Quadrajet tuning

[rocks]

Have the following Edlebrock 1910 quadrajet. Pontiac 455, 10.0:1 compression, iron 4x heads, performer rpm, headers, very large mystery cam. I think it may be a comp cam 292 or around there. The car has to idle at 900-1000 rpms. Get about 14-16" of vacuum.

I decided to put a velocity stack on it for fun, this has required multiple jet changes from what I had, it had a 10x2" air filter. Air filter off it would run around 11.0:1 afr. Filter on it was to lean.

With the velocity stack i started with 72 main jet and CH secondary rods.
It now has 74 main jet with DA secondary rods.
This seems to provide a very responsive throttle, almost instant secondary opening, as in I cant tell when when the transition happens it just does. The car launches very hard now.

My afr at WOT is around 12.0:1. Idle afr is 13.5-14.0.
Cruise is 13.0-13.2 with the APT screw all the way down.

Can I change the rods on the power piston to fix my Cruise AFR?. With a 73 jet I can get it to 14.5 on the highway and it stays there, but with a 73 main jet, its to lean at wot, and throttle response is bad.

I currently have 71-74 main jets, CE,DA,CH secondary rods. Ordered a CK secondary to try it out.

What am I missing? I did go through the carb with all new gaskets and accelerator pump from cliffs.

[77cruiser]

What rods are in it? Yea a bigger rod & then you will be able to tune it in with the APT.

[Cliff Ruggles]

Yes you can install a larger primary rod to provide more control with the APT system.  Edelbrock should offer them, we don't stock those here......Cliff

[rocks]

I found the rods at jegs, can only get three out of four of them. One set is not longer made it seems. So ill be ordering the two bigger rods for now. Thanks for the help. Eventually im going to do fuel injection with megasquirt and get a cowl hood for it.


Some pictures of it.

https://photos.app.goo.gl/UT1r7T1iDmdDgl8h1
https://photos.app.goo.gl/VcPaiehSsaX0xgkc2

[Cliff Ruggles]

No interest in fuel injection here, too costly and same end result as a carburetor aside from you may have to "pat" the throttle once to set the choke for a cold start.

When tuning with your q-jet I would establish the ideal primary jet size FIRST.

It should have been 72, 73, or 74 for that carb number.  It has small MAB's so we often find the factory installed 74's just a tad rich at heavy part throttle when mated up to metering rods with .036" tips.

You should not have experience "bad throttle response" by simply moving from 74's down to 73's as the 50M metering rod could simply be raised up to compensate and delivery the same part throttle fuel as the 74's were with the same rods.

Once we determine the ideal jet size we move to the primary metering rod or APT, depending on carb type.  Once the APT is dialed in we go to the track and do some full throttle runs.  I take along precision machined metering rods and find out which ones it likes the best by ET and MPH. 

I may use an A/F meter for some of this tuning but really don't use it for much more than reference. 

I also spend considerable time with the distributor and the vacuum advance when part throttle tuning as you can pick up considerable efficiency there by adding timing at light engine load.  Not at all uncommon to see A/F ratios well into the 15's and the engine run flawlessly once the APT is dialed in with the proper amount of timing.

Each engine will be a little different, so looking at A/F readings on a wide ban only tell me where I'm at for A/F, not particularly what the engine is the most happy with. 

For WOT I typically stay "fat" just to keep EGT's down and reduce any potential for detonation.  You aren't really chasing much power or vehicle performance there anyhow.  At the track I've tuned from "pig" rich to so lean the engine started to surge a bit and the entire spread of ET/MPH from one end to the other was less than .2 and typically no more than about 2-3mph. 

When tuning for WOT with a Q-jet you will find it very difficult to swap around factory secondary metering rods and get the results you are looking for.  Too  many different rod diameters, tip length, upper section tapers and included angles leading to the tips.  You can actually go leaner in A/F going to rods with smaller tips depending on the tip length and included angle leading to them. 

We support quite a few dedicated racers who use Q-jets, and most if not all of the ones that are record holders or running at the head of the pack will have a full set of custom machine secondary rods, and a weather machine, and detailed notes to go with their documented runs........Cliff

[rocks]

What I mean by throttle response is the afr ratio from the accelerator tip in. With the 73 jet when I would go say 1/4 throttle cruising to 1/2 throttle the afr would climb to 14-15 before settling back down, this was giving me a lean bog and delaying the opening of the secondarys. Now I can't tell when it opens because it is smooth. With the 74 it drops down into the mid 12s before the secondarys open then goes to around 11.8-12.0. A 11.8 - 12.0 afr is usually referred to as an aggressive tune on a turbo car and a safe tune on a NA car. With the way the Quadrajet works I see it in my mind as a boost transition on a turbo car, the hardest part of the map to tune. Ramp up timing to bring the car on boost and bring it down fast enough to keep the turbo spooled without melting something.

I have always tuned with widebands. On my 350z it had dual widebands. I have had two cars with megasquirt systems. I have also melted down a few turbo cars in the past from not having a wideband.

When I do fuel injection it will be port fuel injection not a throttle body system like fi tech or the holley. It's nice being able to set a target afr and the car will correct itself by referencing the wideband. Megasquirt also has a very good autotune function now.

[Cliff Ruggles]

The velocity stack may be part of the reason, not sure.

I don't see that much of a change from one jet size with the 1910's or other later model SMAB units set up in a similar manner.

I have ran into tuning issues on multiple occasions messing around with air cleaners, Shaker assemblies, different height air filters, lids of different type of construction, etc.

The WORST results I ever obtained were trying to tune with a K & N X-Treme lid, my set-up absolutely HATES them and they require quite a bit of additional fuel from the accl pump and primary side to offset a lean condition going quickly to heavy or full throttle.  Problem is that even with considerable tuning I was never able to get where I needed to be or the engine liking it much.

As for fuel injection set-ups, the only ones I've had any success with is when we had 8 bungs welded into the intake and used 8 injectors, one for each cylinder.  The TB set-ups are pretty much hopeless, and after all the money spend and time put into installing one, about the only difference you'll see between it and a well set up carburetor is that you don't have to "pat" the throttle to set the choke on a cold start......Cliff