mobird

Ok so I'm hoping some of the real knowledgeable tech guys on the forum can weigh in on this (preferably people who have spent some real dyno time playing with timing on different types of motors).

My motor is a stock bottom end, 4- bolt main 350 (-18cc dished pistons) with Brodix IK200 aluminum heads (64cc chambers) and .015 shim head gasket to help raise the compression on those cast dished pistons. That gives me roughly 9.4:1 compression. I'm running a Lunati 268 cam (227/233 duration at .50", and around .500" lift)

I have my timing set at 36 degrees total (20 initial, 16 mechanical) and I run 91 octane no-ethanol gas. What I"m trying to figure out is how much timing would be optimal for power. I do plan to get it on the dyno in the near future to experiment, but I'm curious how people do it without dyno time. I've heard some of the old methods of "turn it up till it pings, that back it off two degrees", but I doubt that is accurate for finding the optimal timing for max power.

In doing some reading, I've seen some references to lower compression motors with poor combustion chambers (mine is a poor piston design with a head that has a great combustion chamber) needing more timing, closer to 40 degrees. But I've also seen guys with 10:1 compression motors (not that much more than mine) making optimal power at 32 degrees. I assume the cam events and static vs dynamic compression play a significant role.

So how do you determine close to optimal timing without dyno time? Or is dyno time the only accurate way?

I have spent alot of time tuning my Quadrajet under the assumption that my 36 degrees total was optimal, but I'm realizing that I should really get that timing dialed in perfect THEN go back and fine tune on the carb.

Buccaneer

IMO, there is NO other way to "accurately" tune the motor to optimum performance without a dyno, that's what they are designed to do in the hands of a skilled tuner. How do you know you have the correct/optimum A/F ratio at WOT right now?
Actually...you tune the your VE or fuel first, then dial-in the SA or spark on any dyno.

Tim Ware

You've spent a lot of money on what looks like a great engine. Now its time to spend about $350. on the dyno. The dyno doesn't lie.

MelWff

Everything I've seen on this forum for years says 36 total for cast iron heads and less, about 32, for aluminum heads but nobody can give you the perfect number within that range for your engine unless you dyno it .

Tim Ware

I want to add something that might help. Dyno "tuning " is just that. Tuning of the engine. In my case, I don't go for maximum horsepower. The dyno can tune the engine for much better & smoother drive-ability. That is, all the time we drive the car around at normal speeds. I had a Road Runner that was built-up & could be a little grumpy until it was tuned on a dyno. Same horsepower, but easier to drive.
Hope this helps.

mobird

True true. I've been such a DIY guy on this car (just did my own alignment last week, learned to tune the Qjet using Cliff's book and some advice from Lars, including drilling out the idle restriction tubes, changing the main air bleeds, drilling out the mixture holes, messing with APT and hanger height etc...) that I want to get it to run as best I can at home. But I guess if I am REALLY wanting to find optimal performance, I should go take it to a dyno shop. I am verifying my A/F with a wideband 02 that I welded into the exhaust, excellent tuning tool!

mobird

Interesting. I assume this is because a more efficient combustion chamber requires less timing advance. But that could also be offset by my less-than-optimal dished pistons (not exactly a great design for combustion).

I think I answered my own question: I just need to bite the bullet and go get it dynoed.

To be honest, it probably won't be that expensive since I won't be having them touch the carb (doubt I could find someone around here who knows how to tune the Qjet anyway, and I can make any changes I may need). So the dyno runs would really just be to verify timing. I would think 3-5 runs would be sufficient. One baseline run with my current 36 degrees. Drop it a couple degrees and see how it responds, raise it a couple degrees and see how it responds. Then a run at the end to verify the change in power. I really just need someone to operate the dyno for me while I test haha.

mobird

Good thought. I have been putting alot of effort this year into increasing driveablilty. In this case though, I really only want the dyno for the max power side (and to possibly see if I have any major power dips under the curve). Since I have the wideband 02 in the exhaust, I've already worked through most of the rich or lean spots throughout the powerband. And any potential improvements for partial throttle I can play with with my adjustable vac cannister and the APT screw on the carb.

Buccaneer

You are absolutely correct. You can not effectively tune a motor with a narrow band O2, it doesn't have the resolution like a WB does. Are you shooting for roughly 12.6 at WOT? Good luck with your tune in the future.

stingr69

The 36 degrees works best for the old original wedge combustion chambers. The newer style combustion chambers need less total advance because they burn faster. This is a good place to start because you will already be in the ballpark.

mobird

I have it pretty well dialed, depending on engine speed and such the WOT A/F is between 12.2 and 12.8 (sometimes it dips a little richer to like 11.8:1 for a split second).

Idle and cruise are both around 15:1, so I could probably lean my cruise out a tiny bit more.

Thanks!

mobird

So question on that though, my heads have a nice efficient combustion chamber, but my pistons are the not-very-efficient huge dish with a ridge around the edge. So my actual combustion area may not be as simple as the efficiency of the head!

Hence the dyno time haha, but I am curious about the science behind it.

gkull

Fuel atomization and exhaust gas dilution have the biggest effect BSFC (Brake Specific Fuel Consumption) Which means how many pounds of fuel are needed to make HP. Another term you need to understand is VME ( Volume Metric Efficiency) Or how much HP are you producing per CI.

The shop had a dyno and I helped dyno tune motors that had to be ready to go when the owners showed up at the track. The boss never tuned for the highest numbers. He always went for the highest average hp and VME along with the best BSFC.

In a carbed motor fuel atomization is all about starting with the tinniest fuel droplets suspended in air and keeping the intake track at a high speed without turbulence to keep the atomized fuel from creating larger droplets. You have to start out with high air velocity over the best downleg or annular type boosters with correct air bleeds emulsifying the incoming fuel. Then your manifold has to continually decrease in size on the way to the intake valve creating a ram effect during the intake opening. You can't have smaller intake port than the head port. The cylinder head, piston design, and exhaust tuning determines how low the exhaust gas dilution is. It is just reasonable to conclude that the higher the C/R the less exhaust dilution. Intake ram effect and valve overlap can actually be sending fresh fuel out the exhaust pipe. tuned exhaust can create both positive and negative pressures in the exhaust port over a limited amount of RPM. Free flowing dual exhaust and side pipe system can decrease the exhaust pumping losses to a motor. But the ultimate is a correct primary diameter and length going into a merge type collector of the correct dia. and length to open air.

The cylinder head design with correct quench causes atomized fuel to become a plasma at the point of ignition. That is where you get the highest efficiency burn and that is at the point of the highest TQ on a dyno run.

That is why an engine can run very well with 2000 cfm using 8 intake stacks. Where a stock type dual plane is over carbed with an 850 cfm because of the poor port design. You want the best tuned carb, a tuned intake port and heads, compression within reason, and a tuned exhaust system. That is why I went to single plane intakes to match the heads. Tuned long tube headers and three inch exhaust system for street and header collector extensions for open to air drag racing.

Watch the important numbers not just A/F ratio.

427Hotrod

And READ the plugs. Give the motor what it wants...not some arbitrary A/F ratio or timing figure. All of that is great to start with...but every combo is different.

The pistons you describe aren't going to be as efficient as a true 'D" dished one that matches chamber. There really is very little quench with OEM dished pistons...so that's out the window. Things that impact oxygen remaining in the exhaust (big cams) can make "normal" numbers inaccurate for what it's going to take to make that particular combo run.

JIM

2mnyvets

Just thinking out loud. There has to be other side effects to the wrong ignition point. We all know that too early ignition leads to ping. I would suspect that too late ignition would result in higher exhaust temps, and possibly more CO, HCs or NOx Temperature would be the easiest to measure.

Jebbysan

The IK head does not have the fast burn chambers on them so 36 degrees is a good start. The only real way to know what timing to use is to read the plug strap after a hard full throttle 1/4 mile run. Too little timing will see the "burn" point higher up and over the strap.......too much will pepper the strap and the "burn" mark will be way down the side of it. This is the exact procedure we used on our race engine on a stand alone dyno years ago and it works......we would read the plug by pulling a "sweep" run at about 3/4 load (water) to see where the timing was at to get a ballpark before throwing everything at it.
The engine will basically want what it wants.......no set rules about where it should be given a combo. As Jim said above, your quench is horrible so it would not surprise me that you would actually end up with less total timing due to this.........
I built a 350 for my Dad 20 years ago off of a Goodwrench crate motor.......threw everything out except the short block and installed Edelbrock heads (Performer RPM).......this had the same crummy pistons in it that you have......but it ran very well. With a 64cc chamber I ended up at 35 degrees for max power. This was with a 292H Magnum Comp Cam.

Have Fun

Jebby

CA-Legal-Vette

ronarndt

Shop around for dyno costs My local shop in Manassas, VA charges $75 for three pulls and $15 for each additional pull. Cost is for HP and torque results only. A/F reading is add'l $15. If they do not have to do any of the timing adjustments, there is no charge for the car owner to quickly change the ignition timing between runs. With what you are looking for you should spend less than $200.

mobird

Thanks for the info! So are you saying you use a single plane intake because the current market dual-plane's don't have the right intake port size to match the heads?

mobird

Will do. When reading the plugs, I'm assuming you do the hard pull and then shut the car off and coast to a stop so you can pull the plug and see the WOT color before it gets discolored by the idle circuit mixture?