I'm not trying to start another L98 vs LT1 war, they're pointless. I just found these results and thought they were kinda interesting so I figured would share.

A friend of mine has an IROC with a TPI and MAF computer on it currently. He eventually plans to convert it over to a speed density computer and a HSR intake. It's currently a heads / cam car so we know the TPI is choking it to death, but that's not the real point. Point is I was playing with TunerCat trying to make him a good chip that will at least get the car running with the SD computer. I was interested in putting my VE tables into his tune, but I can't just copy and paste because the VE tables are different dimensions from my DA computer to his 8D. It can be done cell by cell, and even at that you have to extrapolate some stuff, it's the perfect job for excel.

Anyway, in playing with excel I got it all set up so I can just plug in a set of VE tables from a DA computer and it'll do what needs to be done so I can just paste it into an 8D table. To test it I plugged in the stock VE values from a stock 5.7L TPI car and tossed in the numbers for a stock LT1. Curiosity got the better of me so I made one more sheet that simply compares the numbers.

Below is a picture of what I found. What you are looking at are LT1 values, formatted for the 8D computer and then the TPI values subtracted. In green are numbers where the LT1 has a higher volumetric efficiency and in red are numbers where the L98 has a higher volumetric efficiency.



Notice anything interesting? Just look at the power band where the L98 has a higher VE's programmed and you'll see that it's right at the L98's torque peak and where the L98 would beat an LT1 on a dyno as well.

Ultimately I wasn't sure if I was really looking at apples to apples. I mean does a value of X in the 8D computer mean the same thing in the DA computer? Just looking at that picture, I'd say it's pretty dang close. Anyway, I found it interesting, I hope you do too.

 

hmmmm!

 

The only thing I find surprising about this would be that the red isn't a little fatter. Because that would be closer to dynos/sims I've typically seen of TPIs.

Engine simulations with EAPro have shown me that short-running intakes (like the LT1 intake) have better VE low and high. In the low range, it's because short-runner intakes have less "pumping loss". Shorter pulses of air are easier to "load" into the cylinders at low rpms -- because it takes less energy to move a smaller air pulse (which has less mass). At higher rpms (e.g., above 2500rpms), VE is best where wave harmonics of the intake fill the cylinders best.

A TPI (with it's longer runner) catches the 3rd order harmonic (of intake reversion) at those mid-rpms. It fills better and has better VE -- at those rpms. As it moves above that harmonic (rpm), the wave disappates -- and so does VE.

Short runner intakes catch the 3rd order harmonic at higher rpms. Plus, they typically flow more air anyway. So, their VE is better at rpms above the TPI. The next harmonic is out of range for motors staying under 6k rpms.

I know Nathan already knows this stuff. But for those wondering.... whoop, there it is.

BTW: Intake reversion is the air which gets pushed back into the intake during the intake stroke. The intake valve has to gradually shut because it's on a smooth, curved surface. More power is made by shutting it AFTER the piston hits BDC. As it moves back up, the valve finishes closing -- but not before "reverted air" makes it's way back into the intake.

Or maybe what I see is just coincidence.....

gp

 

The pressure wave reflects at the open end of the runner/plenum interface so doesn't the pressure wave travel back down the runner and bounce off the closed intake valve? Of course there is damping but does not the pressure move up & down the runner till it is damped out?

Of course the "pumping action" happens if the intake valve happens to be open when the pressure wave arrives.

Of course the "real world" is a bit more complex, especially in 1st gear where the engine is reving very rapidly and produces a constantly increasing frequency of the pulse at the valve.

 

We have to remember too that there is a lot that plays into VE besides the intake. The LT1 had better exhaust, a different cam, higher compression, more timing, etc. Perhaps if everything but the intake was held equal we might see the TPI come out on top in a broader range.

I just thought it was really cool to see some numbers in the computer that more or less support what we already know.

BTW, I should mention that the numbers here are from a TPI F-body and a LT1 Corvette. Since a TPI Corvette performed better than a TPI F-body, and an LT1 Corvette Performs better than an LT1 F-body, perhaps that is skewing the results a little bit towards the LT1's favor. Let me see if I can did up the VE tables for a 91 Vette, that might look a little different.

 

Ok, it doesn't look like I've ever tuned a 91 Vette so I didn't have a stock table for it, but I do have one for a 90, I imagine they'd be damn near identical.

Here's how it compares to the LT1. Notice it is slightly different. The TPI has the edge in a wider load range at 4000 RPM. Above 4000 notice that even though it is still less than the LT1, it's not nearly as much less as the Camaro. I can't help but feel like this is mostly a function of the exhaust.

 

All good questions. Unfortunately I'm still learning....

To what you posted above, I'd add:

Why doesn't reversion mostly disappate when it exits the top of the runner and goes into the plenum. I can't see very much bouncing back down the runner. So, I assume it's a oscilating "war" between pressure in the plenum and runners. IOW, the pressure wave is held in the tube by "counter-pressure" from the plenum. ???

I've read 2nd, 3rd, and 4th order harmonics are the only ones that have much of an effect. IIRC, 2nd order retains about 10% of original pulse strength, 3rd order = 7%, and 4th about 3-4%. 5th order is SO weak, I believe the wave is considered disappated by that point. 2nd order isn't helpful/useable because of the conflict between tube length and rpm. (For example, the 2nd order harmonic occurs in the lower 6k rpm range for TPIs. But, HP has dropped SO low by 6k rpms, that going higher to hit it is -- meaningless.)

3rd order is the main variable. NASCAR tunes for the 3rd order harmonic wave. This extra 7% is why runner lengths increased again with the LS-series motors. They wanted to leverage that wave.

Your last point isn't something I've though of. But, the point seems valid. One thing I've heard recently is that long-runner intakes may run better with MORE duration on their intake cam lobes (than exhaust lobes). That's so the intake valve is open a bit longer -- just to catch more of the reversion waves. (And, less exhaust duration might work better if/when exhaust system flow is sufficiently good. This insures minimal HP bleed-off from too much TOTAL duration.)

 

Since the premise of this thread is (mostly) about the difference in intakes/tuning, the intake is elimated. The next biggest difference has to be the exhaust. But, I might add the heads as a factor too. They move extra air as well.

 

Now I've seen people claim that the L98 heads flow every bit as well, if not slightly better than the LT1 when comparing stock for stock. I have never compared them myself.

 

Gregg I think this thread was meant for you, it has been a while since you focused on tpi

 

This is why the zr1 used a variable length runner as do many modern engines.

 

113 owners not doubt.....There's no way they're the same. Close maybe.

By some charts I've seen, the LT1's may flow 15-20cfm more.

(Maybe that'll start something!)

There...I didn't mention TPI. Ooops.

 

As far as reversion is concerned, you have to think about other cylinders operating as well.

Most people think about one cylinder when they think about valve dynamics and reversion. If you draw an SBC firing order, then sit there and stare at it while going through the valve events in your head, you'll see that the pressure wave from the intake valve closing event enters the plenum while another intake valve is in the meat of it's intake stroke. Sometimes, this is happening with cylinders across from one another, so the effects of the reversion wave are minimized and could be possibly utilized to create power.

I don't have the knowledge to figure out how fast the reversion wave is traveling, how powerful the wave is, or really to quantify any characteristic of the wave; but it would appear that on a TPI car the wave is working at the TQ peak. With the small cam of a stock TPI, and the long runners, is the wave really that significant? Hell if I know.

Another thing that most people don't consider is what is happening on other cylinders during valve overlap. In my opinion, you need to consider the idea that while both valves are open on one cylinder, the intake stroke on another cylinder could be pulling hard enough to suck exhaust back into the cylinder that is trying to empty itself. The CC306 is a great example of a cam with this problem.

If you use a dual plane intake, you've isolated half the cylinders from the other half, so you can do different things with over lap and make it work. A TPI, with it's long runners, also acts like a sort of isolator because the effects of one cylinder must travel so far to reach the valves of another cylinder.

Good stuff guys, keep it coming.

 

While you're sitting there looking at that picture, notice that the ports in the plenum don't really oppose each other. The driver's bank is set quite abit behind the passenger bank. Ports 1&3 are across from ports 6&8. But, it is an interesting point you make. I've considered just the opposite though.....

I wondered if opposing tube suction would LOWER the amount of air to the opposite side. And, that bigger plenums minimize that effect (ala superram).

That's pretty easy to figure out. Replace a TPI intake with a short runner intake and observe how linear the HP graph becomes. Anything above linear response is a result of long-tube harmonics.

I should also point out, anything below it, is also due to the same reason.

Well, that might be part of the "bleeding of hp" that more duration (of cam) creates, but I wouldn't guess it significant. When you set off an explosion in a cylinder, then open the exhaust valve, there will be a tremendous amount of force behind gas expelling into the headers/manifold. Most of it has left by the time the intake opens. The momentum of escaping gas seems likely to be stronger than suction at another cylinder -- especially when it's "easier" to get air thru the TB vs sucking it back out of another tubes exhaust.

Also, more duration helps create more power (at higher rpms anyway). At lower rpms, it has a negative effect because piston speed is slower and valves are open longer. IOW, less of the explosion is applied thru the rod before valves open.

Don't you just love it when someone helps to make a point! Add this to what I said above.

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Many readers could have formed the opinion that I'm a TPI-Lover. Actually, not so much. I do like the torque bubble it creates and the corresponding impression of a larger motor. When combined with the fact that we must drive under 70mph (hwy) and 45mph (city), the TPI is a neat trick.

Without using a turbine/gas, it adds power in the mid-range rpms. And, there's nothing to replentish or wear out! Realistically, for non-raced cars, this extra power is in the range where we drive. It's a win-win.

For people wanting "stupid power", the TPI won't cut it. You need heads, cam, blower, and/or "juice". When you want more than 250-300 hp, you really need to look at these alternatives. Or, at the very least, get a bigger TPI!

Also, when you hit a certain power range, it makes more sense to spread the power -- so you can hook at lower rpms and pull better at higher ranges.

See, I can change gears! Gears? Did someone mention gears?

gp

 

Yeah I wouldnt buy that unless both were tested on the same day/apparatus. '113s are close but should flow just under.