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Discussion Starter · #1 ·
Hey guys looking for some input here. First question regards lifters in a SBC. Currently I have a lunati voodo hydraulic flat tappet cam in my 383 stroker. I am using with it a set of lunati hydraulic lifters. The engine is coming out and being rebuilt. I am thinking of switching to a set of mechanical lifters instead of the hydraulics. Question 1 is would I be able to use a set of mechanical lifters on this cam or do I need a mechanical flat tappet cam or is there no difference between a hydraulic flat tappet cam and a mechanical flat tappet cam? Question 2 being, with all other parts remaining the same, what main differences would one notice between using mechanical flat tappet lifters opposed to hydraulic flat tapet lifters? I know mechanical lifters are louder but would there be any difference performance wise? I would imagine mechanical lifters would lower the engines power because they use a clearance between the rocker and valve stem thus both the cams lift and duration would appear to be smaller since that clearance would need to taken up for the first few degrees of duration. Would this not shorten the duration making the cam profile different? My next question regards cylinder head intake port volumes. I know the larger your intake port volume the higher up in the rpm band your engine will make power and the smaller the volume the lower in rpm your engine will produce power. My question is, with all parts being the same and a moderate cam, which head would offer more power down low...... a head with 195cc intake port volume or one with 220cc intake port volume. I understand once you reach a certain rpm the smaller port will not be able to flow enough thus the bigger port will offer an increase in power but how is the power affected down low where both ports are capable of flowing the amount of air needed?? Anyways any input here would be greatly appreciated as always.

Thanks,
Keith :welcome:
 

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1. The profiles are different. To use mechanical tappets on a hydraulic grind or hydraulic tappets on a mechanical grind is to invite failure.

2. Mechanical lifters are normally chosen if you plan to operate the motor into a rev range where hydraulic lifters would pump up and stall the valve train (float the valves). Don't worry about losing lift at the lash point. If that's your worry, use a cam with more lift or change the rockers to a higher ratio.

3. For any given motor, going to larger port volume will slow down the mixture and you could lose velocity to a point where the cylinders would not be filled as well as they would have been with a smaller port volume, assuming equally efficient port design. A rule of thumb for street/strip motors is cubic inches times 1/2 for intake port volume. Example, 383 X .5 = 191.5, with 195 being the closest commercially available intake port size. Again, this would be for a street/strip motor with a rev limit of maybe 6000 rpm's. If you were building a dedicated race motor that might rev to 8000 or 8500, then the motor is going to pass more air and you wouldn't necessarily be concerned with low rpm cylinder packing, so you would use larger volume intake ports to prevent stalling of the port at the higher revs.

I haven't done this before, but let's see if we can use a little math to figure this out.

A well built motor will have a volumetric efficiency of around 100%. That means that a 383 turning 6000 rpm's will pass 665 CFM. (383 x 6000 /3456). A not-so-well-built motor might have a volumetric efficiency of 80% (383 x 6000 /3456 x .8) and a really well-designed motor could have a volumetric efficiency of 110% (383 x 6000 /3456 x 1.1). I don't know for sure how valid it is, but 100% is what I shoot for when I'm playing around on the DynoSim and putting combinations together. That, along with dynamic compression ratio and BMEP. One of the other goals I shoot for, on a street motor, is making 400 ft/lbs of torque at 2000 rpm's. For a 350/383 street motor, that's a tough thing to do and really takes some searching for the right parts.

Now, if we divide 195 by 665, we arrive at a factor of 0.293 (195/665 = 0.293233). So, let's play what if and apply that factor to a 383 turning 8000 and see what happens. 8000 X 383 /3456 = 886 CFM. Now, if we multiply 886 by our factor of 0.293, the math tells us that we should be using an intake port volume of 260 cc's.

Playing down on the other side of the scale, let's say that we're building a motor for towing and the rpm's will be limited to 4500. 383 X 4500 / 3456 = 499 CFM. 499 x 0.293 = 146 cc port volume to maintain adequate flow and velocity.

You mentioned 220cc ports in your post. Let's look at what CFM would be required and thus what revs would be required to support that intake port size.....
If we divide 220 X 0.293, we find that this port would support 750 CFM. A 383 running 6775 rpm's will pass 750 CFM, so if you wanted to build the motor to operate at 6775 with a 220cc intake port, using the proper valvetrain and reciprocating components, including a solid flat tappet cam or solid roller tappet cam, you would conceivably have a valid combination.

Now, there are quite a few individuals on this board who are sharper than I am and will most certainly shoot holes in this hypothesis, but it's the best I can do to try to explain for you how stuff works. :D

BOTTOM LINE, IN MY OPINION:
Err on the side of caution. If a 383 will make the most power with 195cc intake ports, it doesn't make any sense to use 220cc intake ports and slow the mixture down in the runner at lower engine speeds. You also need to take into consideration that the motor isn't always operated at max power in a street/strip application. You could conceivably have a faster car by using a smaller 180cc port volume, because you might make MORE POWER UNDER CURVE, all the way up to the point of MAX power. In other words, the smaller volume might keep velocity up better than the larger ports at 2500, 3500, 4500 and 5500 and pack the cylinders better while the intake valve is open.
 

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Discussion Starter · #3 ·
WOW..... thanks for all the info!! I was thinking along the same lines as you were there however I didn't know the equation or math that is used in determining this. I can see how the smaller port would have more velocity at lower rpms but then would reach its MAX velocity at some point and wouldn't flow any more past this rpm. I currently have a set of AFR aluminum heads with 195cc runners and a holley 670 carb...... using the numbers you provided these would work well together up to 6000rpms in a 383. I agree that anything more than 6000rpms for a 383 is getting very dangerous as I have been told by more than one person that 383's don't like reving high at all and are prone to failure if reved too high. Moving onto the lifter portion of my question, the reason I want to use mechanical lifters is because I tried and tried and tried again to properly adjust my hydraulic lifter pre-load and I just couldn't do it. I used 3 different methods and still couldn't get them right. I would much rather use a feeler gauge and set lash since this is much easier to me. However I have a hydraulic flat tappet cam so if I do go the mechanical lifter route I will need a new cam and lifters :sweat: . Anyways thanks for your help if anyone else has anything on these topics don't hesitate!
 
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