So if I put the crank sprocket on the +2 slot I'll have 104.5* intake centerline. The 2 most important events - intake closing and exhaust opening will still be pretty close to the cam card spec.

I can't see a detrimental effect in closing the exhaust valve a little early. I believe the overlap is in the ballpark for a turbo motor. As you say, it seems like the additional intake duration would be beneficial even with boost.

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Seriously not raggin' on anyone at all, but the exhaust duration is so far off the card numbers that something is up..... I've never measured one that is more than 2-3 degrees off the card. Wrong cam? I'd be wondering about it all at this point. JMHO

I agree it might be worth a call to Howards. I also have not seen numbers that far off from a cam card before.

Lou

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Home of Slant6-powered fun machines since 1988

Yes, it's obviously not as advertised but as I read more about cams and cam timing for turbo applications it just might be better for my project than it would be if it matched the cam card exactly.

What I have is a somewhat exaggerated 'reverse pattern'. I gather that most reverse pattern cams will have 4*-8* less exhaust duration. I've got 17*

I'll call Howard's and see what they have to say.

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Cam card numbers are at the cam, at a specified amount of lift, typically .050. At the valve adds the ratio of the rocker to the figures, and throws the numbers off.

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If that is true then it is even farther off.

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Once you are on the lobe, you are not working with linear measurements related to lift and degrees, slope angle messes that up. A degree or two can be a few thousandths lift. The start of the lobe is more gradual than say, half or 3/4 of the way up.

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An officialis Tyrde-Browne Racing membrum

Well, I take that back.

Measuring directly off the ends of the pushrods I have new numbers. These are measured at 0.050" of lobe lift.

Intake opens - 1* BTDC (1* ATDC)
Intake closes - 33* ABDC (35*)
Exhaust opens - 45* BBDC (43*)
Exhaust closes - 12* BTDC (9*)
Intake duration - 214* (214*)
Exhaust duration - 213* (214*)
Overlap - minus 11* (minus 10*)
Intake centerline - 106 (108*)
LSA - 112.25* (112*)

Right on! Thanks!

I'm still trying to wrap my head around why measuring at the lobe made my exhaust duration figure increase while the intake duration decreased.

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Whew....my brain feels better now LOL

For me, it is so easy to reverse a number or list a 'before' as an 'after'. You may never know what happened before: I just take it as a given that if I am way off from the cam card anywhere, I need to do it again. The openings and closings are all consistently advanced now, consistent with the ICL, so that is a good sign that you got good numbers.

Looks great. I would still recommend 104 installed centerline, but 106 might be fine too. There are much less concrete data and results for boosted 225 engines.

Lou

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FWIW, in thinking about the advance exhaust closure a bit more, one thing to keep in mind that with a turbo, there is exhaust back-pressure in the form of the turbo impeller that you can't get rid of like in a NA engine. The piston has to be used to push out as much exhaust gas as possible. So getting too far advanced would be detrimental to that. But with this cam, the difference between 9 degrees BTDC exh. closure (4 degrees advance or 108 ICL) and 13 degrees BTDC (8 degrees advance or 104 ICL) in terms of being near the top of the piston stroke is miniscule, so I am reasoning (guessing?) that it won't make much difference there. Similarly on the intake; you're gonna be around TDC regardless in this range of cam advance/ICL settings, and there won't be much change at all in cylinder filling with turbo pressure.

This cam with it's negative overlap is different from standard NA cams for sure.

Thanks guys, I don't imagine there will be all that much difference between 106 and 104. I'm going to go ahead and set it at 104 and assume that as things wear it may end up at 105. One thing I read from a few different sources (not sure if I really buy it) is that opening the exhaust earlier gives a bit more velocity to the exhaust gasses to help spin the turbo. That sounds somewhat plausible and nudges me over the edge for advancing to 104* ICL.

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Yes, the velocity of the pulses is common knowledge to make a turbo come up faster. FYI the biggest things I did on my Starion 2.6L turbo to make it spool MUCH faster was to:
1. Buy a turbo with the impeller tweaked specifically for better low flow behavior (16G super)
2. Then I increased the true static CR from 7.3 to 8.2; that really took advantage of the tweaked impeller, plus made the engine torque better as the turbo was coming on. With this 2nd mod, the turbo is spinning and burbling out the exhaust even at low idle. With the higher CR, the engine still does not detonate until around 17-18 psi.

Well, also keep in mind that this is measured@.050, not advertised (You may have caught this, dunno)

My 264*(219@.050) cam set on a 109 LSA only has 1* of overlap according to the cam card. It really has 46* when you're talking adv. If my cam was ground on a 110 like the normal grind it'd have negative overlap@.050 as well. His cam has around 25* advertised overlap (according to the cam paper).

Yeah, I thought about that... later, of course....LOL but did not think any one would be interested in any edits of my ramblings. (And flow below .050 is relatively small so the net effect is low). You are of course 100% on the money. Tnx