Stock Cam Specs?

El Paso Bob asked:

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Dear Bob (and the Group),

Before answering your question there's a couple of points you should keep in mind. First off, ALL VW cams, from the 1300 on up, are the same. SP, DP, bus, bug, early & late, 1300 thru 1584 all have the same cam.

Secondly, VW uses a 'split' cam in that the lift for the intake is different than for the exhaust. Be sure which one you're measuring.

Finally, taking manufacturing tolerances into account (ie, averaging), stock VW cam runs about .0.297 for the intakes, 0.285 for the exhaust, MEASURED AT THE CAM. Running stock rockers? Proper geometry? Then you can mulitply that by

1.1 (ie, 0.322 intake, 0.310 exhaust).

Which doesn't mean squat when it comes to adding ratio rockers to a non-stock engine. What matters is your deck height and if the heads were flycut when you built that 1776. Odds are, you can run 1.25:1 without any problem. But the odds are equally good that you won't see much for it unless you've made other modifications that will allow the engine to take advantage of the additional lift. Main problem here is that the torque-peak for the stock stick is around

2000 rpm. You can fiddle with that a bit by altering the cam's timing slightly (ie, about 4 degrees) but that will simply shift the bottle-neck to another part of the system... exhaust, intake or what-have-you.

You may find some of the cam poop on the AirVW Group of interest (ie, over on Yahoo. Look in the 'Files' archive.)

-Bob Hoover

Bob, TYVM for your reply. I'm currently running an el cheapo Bugpack cam and the cam card indicates 0.378" for both intake and exhaust; albeit at different times/degrees. I'm currently running 90.5mm p&cs with .068" deck. I had the heads (CB Magnum 044's) flycut for 53cc's for a static CR of 8.25. BTW the heads wereslightly ported/polished and the combustion chambers were slightly reshaped to unshroud the valves. I used an adjustable pushrod to set the valve geometry. Also have a 1 1/2" Tri-Mill merged collector header. Just wondering if 1.25s or 1.4s were the way to go. Your opinion would be appreciated.

El Paso Bob

71 1776 Baja

I'm not Bob, but you can NOT successfully run 1.4s with that cam. 1.25s will be OK, provided you set the rocker geometry and pushrod length (it will change with the different rockers).

John Aircooled.Net >

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Dear Bob (and the Group),

Sounds like a nice engine.

Based solely on your description I don't think ratio rockers would be justified. You already have larger than stock valves and the amount of lift is more than you need at practical rpm's. Plus the amount of flycutting can't be accurately determined from the chamber volume; 0.386" of lift, times 1.1, minus any geometrical losses, the edge of the intake valve could already be pretty close to the top of the piston.

Or mebbe not.

With a bore of 90.5 and a stroke of 69 you have a swept volume of 443.6. Deck height of .068 equals 1.73mm for a deck volume of about 11.1cc. If your chambers are 53cc then your static CR is about 7.4:1

All else being equal, even an 11% increase in displacement probably can't offset the 5% reduction in compression ratio, especially if your cam is meant to enhance rpm rather than torque. (You didn't mention the specific timing, overlap nor if the thing was installed with any advance or retard. Lift alone doesn't mean much unless you know if it's measured at the cam or the valve, and if at the cam, what amount of checking clearance is included.)

If you were considering ratio rockers because the thing doesn't seem to be putting out the power you expected, odds are the problem is in the cam plus the reduction in CR. (Stock is about 7.7:1 for the Export-model crate engines from Mexico.) Most after-market cams for streeters moves the power band up around

5000 rpm, dictating the need for different gearing, tiny tires, etc.

If that's the case then a stock stick might be a better choice, possibly with ratio rockers if they'll fit. But it would be a good idea to cc the heads yourself. You might be in for a surprise :-)

-Bob Hoover

Bob, You ran a few more numbers than I did. Certainly gives me pause for thought. Didn't want to tear it down, but judging by some of the other work I've had done, I just think it might be justifiable.

El paso Bob