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Intake valve size to exhaust valve size ratio

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Thought this would be a good topic.


Both pi and npi have the same intake valve size, but the pi has the bigger exhaust valve. Doing the ratio math puts the pi head exhaust valve size to the intake at 80% while the npi is 78%.


Now I havent measured the actual exhaust hole size of each head, but I will early tomorrow just for this thread sake. Ill also add the pi intake hole is .050" smaller than the npi even though valve sizes are the same.


{I have measure the exhaust valve holes of both heads and comes in at 1.120" for the npi head and 1.200" for the pi head}


Exhaust blowdown is an excellent topic and fun to research. How the engine blows down once the exhaust valve hangs open is really where the power is and of course its the camshafts bumps that determine how, where and when the valve opens.


Exhaust blowdown begins directly at exhaust valve lifting off the valve seat.


Performance "off the shelf" (<remember that) cams has the exhaust valve opening very early and for good reason. If any of you have followed me youll know I campaign off the shelf cams are your best bet, cause once you learn the benefits,  you can send them back for regrinding to your particular build and this is the exact reason why cam companies do this. 

Edited by 96blackgt54
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You always have something interesting to throw at us! I love it!


How would one go about finding the valve open point that is needed to make the exhaust open at or slightly after bdc? Would we map the cam and have a degree wheel on there, then find bdc and tell them we want the exhaust valve to open at this point?

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You nailed it. Only problem is the modular valve train isnt 100% direct mechanical. Unlike a pushrod cantilever operation, the follower does just that. It follows! The valve spring or the lasher.....one of them has to give in before the valve opens and with constant oil pressure being applied to the lasher, this alters its caracteristics. High rpm means more oil psi.


The lasher does have a zero lock point and works while oil psi is available,  but untill its lock point happens the amount of psi changes its resistance to the lock point. Higher pound valve springs changes this as well. 


Ones got to think the follower just floats. Even though most of the movement is spring side, a float happens untill the lasher locks.


Now add in cylinder compression psi causing resistance for the exhaust valve to open. The 2000psi pushing that valve shut has an affect on the components as well. Again altering valve timing.


One can see how complex the modular valve train is a head scratcher.


To answer your question, im not really sure if I can. Altering LSA (lobe separation angle) from 116° to something like 106° is 10° at the cam and °20 at the crank. In other words delaying the exhaust 20° opening. This is an easy to look at scenario, but would require regrinding the entire lobe to achieve that desire LSA. Imagine a pic of both lobes and the exhaust rotates toward the intake. Intake needs to remain the same. Thats why when observing cam profiles the intake duration is usually the part#. Choose your intake duration and special grind the exhaust.


Let me say this. Old drag racers would keep track of 3 to 4 cams rotating them out at the track trying to find the sweet spot. Since these cams are in block cams, the lobes were tiny, unlike our giant lobes. And most of the inblock cams were flat tappet as well being super forgiving to irregular surfaces. These racers would hand stone the exhaust lobes opening points. Not really knowing exact measures of the cam after stoning or if each lobe was spot on they knew what works. Simple tracking like scribing a reference point on the lobe side was all they had. Paint, dykem, marks and such kept their hand stoning within a margin of error.

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Take this camshaft front end view. The red is exhaust lobe. The exhaust lobe is first to operate the cylinder. The highest lift points in the direct center of both lobes is lobe separation angle. Now picture motioning the exhaust lobe rotating counter clock wise keeping the intake stationary.


This is tightening the LSA which in turn has delayed exhaust opening and increased overlap lift, but reduced overlap timing. 


 If you have to, print a few of the picture and make lobe cut outs....design a cam...lol. Maybe a follower too. Youll gain a better understanding of whats happening and when it happens.


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