Results if I join Viper plenums..?

[Viper488]

I'd like some educated guesses about what would/might happen if the Viper's plenums were connected. There is a core plug at the back of each plenum. I could join them with some plumbing and have a solenoid actuated valve in the middle opened by a rpm switch at 4500rpm or so.

This question doesn't have to be Viper specific,and the rpm is just a 'for instance' ..

Anyway, let's stick with the basic configuration of 5, or 4 (doesn't matter I guess) cylinders per side. If the longish intake runners aren't changing their length, it doesn't look like a first thought that the powerband peak would be raised or extended, though the plenum volume would effectively double. The route from one plenum to the other might be less than 2" in diameter

The rear most cylinders are known to run a little rich anyway since they're waaay at the back of the plenums, so they might welcome the added air reserve at high rpm.

Then again might coupling the plenums throw everything off reduce cfm into the runners and I'd lose power?

More importantly if I did try it once would there be a danger of some near immediate catastrophy as I winged past 4500rpm and the connecting valve opened?

Near stock Viper engines run little vacuum at WOT which is probably why they don't respond well to larger throttle bodies.


I just don't want to have all Hell breaking loose inside on that first attempt(!).

Might be something better tried on a dyno first rather than on the road.

 

[PFM]

Viper488,

If you look at the later Porsche stuff there intake manifolds have a valve that opens to add to the connected volume between banks. The larger the plenum volume the higher the RPM tuning point. If the Viper has a volume already tuned for high RPM power, then when you double the volume you may move the tuning point beyond your usable RPM range. If you are really lucky and the plenums are tuned for low RPM torque and for drive-ability then the connection could tune to a higher RPM and help the top end power out. As for hurting something for a test I doubt this would be a large enough change to do harm. I think the connecting conduit would need to be quite large to get the tunning affect you are looking for.

Good luck,

PFM

 

[patprimmer]

Agree with PFM in principal, but a 2" connection will reduce the volume sharing significantly.

as more than one cylinder will be drawing from each side simultaneously, the connection should be at least as big as one port to be really effective

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I think we can guarantee that nothing untoward will happen - the mixture will still be set by the mass flow rate sensor.



Cheers

Greg Locock

 

[Viper488]

The 'later Porsche stuff ' has a computer programmed for whatever goes on when the two plenums are joined. The Viper doesn't, there's still a chance for the sudden increase in plenum size to to be out of the f/a mix the open loop programming is set for. No?

There's only air in those plenums, so the only thing the joining is going to provide is air (lean)without changing the programming.

The whole idea is to gain flow. If there is a gain in the dry flow from the plenum through the runners, it's going to ceate a lean condition isn't it? Of course if it does go lean I know it's been worth the effort and can upgrade my comupter settings to take advantage of the newfound flow into the runners/cylinders.

Okay PART II:

Would just bolting up a couple throttle bodies or something similar (with small air filters) onto the back of the plenums (1 per side)be any different than joining the plenums? I don't mean the plumbing involved of course, I mean the net result.
Same thing isn't it? Allowing more air at acertain high rpm.

GregLocock (Automotive) Jul 31, 2004
I think we can guarantee that nothing untoward will happen - the mixture will still be set by the mass flow rate sensor - If the computer's in open loop, and with Vipers using TPS, I don't think there'd be any fuel added

 

[ivymike]

If the computer's in open loop, and with Vipers using TPS, I don't think there'd be any fuel added
don't they use a couple of scaling factors to increase/decrease injector duration over time, to compensate for eventual degradation in injector performance? If so, I would think that a little bit of driving with 02 sensor feedback would get the fuel scaling factors set just about right for your new scheme... but I dunno for sure.

 

[GregLocock]

At least in the cars I know of the air/fuel ratio is set to a (variable) fixed value in open loop, but MAF is still used to set the quantity of fuel.

Cheers

Greg Locock

 

[PFM]

Viper488,

"There's only air in those plenums, so the only thing the joining is going to provide is air (lean)without changing the programming.

The whole idea is to gain flow. If there is a gain in the dry flow from the plenum through the runners, it's going to ceate a lean condition isn't it? Of course if it does go lean I know it's been worth the effort and can upgrade my comupter settings to take advantage of the newfound flow into the runners/cylinders."

Not exactly, the object is to tune the plenum volume as in a Helmholtz Plenum. This to develop a pressure wave at the intake port timed for a given RPM. If just pure air flow is what you are after check for vacuum in the plenum at WOT and max RPM if you have some vacuum you could use a bigger throttle body if no vacuum you are already there (as big as needed). If all this does gain some increased volumetric efficiency the EFI should figure out how to give it more fuel.

If you want some plenum info take a look at How to build Horsepower Vol. 2 by David Vizard. It has some formulas etc. If you are a real glutton for info try "The Internal Combustion Engine in Theory and Practice" by Charles Fayette Taylor, old but real. You can also look at the SAE books as well.

PFM

 

[patprimmer]

The plenum does not generate air. If more air flows from the plenum into the cylinders, it must first flow into the plenum through the MAF, which will meter the appropriate amount of fuel to maintain the open loop A:F ratio.

Unless the plenum is to small to begin with, I would not expect a noticeable gain by increasing it's size, however if it is really small, a large gain might be had. It is all relative, a balance of compromises, and results in diminishing returns

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[WilliamH]

In my experience, if the OE plenum or runner characteristics are altered in any substantial manner, the whole system can suffer. For instance, if additional mass air flow and correct fuel are available to the cylinder there is no guarantee it will result in more torque. I have seen this additional mixture just exit the exhaust valve during overlap. EGT goes up. This is assuming no other engine parameter changes are made. (Same exhaust, cams, etc.)

As to the OE engine management system correcting for the additional mass air, even if the mass flow meter senses such, the ECU map has a lean stop and a rich stop. These are the limits the mapping was programmed for. There is no reason to add cells to the spread sheet that exceed the OE engine parameters by more than a safety percentage. In some cases we will put the engine management system in "limp home" mode if the stops are approached. "Something must be wrong".

If this engine is equipped with a UEGO type (wide band) Oxy sensor, you can do a tremendous amount of testing, safely, if you also install an after market ECU.

Will

 

[patprimmer]

For an engine to run, there must be some way to meter the required fuel, even in open loop.

I believe that different systems use different methods, but the MAF sensor is a popular method, but I do not know how a Viper is set up by the OEM.

To metre the correct fuel for various altitudes, temperatures, and RPM at WOT, a few sensors must feed the stock ECU information. As far as I know, the MAF is one of these sensors. If more air does flow into a cylinder, more air must flow through the MAF, and more fuel will then be metred. If this is half ways decent in maintaining a maximum power mixture, more power will result from more air, however, it cannot be presumed that a "balance tube" connecting the two plenum in this case will result in more airflow to the cylinders.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Viper488]

Last things first, the Viper does use a MAP sensor, but not a MAF sensor inline near the throttle bodies. Any additonal runner flow into the cylinder would seems to be mostly uncompensated for by the computer.

One individual within DChrylser who would know such things had this to say about computer compensation for injector degredation:

The US06 portion of the EPA/CARB tests require the engine to run stoichiometric air/fuel ratios (closed-loop) at all times with the exception of those areas of the engine map where engine component/catalyst temperatures exceed their operating limits (open-loop) and then no richer then LBT +6% (Lean Best Torque).
The adaptives allow all of the stackup variances in the components and the crazy variety of gasolines that are required in different parts of the country to be accounted for and allow true stoichiometric operation every time the engine starts. Otherwise, the air/fuel ratios would potentially be out of the stoichiometric area and the pcm would have to relearn after each startup.


PFM, I've read that 4 cylinders is about the max for a true Helmholtz Resonator effect. I can see the point with 5 cylinders pulsing away-at least if the entire intake tract out to the airbox is to be considered.

So I'm left with the fixed runner length. I could add some volume to the top of the plenum, but it'd actually be easier to either join the plenums or just add small throttle bodies on the back of the plenums to admit additional air at high rpm.

 

[GregLocock]

MAP is no help at full throttle, since it will be near as dammit 0 at all speeds.

I do not believe the Viper could run true open loop fuel control at WOT - that is basically delivering x grams of fuel per second no matter what the speed. So, if it ias then at 2000 rpm it will be running at say 4:1, and at say 6000 rpm it would be 12:1

That, frankly, is absurd.

So, it must be running some form of MAF or volumetric flow rate measure to set the nominal fuel requirement.

Cheers

Greg Locock

 

[GregLocock]

Well, I suppose it could be taking the rpm and using a lookup table of VE to set the fuel flow rate, but that is dangerous - drive up a 3000 ft hill and your mixture will change by (?) 10%

Cheers

Greg Locock

 

[Viper488]

On another forum I've received this latest reply from that same DC source: The pcm will adapt those areas of the map that operate in closed-loop only. The open loop areas will not. If the shape of the volumetric efficiency curve changes due to hardware modifications there will be some areas that are rich and some that are lean. This will occur even within the closed loop area. Ignition timing may also be retarded or advanced (with respect to what would be the optimum). There is no way the end user can correct this with the stock pcm.The reason this occurs is because the software interpolates spark and fuel along a straight line from map/rpm breakpoint to map/rpm breakpoint. The breakpoints are clustered around the inflection points of the VE curve to make a smooth as possible curve. If the inflection points are now different due to engine modifications, the interpolation will not occur along the new VE curve and spark and fuel delivery error with respect to the new VE curve occur." I'll take that as a 'No'..