Choose a camshaft

[SGKent]

over the last few years I've seen quite a few questions pop up on camshafts. Thought maybe it might be worth to share some thoughts learned from years of racing that could help someone choose a camshaft. I am not making recommendations for a specific vendor or grind in this thread - just some general thoughts to help choose.

To know what camshaft to choose, one must first understand their purpose and the operation of a 4 cycle engine. The four cycles are intake, compression, power, and exhaust. The air and fuel does the following during these cycles

Intake - air and fuel is drawn into the engine
Compression - the air and fuel is compressed and heated
Power - the air and fuel is combined making more heat that provides power to the piston to move the vehicle.
Exhaust - the spent hot fuel and air is exhausted out of the engine to make room for the next incoming charge.

At least two valves are needed to control the air flow and to contain the power so it can be used. The position of those valves is different for each part of the cycle. They are the intake and exhaust valve.

Valve position during each part of cycle in general, there is some overlap between parts, eg., exhaust opens near end of power stroke:
Intake - Intake open and exhaust closed
Compression - both closed
Power - both closed
Exhaust - Intake closed and exhaust open

To open and close the valves a camshaft is used on the VW engine. Lifters ride on the camshaft lobes which in turn transfers the movement thru pushrods to the rocker arms which then open and close the valves. Springs attached to the valves provide the pressure to close the valves.

The quality of the camshaft is one question that someone buying a camshaft should ask about. Companies like Isky, Engle, CB Performance, WebCam, Scat are known to make reasonably good camshafts as to the quality. The lifter and camshaft need to be the same quality steel so that they wear evenly. For this reason it is common to get the lifters and camshaft from the same company.

Each valve has a length of time that it is open. Some open earlier in the cycle and some open later. This is called the duration of the cam. Some camshafts have different durations for the intake than the exhaust. This is called split duration.

Choosing duration. Many people think of the air moving in and out of the cylinders like taking a breath. At low speed it probably is as the air is moving slow and has very little momentum. However at higher speeds the weight of the air gives it momentum so it is more like a wave. Think of a wave of water in a pool. It travels across the pool until it hits the side then it reverses and travels back the other way. Each molecule is like a little billiard ball hitting a rail. It bounces off. If you roll a bunch of billiard balls together they will hit the side together and bounce off together. So when the air moving into the cylinder at high speed hits the back of the valve as it closes that wave of air is reflected back towards the carb or plenum. There it reflects again. If the camshaft is properly timed in duration, the valve is just opening again during another cycle as that wave arrives again only this time the air rushes into the cylinder and packs in more air and fuel than could be drawn in. Then the intake valve closes and traps the air while at the same time another wave is being reflected back towards the carb or plenum for the next engine firing.

At high speed this effect becomes more noticeable. Thus the velocity of the air becomes just as important as the width of the column. A narrow high speed charge of air may pack more air into a cylinder than a wide slow moving column. At low engine speeds the camshaft duration can be short as air flow speeds are slow but at high engine speeds the camshaft duration must be longer and it must lead the incoming charge more. The actual air is effectively sloshing back and forth in the intake and exhaust tubing, and the valves are harnessing that energy. Some high end racing engines will even tune the length of those tubes to help tune the length of the sloshing to time it perfectly. That is what headers and intake stacks are designed to do.

Here are some equal length headers - notice how much trouble the builder has gone to in order to keep the tubes the same length and short. This is to get max flow at high RPM.

Here is the intake side tuned by adding velocity stacks that are tuned in length

So what has all this got to do with my VW engine?

It operates the same way. But there is a difference - you probably won't get that engine up into the 10,000 RPM range or more where a slight difference can make a few horsepower more or less. You will be operating in a much lower RPM range. That is actually the VERY FIRST THING YOU SHOULD ASK YOURSELF! WHAT RPM RANGE WILL I RUN THIS VW BUS ENGINE? This is a VW Bus and not a Bug or Sand Rail. It is not a 914 or 912E.

Will you run it at lower RPMs - 3,000 to 4,500 mostly or are you going to go for an engine that peaks at 4,000 to 5,500 RPM? Will you want even more RPM? Remember that with higher RPMs comes more times that your engine can cram air and fuel into that cylinder and burn it, however with that comes more vibration and wear. Or will you opt to run with a lower RPM and less horsepower but try to capture as much torque as you can? You don't have a dual range camshaft like my Acura RL engine. It has a variable camshaft - one set of lobes for low RPM and one for high RPM. That technology was not around when our VW motors were built. You can only choose one camshaft duration.

So generally here is what you might consider. About 280 - 285 degrees total duration (zero lift) is the max duration you can get and still have a solid idle. Beyond that the idle will start to lope. At 280 degrees you will find that the peak HP comes about 5,500 RPM. A bit high for most VW buses. Probably you'll notice the lower RPMs feel a bit flat. If you are running a single carb you won't even be able to flow the amount of air needed at that high range so you'll end up with no low RPM power because of the cam and no high power because of the carb.

General terms ADVERTISED duration (at zero lift) - These numbers will vary from manufacturer to manufacture based on lift and profile. Ramp speed and other variables make it impossible to compare two cams even of the same duration unless you put them on an engine dyno. Even then different temperature and humidity conditions can cause differences. The numbers below are general suggestions only. One manufacturer might suggest that their 255 degree cam will provide power from 1000 to 4000 RPM while another may suggest that their 255 degree cam will provide power from 1500 to 5000 RPM.

These are my observations and my opinion only. Refer to the manufacturer's spec sheets for their opinions.

Above 285 - 290 degrees racing only and will extend to 320 - 330 degrees

270 - 280 degrees total duration will give you a peak about 5,200 - 5,500 RPM. Low end below 2500 RPM will suffer

250 - 260 degrees total duration will give you a peak power about 4,800 RPM. Low end below 1500 RPM will suffer the closer to 260 you get.

240 - 250 degrees will give you peak power about 4,000 - 4,400 RPM. Low end below 1000 will suffer. Tuned properly you can extend peak power to about 4,800 RPM with about 250 degrees without losing torque at the bottom end. About 250 to 260 degrees is my choice for most bus engines. In my opinion only, if you are in a smog state and subject to smog laws you want to stay closer to the 250 number or lower. Certain engine builders are able to extend these ranges thru careful tuning of engine parts - air velocities, port and valve sizes, combustion chamber shape etc but this is beyond most novice engine builder's abilities who are buying a drop in camshaft.

Below that duration you'll have a constant speed industrial engine or what you would expect to see in a family V8 that drives for fuel economy only.

PLEASE keep in mind that the engine will pull past its peak RPM, you just won't feel if pull as strong as the torque falls off quickly. An engine that has peak power at say 4500 RPM doesn't just get to 4500 RPM and freeze there unless you are climbing a hill with a heavy load - it will continue to accelerate to maybe 5,200 RPM before it poops out as power falls back off.

Lift is also important to consider because the wider the valve is open the more air can get by it. However more lift means more wear on the guides and springs, and more risk of geometry problems. Personally I like a moderate to stock lift combined with swirl cutting and polishing the valve backs which lets much more air by them. Basically the back of the valves is streamlined so max air flows past it. That way I get good air flow and minimal wear. I can also worry less about a valve tagging a piston at high RPM. (valve float). Again - this is an engine built for my bus, not a race car where 2 or 3 HP might be the difference between winning races and finishing 5th.

I've said a lot and it might not make a lot of sense but for a bus - which is basically a utility truck, running between 3000 and 4500 RPM is probably where you will spend most of your time, with occasional bursts to 4,800 - 5,000 RPM getting on the freeway. In my humble opinion around 240 - 250 degrees duration will give you this performance without losing the bottom RPMs. If you see yourself with dual carbs and ported heads, racing around town at 5,000 t0 6,000 RPM then you would want to be up in the 270 - 280 degree range.

I might add that the only way to really determine performance in a finite way would be to dyno many different engine, carb, cam and exhaust configurations to graph each, then drive the top performing ones in different conditions to see how they wear. That is beyond the ability of all but professional engine builders and it can take hundreds of thousands of dollars to make an adequate study. You also need to be keenly aware to READ the details when looking at duration. Most cams are advertised at both total duration and duration at .050 lift. Make sure you are comparing apples with apples. A cam that is 247 degrees total duration might be 210 degrees at .50" lift at the valve.

As my last thought. As you lengthen the duration of the cam, the engine has the ability to make more power as the RPM range of the engine rises. Torque suffers at lower RPM's but HP increases at higher RPMS. Make sure if you go to a higher duration camshaft that you also make the necessary changes to the intake system - porting the heads, dual carbs of an appropriate venturi size, larger valves etc., and that you also open up the exhaust to get rid of the air and fuel your engine has burned. If you don't make the necessary changes to the intake and exhaust then you get the worst of all worlds - low torque at lower RPM because of the camshaft and low torque and horse power at high RPM because the engine cannot breathe.

Links to some cam manufacturer websites. No one is intentionally missed. I did not include one manufacture as I do not know if they are offering cam kits right now or just full engines. They can be contacted at http://www.aircooledtechnology.com/

Webcam http://www.webcamshafts.com/
Engle http://www.englecams.com/
Schneider Cams http://schneidercams.com/
CB Performance http://www.cbperformance.com/SearchResults.asp?Cat=12
Mofoco http://www.mofoco.com/category/VW_Camshafts_Cam_Parts/c134
Isky Cams http://www.iskycams.com/
Crower Cams http://www.crower.com/index.php/camshafts.html?cat=127
SCAT http://scatvw.com/master/camshafts/

[Lionhart94010]

RE:

[Wildthings]

And your next post will be on the intricacies of cam lope centerline?

[SGKent]

[SGKent]

[Lionhart94010]

[SGKent]

[Lionhart94010]

I’m in the middle of home renovation, my cam cards are buried somewhere in my unpacked boxes form four years ago; but I do have the information I entered into Denotation 5 for my Scat C-25 Cam…

https://www.thesamba.com/vw/gallery/pix/1120601.jpg

_________________
Current VWs 71 T2 Westy SO-72/6(Miami), 71 Crew Cab, 2015 GSW TDI
Other owned VW’s 59, 68 1500s, 69 & 71 Bug’s; 72 & 73 S-Bug’s; 67 Westy, 67 Deluxe, Other 71 DC, 72 KG GT that now lives in Australia, 12 JSW TDI, 2015 GSW TDI, 2023 Tiguan
VW technical information sights
thesamba - www.ratwell.com - www.shoptalkforums.com/ - www.vw-resource.com - http://www.type2.com/
http://bobhooversblog.blogspot.com/ - www.aircooled.net/gnrlsite/resource/articles.htm

[SGKent]

[quote="Lionhart94010"]I’m in the middle of home renovation, my cam cards are buried somewhere in my unpacked boxes form four years ago; but I do have the information I entered into Denotation 5 for my Scat C-25 Cam…

It indicates you are running a turbo. That hardly compares to the average bus owner here.

[Lionhart94010]

Blower type: = None, Just an unused option...
_________________
Current VWs 71 T2 Westy SO-72/6(Miami), 71 Crew Cab, 2015 GSW TDI
Other owned VW’s 59, 68 1500s, 69 & 71 Bug’s; 72 & 73 S-Bug’s; 67 Westy, 67 Deluxe, Other 71 DC, 72 KG GT that now lives in Australia, 12 JSW TDI, 2015 GSW TDI, 2023 Tiguan
VW technical information sights
thesamba - www.ratwell.com - www.shoptalkforums.com/ - www.vw-resource.com - http://www.type2.com/
http://bobhooversblog.blogspot.com/ - www.aircooled.net/gnrlsite/resource/articles.htm

[Wildthings]

[Chris_914]

Mine was simple.
Web offered two solid cams for stock fuel injection, Stock and 73.
Stock has no overlap and the 73 has a little.
After reading up on overlap, it basically said if you have a restrictive exhaust, like I do, you want as little overlap as possible. SGKent did get me thinking in the right direction and I followed up with research of my own.

Nearly 300 miles on the new engine and I am happy with the choice.

914 will get the 73 when I tear in to it to remove the CB drag racing cam I put in and thankfully never ran.
_________________
'75 Westfalia Camper FI 1.8L
'75 914 2.0L

[Jake Raby]