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Old 10-14-2009, 01:51 PM   #21
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Thinking more and more on this...

ALL of the overlap in the stock cams is under 0.050" lift, which must mean that there is some significant airflow down there... otherwise why would we be worried about it?

Stock BP05 cams have 26.5* separating the intake and exhaust points at 0.05", yet they have 19* overlap

Stock BP26 cams have ~35* at the same point, yet they have 10* overlap

Now with the 505+2.5 cams set with the same LSA as the BP26 cams I only have 24.5* separating the 0.05" points... which would likely mean significant overlap of 20*+

So, I guess I need to open up the LSA even farther... I'll probably leave the exhaust cam where it is and retard the intake. IIRC, retarding the intake shifts the powerband up in the RPM range, which will hopefully increase my 6000+rpm VE. It falls off HARD, I'm down to like 84% VE by 7k at 23 psi. At 5k, same boost level, it is at about 112% VE.
Nothing is happening until .050"! The reason there is duration and lift before .050", is because the cam has to be working before anything starts to happen. Also, the valve is not open enough below .050" for air and fuel to be flowing in and out of the cylinder head. That is what you want, air and fuel to be moving. Advertized duration is not when things are happening. When the valve opens is when it happens. That is why the .050 specs are so important. It also gives you an apples to apples comparison between cam manufactures.

The cams that you were refering to will have a lot (or all) of the overlap long gone by the time the valve opens.

However, overhead cam motors with no rockers, will open the valve sooner than any motor with rockers. That is part of the reason the cam specs seem small.

Do not get too caught up in it until you tune. It will make you crazy.
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Old 10-14-2009, 02:25 PM   #22
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well then we essentially have negative overlap is what your saying? Intake and exhaust are never open at the same time?
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Old 10-14-2009, 04:04 PM   #23
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well then we essentially have negative overlap is what your saying? Intake and exhaust are never open at the same time?
That wouldn't surprise me on a stock cam or a small 205 @ .050 size cam. Stockers have minimal overlap for the sake of emissions. A 205 is a conservative grind, so a minimal overlap doesn't surprise me there either.

The '94 to '97 exhaust cam has a similar duration (205 @ .050) but not as much lift. This is why the "exhintake" cam swap is common (that and it is cheap). Your increased lift on both cams should certainly show some flow benefits over the exhintake mod.

And don't worry that I called your cams "small." Turbos like larger than stock cams a little but don't go well with really big cams. It comes back to the overlap problem. But hey, since you've run the numbers and the overlap looks like a non-issue, I think you will be just fine. I would start with them "straight up". Then later I would definitely dyno tune those cams in for the max benefit. Having better cams may raise your rpm range enough to make you happy without retarding.

And yes, retarded intake cams remove low-end torque and shift the torque slightly up the rpm range (200-400rpm). Not only is it common engine builder knowledge, but I've simulated it enough on the computer program "Desktop Dyno" to make anyone a believer. (BTW my '68 GTO has a cam with a duration of 231 intake and 240 exhaust @ .050, which would be terrible for a forced induction application, or even a small displacement application. Also, Miata heads flow so much better that the valves don't need to be open so long to make good VE numbers.)

As it was stated with regard to "advertised duration" numbers, they are measured differently by different manufacturers. I know some are measured at .006 lift and some at .001 or even a few at .010. Numbers at .050 are a standard that is universally accepted and always published, so they are the best measure of apples to apples.

Good luck.
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Old 10-15-2009, 10:50 PM   #24
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you should try to pick up a set of hks 256s. in my 1.6 they made a world of difference in the mid range to high end power, in fact they were good for 22whp with absolutely no other mods to the car. 114.9whp on an otherwise completely stock 1.6.
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Old 10-16-2009, 09:35 AM   #25
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N/A != boost
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Old 10-16-2009, 09:42 AM   #26
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you should try to pick up a set of hks 256s. in my 1.6 they made a world of difference in the mid range to high end power, in fact they were good for 22whp with absolutely no other mods to the car. 114.9whp on an otherwise completely stock 1.6.
Thats funny because my friends J-limited had 264s and only had 116whp and without a strut brace the car wouldnt idle at traffic lights. They only moved the power really but the profile didnt exactly suit compared to other 5 developed ones
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Old 10-16-2009, 01:50 PM   #27
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Thats funny because my friends J-limited had 264s and only had 116whp and without a strut brace the car wouldnt idle at traffic lights. They only moved the power really but the profile didnt exactly suit compared to other 5 developed ones
You lost me. How would a strut brace help a car idle?
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Old 10-30-2009, 10:46 AM   #28
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We don't actually know why it helps but without it the car dies at traffic lights. With it there is nothing but a louder growl and slightly shifted powerband.
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Old 10-30-2009, 11:14 AM   #29
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What what? Sounds like bollocks to me, Proof or it never happened!
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Old 10-30-2009, 11:50 AM   #30
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yeah... that kinda sounds like BS to me LOL
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