According to tests that were conducted, (as stated on another list) a variance of 35*C is worth app. 1 FULL a/f ratio number. BTW, in Fahrenheit that equates to 95*. If in doubt as to the accuracy of this post, maybe FJO will be willing to expand more on this.

 

When you are playing with 0.5 in. Hg vacuum ( about .26 psi) in an inlet, resonance tuning that can achieve 5 psi positive pressure pulses near intake closing point appears to help fill the cylinders quite well. In an SBC engine I am familiar with, I believe this is the reason it achieved over 1.4 lb-ft/cubic inch at torque peak (over 525 ft-lb from 365 CID) NA, 92 octane, carbureted.

The single plane manifold used (Super Victor) was chosen for it's approximately 5-6 in runner lengths. Shorter runners (4-5 inches) couldn't be made to tune in the 2500-6500 range this engine was designed for. LT1 length (3 inches or so) seemed even worse.

IMO, resonance tuning length doesn't really change much from wet flow (carbureted) to dry/mixed flow (LT1, LS1) where fuel is injected prior to the inlet valve. Maybe it does. Any input?

If we are going for more power on an NA LT1, I think inlet tuning is very important.

I just don't have a good solution in mind as to how to economically modify an LT1 for longer runners and still fit under the hood. IMO, GM had the same problem when the LT1 was designed to fit the Corvette; they sacrificed some tuning. Remember this was the follow-on to the Tuned Port long runners which were probably designed for low/mid range torque on 305 CID, and were two small and too long for higher rpm larger displacement engines.

My highly opinionated $.02.

 

From what I saw with the tuning of my engine (by others ), the air distribution isn't all that bad - but that is the LT4 intake, with internal mod's by CNC Cylinder Heads. Even with the ability to trim individual cylinder fuel rates, it wasn't necessary in the intended peformance range (5,000-7,200rpm). Again, this is a fairly mild cam application, so maybe that reduces the problems. But even when adding a 300-shot of dry nitrous at the outlet of a WS6 airbox showed absolutely no measurable maldistribution. It wasn't necessary to do anything with the individual injector flows to compensate for maldistribution. That tells me the problem may not be as bad as Trey's initial post would have us believe.

As far as spraying nitrous through the back of the intake, where it would not have the benefit of turbulent (velocity through the TB) mixing with the incoming air, it would seem to be ripe for disaster. We looked at a conventional "plate" behind the TB for dry nitrous addition, and it was terrible.... huge maldistribution of nitrous to the eight cylinders.

 

Not to say that the condition cannot be improved upon but all in all... I agree. The distribution problems are much less a problem than say a wet-flow intake manifold of the same port configuration.

No, the issue I'm speaking of is in regards to a "siamesed" port configuration and pressure wave interaction that leads to irregular a/f distribution, which is a greater factor in this design. This can be seen in any siamese port intake manifold and I encourage you to design and test to see the true differences in each. You will not see the same variation in an individual port configuration. At least I haven't in many years of design work now. And the distribution problems I've seen are usually observed in 5-7 and 1-2, not just 7-8. I'd be more than willing to share my research with those who have actually performed research in this area. Otherwise I don't get much out of this and neither do you. A book suggestion would likely get you a higher degree of information... on a theoretical level anyway. Then you just have to apply it.

The issue here is helped with an increase in plenum volume. Especially true of wet-manifolds but still good practice in dry manifolds. You'll see this approach was taken in the LS6 intake's development. And Fred is right in his assumption, the problems only get worse with larger camshafts (greater overlap periods).

Take care

 

If I was going to build another naturally aspirated LT1, I would try the TPIS miniram. Same basic design as the LTx manifold, but instead of straight/twisted runners it has drastically tapered runners (the top of the port at the plenum is much larger than where the port meets the head).

I do think the LTx manifold works very well though. My 383 made 453rwhp 416rwtq with ported production LT1 heads and intake. I think 500rwhp out of a solid roller stroker with ported production LT4 heads/intake should be attainable (even with the stock PCM and it's 7k limit).