Cam Design Pros Weigh In

One comment I'd make is that on engine shows where they talk about LSA (like Engine Masters), they never give the whole story as LSA only defines the relationship between the intake and exhaust, but not how they are related to the crankshaft. Another example is this article from Motortrend:

https://www.motortrend.com/how-to/camshaft-shootout-lobe-separation-angle-tested-explained

where they compare three cams of different LSA, but how they decided to set each cam relative to the crankshaft is never disclosed. They do give enough information to figure out how things were set (by giving the intake lobe closing value for each cam), but never why those particular positions were selected for each cam:

101 LSA cam
intake cam peak lift = 101 deg ATDC
exhaust cam peak lift = 101 deg BTDC

107 LSA cam
intake cam peak lift = 107 deg ATDC
exhaust cam peak lift = 107 deg BTDC

113 LSA cam
intake cam peak lift = 109 deg ATDC
exhaust cam peak lift = 117 deg BTDC

You've got a ton of overlap so I'd have to think:

1. your idle vacuum would be very low

and

2. a lot of the intake charge would be escaping directly into the exhaust (i.e., your MPG is terrible).

Are you seeing those effects, and how did you decide on the settings that you have now? (It looks to me that, relative to a late euro carbed 308, you sort of split the additional intake duration on both sides, but all of the extra exhaust duration was added to the closing side = huge overlap increase.)

 

I would keep the intake where it's at and advance the exhaust 10-12deg.

 

Anything from where you are would be a plus . I believe in boosted applications most manufacturers reduce the overlap (compared to the same NA model) to give a smoother idle, more idle vacuum, and better MPG (because they are getting plenty of power anyway from the boost). The GTB/GTS (328) Turbo cam specs aren't much different from the 208 Turbo specs that you posted:
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Setting the intake and exhaust lobe centers closer to the same positions as the GTB/GTS Turbo specs would be a quasi-reasonable place to start IMO (unless you have some specific application requirements).

 

I don't know the specs of a 208 turbo turbocharger but I would guess it needs to be rather small to have any kind of response on a 2 liter V8.

That would result in rather high turbine drive pressure at high rpm meaning it would tolerate less overlap.

IMO, not necessary to reduce overlap that much on a supercharged engine, especially a low psi centrifugal which doesn't make much/any boost at mid/low rpm.

So to me, if you go too far on advancing the exhaust you will soften up the low and mid range with no high rpm benefit and possibly no real gains in drivability.

 

Which just shows that huge duration is not always a good thing in all cases. I wrote "closer to", not "exactly the same as" -- so your suggested 12 deg might be a reasonable thing to try (even 8 deg would put the lobe centers at about the same locations as a NA carbed euro), but the bottom line is your BSFC/MPG is going to be horrible at higher RPM unless you use a less duration cam. Also, it's usually a good idea to have your power brakes work at idle .

 

My thought for what little it's worth is those cams are quite mild and timing quite odd. Its generally best to talk about time by lobe center and most all engines want the intake in the 102-110 range and the exhaust in the 106-114 range. There are a few reasons to go out of that range but normally they are about trying to compensate for something else that is suboptimal. I would suggest putting the lobe centers at 108/108 or maybe106/110 as a starting point and you'll be much happier....your current exhaust cam timing has got to be costing you 30+hp

A long duration cam would have your durations at .050" lift and 300-320 degrees at 0 lash so what you have is relatively mild and no concern at all with your 6psi as that is not enough boost to have much impact on cams needs. Just as a reference point, the headers are creating about 5-7 psi vacuum during overlap in the tuned region so its perfectly normal to see differential pressures like that. The SAE standard for measuring seat to seat cam duration is at 0.006" lift and anything below that the valve is considered and acts closes as flows are too low to be meaningful. And that means actual lift, so lash plus 6 or 14 thou. with performance cams in the US we usually talk about duration at .050" lift, in Europe you often see 1mm used (.039") and if I'm remembering right the ferrari pollution cams are 220 deg at .050". the early non pollution are 240@050 and P2 [email protected], then add about 40 degrees to get the 0 lash numbers so you are right around the stock pollution/CIS cam specs duration wise I think. For comparison my frankenferrari pulls good vacuum and idles smoothly at 700rpm with 246/[email protected] (at the valve - cam is 241/247@050 with lash), 273/[email protected] valve lift and 0 lash at about 290/300 and I have the timing at 104/110 and would thing twice about that timing with 12-15psi boost but would probably roll them a bit to maybe about that to maybe 106/112....very little change because the engine, cam, intake, and exhaust are a set so its more like there is the right place to set them so everything works well together and there is everywhere else.

I could plug it into the simulation software (dynomation6) but I've done a ton of 308 simulations and I'm pretty confident it won't suggest anyhjitng different....ferrari got the lobe centers right.

 

I just checked my files, I think those are the Web Cams CIS grind. Not sure why someone did that but 108/108 should be about the best.

 

hmm..something is wrong in my notes, I just checked the Web cam site and this is there 78-82 recommendation which is similar but more aggressive that what you appear to have


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No no.

at 700 rpm its at 40kPa so about 18inHg vaccum.

The 5-7psi thing is how much vacuum will be seen at the exhaust port at about peak torque rpm when the headers are working their best

You can get a much better idea of how aggressive you cams are is you measure the duration@ 0.050"lift....that is a standard that is used. 200-220 are OEM driver type cams, 220-240 is a nice street cam, 240-260 hot street, 260-280 race as a basic guideline. But again there is nothing about you cam specs that is anyway concerns my about a carb engine with 6psi boost or would cause me to want lobe centers different from ferrari OEM numbers

 

Just for reference, this is kind of what port pressures look like but it changes with rpm. Here its almost -7psi at EVC
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Same engine (franken ferrari) with about 6 psi screw type blower to get boost everywhere. top graph is the hp & boost. Then the data you can see in the 2nd from fight column the charge loss to exhaust....there is some but its not much.

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Same engine no boost for comparison so eyeballing it looks like 6psi about doubled the exhaust charge loss
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That seems very wrong. retime the cams and you should see 15-20inHg and it will run a whole lot better I'd think.


I just realized that I'd misread you chart and it was at .020 lift, for some reason I thought those were seat to seat numbers. Was it measured right on the cam or 0lash on the bucket of normal lash on the bucket?

 

Last thought. My guess is it will be way rich everywhere as it will be pulling more vacuum and flowing more air. I'm no weber expert but that is what I'd expect with other carbs I've played with....more air makes a bigger pressure drop through the venturi so it goes rich

 

how did you decide you were at the point? like a .001 feeler gage or when you saw a dial indicator start to move? I normally shim to 0 then read the 020 or 050 or whatever on the dial indicator.

Here is my whole cam card as a reference so 269/275@020 and you can see how duration changes with measured lift. Remember too that mine is a 4V with very large valves (34/28) so this cam acts about 10 degrees bigger in my engine than it would in a 2V.

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Looks quasi-reasonable to me for dealing with the increased cam durations that you have. The downside would be the exhaust valve opening a little early and the intake valve closing a little late, but don't think you can do much else without having huge overlap and very low idle vacuum and very poor MPG/BSFC (which you'll probably still have a little of both of anyway ). Please post you results/feel when you get it all running.

 

It's probably going to run a whole lot better! I would still sugest moving the intake closer to 106 or 108 but what you have should be just fine imo.

 

Isn't 25 degrees 1 tooth on the belt or am I remembering wrong? Just wondering if that might explain that odd exhaust cam timing?