Test and tune

[Cliff Ruggles]

Did you rebuild your carb following one of the recipes in the book? Need some help with tweaking it for maximum performance? Trying to dial it in and get your mileage up? Need a few more tenths? Want advice on a combo change? New intake? Spacer? Jetting?
Post it here.

[JimC]

I am intrigued with the idea of Q-jets working like FI.  I have 3 Q-jets, I am wondering which would be the best platform to start with?

     17081226
     17082226
     Edlebrock 1904

I have your book and 3 other Q-jet books.  Right now, I have a Holley on my truck, but I believe I can do better.  My vehicle is an 81 Jimmy.  I have a new 249hp 350 crate motor with an EPS intake and a 570 Street Avenger. (I have 2 more Holleys).  I still use the cast iron exhaust manifolds, but have gone to 2" dual exhaust.  Since the warranty is still good on my engine, I have not made any real changes to it.  I have, however, changed the drive train.  It came stock with 2.54 rearend.  (GAG) Now I have 3.73 front and rear with LSD's.  I am only interested in fuel economy and torque.   

So where from here?

Jim

[Pontiac Collector]

Hi Cliff, I am working on dialing in my 68 Firebird carburetor you built for me a few years back. Time flies.... Anyway, I am trying to work through what I perceive as a lack of WOT performance.  I wont' go into detail on the motor unless you need it, but it is basically stroked 400 (462ish), Eagle crank and rods, Comp Camps roller, mildly ported 16 heads, stock intake, doug's headers, 3.55s, and a Cliff's 7028265 built Q-Jet, altitude 5K in the foothills of Colorado.  I figure it should be just a little scary, but it really isnt that impressive.  Anyway, I have an AFR wideband O2 setup from innovate motorsports to get a good AFR read.  It runs like a top, idle, part throttle, no stumble, etc...  I took some AFR readings this weekend and at cruise it is in the 14s, but when I mash it, the AFR meter dips into the 9's and works its way up to the low 11s over about a 5 second span of WOT.  The choke pull-off rate is about 1-1.5 seconds. The car seems to respond immediately and then just goes flat on the way from 3K to 5K RPM.  If I can trust my equipment, I am definitely too rich.  I know you tweak these in to a customers specs (including altitude).  Where should I start?  Secondary metering seems to be a likely choice, but I want to get your take.  I can share the innovate logs if you want ( no RPM data in them yet, just AFR...). I think the software is free from innovate motorsports.

[Cliff Ruggles]

I agree, start with slighlty leaner secondary metering rods. 

Even with that said if you are around 11 to 1 A/F ratio, you aren't leaving much power on the table. 

I've changes metering rods at the track on back to back runs, going from "pig" rich to slightly lean, and the entire spread of ET is never more than about a tenth of a second.  I run my own engine slightly rich, using .041" secondary metering rods, which are pretty close in the cool/cold months, but pretty rich in hot/humid weather. 

When drag racing, it simply makes ET predictions easier as evening approaches and the temps cool down some.  On the few occassions I've chased ET and MPH with metering changes, it's just too difficult to accurately predict how much the car will pick-up during the day and into the evening, not to mention knowing exactly which metering rod to install for the next round?

What are your engine specs, compression ratio, camshaft, etc?.....Cliff

[Pontiac Collector]

Cliff, here is what I have done so far. 

I noticed quite a few lean misfires on my AFR readings (the LM-1 really takes the guess work out of this effort) so I installed slightly hotter plugs NGK v-power XR-5s and gapped them to .40 where they were at .35 with NGK v-power 7s.  I have a pertronix flamethrower coil so I felt that was probably a good gap.  I have absolutely no lean misfires at all, so I guess that was a good start.  Incidentally, this motor will run @ 150-160 F all day long... 

I went ahead and changed from a B hanger to a L hanger when I changed my plugs (I know changing two things at once isn't good science).  If I'm not mistaken, this will only modify the rate of change of the AFR while the secondaries progress to fully open.  Between those two changes I noticed a much better 3K-5K acceleration and it pulled really strong.  My AFRs seems to hover around 12-12.5 once I get past the rich dip which is around .5-.7 seconds.  I have continued to use the metering rods you sent with the carb. I am going to put the B hanger back in and do some more testing because the final AFR should be about the same I would think.  I also want to re-validate my original results. 

Since I am greedy I would like to address the rich dip to an AFR of 9.0 as I am pretty darn confident I can feel this in the transition to WOT.  Essentially, it is a very short duration power drop before it cleans out (this is not a stumble).  I moved the primary side accelerator pump lever to the outer hole in an attempt to reduce the pump shot, but that didn't really do much to the measurements or feel.  I am not sure how to experiment from here to see if I can tune this out.  If this is more a function of the secondary accelerator ports, could I close off or reduce the slot size in the air valve flaps as a test?  If so with what?

I am interested in your thoughts.

Per your previous inquiry,  my cam specs are the following.  I don't claim to understand them all....
                           
BTW, Kauffman put this combo together for my local builder.

Comp Cams roller grind# 3315/3316    HR112+4

                                          Intake                         Exhaust
Gross Valve lift                    .510                           .520
Duration @ .006 tappet lift   281                            287
Valve Timing @ .050          7 BTDC 43 ABDC          54 BBDC  2 ATDC

Specs are for cam installed @ 108.0 Intake Center Line
Duration @ .050                   230                             236
Lobe Lift                              .3400                           .3470

Lobe Seperation 112

~462 C.I.  Eagle Crank and Rods
Ross pistons
Balanced and blueprinted (I am pretty sure it was zero decked and given all the Pontiac performance treatments)
16 heads with mild porting
Stock Intake
Cliff's Q-Jet 7028265
Doug's headers
Muncie M22
3.55 gears

[Cliff Ruggles]

Have you tried loosening up the secondary airflaps slightly using the adjustment spring?  The flaps may need to come open just a tad quicker.....Cliff

[Pontiac Collector]

It had already backed it off about 1/8+ turn from what it was set at delivery, and that made a pretty big difference in what seemed to be mildly sluggish response.  I will have another look at backing it off a bit more.  The reason I hadn't considered more in that area was that I felt it wouldn't really overcome the  rich dip event which occurs immediately upon WOT for a duration that is well before the secondaries are really opening up (a choke pull off rate of 1-1.5 seconds). The dip is quite obvious in the graph. It is a rapid decline from and AFR of 14.7 ish to around 9.8 in a few tenths of a second.  At that point it starts moving from 9.8 toward 12 for a about .4 more seconds.  This is probably too much detail so I will take your advise and loosen it some more. 

I'll let you know how it goes, thanks for your support.

[Cliff Ruggles]

I'm also curious if you blocked off the exhaust crossovers in the heads, filled them with aluminum, or running them open?

I would also run a 180 degree thermostat if you have full control of the engine temps.  With my own engine, it runs quicker at the track when allowed to come completely up to 180 degrees.  If I try to leave at 150-160 degrees is "lays down" ever so slightly and runs slower at ever point on the run.

The higher engine temperatures allow for the A/F mix to atomize more effectively on it's way to the combustion chambers.......Cliff

[FORREST MILLER]

Any chance that rich dip comes from an over-agressive accelerator pump shot???

[Cliff Ruggles]

Yes, but it is not always a negative.  A LOT of fuel is required with any large CFM 4 barrel carburetor the instant you go to WOT to avoid a lean condition, which will result in a "bog", hesitation, stumble, and in extreme cases backfiring up thru the carb and catching it on fire!

FWIW, I've seen folks armed with LM1's and other A/F monitoring devices chase this condition to death, particularily with Holley double pumper and other Holley DP clones, just to find out that they didn't pick up any ET or MPH, or improved 60' times by continuing to lean it out to avoid a quick "dip" in the A/F curve.

Not that there's anything wrong with having some fun with these things, but any and all testing should be backed up with some dragstrip runs to monitor the results to see if it leads to improved performance, and exactly how lean one can go before the vehicle starts to slow up........Cliff

[Pontiac Collector]

To answer your question:

Q: I'm also curious if you blocked off the exhaust crossovers in the heads, filled them with aluminum, or running them open?

A: I am pretty sure they are open.

I have loosened the air flap about as much as possible and still have the tension to close the flap.  As I said before, I did try to reduce the pump shot by moving to the outer hole on the lever arm.  It is running pretty good actually and I agree with you Cliff that I may be making too big of a deal out of it.  It is pretty subtle and the transition feels virtually instant with no bog.  It was just that little .5 second blip I could feel, but as you state it probably isn't diddly for E.T.  The closest place around here to tune is Bandimere, but I am really just street driving the car, not really racing it.  As I stated before, I don't really know what circuit would even tune this out (if any)?

I had considered my lower running temps might actually be hurting me, but it is the first Pontiac I've had that is afflicted with too low of temps.  I think we ran a 165 thermostat (not sure why it runs @ 150-160 then, but...).  Easy enough to change it out for a 180 though if you think there is some power there.

[Cliff Ruggles]

That little "dip" in the A/F ratio is common, I've seen it on quite a few "playback" runs from folks who monitor them then look at the run on a laptop. 

Pretty common practive for many Stock and Super Stock racers these days.  I don't fully understand the graphs, bit have noticed "dips" in them on the launch and often when the transmission makes a gear change as well.

One small item to try is to install a lower hanger for the secondary metering rods.

Another thing that we often overlook is venting.  Some cars, my own is among them, leave hard enough to cause some fuel to splash up and out of the center vent or slot just behind the choke housing in the airhorn.  Early carburetors just have a round hole there.  This also causes a rich dip in the fuel curve, and in extreme cases the engine will even falter for just a moment, then recover and run fine the rest of the run.......Cliff

[Pontiac Collector]

Thanks Cliff, I did go to an F hanger from a B, but have decided to stick with the B hanger as delivered.  My test re-runs showed little difference actually.  The dip was there either way. I never thought about the vent hole dumping fuel, but another idea to check into. Off the topic of AFR, I was just looking at changing my thermostat and was poking around performanceyears.com forums.  It reminded me that I am running a direct (not clutch) mechanical fan.  It got me thinking I may be giving up a bit of power there as well and it might help explain my cars amazing ability to stay cool.  Do you think it also makes sense to consider an OEM style clutch fan.  Going electric won't be an option.

Thanks again for all your assistance and great information!

[Pontiac Collector]

Never mind on the fan comment, I realized that this is a stock 68 Firebird 400 flex fan, so I won't be messing with it.

[Pontiac Collector]

OK, its been a while and I've done a few things to the Firebird.  I've added a 180 (from 160) robert shaw thermostat and moved from a Muncie M22 to an M20.  This car really liked the M20 gearing.  I have almost instant throttle response (only a very very slight power "dip") that I am no longer going to try and chase down.  I think I've done all I can and it is running quite well actually.

Now, for my next project I am working with another Cliff's Q-Jet 7040268 on my 1970 GTO 455.  Before I go further I am at 5K altitude.  Since this is mostly my cruiser I haven't been too worried about throttle response (and my focus was on the Firebird). It starts, idles, part throttles, and cruises perfectly. I am trying to tune out the WOT stumble and have set the secondary air-valve spring to anywhere from no tension to about 1/2 turn where the stumble improves.  What I do notice is the more tension I add, the more sluggish the WOT response feels.  A dead stop punch results in minimal tire spin (street tires) and a weak launch.  Without spring tension (or at least minimal), it seems quicker after the stumble event, but it could all be in my head.  The choke pull-off is about 1-1.5 seconds and the secondary flaps do not have accelerator port slots in them.  I moved the accelator pump linkage to the outer hole last night as well.  Maybe I am asking more than my car can deliver at this altitude, but in California with a different Q-Jet (my car was dyno tuned though), a dead punch resulted in instant tire spin for as long as I desired.   I have also advanced the timing curve about 5 degrees from sea level.  I guess the real question is, should I tune for no stumble via air-valve spring tension and call it good, or work to richen my mixture at the WOT event somehow so I can open the air-valve quicker.