333pg333

Some time ago I started a 3.5L 16v build. It never got finished and is sitting half built in a workshop. At that time I was enamoured with the idea of going big on the basis of having some brief flirtations with 3L 8v builds and their associated extra tq. If I ever finish that build I won’t be going to the extreme of such a big bore however. On an 8v I am thinking that the bigger bore you go, the more limited the motor becomes in terms of inlet valve size. This is based on discussions with 67King as some are aware of. If you want a TQ monster, then sure, go big. If you want the motor to rev beyond 6k and continue to make a bit of power then perhaps something a bit smaller might be the way to go? Perhaps when Shawn makes the new intake then this may help the valve size limitation. Hope so.

KSira

How do you measure deck clearance with hydraulic lifters?

refresh951

Do you mean valve clearance? When I "clayed" to check that the motor was non-interference (meaning valves will not hit pistons at any crank angle) I used lifters that were pumped up.

refresh951

I guess I would think of it more as displacement limited as the more displacement, the more mach index comes into play on the 8V. The larger the bore however, the bigger the valves can effectively become from a physical and shrouding perspective.

From what I have researched, it may be that the mach index hit to VE may not be as severe as the hit to VE on the top end caused by longer runners. Thus the new intake and dyno testing. Hopefully have the intake done in about 10 days.

V2Rocket

Thoughts on a 2.5 16v motor, sleeved for 4" Chev pistons, and a 944 3.0 crank ground to 93-95mm stroke?

Slight overbore would help the valves breathe better, without taking out so much material that the cylinders move too much and the 16v head is apparently stiffer than the 8v. Long stroke for added displacement for a street motor...~6000RPM is all I need.

Would work out to 3.0 or 3.1L or so.

refresh951

Your basically talking my current 3.12L (4.03" bore, 94.75mm stroke). I am very happy with it but I am trying to get a bit more top end from about 5000-6500 rpm. It really performs amazingly up to about 5100 rpm. Hopefully the intake will show some improvement. The 16V head would be ideal as it would greatly improves flow and mach index. I would still go short runner/large plenum for the intake however. Duke's Green Hunter 16V motor ran a Supra style short runner intake and I think the performance across the whole power band was really outstanding.

URG8RB8

Great thread that should be a Sticky!

67King

Actually, the opposite is true. Two phenomena. Mach index (aka Z-factor) is a ceiling on airflow, the runners are just tuning, which means pressure. So you can tune your intake to have a higher pressure at EVO, but you may not be able to get the air in there because of velocity, no matter what you do. As long as you aren't limited by velocity, you can tune your intake to tune higher in the RPM range, meaning higher pressure at EVO.

The intake tuning decline is gradual and more or less linear, both up and down. The mach index limitation has no ramp going up, it just is like a cliff on the downside.

Putting it all together, mach index is primarily a peak power consideration, and intake tuning is primarily a peak torque consideration. So the approach I take is find out where I want my peak power, size the valve accordingly (or if you are physically limited, determine peak RPM available from mach index), and then design the intake to tune at an appropriate RPM point below that to get a good, broad curve.

KSira

Yes, that's exactly what i meant. How do you pump up the lifter? I have been told that the lifter will flex and have to use softer a spring or a solid lifter.

V2Rocket

I think the 16v head would probably solve the top-end issue, but I will be interested in your findings with that manifold.
I have a set of cut-off 8v runners that might be fun to mess with someday...

refresh951

As always I appreciate your time and knowledge. What do you make of this curve I have posted a couple of times? The runner lengths effect on VE looks to be non-linear and with long runners like the stock intake (440 mm) the dropoff in VE at high rpm can be dramatic (about 20% VE drop from 5000 to 6000 rpm).

The mach index downside looks like a linear drop after 0.6 of about a 25% in VE from 0.6 to 1.0.

67King

Look at the curve with no intake pipe, and the "baseline" curve is not completely flat, either. There are a lot of other things that will impact the shape of the curve. Other things will affect tuning of hte intake, the exhaust system will tune, the friction of hte engine is not contstant. In reality, it is sinusoidal, as it is a standing wave. As you move off of the peak, you don't fall off drastically, but rather, gradually. Shouldn't have said linear, but the main point is that the mach index is a true limitation. Think about it this way, your intake defines your peak torque. If the fall-off were hard, your peak power would be MUCH closer to your peak torque, but in most applications, there is a substantially large gap between the two.

Contrast that curve with this one. Notice how the curve is completely flat (it is normalized) when Z<0.6, but when it hits 0.6, it just goes right down. It won't go straight down, as the X-axis will be directly proportional to RPM. You can still turn RPM above Z=0.6, but you won't be flowing more air (and Z will go down proportionately):


I need to see if I can dig up some stuff on the intake, probably won't be soon, but again, the resonance is a standing wave, so it is sinusoidal.

refresh951

Great info Harry as usual. My torque drop from 5K to 6K is over double my worst case calculations for Z factor. The Z factor hit is inevitable with my current head but I am in search of the rest of the drop. I think the runner length could be about 40% but the dyno should tell me for sure. Hope I get something out of the effort as making an intake manifold is not easy.

BC

Take the lifter apart and stack some metal washers under the button to above the normal measurement for the lifter, and you will see where you are at more than normal lift with the clay on the piston.

KSira

Great tip. Thanks!