I was just out playing with base timing and thinking about tuning in general when a question came to mind.

I've noticed that, during a WOT blast from 0-70 say, fuel pressure begins to drop off as manifold vacuum begins to approach normal. This shows up on an O2 sensor monitor as the reading getting lower, which means the A/F is leaning out. During these road tests the vac line was connected between the AFPR and the plenum port, as usual.

It seems to me that on the 1/4 mile track we need for fuel pressure to hold constant, especially since the engine needs full flow and so pressure up near the shift points.

Since the vacuum control of the FPR is a compromise for overall driving, I'm wondering if we would benefit from removing the AFPR vac line and plugging the port at the plenum, for track use only of course.

What are some thoughts on this idea?

Anyone experiment is this area?

 

I've tested mine at WOT and it is rock steady at 46 PSI, only 4800 RPM though. I think you shift at higher RPM? When it gets warmer I can make some runs at more RPM. It's back to snowing here.

 

The fuel pressure with respect to manifold pressure (or vacuum) should remain constant. That is the purpose of referencing the regulator to manifold vacuum.

If the measured fuel pressure is dropping at WOT and the vacuum is not increasing (which it shouldn't be unless the throttle body is too small), then there is a fuel supply problem.

 

I assume that was with the vacuum line connected to the FPR. Indeed if manifold pressure ramained constant through that RPM range then I would expect pressure to hold too.

My L98 pulls to 5,200rpm on the 1-2 shift and I've seen pressure drop off a little before the shift point as well as seen the A/F lean out a bit. (BTW, I'm running a 52mm TB, ported MAF and building about 295chp, so there should be enough air flow potential)
I was NOT monitoring manifold vacuum at the time.

With the vac line disconnected this did not happen, indeed pressure remained rock steady through the upshift.

Soon I plan to make another 0-70mph blast with the AutoXray to check for knock counts; I'll check the O2 sensor reading to see if it still leans out near the upshift.

 

Assuming that there aren't any knock counts registering - i.e. timing being pulled- (or any damage occuring), wouldn't a leaner a/f ration mean more power? (I don't know I'm asking, its what I thought was true).

 

Since the stock 88 tune is a bit on the rich side (Target AFR during PE is 12.4:1) there may be some benefit to enleanment.

If the manifold pressure really is dropping (vacuum increasing) due to some sort of dynamic throttling effect as the revs drop during the shift, it would still make sense for the fuel pressure to be compensated accordingly to the current manifold pressure.

Of course if its going too lean as this happens, than it may merit some extra compensation in the PE tuning for the specific rpms in question.

 

From the curves I've seen there is some latitude in A/F for peak power. A dyno operator told me that my 12.6:1 was just about perfect, though I did see some leaning in the A/F plot.

Indeed I've been trying to reason through this idea an your argument makes sense unless, as you mention, the A/F drops off at some point.

It looks as if it's time to make some runs w/ & w/o the vac connected and compare O2 reading trends.

 

Jim, this is a good idea, let's try it out at the track w/timeslips.

I have never even scanned my car during a run w/ the new setup.

Vic

 

Just curious, what are you running for WOT FP?

 

Vic I'll bring the scannner but, after my -.004sec 1st round loss, I don't think I'll mess around with scans on the 14th.

As I haven't done any air flow mods since last I adjusted fuel pressure, I believe I'm still running 48psi w/o the vacuum connected, i.e. WOT.

BTW, here's an example of what happens to the A/F ratio during a dyno pull to ~5,200rpm, w/ the vac line connected.

Note that, shortly after goint to WOT, the A/F drops to ~12.6:1 by 3,800rpm but drifts up to nearly 12.9:1 by 5,200rpm.

This behavior should show up on my scanner as the O2 sensor value dropping from the low 900mV range at launch to the high 800mV range by the 1-2 shift.

 

At some point in November, my fp went up to 50 @wot I readjusted to 45 picked up my lost time in the 1/4 (NA)

 

What engine mods did you have before & after the change in pressure??
What year Corvette?
Same launch weight?
Same weather conditions?

 

Well some interesting "test data" from the AutoZray scanner:
Table A - W/ Vac line on AFPR
TPS.......RPM......MPH......O2
1.35......1,610.....16....... 820
4.66......2,180.....20........970
4.70......3,720.....28........900
4.70......4,700.....40........900
4.70......5,580.....49........900____(1-2 shift)
4.70......3,570.....58........940
4.70......4,030.....65........920
4.70......4,270.....72........920
4.70......4,820.....79........890
1.27......4,700.....83........780

Table B - W/O Vac Line on AFPR
TPS.......RPM......MPH......O2
1.37......1,750......11.......800
1.99......1,930......17.......790
4.70......3,200......24.......810
4.70......4,700......38.......820
4.70......5,890......48.......830
4.70......3,590......56.......860____(1-2 shift)
4.70......3,840......63.......860
4.70......4,620......70.......870...(yse this is really in the scanner)
4.70......4,530......76.......900
4.70......5,120......82.......900
1.17......4,430......86.......780

Table A is pretty much what I expected, including the juke in O2 sensor reading around the 1-2 WOT shift.

Table B holds the real surprise to me with the low and slowly climbing O2 sensor values??? I can't explain the 4,620rpm data point other than to treat it as an anomaly; yes I double checked the scanner values.

I wanted to make another "pass" with the vac line disconnected but Sun. traffic was heavy so I called it a day.

Though I cannot yet explain the data in Table B, I want to repeat the tests to see if the data is reproducable.