This is a post from STF a while back, thought some of you might be interested...

The Elusive Perfect Turbo Cam

Did I get your attention with that title? Good, because we are all here for the same reason, to make power with forced induction. Really forced or "artificial" induction includes various kinds of supercharger, not just the turbine-driven variety, as well as what I have learned to call "chemical" supercharging, including but not limited to Nitrous Oxide, Propalyne Oxide and even NitroMethane.

Each of these are mthods of getting more oxygen in the combustion chamber, because as we all know, you need more oxygen to burn more fuel, and burning more fuel makes more power, efficiency not withstanding.

But most of us around here are turbo nuts, so let's talk about valve train timing events with regard to turbochargers, when compared against normally aspirated engines, and perhaps even other methods of forced induction.

"I will write in blocks, so that my post won't disappear all at once like Joe Sumen's tend to do after he spends so much of his valuable free time sharing."

In reality I'm hoping to lure some of the big brainiac guys out of hiding, including Joe, Bob, Marty, JC, Mike and even our international brethren from across either pond.

This is my "Buzz Phrase" block. Let's see if I can remember them all, I have so many. All of the turbo building theory I have is based on these Buzz Phrases, for what they're worth.

- Boost numbers without flow numbers mean squat.
- Boost makes heat. Heat = bad.
- More boost makes more heat.
- More efficiency makes less heat
- An intercooler is always worth the effort.
- Build an efficient engine to resist detonation and let compression fall where it may.
- Don't let your turbocharged engine become a low compression pig.

Your camshaft should be determined by your intended use of the engine. If it's a daily driver that has to last 100,000 miles, you are going to choose a different cam than if you were running 60 1/4 mile runs in a season before teardown. This doesn't change whether you are running a turbo or a blower or just carbs. So, all this talk about stock cams being the best for a turbocharger, well, it's true, but only if a stock cam will give you the best performance while you are not on boost.

The Lauffers don't run a stock VW cam in their record setting dragster. Why should you run a stock cam in your weekend warrior? There are very good reasons why the Lauffers don't run a stock cam. You are less likely to have good reasons, even if it's only a weekend warrior.

So, first you have to decide what you are going to use the engine for. This includes what car it is going into, how many miles a year you intend to put on the car, how many miles will be on the car before teardown, and how many miles will be on city street, highway and on the strip. Hey, it's your engine. You can deal with ratty idle in stop and go traffic if you want. That's not up to me. If you don't want to evaluate yourself and your intentions, maybe you're reading the wrong thread. This is about configuring an engine to be the best for s specific application, so unless you have one, sorry, can't help you.

To oversimplify, you want a cam, and in fact an entire engine, that will perform well while not on boost, at between 500 and 1000 RPM lower than you intend to run on boost. If it's a city street car, you'll need quite a bit of low RPM torque. If it's a southern California car, with many highway miles ahead of it, you'll want midrange torque for easy highway speed cruising. If it's a weekend warrior, or a show-off car for those with a grudge, you can still build a spitting and angry high revving car, but there will be a price to pay. If it's an all out race engine that will never see the street, what are you doing reading this for? Get our in the garage and get back to work!

The main variation from a normally apsirated cam will be the timing. Remember I said that you want a cam that performs best a few hundred or a thousand RPM lower than you intend to run. That may be a little general, but it implies that you want to start with a shorter duration lower lift cam, so get that right out of the way first. If you're choosing fmor a list of cams, see the one that puts you where you think you want to go, then move 3 steps lower, at least. Resist the urge to go for a bigger cam "because the toher one is too small." No, it's not.

So what about valve timing? Simple.

The air/fuel mix or intake charge won't flow into the chamber unless there is a force making it go there. Not being an engineer I always assumed that the piston moving down in the cylinder creates a vacuum which SUCKS the intake charge into the cylinder. WRONG. The vacuum is created, but the atmospheric pressure (roughly) waiting in the intake port PUSHES the intake charge into the cylidner to fill the vacuum. I'm not sure how much difference this change in thinking really makes, but it does lead into the next step fairly well.

Before the piston pulls down to create the vacuum for the intake charge, it has just finished pushing up to force the exhaust gasses out. And we've all heard the story before, that gasses have mass and moving mass has inertia, so that even when the pressure in the port and the pressure in the chamber are equal, because the exhaust gasses are moving out the chamber into the port they will continue to move until the pressure differential becomes great enough to cause the gasses to stop and turn around. And we all know that it it is this evacuation of exhaust gasses that effictively siphon more of the exhaust gasses out than would volumetrically go. And we all know that since there is valve overlap the intake valve opens before the exhaust valve closes and some of the siphoning effect draws the intake charge out as well, jump starting the cylinder filling action, and if done just right can actually create a volumetric efficiency of greater than 100%.

And that's just at 14.5-14.7 pounds of pressure, standard atmospheric pressure.

If we increase this pressure, then things happen just the same, only ampified or accelerated. The amount of exhaust gasses increase, because we are running the intake at pressure. The mass of those gasses is of course increased. At whatever velocity the exaust gasses move out the port the gasses have greater inertia. That greater inertia imparts a stronger siphoning effect on the chamber and the intake. Now is the moment of truth. We have a stronger siphoning effect out the exhaust valve. We have higher pressure in the intake port. Given the same overlap characteristics, an engine under boost will shoot more intake charge out the exhuast pipe, and that portion of the intake charge will not burn.

But what if the pressure in the exhaust port is already higher than the pressure in the chamber, to the point that there is no siphoning effect? That's bad. It is unlikely in this instance that the intake port pressure will be greater than the exhaust port presure. It's possible, but unlikely. If it were then the intake pressure would force it's way in, displacing some of the exhaust mix out the exhuast port, and a small amount of intake charge with it, but the remaining mix inthe cylidner would be dirty. Not good.

More likely is that the exhaust pressure is greater than the intake pressure and the hot dirty exhaust gasses push their way up into the intake port, fouling and pre-heating the intake charge. Not good at all.

What to do? Well, here is the crux of the debate between almost everyone on this forum regarding turbo cams. The goal is to limit, reduce or eliminate exhuast gas reversion AND an excessive amount of over-scavenging of the intake charge.

Then throw in a new set of specs. Remember when I said you want a cam that is best at a lower RPM than you intend to use? Remember when I said that if you want a city car you want low RPM with high torque. Well, you do, but you don't want a low RPM high torque cam.

Huh?

Many low RPM high torque cams, although they have short durations, have a lot of overlap. In normal aspiration they need that extra overlap to scavenge the chamber, but don't want the extra duration creating reversion up the intake port when the piston pushes the intake charge back out the open valve. So they reduce the lobe centers of the cam.

If we were to use one of these cams, our problems would be exacerbated (is that really a word?). The additional overlap would almost certainly allow a large portion of the intake charge to escape out the exhuast port, and a large portion of the exhaust gasses to push into the open intake valve. So we are stuck using a low duration cam with medium lift and wide lobe centers.

It is entirely possible that you could create or configure a system in which the exhaust pressure is always or almost always a little less than the intake pressure. If you do that you can get away with a little more overlap, similar to a normally aspirated cam. But if the pressure differential is large, in either direction, large amounts of overlap will eat your power up and may kill your engine (heat = bad).

Good Point. We should all have turbocharged engines with equal sized valves right?

Wrong. Boost numbers without flow numbers mean squat. We are talking about VW aircooled engines, so maybe, yeah, we could use larger exhaust valves. But don't go assuming. Figure it out. Flow your heads. Flow them at tenth of inch intervals until .550" (if they go that high). If you've got a 85% exhuast to intake ratio, you probably don't need larger exhaust valves unless you are going to do something funky with valve timing.

Your engine is a whole. We can talk cams all day and never settle on a consensus because when you change valves, intakes, combustion chamber shapes or even compressions you are going to change the interaction of all parts. Unless you are willing and able to buy, modify and test several different designs, you're going to have to go with generalizations and leave some tolerance for error.

So the elusive perfect turbo cam is so elusive, if it exists at all, because the configuration of every engine is different, as is the driver and intended use and enviornment in which an engine lives.

But we can agree on a few things, hopefully.

Build an efficient engine that resists detonation and let compression fall where it may. Who cares what compression the engine is at so long as it doesn't ping. It does effect overall power, but everything does. Don't lower compression "just" to run a turbo because that's not the only way to do it, and might not even be the most efficient way. Why not leave the compression and retard the cam instead?

Eh?

Retarding the cam would delay the moment when the intake valve closes. It would create reversion of the intake charge up the intake port.

But not always.

At low RPM, when ignition timing is so crucial, the intake charge would have plenty of time to fill the chamber and then be pushed back out the chamber, effectively reducing the compression seen by the intake charge once the valve closes.

But the intake charge has mass. The faster it goes the more intertia it has. The more inertia it has the more force it takes to push it back out the intake valve. At some point in the RPM the cylinder pressure is so much greater than the intake port pressure that the intake charge stops and starts to turn around at the exact same time that the valve snaps shut. Below that RPM you have some amount of reversion, reducing the effective chamber pressure seen by compression. Above that RPM the chamber doesn't have time to fill completely, reducing the effective chamber pressure seen by compression, bot no different than a normally aspirated engine.

The timing of the moment of maximum volumetric efficiency can be placed in an RPM range where the engine is less susceptible to ping, typically between the middle ranges of the RPM band. Where your middle is will change based on the decisions you amde when this thread started.

But what about the exhaust valve timing. Retarding the cam would delay when the exhaust valve closes, allowing perhaps the intake charge to escape.

I suppose it could. Another reason why the lobes should be widely seperated. In fact we should do all we can to limit the potential for that, as well as limit the potential for dirty exhaust gasses to creep into the intake charge, like party crashers.

So we have this cam that we've retarded to some point, such that the intake vlave closes late, allowing intake charge reversion back into the intake port. Well, everything is retarded in this cam, mauybe even the idea of it is retarded. But the exhuast timing is retarded as well. The exhaust valve stays open for a longer period before the intake valve opens. During that time the exhuast gasses have had time to exist and siphon. With the widened lobe centers we have given the exhaust ample time to escape before introducing the intake charge. The initial high pressure just outside the exhaust port has had a small amount of time to get out of the way, push it's way past the turbine, so pressures should be down some. It is possible, maybe even desirable, to have the piston begin it's downward motion to a point where there could be some reversion into the chamber. What this means is that the pressure inside the chamber is now less than the pressure in the exhaust port, and probably even less than the presssure in the intake port, due to the pistons motion past TDC. This will minimize the potential for exhaust gasses to get into the intake port, basically admitting some defeat by allowing a controlled amount of exhuast into the chamber but gaining success by preventing the gasses from getting into the intake port.

Efficient exhaust port and pipe design will minimize the likelihood of exhaust gas reversion inot the chamber. Merely take the normal precautions to prevent exhaust gas reversion.

I'm treading into territory I'm less familiar with, so patience please. I am taking these recommendations from others. I am not a head porter.

Geography of a port for this dicussion is:
- The top of a port, or UP in the port is near the valve.
- The bottom of a port, or DOWN in a port is near the exhaust flange.

To help reduce or discourage exhaust gas reversion, exhaust valve seats should have a ridge at the transition to the port. A valve/seat combination that has poor flow at lifts below .100" might actaully be preferable.

Exhaust ports should be cavernous just below the valve but restrict rapidly as they near the exhaust flange. They should go from being round and open up near the valve to wide and flat down near the flange. Curves or bends should be large radius when possible.

The primary exhaust pipe should be larger than the exhaust port exit, and in-line with the port's orientation (not necessarily perpendicular with the face of the flange). The short radius of the port should meet the inside edge of the primary tube, while the long radius of the port should fall short of the other inside edge of the primary tube, creating a "step" in exhaust flow no less than 1/16", preferably 3/16" on the long radius side.

Exhaust pipes should be coated to reflect heat energy back into the exhaust flow, and should be wrapped with insulation on the outside to prevent conductive heat transfer out of the exhaust system.

This setup creates a venturi effect out the exhaust valve through the port and into the exhaust tube, while maintaining heat energy necessary for quick turbine spooling.

Velocity is equally important, so keeping exhaust tubes small and short are preferable to big an long. Resonance tuning may be effective, but any gain would be miniscule compared to the potential for engery loss due to added tube length. Satisfy yourself that equal length tubes are sufficient.

What's this got to do with the perfect turbo cam? Well, cam's have an exhaust lobe right? They effect the exhaust valve right? So there you go.

Rod ratio.

I love this one.

Nothing gets folks so worked up about nothing except perhaps compression.

What rod ratio shoud I run? Uh, how about who cares?

We are dealing with a VW engine that has from the factory a 1.96 rod ratio. Don't tell me about long rod ratios. That's long.

Rod ratio has a couple of things it really influences, well, three that I can think of off the top of my head. Intake change, combustion and exhaust gas expulsion. That's all. So it can't possibly be important, can it?



If you run longer rods your engine will not get going until higher RPM, right? Well, maybe. What a higher rod ration does is average out the piston speed. There's a lot of geometry involved, and John Connolly has written an EXCELLENT articel in VWT a year back or so describing the effects of changing the rod ratio, so I won't even TRY to repeat that. Find it. Read it. Know it.

Short version is, a longer rod ratio slows the piston near TDC and speeds it up near BDC when compared with a shorter rod ratio. One bit I'll repeat from JC's article that struck home with me, but in my own words. If a 100mm diamter crankshaft is rotating 60 RPM then each second the outside of the crank travels 100x3.1416 mm, or 314.16mm. If you have a 100mm rod attached to that crankshaft, how far will that piston travel in the first 90 degrees of rotation, the top half after TDC? 100mm. How far will it travel in the next 90 degrees of rotation, the bottom half before BDC? 0.0mm. How far will the piston travel in the next 90 degrees immediately following BDC? 0.0mm. How far will the piston travel in the last 90 degrees before TDC? 100mm. The average speed of the piston is only 50mm/sec. The peak speed is 400mm/sec, or some such. It has 1/4 of 1 second to travel 100mm. Yet it is at a dead standstill for 1/2 second at a time. The G-forces at TDC are as high as they can be.

If the intake valve opened up a HUGE vacuum signal would be created all at once, but the intake valve would have to remain open long after 90 degrees ATDC to let the intake charge catch up. Same with the exhaust valve on the exhaust stroke.

Take a longer rod, like a 200mm rod for a 2:1 rod ratio, just a hair over stock VW. The piston is moving in a much more even Bell curve, with much more movement around BDC and much less movement around TDC. This reduces stress on moving parts and reduces the vacuum signal. The exhaust gasses begin being pushed out the exhaust valve sooner and more gently and the piston remains near TDC longer to keep cylinder pressures up as the exhaust gasses escape, better evacuating the chamber in preperation for the intake charge to come into a clean room. The piston is not moving at as much peak velocity, but bof couse the average is the same. That means since the piston spends more time near TDC it spends less time at BDC. The piston is still breathing in longer into the stroke.

Whow cares? We all know this already. Right? What difference does it make?

It reduces the peak velocity of the intake charge The slows are faster than they are in a short rod engine, and the peaks are not as fast. Ports become restrictive near peak velocity so if we can reduce the peak we can rev higher, or rev the same with smaller ports before we reach the restrictive point. With faster slows we don't have to worry about fuel seperation at low flow speed (as much) so we have better low RPM power. We have a broader power curve and a more efficient burn.

The longer rod also changes the degree of rotation at which the piston will exert maximum force on the crank. This may be a minor issue, but, it is still important. A longer rod will delay the moment at which maximum leverage is available on the crank, therefore the ability to time peak cylinder pressure with maximum leverage is crucial.

"But I thought you said 'Who Cares?'"

I did. We have VW aircooled engines. We have to work within the limitations of our engine. You can run any rod ratio you want so long as you are willing to invest in the parts to make it work and are willing to modofy your engine space accordingly. The fact is, anything much more than stock stroke and you get into a really wide engine. There are very few pistsons with very high pins. B pistons really aren't all that high. That limits your ability to run a long stroke with a long rod. After all, if you run a longer stroke you have to run a MUCH longer rod to compensate, much less INCREASE your rod ratio. So it's a matter of economics. If you run a 76mm crank you need a 6.0" rod to maintain stock rod ratio. If you run a 84 crank you need a 6.6" rod to maintain stock rod ratio.

That's madness for most folks, and wisely so. The gains in power from making your engine more efficient is minimal compared to the gains in power by making your engine BIGGER. Besides, the VW engine's stock ratio is well outside the norm considered ideal in other engines, even high performance engines, so compromise for power's sake is acceptable for most of us.

By the way, there are some very fast racers out there, who refuse to be recognized, who are running 7.0" and 7.2" rods. I'll let you investigate them on your own. Then you can tell me that rod ratio doesn't matter.

So you've got all this pressure in your combustion chamber. What do you do with it?

Let's see.

Let's say you weigh 200#. Stand on one foot, and on one toe if you can. That's a lot of pressure on that toe, even if it's your big one. You're placing the same amount of pressure on the ground, but that area is compact, and effectively amplified. See how high you can jump like that. Probably not very high. If you really did, you might overstress yur toe and break it, or colaps some other part of your anatomy.

Now stand on both feet, flat. It's easy on the parts, with few pressure points. You are exerting the same amount of force on the ground, but it is much more comforable. Jump. Much higher. Yeah, I know, it's hard to jump on one toe because of many external factors like balance and coordination, but you get the idea. You are better able to exert force on the ground because that force is distributed more widely. You surely won't break anything unless you are so uncoordinated that you fall and hurt yourself (Old folks like KaferDave might not want to trythis one ).

My point is this: Surface area matters. 1000# of pressure on a 90mm piston will not be able to move the piston as efficiently as a 94mm piston, which will be less efficient than 1000# of presure on a 101.6mm piston. The higher your pressures are, the more a big bore engine will excell.

You see examples of this everywhere. Hydraulic jacks take high pressure from your little cylinder and divide it equally over a much larger area to lift the car. Firearms manufacturers are moving to wide base cartidges to push small diameter bullets faster with less powder and less pressure. Vacuum salesmen show you how they can pick up a bowling ball with an inverted funnel, when the same trick with the bare end of the vacuum would fail miserably. Use it to your advantage.

I won't go into stroke, because we all know that a longer stroke is like a longer lever, and an increase in leverage means a reduced force over a longer period of time can induce a greater torque force on a point than it can on a shorter lever.

But here again is one of those catches with our little engine. The increase in efficiency may not be as great as an increase in leverage. It's the balance between efficiency and displacement/leverage where most apples to oranges arguments get lost.

Back to that cam. Hmmm. Tricky little sucker, keeps getting me distracted with all these unrelated and unimportant things like rod ratio, bore to stroke ratio and exhaust configuration. Why can't we just focus on the cam for a while.

Ok. Shoot.

We've covered a lot of info already. There's a lot more to cover. But I'm not certain I'm the best qualified to continue. After all, we've had discussions recently with Turbo Bob, who in my opinion is on to something, and Marty Staggs, who has maybe the most awesome roadster I've seen in a long long time, and Joe Sumen, the fellow I want to work for, and John Connolly, one of the few who have made an effort to bring innovation to the VW aftermarket industry, regarding boost vs. compression vs. ignition timing. Rod ratio is tossed around like a hot potato that few want to open. Experts on head porting and configuraiton are a tight lipped group, not wanting to sell the farm for the price of an egg, and rightly so. Then there's Danimal, who is a pretty good guy when you actually get to talk to him, depsite his loathing for what he calls "tractor engines," but what he brings to the table is the requirement to back up what you say with something more than "It just is," or "I said so." Which at the moment is something that I cannot do.

What about the intake port? Don't turbos like smaller ports and valves?

Doesn't 30 lbs boost make more power than 20 lbs boost?

Riiiiight.

Boost numbers without flow numbers mean squat. Anyone can run 30 lbs boost through a straw. That doens't mean you are going to make power.

That said, there is a balance between too large and not large enough, just like an aspirated engine. Remember that you want to build an engine that is most efficient a few hundred up to a thousand RPM lower than you intend to run with the turbo. Just like the cam, you want to pick something smaller than where you think you want to be. It's all about velocity. Too big of a port and you don't get the mixture benefits of the "organized turbulence" of good velocity. Too small and, well, you've got a restriction.

There is some debate about how fast it optium velocity. That's fine. It's peak velocity we're worried about, the restriction zone. If we've got a short rod ratio, then we need larger valves and ports, since we're going to have a high peak velocity but want to keep durations short. If we have a long rod ratio then we can reduce the port and valve size, and still maintain good flow up to peak. It's almost like a longer rod is a longer lever over the intake charge, less force over a longer duration excerting a stronger total force over the mass of the itnake charge.

But all this is about the absolute amount of air you are getting into the cylinders. Rod ratio isn't what's important, or even port size. It's about cylinder filling. You CAN have big ol fat heads on a turbo, but if they flow a lot then you are going to have reversion at lower RPM unless you compensate with lower lift and/or shorter duration. If you have tiny heads you are going to have a flow restriction early, so you have to allow extra time for that intake charge to get moving, extra duration. Just adjust your engine parts to meet the flow requirements of your engine desires. Somewhere in the middle is where your engine probabaly SHOULD be, but when you hear folks say "I run mega super welded and CNC ported moster heads that flow 300CMF at 3 inches water and my turbo makes great power at 5000 RPM," you can be sure that he is either loony, or that he's running a 180 duration cam with .300" lift at the valve.

IMO, there are some laws of physics that point towards the kind of engine you build for a turbo application.

Bore: More surface area means more total force pressing against the piston with less pressure per square inch. Therefore, big bores make more power than small bores, and with less stress for the piston and head.

Stroke: Double edged sword. Long strokes increase the levereage placed on the crankshaft by the pistons, but increased piston velocity places more force on the reciprocating parts. This effectively reduces your desirable RPM range. Short strokes give short rods act higher rod ratios, increasing efficiency, and reduce piston velocity increasing desirable RPM range.

Ports: The lower the rod ratio is, the larger the ports must be, regardless of bore or stroke. Longer rod ratios allow efficient use of small ports and valves without restricting peak RPM power.

Valves: Valves should be sized to flow comperable to the valves at relatively low lifts, under .500" in any event. Bigger valves allow lower lifts while smaller valves require higher lifts. If the valve must lift greater than 1/4 its diameter to flow well, then it is too small.

Cam: Well, that's why we're here, isn't it. The cam to use is determined by your intended use of the engine, and all the parts that interact with the cam, which are determined by the cam. There is no one right answer. It depends. Almost any cam will work if you engineer every other part around it to make it work. If you choose a cam first, then the remainder of your engine should rotate around the cam choice you made.

Once choice leads to another based on the direction you are trying to go. Choices are made for you based on the first couple of things you say you want. By not going with where the path is leading you you will be forced to make compromises in design that will reduce your potential for horsepower.

Ignition: What, you're kidding me right? I barely retain the turbo stuff. How the hell am I going to remember anything about spark? That has as much or more to do with intake charge mixture quality and combustion chamber shape than compression. Talk about a tangled web. The only thing I know about figuring spark is put the thing on a dyno, get yourself a knock sensor and advance it until it knocks, then back it off 2 degrees. Rev it up another 100 RPM and do it again.

Compression: Hasn't this already been covered? Simple. Less compression allows more boost. More boost means more oxygen should be available in the chamber which means more fuel can be burned which means more power can be made, but don't expect it to get out of it's own way until after it spools. Don't let your turbo engine become a low compression pig. Yeah, the Nail is a pig. Uh-huh.

It's all in the application. Your engine is a unit, a whole. Build it like one.

----------------

Rod ratio....

Most people go overkill and beleive that the character of the engine will change so drastically with change of rod ratio.
Let my quote stay were it belongs and that is with this part of the forum, Forced induction.
Put the longest rod that you can fit practically and afford in to your forced induction VW.
Forget the character change. It is there, but how noticable? Will it make a big difference in lag? Nope, I do not think it will make a big difference. Most other items in terms of sizing and fuel delivery, ignition will make much more difference.
Why a long rod in my school?
Friction losses, longevity of rings and sidewalls forces (causing friction) on pistons.
Your forced induction engine is already close to many limits and do not add insult to injury on your design by too small rod ratio.
The only balance I can see is the one of very high rpm were you want to limit weight in all places. ....but do not do it, on a design with short rod.
Well, most Turbo or S/C engines should not rev like a NA anyway.
Me? I run a 5,9" on a 82 stroke with supercharger. 1.83 ratio.

These are just my thoughts on rod ratio. To much freakin about on what damage a long rod will do. I do not beleive it hurts that much and that it gains more in the long run on most of our engines.