Hey guys. I am trying to learn how to tune my 408. 9.5 compression, 235/239 @050, 535/550 lift with a Holley Sniper feeding the fuel and controlling the timing. The engine was dynoed with a best of 12.8 a/f and 35 degrees of timing at wot. I am trying to learn more of how to fine tune the cruise settings. Right now it seems to like 13.5 idle, 13.8 cruise a/f. I started the cruise at 15 and it felt like it was “trailer hitching”. Couldn’t hear it, but I could feel it. I dropped the a/f .2 at a time, the 13.8 feels the best. I looked at the plugs when it was about 14.5, the electrode was a grayish. Right now I think I am good with those settings for now. I have read a lot about the timing. Holley uses kPa, it cruises between 35-40 kPa (converts to 10.3-11 hg). This is cruising at between 2500-3000 rpm, which is from about 50-60 mph. Right now the cruise timing is at 36, I am trying to introduce more of a vacuum advance in that area. From what I have read I should be able to add a good 10+ degrees of timing into that area. I’m going to attach a screenshot of my timing table, does what I think make sense? I can adjust as I go, but is adding 10 degrees sound like a place to start? And I know, there’s probably some head scratching blue areas it there, I’m trying to figure this out.
10 is safe. Hook a vacuum gauge up and keep bumping it up and you will find the sweet spot. My 10.5:1 340 with a 236@.050 cam likes 50* at cruise 70mph 3200 rpm.
If you learn anything. Let me know. I'm fixing to fire off a brand new build tomorrow or the next day with a complete Sniper/Hyperspark system. Have about 8 wires to terminate , build a new coil wire, machine a new throttle bracket. Then double check everything. Sure would have been nice if they would have used some of the same dimensions. Change one thing and ten things changed. Fingers crossed.
Most Mopars will tolerate vacuum advance into the fifties. Rick Ehrenberg/ Mopar Action has covered this repeatedly in the last decade (for carb systems but same methods apply) so if you visit their website and look in the tech archive you should find it. I've been wondering, since magnums (which is what I'm converting to) don't have knock sensors that I know of, how do you know when you've gone too far? They say if you can hear it, it's already been too far for awhile.... Also, be careful tuning by feel. I've tuned many motorcycles that had huge holes in the torque curve that you could fill in and gain a ton of power, but bike didn't feel as fast, because when it was running out of that hole into clean power, it felt like it was "getting into the cam". If the combo is built right and you get a flat torque curve, it'll feel like it runs right everywhere.
You need to slowly increase vacuum timing and read the plugs with a magnifying glass looking for shiny metal. At steady cruise the primary throttle blades are barely open so most of the fuel is coming from the idle and transition slot circuit. Tuning to max manifold vacuum at cruise is the key. A mighty vac hand pump can be used if you run a vacuum line from the distributor can inside the car to see how much advance the car likes.
The AFR you are zeroing in on are in line with what would be expected from a hot rod. Here's the real deal Desired Carburetor AFR Characteristics At Different % Load (just close the pop-up if tapatalk asks you to join) The one caveat here is to be somewhat skeptical of the AFR interpretation by the wideband O2. Will get back to the is in a minute. As Jos mentioned factory Chrysler engines added enough advance under light throttle conditions to bring the timing into the low 50s BTDC. Thing is now you have a different stroke ratio so I don't know how that all plays into the dwell around TDC which in turn has some effect on ideal timing. Frankly that seems quite low. If its really that low then fuel distribution is going to be poor and the mixture will want to be somewhat richer. Richer mixture will burn quicker than a lean one, so less timing is needed. Timing advance guru input needed The reason to be a little suspect of the AFR number is the WBO2 interpretation is based on the assumption that the combustion is uniformly completing in the predicted way. With the low manifold vacuum that's fairly unlikely. If you're interested in the in depth version, read here Motorsports Village • View topic - How a Wideband gets tricked to read wrong AFR
I punched it into some google calculator, didn’t know what it should be. Using the table Mattax used, looks like around 19.5-21. Not sure yet how that helps me but still trying to put all those pieces together. Thx!
edit. I think you're on the right path. See if Holley has left side of the table as Vacuum, or absolute pressure. I could see them doing vacuum, but my understanding is that pressure is usually shown directly from the MAP sensor. In which case atmospheric pressure is around 101 kPascals at sea level. If so, then maybe that's the issue, not low vacuum. Assuming your table is showing MAP, then vacuum would be roughly the brown numbers
IF that's MAP on the left side, and my conversions are ballpark correct, then additional advance for lean burning mixtures should be possible in the range of the lower box. Definately no vac advance below 5"Hg, possibly not any below 11"Hg. For example, lets say you want to experiment by start at 50 * BTDC at highway cruise. So where I wrote "Full vacuum" add 15 degrees to the base timing on the table. And note there is no vac advance at idle rpms.
I I had trimmed the original chart, here’s a pic of the whole thing. Computers locked up so sorry about the crappy pic.
Yea. That's the deal, its MAP not vacuum. Find a converter that lets you do that, or just use the table in the other thread to ballpark it. Eventually you may start thinking in terms of MAP but for your own sanity probably best to have conversion handy. The conversion table assumes 101 kPa is atmospheric pressure.
Ok, sounds like while the turkeys in the fryer this afternoon I study up on MAP. But I think I am heading in the correct direction. This is a table I've been playing with the last couple days, haven't ran it in the car yet and don't want to go to big to fast. But sounds like I'm going the right way. Thanks again, this is all new stuff for me.
The stuff I linked to should help. Except maybe Shrinker on how a wideband gets fooled. That might take several readings and a few days. LOL. But that's not neccessary to set up the timing table. If you're asking for my comments on the table you just posted- Experiment with one thing at a time. Look at the spark plugs now to see what they show under the current conditions. Adding timing to the baseline (atmospheric) is the most likely to cause damage to the engine. I wouldn't touch that or anything near that until you have an opportunity to put the car back on a chassis dyno or maybe the 1/4 mile if you are careful and are able to back off the throttle during a run. See the example in post #10. Many Chrysler high performance v-8s needed enrichment when vacuum dropped below 8.5"Hg Some of the GM high performance engines needed to be rich below 10.5 "hg. Assuming your hot rod is going to have similar or worse part throttle fuel distribution and combustion characteristics is the safe bet. In which case, leaner mixtures and additional lead time should only be added when vacuum is higher than 8.5 to 10.5"Hg. Hopefully that clarifies why I boxed out the area around 10 to 11 "Hg for the beginning of vacuum advance.
If you hear pinging at WOT then back off right away. Whther you look at one plug or all depends on the situation. First go around I'd certainly look at all as some may look different
