jcorenman

This is correct. The ISV is a voltage-controlled rotary solenoid which opens a bypass valve, and is controlled by the LH in response to engine RPM vs. a target-RPM.

This is true, not because of any "reflection" but simply because you are isolating the cylinders from each other. None of the resonances (intake or exhaust) work in slow-motion. What happens with a single throttle is that the plenum is under some vacuum, the exhaust is at atmospheric or higher, and if you open both valves at low RPM's then the exhaust gas flows backwards and fills the plenum. Does this literally cause reverse flow in the MAF? I don't know, that's one theory but not relevant here I think.

The whole point of the Alpha system is that getting rid of the MAF allows intake arrangements that don't lend themselves to single-point flow measurement with a MAF, such as ITB's. Sure, ITB's with a common plenum can be measured upstream of the plenum with a MAF, but that's not ideal and not always convenient.

No argument, MAF's work fine.

And so does Alpha, we've driven some 25K over the last 18 months without a MAF. The point is that you don't need a MAF, and there may be more design choices without. Heck, even putting a proper bell-mouth where the MAF is would be a great help.

Cheers,

Lizard928

Tuomo,

MAFs have benefits and pitfalls too.

One benefit is that any mods can be bolted on to increase power and the additional air is easily accounted for (X-pipe).

A serious downfall (for me), is that the engines rpm/sec is much different in 1st to 5th gear. Because of this the rate of airflow over the hotwire changes from 5th to 1st. So if you tune for 5th, 1st will likely be overly rich. This has been very apparent in a number of cars I've worked on. Another downfall is that if the MAF becomes disconnected from the intake, or you get a vacuum leak your vehicle goes totally lean, or pig rich (if boosted) making the vehicle unusable, or highly prone to failure.

My preference is to standalone MAP based systems which only measure manifold pressure as if you get a vacuum leak, your idle goes up, but your fueling remains the same. Or if a boost pipe pops off, the engine will still run enough to get you off the road and to safety.

ptuomov

I mostly agree.

I agree the motivation for using Alpha-N in some plumbing situations.

I think it's extremely unlikely that a single MAF attached to a single plenum of a V8 will see literal reversal.

I think that higher order resonances work at lower RPMs as well.

Yes, I agree that one of the reasons for the lack of low RPM oomph of some "manly" cam engines with single-throttle plenums and very short runners is that low RPM reversal sends some inert exhaust gas to the plenum which is then sucked in by another cylinder. If you use a MAF to feed the plenum, however, the mixture will still be correct and exhaust gas is simply recirculated which shouldn't hurt the idle. It's just inert gas there in the mix, much like nitrogen in air. And the same thing happens with ITBs if you floor the pedal at low rpm.

Another problem with big cam, short runner, single plenum, port fuel injected manifolds is that, at low RPMs, the reversal from the head can kick out an unpredictable amount of fuel to the plenum and the next cylinder. ITB systems that have the injector between the head and the throttle plate fix this problem. There's a design tradeoff there, however. The peak power grows with the distance of the injector from the port up to about (impractical) three feet. That's why some race engines have the injectors way upstream pointing to the runner, resulting at a higher peak power but worse idle. I recall seeing a sports bike (Yamaha?) with double injectors, one at the port for low RPM and one pointing to the bell mouth for high RPM.

So who knows why but big cams seem to work better with ITBs. no disagreement there.

My understanding of how tuning should happen and how I've tried to do it as follows. Tune the steady state first, with either pure load dyno or with the highest gear and the full break setting. When the steady state is done, tune the transient conditions. I remember believing (but do not know) that the LH algorithm for transient conditions computes an approximation of the pooled fuel "tau" in the port and should correctly map the transient condition for a wide range of engine accelerations. That is, the acceleration enrichment is larger when the acceleration is faster. It would be nice to know whether that's actually true.

simos

One of the most restrictive component on intake system is the MAF. There are numerous testing done around this. This is why I would consider switching to use MAP.
I wonder how many PSIs of boost will be lost in stock 928 intake setup...

http://autospeed.com/cms/title_Elimi...2/article.html

http://forums.mazdaworld.org/147-tec...surements.html


For the Turbo setup, the wery best intake will be the WRC style one. The air flow to turbo in WRC engine is being limited with specific restrictor plate
and therefore the engineers need to find "lost" power from wherever it's possible. There must be a reason to use this kind of plenum...
Check the CFD pictures of those links, look how even is the flow velocity on each intake tube.

http://honda-tech.com/showthread.php?t=3016531
http://forums.hybridz.org/index.php/...anifold+%2Bcfd

This would be the way to go for V8 T or TT as well. Local shop is doing DIY kit. 2 x separate WRC plenums, having separate throttle plates or connected to one larger TP.

http://www.amworks.com/pictures/WRCWEB.jpg
http://www.amworks.com/pictures2/EVOWRC2.jpg

On NA engine, it may not beat ITB's but are wery close...

PorKen

This has been my dilemma, too. MAF voltage vs. RPM is only correct in one gear.
The auto makes it even worse, with the torque converter making a constantly variable gear ratio.
For my own tuning, I have decided that autos can't be used for making a fuel map from scratch.

What's needed is a speedo input to select a map, or a correction factor, for each gear.
Plenty of room for those maps in LH2.3.


If only I had a S4 5-speed to start tuning with/coding on.

ptuomov

On a turbo car with a custom manifold, MAP is probably a good choice. They've used that in production turbo cars frequently. Design flexibility is certainly there.

I have a question about those intake manifolds. If one just bolts on two of those manifolds, one on each bank, they aren't going to work very well for two reasons. First, the turbos aren't exactly identical and the plumbing is not symmetric. Therefore, the boost would not be exactly the same on both sides. This would require a separate measurement of boost from both sides or a small balance tube. Small balance tube is not a problem. Second, even if the boost would be measured separately from both manifolds or there would be a small balance tube, the firing order is such that the pulses are not equally spaced per side. (The Audi V engines in one of the links has an equally spaced firing order per bank.) That's why I think one needs / could benefit from a single plenum or at least a very large balance tube. If one puts a large balance tube between the plenums, will any of the CFD analysis be valid?

The conical feeders to the plenum are a great idea regardless, and two of those should work well on a single plenum system no matter whether there are eight or two throttle plates.

danglerb

I don't want to hijack the thread, but thanks to those explaining reversion. We got into maybe two or three levels of more complexity and detail on Speed Talk without consensus on exactly what is happening other than flows dynamically change sufficiently to create a mixture that won't fire.

ptuomov

You guys have tuned way more of these cars than I have, so obviously I am missing something. That said, isn't the correct diagnosis of this problem that the base map is too lean and there's too much acceleration adjustment? And isn't the correct remedy for this (1) to turn the acceleration adjustment off or to a very low level and then tune the base map under a constant load which will make the fifth gear work well and (2) then after the base map is good for steady state adjust the acceleration enrichment to make lower gears work well under acceleration? What am I missing?

Lizard928

Tuomo,

The accel enrich is a very momentary item. Think of it as an accel pump on a carb. A heavy stab of the throttle and it pumps (do to speak), but a slow roll on the throttle and nothing happens.
I do use steady state (mustang eddy current) dumps for tuning going up and down the ranges. And I use 4th gear tuned ever so slightly to the rich side so that 5th doesn't run out of fuel. 5th gear is too tall for most dynos to touch redline on 5th gear.
A speedo input to pull fuel out at low speeds would be a great idea. Or doing the speed density alpha N tuning as that will remain constant regardless of rpm/sec.
But alas that wouldn't work for boost.

I have my turbo car running VEMS w/ COP now so thankfully not an issue anymore on that car. But it's still a fight on a stock car.....

ptuomov

Here's an explanation of the problem that the acceleration adjustment parameters seek to solve:
The only way in which the gear can matter is load and engine acceleration. I can't think of any other reason why a car that has a correctly tuned base map for steady state operation at all loads would behave differently in different acceleration rates than the acceleration adjustment not working. The likely reason for it not working is that the acceleration adjustment parameters are not set optimally.

Hilton

You know you shouldn't get that '88, and should hold out for an '89. Think of the cool stuff you could do with the digital displays

dprantl

There may be an issue with the acceleration enrichment on the stock 928 LH 2.3. When I set the base value in fuel params to even -64 on ST2, I don't see any change under transient load conditions.

Dan
'91 928GT S/C 475hp/460lb.ft

sendarius

Put me on the list NOW. I suspect however that playing with the digital display will require changes to CPU code, not EPROM data.

I asked John Speake about the digital display's fuel consumption information. I posited that the LH is using injector open times, and knowledge of flow rates (stored in a constant in the computer somewhere) to calculate the instantaneous consumption display.

He advised that this was indeed how it worked, but that the constant is NOT stored in the EPROM, so Sharktuner can't change it. This means that installing bigger injectors results in several of the display's functions become no longer useful.

ptuomov

JDS guys could probably tell us wheter it works and how it works. Maybe even put some equations in the ST2 manual. The coolant temperature is one key determinant of pooling of fuel on the port walls and therefore what the parameters on that screen do should and probably do depend on the acceleration enrichment map that varies by coolant temperature.

Here's the current acceleration adjustment documentation:

John Speake

Accel enrichment is very complex in how Bosch have designed it. On the first big stab of the throttle, you get the enrichment as set by the parameters available with the ST. Imeadiate subsequent stabs at the throttle don't trigger the enrichment. I have observed this on the test jig. There appears to be a complex time/size and rate of load increase algorythm. We have never had the time to investigate nor any pressing need as I haven't had any problems tuning fuel over the rev range.

At present we are unpicking the ISV system design which is also very complicated.....