Technical CIS operation & Cam Timing w/ CIS Discussion

[camgrinder]

Interesting thread.
The 2.7 cis pistons are close with SC cams. I believe the 964 cams are get too close on the exhaust side. I usually tell customers they will require piston valve pocket machining.
911 Rocker arm ratio modifications are very very expensive. I have seen a few people try it, but the results have been minimal.
Adding duration to the exhaust side of the camshaft will decrease exhaust port velocity, and add to the overlap problem. Even though the exhaust port only flows 75-80% of the intake port, the port size is big. I have only seen 2 "single pattern" cams work, one is for a turbo the other a factory racing cam.
I think by increasing the compression ratio, moving the intake closing and exhaust opening points, and keeping the overlap to a minimum a good HP increase can be had. I made a post a couple of months back with a few cam timing charts attached, for some reason I couldnt find it this morning.
Valve lift... Most 911 heads flow past .400" to .450" valve lift. After .450" the valve is no longer the restriction in the port. Some prople will say its a waste of time lifting the valve further than .450". I believe the opposite. During the intake cycle, the point where the connecting rod and crank are at a 90 degree angle, is where maximum piston velocity is seen. It is usually 75 to 80 degrees atdc (after top dead center )(depending on crank stroke and rod lengths). The pressure differential between the cylinder and atmosphere is highest at this point during the intake stroke. But the valve has to wait until 112 degrees ATDC to achieve full valve lift. By opening the valve to .490" at 112, the valve will be open further at 75 to 80 degree ATDC.
Of course if you find severe turbulence during your flow test and airflow decreases after a certain point as you open the valve, you will have to restrict the total lift or fix the port.

[MotoSook]

Excellent! Now we have John in the discussion!

[911sTarga]

Here is a bit of information that I've dug up so far. A big thanks goes out to John and his website for most of this!

These numbers clearly show that a cam with a wider center line is the "key" to CIS camshaft design. What I really need know are the opening and closing events. That way we can tell overlap and get a good idea on when these valves are starting to move.

Anyway, I think we're starting to get somewhere!

C/L –
CIS = 110
S = 98
SC = 113
964 = 113
Web 20/21 = 113

Duration (@ .050”) –
CIS = 220°(I) / 200°(E)
S = 263°(I) / 235°(E)
SC = 228°(I) / 218°(E)
964 = 238°(I) / 226°(E)
Web 20/21 = 238°(I) / 226°(E)

Lift
CIS = .405"(I) / .350"(E)
S = .455" (I) / .395"(E)
SC = .450"(I) / .395"(E)
964 = .470"(I) / .430"(E)
Web 20/21 = .485"(I) / .452"(E)

Information like this should be stored in a database on this site somewhere. I honestly think this thread will help people understand cam choice with the CIS by the time we're done. I also hope it'll show what other modifications can/can't be made to make a CIS equipped engine really run (without hearing "CIS sucks, just trash it and buy carbs")


Also, here is the lonk to that thread John.
Carrera 3.0L Race Engine Questions

By the way John, I like the cam you named "Custom #2" in that thread. #1 might be a bit easier on the CIS though. I wouldn't mind trying both designs out, but man those opening and closing points sure have shifted around a bit.

Thanks!

Jay

[camgrinder]

Here is a simple timing card I made in Excel.
Timing Card
The yellow fields can be changed and the file does the math.

[911sTarga]

That is a GREAT file! Thanks!

Would level of compression would you suggest with the custom cams you described in the other noted thread? (2.7L narrow body) I'm thinking somewhere around 9:1 to 9.5:1?

[emcon5]

Originally posted by 911sTarga
Information like this should be stored in a database on this site somewhere.

Much of it is, as compiled by John Luetjen, in one of the links I posted above.

Here it is again:

3.0 CIS cam grinds, 964 vs. web 20/21

Tom

[camgrinder]

Compression ratio should be the last thing chosen. It is based on fuel octane, volumetric effciency, engine loading etc.
Remember, nothing in the cylinder is compressed until the intake valve is closed. As you can see by the timing card, the valve isnt closing until 50 plus degrees after bottom dead center. You lose 30% of your swept volume. A typical 10-1 static compression engine will have 7-1 dynamic.

[island911]

Originally posted by camgrinder
. ..
Remember, nothing in the cylinder is compressed until the intake valve is closed. ...

uhmmmm, last I remember, air is a mass, and subject to inertia. (read: lots of matter in motion on the way in . .. reversing that direction takes time ... why we (advance) time these things)

[911sTarga]

I understand clyinder bleed and the effects of overlap. That is why I asked about a recommended compression ratio with regards to those cams you engineered. Your statement actually backs up my question about a recommended compression ratio too.

Originally posted by camgrinder
nothing in the cylinder is compressed until the intake valve is closed. As you can see by the timing card, the valve isnt closing until 50 plus degrees after bottom dead center. You lose 30% of your swept volume. A typical 10-1 static compression engine will have 7-1 dynamic.

The most important valve event and the most sensitive to the induction system used on ANY engine is the intake valve closing. The more fuel/air mixture that can be forced into the cylinder, the higher the performance will be. So IVC is normally delayed until well into the COMPRESSION stroke. But if IVC is delayed too far, the building pressure in the cylinder due to the piston upswing will exceed the induction systems ability (through pressure waves and gas molecule momentum) to hold back the pressure and fuel/air will flow back out of the cylinder (reversion).

As with the exhaust, a pressure wave will be generated in the intake as well. In this case, an expansion wave is generated, although will less amplitude than the exhaust pressure wave. The strength of this wave will be determined by the amount of suction that can be created in the cylinder resulting from the piston downswing and the exhaust scavenging wave.

When the expansion wave reaches the end of the intake runners (the plenum in the CIS), it is reflected back as a compression wave. By the time this wave reaches the cylinder, the intake valve is closed and the wave bounces back out. This wave continues to oscillate in the intake system until the next time the intake valve opens (this is what plays havic on the sensor plate).

Since the length of the intake runners are typically significantly shorter than the exhaust headers, the frequency of the pressure wave is considerably higher – usually two to three times higher – so by the time IVO occurs, the wave has bounced back and forth several times.

As with headers, the intake system must be tuned for a particular RPM to deliver the most benefit from this pressure wave oscillation. The air horns on some induction systems (Webers, TWM, Kinsler) are designed to spread the reflection wave so that it will provide benefit over a broader RPM range.

The area most cam companies error on is the exhaust side. Simply put, on an N/A motor the intake aircharge is not assisted. (leaving wave dynamics of the aircharge out for a moment).
After the combustion stroke there is tremendous pressure in the cylinder. As soon as the exhaust valve cracks open it flows a LOT of air. It's basically boosted out of the cylinder if you want to look at it like that.

Having the exhaust valve open too early not only costs heat (power) and velocity through the exhaust runners, it also empties the cylinder before the intake valve is open enough to take advantage of the pressure differential. (in a limited overlap camshaft this is especially true). This causes exhaust reversion and is one of the key factors in surging problems. When the airflow reverses course, it is loosing a lot of it's inertia. Typically this is overcome before peak torque however. So only low-speed issues are present.

This is why I wanted to discuss this "CIS vs camshaft design" in detail. It's nice to see people who have the knowledge/hands-on experience jumping in and giving their suggestions/knowledge. I'm hoping we can design a few cams/CIS mods to allow us to finally make some power.

[randywebb]

I'll bet you guys can improve on the ramp profiles that P AG generated in the mid-70s for these cars too. This would be true even if there was no cheap way to improve the lift... again, whether it is worth it or not....

[camgrinder]

Originally posted by camgrinder
Remember, nothing in the cylinder is compressed until the intake valve is closed. ...

It should have read...
Remember, nothing in the cylinder is compressed by the piston until the intake valve is closed.

It would be interesting to see how much pressure is in a cylinder right before the valve closes. I would think at peak torque on an engine with 100 % VE you might see some pressure.
Not too many N/A engines are seeing 100 % VE.

I agree with all of 911sTarga's last post. Lots of good info there.


I try not to recommend compression ratios for camshafts as an absolute until I know more about the engine combination. Someone can read it on the board, order pistons and then find out the previously quoted ratios were with twin plugs and race fuel etc.


Most 911 cams are usually 10 to 20 degrees shorter on the exhaust side. I think you will find carb engines like a late exhaust closing and cis and motronic engines prefer an earlier closing. Actually I should say Weber carbs like the late closing. PMO Carbs are somewhere in between. You should do some airflow and velocity testing at low lifts. I think piston shapes and the airflow at low lifts can be manipulated in the right direction to help increase the scavenging effect without increasing the overlap.

[randywebb]

FWIW... VE = volumetric efficiency, right?

[patkeefe]

Correct, Randy. It can be 100% or even greater, but very difficult on an N/A car, as John D says. I found a short practical discussion of VE in this link

http://www.installuniversity.com/install_university/installu_articles/volumetric_efficiency/ve_computation_9.012000.htm

Pat

[911sTarga]

Originally posted by camgrinder
I agree with all of 911sTarga's last post. Lots of good info there.

Thank you very much for the kind words. It means a ton, especially coming from someone like yourself!

Originally posted by camgrinder
Most 911 cams are usually 10 to 20 degrees shorter on the exhaust side. I think you will find carb engines like a late exhaust closing and cis and motronic engines prefer an earlier closing. Actually I should say Weber carbs like the late closing. PMO Carbs are somewhere in between. You should do some airflow and velocity testing at low lifts. I think piston shapes and the airflow at low lifts can be manipulated in the right direction to help increase the scavenging effect without increasing the overlap.

I think I know where you're coming from with the CIS camshaft theory. Have you tested any different profiles on the CIS yet? We might have to talk about doing a CIS cam project together. Since Babe (Erson) passed away a fews years back, I don't have a specific shop that I use anymore. I know his old asst. Steve (Long) started his own deal afyer Babe passed, but I haven't tried his personal work yet.

Patkeefe,

That is an excellent link for VE info! I was just about write "way too much info" until I clicked the link. Saved me a ton of writing!

[camgrinder]

Halfway down the page here:
Combustion pressure graph
there is a graph of combustion pressure. I like this graph because it is taken from pressure sensors, not computer modeling software.

I have done a few "one off" cams for cis engines. I made one cam with the Super cup intake lobe and 964 exhaust lobe on 115 centers. The customer is still in the building process, I think he has been down a year or more now. IF I remember right he was builing a cis 3.0 into a 3.2 with 10.3-1 compression. My idea was to bleed off a little cylinder pressure with the Cup intake lobe and widen the lobe centers to 115 to help reduce the overlap.
I think most companies shy away from changing lobe centers and mixing cam lobes etc. But I have no problem doing it.

The most aggressive I use for the 2.7 and 3.0 (8.5-1) litre engines is the 964 profile and the 3.0 (9.3-1) and 3.2 litre the DC20 profile.

[klatinn]

Hi,

Just an add-on to that VE link (sorry, may be a little OT).
If you know the fuel mass flow rate (inj. duty cycle and rated inj. flow at 100% at specified or measured fuel pressure for EFI) and you know AFR, you can also calculate mass air flow and consequently VE. (air_mass_flow = fuel_mass_flow * AFR)

On a CIS it would be very interresting to characterize injector flow rate vs. injector pressure. From logging that pressure, AFR and IAT you could then also calculate VE without a characterized MAF sensor and it's associated laminar flow requirements.

Regards,
Klaus

[randywebb]

-- I like this graph because it is taken from pressure sensors, not computer modeling software.

As Billy once said:

"When you measure, you know ... something."
- William Thompson (Lord Kelvin)

for whom the absolute temperature scale is named.

[911sTarga]

John,

I would be very interested in any updates you hear on that engine. I'd love to know how the CIS reacts to the cam/compression.

I am going to stay away from a larger engine for now and practice these ideas on the smaller 2.7L. These are some of my thoughts so far. All input or ideas are appreciated.

*Custom ground cam that is close to the "964" or "20/21" profile, but with 111° centers.

*Custom intake plenum or build a spacer for the air door. My main goal is to increase the plenum size to help avoid reversion, but also keep it small enough to save low speed operation. I can easily make one from Carbon or 6061. 6061 might be better, as CF might create added harmonics.

*Have a custom set of pistons made by some friends at Arias. I really need to do some flow testing on the head and the CIS as a unit before we get this far though. Compression ratio will be close to 9.3:1 (static).

*Custom designed header system using a different collector system I designed back in 1999. Worked awesome on the inlines, but I'll have to rethink it with the firing order of the Porsche. (162435 vs 153624)

*Plus the engine will have the obvious upgrades to the heads and such, but mods will be based off of flow data and not past Porsche thinking.

My main goal is to concentrate on combustion efficiency, I/E port velocity, and I/E port scavenging. I know that this idea and testing seems dumb to most, but I'm luckly enough to have friends that'll help me out and/or the ability to make the prototype stuff myself.

If I am wrong, then I am wrong. If I am correct.....oh boy! Just think, we might be able to use a hotter cam on a stock CIS by just adding a little volume to the plenum. Bump compression and/or change the header design and now we can do even more. The limiting factor (besides absolute airflow) for a performance based CIS engine is reversion. If we fight reversion using other methods then just cam overlap.....anyway, we'll see.

I'm wide open for suggestions on this guys and very serious about this testing. My first step might just be a new plenum design. Flow test it against stock and then dyno test it without any other changes. Then change to a 964 cam with a little tighter lobes and see what happens.

Who wants to help attempt to figure this out? Right or wrong, just toss out some idea or theory you have and we'll discuss it here.

Thanks!

Jay

[randywebb]

I think your first move is to get out the STP can and use some of the "Racer's Edge" (modelling clay) to see how much lift can be tolerated at what degrees...

Everything else will be subject to that I think...

[smdubovsky]

Im a little late to the game, but I have the Bosch Kjet manual if anyone is interested. PM me