Don Terrill’s Speed-Talk

I just finished up a dyno test where I used a standard port intake with Trick Flow 270 Mopar big block heads. The 270 heads use the larger Max Wedge port size. The intake had been highly modified by Wilson and is my best standard port intake. It worked somewhat as expected with more torque down low than a MW intake but less power up top. However, the interesting thing was that the standard port intake had a "divot" in the torque curve right around 5200 rpm. The standard port intake made more torque than the MW intake below 4500 rpm but then started to fall behind. At 5200 rpm the standard port was down 60 ft lbs. But by 5800 rpm it was back to making more torque than the MW intake and then didn't give up the lead until 6100 rpm. At 7000 rpm the smaller port intake was behind by 30 ft lbs.

My guess is that there is some sort of reversion going on that really gets buzzing at 5200 rpm. Below that and above that the higher velocity from the smaller intake seems to help things. At the top end of the curve the smaller intake is probably sonic choked. I'll attach a picture that shows the port size difference. It is substantial, the smaller intake barely seals up the ports on the large port heads.

(guess we can't add attachments anymore so you'll just have to take my word for it)

I have witnessed this type scenario with many different brand engines.
I will presume that the intake runners and plenum have different architectures and volumes.
Their wave tuning, both sound and pressure, will definitely act differently; it is not necessarily mass airflow capability or velocities but, it certainly could be part of it.

How do you take the dip out of a torque curve that seems to appear around 3,000 rpm or so in the bigger cammed engines?

A small cam won't have the dip in it's curve or at least as bad a dip..

I'm thinking about the late, great JL's 502 and 496 BBC's with a 252, a 288AR and a 296 in them here, (or whatever?)...

The 252 cam was up a heck of a lot of torque vs the bigger cams around 3,000 rpm if not mistaken, and I am not digging my old mag's out to verify this.

pdq67

Another great example of 'bigger is not always better'.

The dip is made by the step or mismatch in port size for the port velocity seen at 4500 rpm.
Due to the mismatch the motor at 4500 see's the over all tuned lenght of the Intake runner as being only up to the heads Intake flange which is far too short of a tuned lenght for only 4500 rpm which then is making for the 60 ft lb drop in torque!

From the info that you have presented it would seem that you have found out that up to 7000 rpm that the best average power numbers will come by means of a overall port area being between what you have in the MW heads and smaller Intake.

I'll try again to post a picture. Not sure what the issue is, maybe a board issue?? This picture shows the difference between the smaller port and the larger port size. I think the area difference is almost 25%. I measured the internal volume on the intakes and the larger one had 18% more volume. So the physics of the internal flow could be significantly different with numbers like that.

Here is a picture of the test curves. A little hard to see on the screen but the 745 line shows a big dip right around 5200 rpm while the 775 line (big intake) is smooth in that area.

mag2555 wrote: ↑Mon Oct 09, 2017 8:54 am The dip is made by the step or mismatch in port size for the port velocity seen at 4500 rpm.
Due to the mismatch the motor at 4500 see's the over all tuned lenght of the Intake runner as being only up to the heads Intake flange which is far too short of a tuned lenght for only 4500 rpm which then is making for the 60 ft lb drop in torque!

From the info that you have presented it would seem that you have found out that up to 7000 rpm that the best average power numbers will come by means of a overall port area being between what you have in the MW heads and smaller Intake.

You might be right but no easy way to test that. The 240 heads were a little too small on this engine but the larger 270 heads seem to be a bit too big for this size of engine. There is only one large port intake available for this engine and it isn't a really good intake so that makes the problem more difficult to sort out. My Wilson small port intake worked better than the as cast big port intake when I first ran this combo. I then sent the cast intake to Wilson and they improved things a bunch, but it still isn't quite perfect. I'm thinking I'll convert this engine over to port EFI and then run the intake dry and see what happens.

Oh, now I see what your talking about with the heads!

I see that shift as being more to compensate for the paired left and right ports and the fact that bulk of the air mass entering the Head will be along the outside wall of each runner on a single plane Intake.

It also my help with the Coriolis effect on the opposite rotating ports?

This is probably an over simplified answer...... With no data or facts to back it up...... But...... IMO......it's as simple as:

The two fairly different sized csa's of the intake tract end up creating a situation where the effective shock wave tuning range is out of sync with the motor in that rpm range(frequency) where the dip is occurring.

Andy, the pic you posted of the MW gasket on the head may be misleading some here.
How the motor was run was with heads the size of the MW gasket, and an intake manifold with ports the size of the head in that pic.

Big heads, small intake.

I have to believe that 60ft/lb dip at 5200 would have a pretty noticeable negative impact on the 60' times in a typical bracket car with the ubiquitous "5000 stall" converter.

And that 60 ft lbs is also so 50 hp @ 4500 rpm!