Don Terrill’s Speed-Talk

Is there any evidence suggesting that lack of a decel ramp in cam profile could cause valves to fail, drop?

Or can excessive valve clearance do the same?

My newly built engine has a pretty big intake cam. I set the clearances per cam maker recommendation, 0.016". There is barely any decel ramp as shown in picture below:



The old cam had a much gentler valve closing - below:



I could reduce valve clearance and likely the closing would get a little softer.

Should I worry about this, dropping a valve?

Yes, the velocity at which the valve hits the seat, is very critical. It's what causes seat bounce. It's also what pounds the valve seats, and can cause failure.
Tightening the lash, may reduce the seating velocity, unless the cam has a "Constant Velocity" lash ramp. With a CV lash ramp, the seating velocity would be the same, no matter where you set the lash, as long as it's on the lash ramp.

If you get someone to "Cam Dr" the cam, you can look at the velocity at different lash settings, and see if tightening the lash will help.

Thanks Mike.

This is a DOCH intake cam with buckets. I could try to examine the lobe to bucket clearance degree by degree immediately after valve closing to see if there is a ramp to be had?

Right?

Nut124 wrote: ↑Mon Sep 07, 2020 11:18 am Is there any evidence suggesting that lack of a decel ramp in cam profile could cause valves to fail, drop?

Or can excessive valve clearance do the same?

My newly built engine has a pretty big intake cam. I set the clearances per cam maker recommendation, 0.016". There is barely any decel ramp as shown in picture below:



The old cam had a much gentler valve closing - below:



I could reduce valve clearance and likely the closing would get a little softer.

Should I worry about this, dropping a valve?

If it was ground on a master machine...
It looks like the the machine ran into a travel limit and didn't follow the full shape of the master.
I guess this might be possible if the wheel were dressed down so far that it could't reach the base circle.
Surely the grinder would heave heard the klunking sound when it hit the travel limits.

Another possibility is someone tracing masters, there is no limit to the craziness that people tracing and modifying masters will do.

Are you sure the lobe wasn't designed for a hydraulic bucket ?
They have a very short/fast ramp, like what you pictured.

CamKing wrote: ↑Mon Sep 07, 2020 12:50 pm Are you sure the lobe wasn't designed for a hydraulic bucket ?
They have a very short/fast ramp, like what you pictured.

No, not for hydraulic lifters. This cam is made by well known Italian cam maker Colombo & Bariani for the Fiat/Lancia twincam engines. However, it is not a grind I can find on their website. Maybe a "custom" grind for a US Fiat parts importer. Duration specs would put it between their "Medium Road" and "Medium Race" cam, closer to the race cam.

I think I will try to set one valve to near zero lash and redo the opening/closing profile measurements to see what the ramp looks like.

It is harder to do now that the engine is in the car though.

I would think with the seating velocity on the high side like that....... it would be mechanically noisy.

PRH wrote: ↑Mon Sep 07, 2020 1:21 pm I would think with the seating velocity on the high side like that....... it would be mechanically noisy.

Yes, there is definitely some mech noise one can hear at idle. That is kind of what got me thinking about this. The previous build was quiet.

Can you get a 0.016" feeler gauge between the base circle and the bucket and then measure? If so, and you can post degree wheel readings and lift number then we can calculate what the velocity is at different points.

Stan

The fastest, easiest thing for me to try would be to re-shim it in place to reduce lash and see if it quiets down.

Right now lash is 16-17 thou. What should I try? 12, 10?

go to .010"-.012"
That's a big enough jump, for you to tell the difference.

Stan Weiss wrote: ↑Mon Sep 07, 2020 1:30 pm Can you get a 0.016" feeler gauge between the base circle and the bucket and then measure? If so, and you can post degree wheel readings and lift number then we can calculate what the velocity is at different points.

Stan

What kind of max velocity limits are being recommended for valve seating? This Fiat head has hardened steel seats.

What point in the lift curve do you use for the calculation? The last two data points before zero lift or some kind of an average like the last 5 or 10 thou of lift vs duration?

My opinion on oddball engines is that most of the camshafts available are copies from other applications, and sometimes copies of copies, and of designs that weren’t even good to begin with, and also just poorly ground to begin with.

Not a knock on to the OP, but the offerings were small enough in that world and the OEM stuff was so bad, that even poorly designed aftermarket offerings are still better, but they leave a lot to be desired.

Does the lobe transition to the base circle look smooth to the eye?

SchmidtMotorWorks wrote: ↑Tue Sep 08, 2020 9:05 pm Does the lobe transition to the base circle look smooth to the eye?

Yes. Looks perfect.

I reshimmed #1 intake valve to 0.003 clearance and will try to get the degree wheel on the crank to measure the valve closing in very small increments. This is just so I can measure the full cam lobe profile to assess seating velocity at different clearances.

Need to shim the degree wheel 3/8" off the face of the pulley in order not to remove water pump.

No answers to my question about recommended max seating velocity limit values.