Du'oh! :mad:

What lift(s) are all these duration numbers done at? Normal is 1 mm , 0.010" and 0.050", all give differen't duratins for the same cam. I am guessing that most are at 1mm but if its reground it may be at 0.050"

So what's the advantage of the 20/21 cams?
 

Question concerning the above graph: if 6500 is the peak RPM with the 20/21 cams and 6000 is the peak with the SC/964 cams, AND hp differences between the cams (realistically), are pretty small (the 20/21 cams develop a bit more from what I understand), is it safe to say the only difference and/or advantage between 20/21 cams and stock SC cams, is a powerband raise and not necessarily a "real" horsepower increase?

is it safe to say the only difference and/or advantage between 20/21 cams and stock SC cams, is a powerband raise and not necessarily a "real" horsepower increase?

I think there are so many variables in answering that question. The extra lift and duration of the 20/21s could be choked off by too small exhaust ports or pipes, intate port size, valve size etc... or it may not. To hard to tell without dynoing on specific engine combos. Its suprising how many times you dyno an engine and the horsepower and torque dosn't do what you expect. Sometimes you gain a tiny bit in a very narrow RPM range and lose alot everywhere else. This can show up as a technical gain in horsepower so it looks good in advertising, but in real world driving doen't feel anywhere near as good. I am NOT saying that is the case with Web cams. They have a pretty awesome reputation.

i am curious as to how the changing of camshafts affects the CIS system. i have had some experience with fitting different camshafts to vehicles fitted with EFI and found that while some ecu's were smart enough to compensate, others were not able to compensate enough usually resulting in poor idle quality and no real increase if any in drivability.
Also on another camshaft subject i have a 77 carerra 3.0 and i was curious as to the cams in it. i believe it has either 200 or 210 horsepower ( i can never remember, it's not that important except for bragging rights), which is not that far from where the original post wanted to be ( i believe 220-230 hp). just a thought.

The CIS is *not* "smart", in that it has no ECU, but operates strictly on air flow. It's a mechanical system.

CIS does not work well with (radical) cams that have lots of overlap.
The intake reversion pulses, upset the air sensor plate, making the mixture unreliable.

I just picked up a tidbit from one of my recent race engineering magazines that comes into play with a number of the recent posts. The case in point was the Judd LM V10 engine. Apparently the designer was saying that they picked up HP for the LM engine by reducing the overlap of the cam. Why? Because LM engines run with air restricters which especially come into play at high rev's as the engine is sucking more air. Basically as the restictor chokes off air flow, the pressure in the manifold drops (aka: creating a vacuum) which starts to pull the exhaust gasses back into the cylinder and the intake during overlap -- causing a fairly significant HP loss. So they reduced the overlap, which increased the HP by 10's of HP (I don't have the article nearby).

Whenever anyone brings up "radical" cam grinds, overlap and CIS, the discussion focuses on the poor idle charactoristics resulting from intake reversions and sensor plate fluttering. This is certainly true, especially with those people in states with emissions testing.

But the CIS can also interact with the overlap at higher rev's since the CIS sensor plate (as well as the EFI sensor flap and or throttle body) can be a significant intake restriction. As near as I can tell (I haven't tested a CIS engine for this) the CIS engine's are drawing a fairly significant vacuum at WOT -- this seems to be why they generally have "undersized" intake ports ('78 and '79 SC's excluded)

Basically - another reason why overlap should be avoided with CIS cars, and other cars which have any sort of intake restriction.

Does this mean, in a sense, that 20/21 cams work better on 78-79 SC motors than later SC motors?

I've been told if someone swaps late SC heads with early SC heads, 964 or 20/21 cams are a must. Otherwise the low-end's going to be terrible. It's probably the same theory if a person stuck 3.2 heads on their SC motor.

I'm not sure if I would come to the same conclusion since both the "early" ('78 and '79) SC's and the '80's and later SC's had about the same HP. So just swapping the heads alone can't have that big of a difference (all other things being equal). I think that the conclusion would be that if you are going to use the 20/21 cams with CIS or possibly unported late heads, you should specificy a wide cam lobe angle to minimize the overlap. If you have early style heads or 3.2 heads and you are running carbs, then by all means go for a narrower cam lobe angle, but keep in mind that the overlap will still hurt your low RPM performance some. But your peak HP will most likely be a little better.

John,
That is interesting about the overlap, I would have never thought of that.

As for the 78 and 79 SC heads. I heard, or read, or dreamed, that Porsche built the 3.0L with 39mm intake ports as the optimum port size. But the exhaust with unequal length primaries and a cat caused a restriction that did not allow for the potential of the 39mm intakes. So they made the later 3.0L with the 34mm intakes to increase velocity to try to create some lower end for a broader power band. IF this is true, don't you think the 78 and 79 cars would respond to 964 or 20/21 cams better than the latter cars as long as they both were running SSI or early exchangers?

I haven't seen enough examples to know if this is true or not. Does this fit with your analysis?

Chuck; I couldn't tell you. Like I said, I haven't spent enough time going through all of the different CIS engine configurations. As long as the CIS remains the dominant constraint on the engine's airflow, I doubt that changes in the porting (within reason) are going to have much of an affect.

I have been thinking a whole lot on why Porsche increased the size of the ports from the T to the S engine. The heads can support 300HP as is in the T version. Only trouble is the RPM might be higher than Porsche thought they could live with. SO to lower the RPM and still get the power they increased the port size. In other words you can't get stuff in faster than the piston is moving and if you want more without increasing the piston speed you need a bigger hole. The only problem with bigger holes is that it hurts the low end torque. As to the CIS, its a bastard of a fuel injection system, not real compliant with real world restraints. Overlap is the magic that makes an extra 20 percent HP in a highly tuned engine. An engine is a Pulsed system. A look at the fourier transform of the input system and exhaust system will show how they interact. By overlaying the waveforms proberly one can increase the fill and exhaust significantly over a small rpm band. Ie the magic of tuned headers and tuned input runners. Only problem with all this is that it only works on a very narrow rpm band, usually the high end. everything else suffers, a lot.

Jack:

I agree overlap is certainly critical for horsepower and that is the CIS downfall. Usually with all else being equal, a cam with narrower lobe centers and more overlap almost, if not always, creates more torque than a cam with wider lobe centers and less overlap. And, often at not as high of RPM as you would expect. We also know that torque x RPM = HP. I don't understand your port theory however. My understanding; small ports are better than large ports (stronger pulses, higher gas speeds, better scavenging, etc...) until the RPM of the engine or the engine size gets to the point where the smaller ports will no longer completely fill the cylinder. Camshafts also are one of the variables. At that point you absolutely need to have bigger ports to create more horsepower.

I also suspect that the CIS isn't as restrictive as jluetjen suspects. I think Porsche knows enough not to put too big of ports on an engine that has a intake restriction. Boy I could be wrong though!! I could understand however, Porsche not anticipating the restriction on the exhaust side trying to meet emissions, or not wanting to have to retool for it

So I suspect that a 39mm intake 3.0L, with 20/21 cams 9:5 pistons, and SSIs would do better than a 34mm intake 3.0L would. Could be wrong, have been before.

Its my understanding that the poston velocity determines the max intake port velocity. So if the piston velocity is on the low side, ie it is the limitation, not the port size. If you wish to maintain this lower piston velocity, ie not let the rpm go to say 10,000 rpm, but only 6,500 RPM then the only way to get more in the cylinder is to open up the port. In other words the port velocity at 6,500 rpm is the same for either port, 34mm or 39mm.

I am not absolutely certain of any of this but the one point, ie you can't stuff in air faster than the piston is moving. How this relates to port velocity is not exactly clear to me at this point You have a fixed pressure difference of one atm and an orfice restriction (the valve port) Its clear the bigger hole can flow more volume than a small hole. But what all derermines the rate? Given that the std t head can support 300HP maybe there is no difference but marketing??? Help me out on the head flow theory here.

Just a followup, regarding the smog issue with 964 camshafts:

Passed CA "enhanced" dyno smog check with flying colors this morning.

15 MPH, HC, max 141, measured 11
25 MPH, HC, max 113, measured 21

15 MPH, CO, max .99%, measured .13%
25 MPH, CO, max .79%, measured .24%

15 MPH, NO, max 1144 PPM, measured 212
25 MPH, NO, max 978 PPM, measured 173

Tom

Tom, great news.

Cat or not cat, open or closed loop, idle co%, etc?

I'm curious as I'm going in for smog in VA. SSI's running open loop at the moment.

-Charlie

For what its worth, one can tune an engine with a high performance cam ie 280 to 290 degrees advertized duration, open loop, so that it will pass even the most stringint smog test, by huge margins. Only trouble is you can't drive it over a mile or two before it burns up. In other words smogging an engine is one thing, but being able to drive the car, with the engine tuned the same way is another. The good thing is that there are very few avid hotrodders, consequently very few cars that emit more than they should. For the rest of us the newer cars have finally gotten to the point that a car that passes smog still has performance. In fact better performance than one could dream of for years gone by. The older Porsches have one FUNDAMENTAL restriction in updating performance AND still meeting smog. Lack of water cooling. Water cooling is required to maintain a constant temperature, which is necessary to obtain optimum performance. Without it one is left to emitting a bit more than necessary. Fortunately the additional emmission is insignificant, due to Porsches small production numbers. The Zelots will not let it drop but for a practical matter you are not doing anything to the atmosphere that represents any threat, at all, no matter what you do to your Porsche. But still I can drive my 66 Porsche, and legally emit 100 s times more than a newer Porsche with a high performance anything.. In fact it may be illegal to modifiy my old POrsche to emit LESS! Only in CA.

If you have a problem car that almost passes, retard the timing. I am not a tech, therefore it is legal for me to speculate what may or may not help a car to pass smog, whatever you do RETARD THE TIMING. Techs are supposed to check the timing, but they NEVER do. I am retired and make nothing for doing anything for anyone. SO when I say retard the timing I am not benefiting in any way, it is just somthing I have heard works and I am blindly repeating it. Hopefully someone can benefit from this info. I am not or never have been an auto tech. or made any money working on autos.An additional note, one SHOULD never go be beyond the mfg reccommendatins on timing. If you do so it may reduce performance as well as smog.

Stock exhaust, single plug, closed loop. The car truly runs like ****. My cylinder head temps are in the 300° neighborhood, and oil temps are pushing 230°.

I used the Freq valve duty cycle to set the mixture, I leaned it out until the duty cycle was ~50%. As soon as it passed, I drove across the parking lot, unplugged the O2 sensor and gave the mixture screw a 1/4 turn rich.

It is absolutely illegal to modify your old car to emit less in California, unless you jump through the required hoops, something that is impossible to do as an individual.

I would love to buy Tony's CIS to EFI kit, but it wouldn't pass the visual portion fo the CA smog ispection. The fact that a modern EFI with a wideband O2 sensor would run much cleaner is meaningless.

Quote:

Techs are supposed to check the timing, but they NEVER do. I am retired and make nothing for doing anything for anyone.

They do here. In fact he checked it and got it wrong. I timed it to factory spec, 5° BTDC with hoses unplugged, and double checked it just before the test. He entered it in the system as 3°.:rolleyes: Then again he also set a cooling fan in front of the car, and looked for the cat under the drivers seat. He looked confused until I pointed it out to him.

Tom