air through the engine in the most efficient way, but monitoring the flow by boost only is not exactly accurate.

The Corvette Forum is the best way I know to increase typing speed.

Roy

 

A turbo cam will have a completely different profile than a supercharger cam.

With a turbo, you have about 2-2.5: pressure ratio. Basically, that means that if you run 10#s of intake boost, you have 20#s of pressure on the exhaust side before the turbo.

You have to change the cam profile around quite a bit if you are going from one, or the other.

This is all in the name of power, stock cams work fine. Stock, unmodified cars work well too. They just didnt have the right power from the factory

Louis
www.LGMotorsports.com