APT adjustment

[jagness]

Hopefully, here are all the necessary specs for my question: Olds 350, 9.8:1 CR, solid flat tappet cam 239/241 on a 110 LSA. Quadrajet is a 17057553, rebuilt with one of Cliff's kits and his advice on idle circuit and air bleeds: main jets 74, Cliff's fully tapered 44 primary rods. I'll be happy to provide any more info if needed. Initial timing is currently at 15 with 19 degrees mechanical advance all in at 3500 rpm.

Here's my question. I had trouble with it pinging on 91 octane gas. It mainly happened going up steep hills around 1900-2100 rpm (running a 200-4R with 3.90 reared gears). So I started adjusting the APT out. Started at 3 turns out. Didn't stop pinging up steep hills until I got to 5.75 turns out. Is it unusual to need to adjust the APT that far?

Other than this issue, the engine starts quickly, cold or hot, idles crisply with 14" of vacuum at 800 rpm, and literally rips when I nail it from any speed or rpm. Overall, I'm really happy with it.

[Cliff Ruggles]

Are you using vacuum advance?

Climbing a "steep" hill most likely the load is enough and throttle angle high enough the PP isn't down anyhow.  It is only down during light load scenarios.  Any heavy load or when the vacuum drops off the APT isn't a player anyhow.....

[jagness]

Yes, I have a Proformance adjustable vacuum advance connected to full manifold vacuum. Right now, I have it adjusted to add just a couple degrees advance at idle. I already backed it off so much, I figured it wasn’t causing the pinging.

[Cliff Ruggles]

Vacuum advance adds timing at light engine load to make the engine more efficient, burn a leaner mixture, use less fuel, etc.  You can choose to add timing at idle using manifold vacuum and doing so often requires an adjustable VA if/when the engine doesn't produce a lot of vacuum at idle speed.

This would require setting the spring inside the VA according to how much vacuum the engine makes in and out of gear at idle speed so it keeps the timing added for both scenarios. 

When you are driving the vehicle the engine will make plenty of vacuum at light engine load to employ the VA and it will drop out the timing anytime you load the engine and vacuum falls off.

If you are "pinging" with heavier load on the engine I would hook up a vacuum gauge to see what vacuum reading you see when this happens and if it is high enough to apply any timing from the VA unit.  Most likely the pinging at heavier load you are seeing is just too much timing from the mechanical advance and you just need to retard the timing till it stops. 

The carburetors APT system only adjusts the height of the metering rods in the jets at very light engine load.  In all driving scenarios when engine load is light engine vacuum is high so the Power Piston will be in the down position.

My guess here is that you are just running too much timing on the engine for the octane being used........

[jagness]

Okay, thanks a lot Cliff. I’ll check the vacuum under load as soon as I get the chance.

[Kenth]

350 cui, 9.8:1 CR, 110LSA, 239°/241° @ .050" and only 2000 rpm´s up a steep hill is a fine recipe for ping.

Shift gear and run engine at least at 3500 rpm´s up the hill would place the cam in it´s working range and reduce ping.

JMHO

[Cliff Ruggles]

Kenth is correct.  Even though a 239/241 @ .050" cam looks pretty "big" in a 350 build the effective duration is considerably less with flat solid cams.  The "rule of thumb" used to be about 10 degrees @ .050".  The testing I've done shows it to be closer to 15 degrees but will vary some depending on who ground the cam and what lash/ramp they used. 

I'd also add that tight LSA also narrows up the power curve and spikes dynamic compression higher, plus earlier in the RPM range.  So the "recipe" for ping at lower RPM's is certainly there for sure.......IMHO....

[jagness]

Yes, I have the cam lashed at .017/.019 (Engle recommends .018/.020) hot because I knew it would be a lot milder than a hydraulic cam with the same numbers.

I just ordered a small cap Progression Ignition distributor. They give you the ability to create timing maps with a smart phone. It has vacuum input, which can also be used as a boost reference. I can let you all know how well it works. I‘be heard good reports so far, and it’s an American company.

We live in rural northeastern Oklahoma with very curvey, steep hills. In some places, it’s easy to kick it down a gear, but it breaks the 275/60 15s loose really easily. So I usually have to kick it down and then quickly feather the throttle. This engine is amazingly responsive, so the carb is working well.

[Kenth]

Don´t you have a gear shifter lever for your 200-4R?

[Cliff Ruggles]

I've heard good things about those distributors as well.

Even so I've been doing this sort of thing over 40 years and 20 of it full time.  Without exception every single "bug zapping" distributor, module, electronic "conversion" or other aftermarket ignition device that I have used or tested here has eventually failed. 

They will NEVER fail when you are sitting in front of Jegs or Summit Racing, so plan accordingly.

In contrast stock points and HEI distributors have been dead solid reliable and failures are as rare as winning lottery tickets.  There is also not one ounce of power waiting for you IF you know how to tune the stock parts for the engine combination.

I remember decades ago my first experience with an MSD billet distributor.  A good friend of mine broke open his piggy bank to buy one to "upgrade" the stock points distributor we set-up for his race car.  Set to the exact same total timing the car slowed down at the track.  Yes, it ran about .02-03 seconds slower and fell off a little in MPH as well.  We were shocked and couldn't figure it out.  I checked the timing with the engine revved up again and again, but it still ran slower.  An "old timer" at the track overheard our frustrations and walked over and told us that the points distributor by design has a natural timing retard function at high RPM's and it's not uncommon to see a little more performance when using one compared to an electronic unit.

Hum?  I've also back to back dyno tested a factory HEI against an MSD billet distributor with 6AL box and the HEI made more power at every RPM.  Not much, but it was a tad more, not LESS as we are told in all the very well written advertising they do for their products.........FWIW.......Cliff

[jagness]

Yes, I think I’ll keep working on the curve in my HEI and keep it in the trunk just in case.

[Cliff Ruggles]

You can achieve amazing results with a carb and distributor, plus the less complicated and fewer electronics that you put in place the more reliable the set-up will be in long term service.

When you are finished dialing in your Q-jet and HEI about the only difference between that set-up and any attempt at fuel injection will be that you will have to "pat" the throttle once or twice to set the choke and put a shot of fuel into the intake on a cold start.

I've literally spent countless hours tuning, testing, and evaluating the results on several vehicles that I own/owned over the years dialing these things in.  It's remarkable that the final numbers were very close for all of them.  This surprised me some at first, until one realizes that the requirements of a N/A automotive/truck engine are very close. 

Of course this assumes that you have a very well thought out engine build and didn't make too many poor choices for it along the way. 

Compression and cam selection are critical.  I see folks make poor choices in both areas and the engine never has a chance from the day it is fired up to be efficient and make optimum power on pump fuel and thrifty for "normal" driving at the same time.

I know I'm getting off topic just a tad but these things are important and good reading for those who are planning to build a strong running street engine for their vehicle.  I'll be brief here but the most common mistakes that I see are folks LOWERING compression to effectively manage pump gas.  That myth has been around for quite a while now and is one of the most inaccurate statements on the Forums.  Usually those that are making recommendations are regurgitating the proverbial brick wall of 9.5 to 1 static compression for pump gas.

The biggest reason that statement is inaccurate is that the tolerance for octane is based on many factors, static compression only being one of them.  I also see folks quickly ditching stock intakes to buy a high rise aftermarket part and most also buy an antiquated design inefficient aftermarket carburetor for the top of it. 

Many will also attempt an "upgrade" to their ignition system which typically brings nothing to the table anyplace other than most will prove to be unreliable in long term service.  With no exceptions every single aftermarket ignition "upgrade" I've tried here failed sooner or later, most sooner than I was happy with.

Anyhow, I'll sit down now before I end up writing a book on the subject.  For anyone reading this if you take anything away here it should be that distributor tuning is equally as important as carburetor tuning and changes should be made in both areas to find the ideal timing/fuel curves for what you are doing.  So there is really no "magic bullet" with these things and finding optimum settings can take many miles of driving and tanks of fuel, but once you are finished you can button things up and enjoy the vehicle for many thousands of miles........Cliff

[jagness]

Thanks Cliff. That's exactly what I'm still working on. I think another problem I'm having is that not every tank of 91 octane gas I buy is equally good. But I want it to run well on what's actually available.