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Discussion Starter · #1 ·
There has been some excellent dialogue over on...

http://vca2.viperclub.org/forums/showfla...;o=&fpart=1

where an issue surfaced surrounding problems some people are having with VEC2 equipped Vipers.

This is not a criticism or necessarily even a fault with the VEC2 itself - but a problem that some cars are experiencing that nobody seems to have solved yet.

It seems to me that the issue is common enough to justify a think tank on a seperate thread because the solution once found is going to benefit everyone with the problem and it is something I know all tuners and DIY guys will want to be up to speed on incase they ever encounter it.

Sean Roe made an excellent post about this on the other thread and has promised to modify the VEC2 if that proves to be the source of the issue - OR - to update the trouble shooting manual and instructions if there is something that needs to be done outside the VEC2 when installing it that will eliminate this problem.

I applaud this and we need to support him by doing what we can to solve this mystery.

In a nut shell - what happens is that when certain cars affected hit an rpm level - which is different on different cars - it can be 3200rpm, 4500rpm or even up to 5200rpm.... the engine will hit a flat spot and drop power like it does not want to rpm more, or does but sounds terrible. Almost like it drops a cylinder.

There have been some suggestions that dropping timing out at the rpm range affected helps - it will be interesting to see if some have had this problem running factory timing tables or pulling timing OUT eg. on boosted applications.

Let's use this thread to share the nature of the problem, suggestions on what it may be and how to fix it - and experiences that have solved it for you. Together a solution will be found and this is a great use of the forum because the VEC2 is an important asset in this community.

BTW - somebody asked so here is the answer - AEM does not have this problem (to the best of my knowledge so far) - but AEM is not for the novice either and it costs a LOT more than a VEC2. I support both AEM and the VEC2 because they both fill different needs and market segments - we really need them both.
 

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in my case i think this "issue" happened d/t stock injector limitations. im no expert though
 

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Discussion Starter · #5 ·
KingViper said:
V10 MOJO said:
in my case i think this "issue" happened d/t stock injector limitations. im no expert though
Doesn't the S/C kit come iwth larger injectors? Are we talking about the VEC2 system on NA cars or boost cars...or both?
Both have given problems. It has defied most logic applied so far.

To put it in perspective there are many cars running around with no problem at all - but for the few that have this - there are several people now eager to solve the mystery.
 

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WV98GTS said:
Hey Rich, did your stock injectors max out with just the heads/cam or was it the nitrous?
I am not answering for Rich, but we have pretty much identical setups as far as head and cam and the injector issue is what I am trying to figure out. Whether or not my stock will actually flow enough.


I have recorded pulse-widths in the upper rpms anywhere from 22 to 23 ms, so that seems to put me right around 100% duty cycle, which I know the injectors are not as efficient when maxed out.


The one thing I do know in regards to the VEC2 is that timing actually helped my car and the moment I added ANY fuel value whether positive or negative the car began to miss or breakup. After logging more than 100 a/f readings and making over 30 cards, I know what magnifies my problems when tuning with the VEC2.


My question now is what on the car precipitates these problems assuming the VEC2 is functioning properly. Is it sensitive to a certain type of flow or psi from the injectors or what. I am still clueless and in the midst of ruling out things one at a time.

I will be putting on new clips when they get here next week and see how that goes as well as new plug wires. To me it is something fundamental that hopefully I am overlooking. My car ran great prior to the VEC2, but now even after removing the VEC2 I am still having a subtle miss above 5600 rpms even though my setup did not change.


I think I will go back to my day job.
 

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Just for clarification, on my car the stumble was there with stock injectors and still there with 50 lb injectors. When the injectors go beyond 80% duty cycle it becomes hard to accurately add/subtract fuel, this is a problem not related to the VEC issue.
 

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jack and i both are having this issue and we're using the same dyno. however, jack, didnt we eliminate thisissue when we increased the millisecinds from 0.2 to 0.4???

as for setups; jack john and myself all are heads/cam setups for the base and weve all had this similar and mysterious issue
 

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I'm no expert but, if you can compare side by side different dyno pulls from different cars with the same problem you might be able to see some similar conditions even though you have different rpm. Check the AF ratio, could it be possible that your AF goes rich and you don't have enough spark energy to properly ignite all the fuel. This could be why retarding the timing helps, you giving the fuel slightly more time to burn. Just a thought!
 

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Discussion Starter · #10 ·
Jack B said:
Just for clarification, on my car the stumble was there with stock injectors and still there with 50 lb injectors. When the injectors go beyond 80% duty cycle it becomes hard to accurately add/subtract fuel, this is a problem not related to the VEC issue.
Bugger! :doh: /images/graemlins/headscratch

That swap to 50lb injectors fixed the problem on one car at Heffners. Jason will be disppointed to read your experience as he hoped the issue may have related to the 42lb injectors all the other cars that gave that problem ran.
 

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Freeflyer:

On the four cars I did, we first achieved a flat 13:1 a/f ratio. At that point there was no notch/stumble. As soon as small amounts of timing are added the notch/stumble appears. Back out the timing and the problem disappears.

During the pull the a/f remained constant and at 13:1, then the stumble occurs.
 

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jack, also, recall that it didnt happen with a bit of cooldown as well, it was more prevelant with being back to back runs.

what is norm timing at mid and high 40 and 34??
 

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Jack, is there a way you can look at the stock timing table versus the VEC2 modified table? It could be that if the VEC2 table is advancing or retarding timing by X amount the problem occurs. If that is the case it might be a coil driver problem. Maybe the VEC2 can't process the signal fast enough to get accurate and repeatable results.
 

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Discussion Starter · #14 ·
That's a point worth checking!

Jack - I tried to call you as requested but it must be your work number. I'll try again Monday.
 

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I don't know if I totally understand the question. We know the what the oem advance curve looks like and we know what we are adding via the VEC. There is nothing but these two fixed (by rpm/vacuum) values that are adding together.

We are not talking about adding huge amounts. The problem occurs even with 2-3 degrees advance. As was stated the event seems to manifest itself when the engine is hot or getting heat soaked. On the other hand it happens on the road and the engine temperature should be fairly uniform.

I have never seen the problem with the VEC set to no-advance over oem. This seems to be the common denominator. Another point is why doesn't this issue show up on the SC cars Sean has done in his shop. These cars have to run hotter and with more cylinder pressure.
 

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It would be interesting to send a stock PCM to Jason @ West Coast Viper & have him modify the JTEC software to give the advance that you're looking for where the stumble occurs. Maybe Sean will eat the charge to find out if it's a problem with the advance on the VEC2.
 

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well....i can tell you this guys...i made about 36 or so pulls on the N/A car i did....and once i got a decent tune on this car and started getting serious about driveability and my miss was predictable i took 3 degrees of timing out in the 4000 area and 2 degrees out in the 4500 area....now the car never misses any more and runs jus fine......Len
 

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jack..i'm at the house now and don't have that tune on this comp...i'll look at it in the morning when i get to work....Len
 

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You need a guinea pig car that's acting up and switch sides/banks for the dyno O2 sensor, switch coil packs, switch computers maybe, and (sorry to repeat) shut the car off at that rpm while it's acting up. Even if the duration of the problem isn't very long, it's all you've got to work with. Might be worth a look anyway.


If you pulled a plug wire on a good running engine, would it drop the same hp in that rpm range? More/less? (20hp V notch drop was it?) 'Feels like it's dropping a cylinder', drop one on purpose and see how much hp you lose. Just for reference...

Another point is why doesn't this issue show up on the SC cars Sean has done in his shop? I thought s/c cars pulled had to run less advance typically.
 
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