I'm just about to dial in the cam timing on the turbo motor I am building.
It has a Howard's Cam # 751082-12

My question today is should I time it according the the card or should I advance the cam a couple of degrees?

The motor has a static CR of 7.5:1, big valves, porting and port injection.

Here's the cam card:

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Depends on the RPM you are intending to shoot for. Advance puts the max torque lower. Retarded puts the max torque higher. I think the final first gear ratio is important here.

Sam

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I'm not looking for big RPM. Probably 5000-5500 tops. It's intended to be a driver that might occasionally go the the track.

I understand that some advance is good for bottom end. I think that would be a benefit - especially when it's not on boost.

SO the second half of the question is if I do advance it from the standard spec - how much should be good and how much is probably too much?

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That cam has 4 degrees of ground in advance already; you can tell just from the cam card numbers. (The LSA-LCA = 112-108 = 4 tells you that with a symmetrical cam, and the same thing falls out of the open/close numbers.) I would install it straight up; 4 degrees is normally plenty of advance, but I am not sure if you would want any more for the turbo app.

BTW, I would expect it to help the low end boost too by getting low RPM cylinder pressures up and getting temps up a bit, and thus spooling up the turbo more quickly.

Thanks for the input. When I install with the adjustable sprocket on zero I end up with 2 degrees of advance compared to the numbers on the card. Are you suggesting I should retard it 2 degrees so it matches the cam card?

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If 4 degrees of advance is what you want, yes. The cam has 4 degrees ground in per the card. What I can't tell you is if more would be good or bad for this engine; my turbo experience is on 2.6L Mitsubishi's.

Just to check:
So your measurements are showing the intake opens @.050 valve lift at 1 crank degree before TDC, with the lash set cold at perhaps .002" less than spec'd? (Straight-up timing would be 1 crank degree after TDC per the card numbers.)

Or, alternatively, did you find the intake center line at 106 crank degrees, measuring .050 lifter drop from peak on either side? (Straight up ICL would be 108 crank degrees.)

And per the Howard Cams catalog on page 327, they ID 4 degrees as a commonly best advance and 6 as a maximum you'll need in most cases. So they have put their estimated likely 'best advance setting' in the grind.

When I was initially checking at 'zero' both intake opening and closing events (at .050") were 2 deg advanced from the cam card numbers.

I won't get back to the motor for a few days but will check my work again before buttoning it up.

I've read a recommendation to add a little advance to compensate for initial wear of the chain and sprockets. Maybe that would only apply to the stock style chain and not to a roller chain?

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Can't say that I ever measured any after run-in..... mebbe someone else will chime in.

I would install at 102-104. The 225 requires more advance that about every other motor out there for best overall performance (max HP and area under the curve). IIRC, you gain about 2 deg once the chain wears in, relative to the installed number.

Lou

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102-104 ICL is 8-10 degrees advance on this cam.....sure it should have that much? It is a turbo, so the advance effect on the low CR with the earlier valve closing is not going to be the same once the boost starts. I found an article on some 'charged' cams and the ground-in advance was all in the 2-5 degree range. I would be afraid to reach the detonation point a bit earlier with too much advance.

http://www.hotrod.com/how-to/engine/hrd ... d-engines/

The OP might gain some insight by googling 'turbo cam advance'.'

Since something didn't seem to be adding up right I decided I should record each opening and closing event and run the numbers through one of the cam calculators. All measurements were taken at 0.050" lift with zero lash and the crank sprocket at the zero position. Cam card specs are in parentheses.

Intake opens - 6* BTDC (1* ATDC)
Intake closes - 39* ABDC (35*)
Exhaust opens - 43* BBDC (43*)
Exhaust closes - 15* BTDC (9*)
Intake duration - 225* (214*)
Exhaust duration - 208* (214*)
Overlap - minus 9* (minus 10*)
Intake centerline - 106.5* (108*)
LSA - 112.75 (112*)

So the intake closing is a little late but close and exhaust opening is right on the money.
There's an extra 11 degrees of intake duration mostly on the opening side and 6 degrees of exhaust duration missing from the closing side. Overlap is nearly the same and intake centerline is 1.5* advanced.

Since it doesn't affect overlap is the extra intake duration a bad thing?
Am I hosed? Should I run it? Start over with something else?

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More intake duration is better on a 225, at least on NA motors. The exh port flows very well relative to the intake (usually 80%), and most perf/V8 motors are more like 60-65%, so you can use more int dur on a Slant.

I stand by my centerline statement above. If it were me, I would use 104 installed centerline, no more.

Lou

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Home of Slant6-powered fun machines since 1988

Looking at your numbers and assuming you measured at the valve:

The extended intake duration may due to the zero lash instead of the approximately .020" lash. The intake open is advanced and the intake close is retarded from their numbers so that makes some sense

But, the exhaust should show extended duration too with zero lash, but that is not the case; the opening is on-time and the closing is way advanced, so that is like there is more than .020 lash on the exhaust side. So either the cam is way wrong or the measurements need to be re-checked with the lash carefully set.

If the lashes are not at .020" (estimate .022" cold), durations will be in error, and the overlap will be in error unless the lash errors just happen to cancel out. The ICL and ECL should not be effected by lash errors for symmetrical cam lobes.

Did you use a dial indicator for the .050" measurement?

BTW, the Howard installation sheets describe measuring at the lifter but lash would not effect that, so I can't tell for sure if their 214 duration numbers at .050" are at the lifter or the valve. It would normally be at the valve...

I'd retake the measurements and not dump the cam yet! Their machinery is usually much better than ours.....

Measurements were taken at the valve with zero lash.
I've timed a lot of cams (on motorcycles) and am 100% confident of my numbers.

Just to be sure I wasn't missing something I did some measurements using 0.022" of lash but that generated numbers way off from the cam card. Adding in the lash subtracts about 15* of duration off of each end of a valve open/close event. Measuring using 0.050" lobe lift at the lifter would subtract even more degrees of duration.

If all I was doing was adjusting the intake centerline it would not matter but I want to feel as confident as I can that that the cam will work OK for this motor. The goal is to create a motor that is street friendly enough for a daily driver that can also take enough boost to make 350-400 hp without revving to the moon.

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