[David Holmberg]

I'm fairly well versed in Nitrous cams, but I know that Turbo's have a lot of other things to consider.

In my research, I've noticed that there is a lot of talk about keeping the overlap around -8 to +2.

Is this still the current line of thinking? It would seem to limit duration options.

[Travis Q]

That's a pretty open-ended question. It really depends on the combination and its intended usage. The factors that I consider when selectting a camshaft are fuel type, estimated backpressure, converter or clutch, if converter what's the converter configuration (i.e. stator fin angle, number of blades, turbine capacity, etc.), transmisison type/number of gears, engine size, static compression ratio, turbocharger size.

Smaller engines, large turbochargers versus engine size, or engines with overly tight torque converters will need a smaller camshaft with less overlap to help get up on top of the converter.

Engines with more exhaust backpressure will need less overlap to keep exhaust reversion to a minimum.

Street-oriented engines usually require smaller cams to keep the powerband in a usable rpm range. Less overlap spools the turbos faster, which is a good thing on a street car.

Clutch cars or methanol fueled cars can utilize cams with a lot more overlap.

Engines with more static compression ratio will probably need a bit more overlap to keep dynamic cylinder pressures under control.

There's a lot of variables here, and there's no real cut and dry method to use to size up a proper camshaft for one of these applications. You have to rely on your engine builder, tuner, or camshaft supplier to pick the right profile, which boils down to experience with these applications.

I've run camshafts from -22 to +55 degrees of overlap. They all seemed to do the intended job. One thing is for sure: it's better to be too small than too big. This especially holds true with street cars.

Travis Quillen
www.quillenmotorsports.com

[Tom Vaught]

Agree, Travis. Sorry David. Would be BS-ing you to say that the overlap deal is that easy. Supercharger Cams are a lot more straight forward.

Some of the newer engines have Twin Independent Variable Camshaft Timing capability and use electronics to change the overlap and event timing as the engine modes change. Maybe for a race car you could get pretty close, but a street car with boost is a real bundle of snakes.

Tom Vaught

[David Holmberg]

That's why I asked, and included the "current thinking".

It was one of many "turbo expert" websites out there, but I doubted it enough to ask.

Thanks!

[Tom Vaught]

I reread your post and think that they might be referring to the Intake opening point as being somewhere from +2 to -8 degrees. I have had a couple of Crower Turbo Street/strip smaller camshafts that were -1 on the Intake open event.

Tom Vaught

[BruceWilkie]

Very application related for sure. A very very generalized statement would be that a turbo tends to follow what a NA cam does and extends the rpm range at the top. In streeters I've seen a stock Pontiac 2 barrel cam out perform a custom aftermarket "turbo" grind that was supposed to "make big numbers" over that little cam. I've seen reports of the RA3 cam working out rather well too.

Overlap window is what needs to be considered more so than just an .050 number or seat time number. The reason (besides blowing raw fuel into the turbo) you generally need less overlap is because boost gives the incoming charge a greater push when the intake opens . NA you need to open the intake sooner to get the atmosphere to start moving the air column as it reacts to a negative differential created by the exhaust. Optimum is purge the chamber with fresh charge than close the exhaust. Again with boost it takes far fewer degrees of overlap to accomplish this. Wide lsa cam is usually the result of choosing an early exhaust open event. Too early opening on exhaust loses power, too late opening could create too much pumping loss on the upstroke and lose power. Ideally at peak power you want as close to zero intake charge contamination in the chamber tdc dwell and exhaust valve closing/closed as the piston begins decending down the bore.
Also another very general thing to keep in mind is that as you increase boost your engine may want a much later intake closing than it would NA. Depends on the number of degrees in compression you end up with after selecting an intake duration. Again generally, intake closings several degrees later than you would NA can work out ok.

Things like type of fuel and whether or not your running fuel injection can make big differences in what the motor is going to like.

IMO I see no need for ZERO overlap window or any benefit from doing so except maybe lower emissions.

Belt driven blowers(roots or centrifugal) can stand more overlap as you dont have to worry about raw fuel burning up the turbine or intermittant "explosions" wreaking havoc in the turbine. You also dont have the backpressure that a tubine puts into the system. Dynamic compression is probably the biggest thing to watch out for with belt driven systems. Closing the intake too soon here could be problematic. Opening the exhaust too late could lose power to pumping losses. Excess overlap loses charge out the exhaust that could have gone down the bore same as excess overlap NA. (depending on charge bias)

Generally a boosted motor will end up requiring less intake duration for a given rpm power peak than NA. The exhaust duration will usully want to be greater than NA but overlap considerations may take higher priority. Cam lobe design (ramp rates) and rocker ratio can greatly impact results.

Before you try to copy what works for brand xyz keep in mind cylinder head flow curves, intake/exhaust balance, stroke and rod length.

[David Holmberg]

Tom, actually they said this.

Duration: with a log or factory manifold use +6 int duration, and a single pattern with dedicated turbo headers.

Overlap: they gave the formula to determine overlap, and stated that -8 to +2 was safe for turbo motors.

To me that sounded like a blanket statement, and would limit duration.

I remember the turbo Buick use to idle like a kitten, but most of the faster turbo cars I've seen over the past 8 years had quite a lope.

So I thought it was either older info, or geared more for small ci street cars.

I've also spent about 8 hrs over the weekend playing with the new version of Engine Analyzer Pro 3.9. It's rather interesting watching what a small change in cam timing can do with a turbo.

[Travis Q]

The reason for an intake opening point after top dead center and some time after the exhaust valve closes is because of exhaust backpressure.

With an appropriately sized turbine wheel, a street car will have a considerable amount of exhaust backpressure, sometimes in excess of twice the intake pressure. When this is the case, it is undesirable to open the intake valve when the exhaust valve is still open, since exhaust pressure will push back into the intake manifold (reversion). I see this all the time on high backpressure combinations; the intake manifold will be sooty black, sometimes all the way up to the throttle blade! Opening the intake valve right after the exhaust has closed is still counterproductive, because exhaust gases are trapped in the combusion chamber, and the combustion chamber pressure is still greater than intake pressure. So the opening point of the intake valve is delayed until sometime after top dead center; as the piston starts down the hole and the intake valve is closed, pressure in the cylinder diminishes. When the pressure in the intake and the pressure in the combustion chamber equalize, the intake valve can be opened with little or no negative impact on the rpm range of the engine, and a much more powerful engine all the way beyond peak power.

TQ

[62fatcat]

since we are on the topic of camshafts.
i curently run this cam
comp 288/300 seat, 260/268@.050 with .440 lobes(net lift is .700) it is ground on 114-- lobe seperation and it likes a -----109 instaled position.
---best run 8.97@ 154mph with 27# boost, 335cfm e heads and the f-2, a 477 ia2, c-16
---fuel, 24*timming.
i also have this cam
---comp 318/332 seat, 282/292@.050 with .484/.440 lobes(net lift .772/.700) also on
--- 114 lobe seperation.
----this cam ran a 9.44@147 with ported dports(275cfm), (24#boost), a 468 stock
----block combo, 110 fuel, and 24* timing.
the converter stall is 5400, and i shift around 7000-7200, the fall back is about 700 rpm and the slipage is about 5%.

so dose anyone think the bigger cam will make the car faster??
what about the bigger cam and e85?
i have tested the c-16 versus the e85 and the temp drop goes from 310 to 130 under the carb(1/8mile temps), everthing else the same, just the differnt fuel and carb settings settings

thanks,
brad.

[TD2593]

Originally Posted by Travis Q

The reason for an intake opening point after top dead center and some time after the exhaust valve closes is because of exhaust backpressure.

With an appropriately sized turbine wheel, a street car will have a considerable amount of exhaust backpressure, sometimes in excess of twice the intake pressure. When this is the case, it is undesirable to open the intake valve when the exhaust valve is still open, since exhaust pressure will push back into the intake manifold (reversion). I see this all the time on high backpressure combinations; the intake manifold will be sooty black, sometimes all the way up to the throttle blade! Opening the intake valve right after the exhaust has closed is still counterproductive, because exhaust gases are trapped in the combusion chamber, and the combustion chamber pressure is still greater than intake pressure. So the opening point of the intake valve is delayed until sometime after top dead center; as the piston starts down the hole and the intake valve is closed, pressure in the cylinder diminishes. When the pressure in the intake and the pressure in the combustion chamber equalize, the intake valve can be opened with little or no negative impact on the rpm range of the engine, and a much more powerful engine all the way beyond peak power.

TQ

Travis has put this very well.

[turbo69bird]

Travis Q I am very impressed with your thinking on cam specs.

I very rarely read anything on turbo cams that I feel has much merit. Even from the big cam MFG'S

[Tom Vaught]

Originally Posted by turbo69bird

Travis Q I am very impressed with your thinking on cam specs.

I very rarely read anything on turbo cams that I feel has much merit. Even from the big cam MFG'S

Now WHY did you go and post THAT? (:>))) His wife says that he already thinks he can walk on water, feed the multitudes, and can shoot thunderbolts out of his butt on command.

Tom Vaught

[Travis Q]

Thunderbolts out of my rear, yes. All the rest of that stuff, no. At least, my wife says it SOUNDS like thunder!

TQ

LOL

[turbo69bird]

In was just trying to give him the usual god complex.LOL

[BruceWilkie]

Dave I think this thread needs to be a sticky.

While I'm here I'll cut and paste from another thread I responded to. http://forums.maxperformanceinc.com/...=624707&page=2

Re-think how you look at cams, especially Turbo apps.

Intake closing time starts the compression cycle! (Its the point the valve closes not intake centerline that actually matters) Until you approach seat time there will be NO compression. As the intake valve gets very close to fully closed you start building SOME pressure but the intake valve needs to be fully closed for significant pressure to build. The sooner the intake valve closes the more degrees the engine will be in compression. Early closing helps low speed power and limits cylinder fill. Late closing hurts low speed power but at higher speed, inertia of the a/f charge tends to "pressurize" the fill making the charge more dense. This acts like a compression boost at high speed. Too late and you get high speed reversion because the upward movement of the piston overcomes the inertia fill and reverses the flow direction. High speed turbo/supercharged apps can stand a later intake valve closing than NA apps. (edit: doing so will hurt low speed response/power)

Exhaust opening. A 3.75 stroke 6.625 rod reaches max piston velocity at 75.25 deg atdc power stroke, a 4.21 stroke 6.625 rod reaches max velocity at 73.79 atdc and a 4.25 stroke 6.8 rod reaches max at 74.01 deg atdc. Opening the exhaust too soon loses power to CE loss and too late loses power to pumping losses on the upsweep of the piston during the exhaust event. When the exhaust first opens all the remaining charge and pressure "wants" to leave "right now". The more that can leave before the end of BDC dwell the less hp you lose to pumping loss on the upsweep. Thats where a fast lobe ramp helps. This also affects overlap requirements especially NA but also helps a turbo app get by with an earlier than NA exhaust closing point. (edit: At high speed a little bit too early ex open CE power loss usually gets offset by improvement in pumping loss. Not a good idea in a lower speed street type environment. A little bit late ex opening in a street app may not affect much power wise but can affect water temp. Late helps emissions usually)

Travis has some great insight on overlap and turbo's posted here. Take time to understand what he is pointing out here.

Intake opening and exhaust closing points make up overlap. More important is "overlap window" this is overlap exposure overall. As hinted in my earlier post it isnt just seat timing but the overlap window "area under the curve" as well. Many of the factory cams have a great amount of seat to seat overlap but the short lift rate and .050" numbers keep the overlap window net flow area rather small. With a turbo, "too tight" super fast responding turbine and housing selection profoundly affects how much overlap window and more importantly the overlap bias requirement.

For simplification I'll use a single pattern cam example that is 235 deg at .050 and 270 degrees net valve seat time that has a 115 LSA. Installed at 115 ICL it gives the following results. At .050 ivo = 2.5 deg btdc and the evc = 2.5 atdc. This = 5 deg overlap at .050. Seat time is ivo = 20 btdc and evc = 20 atdc resulting in 40 degrees overlap. This is refered to as "straight up" or "split overlap" and there is no overlap bias. With a turbo a tighter turbine is more restrictive than an NA application and you need to change the bias to close the exhaust sooner by advancing the cam timing more than you would need with a "looser" turbine or no turbine in other apps. (Doing so alters the intake closing and exhaust opening so that needs to be taken into consideration when the cam is designed.)
Advancing this cam 2.5 degrees puts the IVO at 5deg btdc and the EVC at 0 tdc .050 cam lift and seat numbers are IVO 22.5 btdc and EVC 17.5 atdc. Notice amount of overlap is the same but now the overlap is biased in reference to tdc. Also note that with 2.5 degrees advance at .050 the intake closing and exhaust open went from 52.5 ivc 52.5 evo to ivc 50 and evo 55, the seat time went from 70 ivc to 67.5 ivc 70 evo to 72.5 evo.
Note also the EVO timing at seat time is improving CE and the IVC is giving more degrees in compression. Based on the max piston velocity timing point this cam will work better with more cam advance and help offset the effects of a tight turbine as well. A 5 degree advance gives a EVO of 75 deg and a 4 degree advance would be 74 deg. This moves the overlap window bias the same amount.

A solid flat or roller cam with lash factored in can have far less seat time relative to .050 cam lift than a hydraulic flat or roller.
Higher ratio rockers DO change what the engine "sees" at .050" cam lobe lift. True total closed seat time at the valve remains unchanged but every bit of movement from that point is altered relative to piston location when you change ratios.

As I've "preached" before you dont chose or design your cam based on coincidental centerlines of intake/exhaust lobes (LSA) or use a coincidental ICL for anything other than cam checking purposes. Duration at .050 is of no value without timing points seat timing and other durations at various lifts. Exact lobe centers dont address ramp shapes and exact lobe center itself at peak lift doesnt matter much to power as the actual starting and stopping of events.

[cgeise]

Travis - am I to assume you believe that a motor on Methanol - at high boost needs more intake duration than exhaust ??- I have a reason for my question - :-)

[BruceWilkie]

Originally Posted by cgeise

Travis - am I to assume you believe that a motor on Methanol - at high boost needs more intake duration than exhaust ??- I have a reason for my question - :-)

Just a guess I havent played with methanol on boost. It may have more to do with the volume of methanol in the A/F charge. 6 parts air to fuel vs 12 parts air to fuel with gas. My approach would be more port volume instead if possible.

Interesting question though because I've been led to believe NA apps on methanol need a much greater ex duration than gas.

Travis? Tom?

[Tom Vaught]

Marty only ran Methanol fuel in his Turbo Engines. I have one of his camshafts (new)
from the old days and the Intake duration is larger vs the Exhaust duration by about 7 degrees at .050" checking numbers. The cam is on a 114 LS.

Tom Vaught

[Travis Q]

Curt -
It depends on a lot of things.

I do not necessarily think that a methanol turbo camshaft needs to be bigger on the intake side than the exhaust. I didn't always think this way, but I have seen some data that indicates to me that some turbocharged engines may well like camshafts that are a heck of a lot wilder than we're used to......

TQ

PS - operating range and backpressure to boost ratio is still king. If you can give me those two numbers, I can tell you within reason what the engine's going to want...

[Travis Q]

Bruce -

6 parts of air to one part of fuel burns up parts.

Three and a half to four parts is much closer.

TQ

PS - I wish 6:1 would work, I wouldn't have to have all this injector volume. Three sets of 160 lb/hr injectors at 80 psi static is a bunch of fuel, and is extremely hard to package. I got one set under the manifold on the Tempest! Six freaking fuel rails, gimme a break. There's enough -8 on this thing to plumb four cars.