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Timing and RPM Questions
I am running a fuel injected, supercharged 302 in the Cobra. I just changed the heads to new Brodix heads. The problem is the motor will run like crap and is difficult to start with the base timing of 10 degrees advanced. Setting the base timing the old fashion way by listening and short tests drives, left me about 18 degrees advanced. I am not detecting any knock with the knock sensor.
Any ideas why the motor prefers the timing to be advanced this far? It ran great before the head change at 10 degrees. I can set the base timing at 10 and re-write the spark advance tables in the computer. Would this be a better idea than setting the base timing to 18 and leaving the tables as they are? Second question is how high does a late model 302 rev before blowing apart? The valves in my old heads would float about 6400 rpm. I forgot to reprogram the rev limiter after making some computer changes and really over-revved the motor. It is still together and I do not want to set the rev limiter to low. Any comments or pointers would be appreciated. Fred |
The new heads have the same size combustion chambers as the old heads right? What about materials, were the old ones cast iron or alum? Base timing (or initial) is much less important than total timing advance. Max rpm is determined by the parts in the bottom end as well as the valvetrain. There is no good reason to spin the motor much beyond the max power rpm, even if it will hold together.
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Do the new heads have larger ports that are flowing slower than the old heads at low rpms? Are the new heads aluminum versus the old iron heads? Maybe your 10 degrees was low? Questions questions. The only fact apparent seems to be that it runs better with more advance. Is that a bad thing?
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Jeb, MrFixit;
The old heads were iron, 64 cc combustion chamber, intake runner volume 135cc. New heads are aluminum, 60 cc combustion chamber, intake runner volume 171cc. My big concern with the advanced timing is detonation due to the high boost from the supercharger. I am intercooled but detonation will still destroy the motor, if I do not over-rev it first. I changed the timing table before so the total advance was not to great at high RPM. Adding another 8 degrees advance, (from 10 to 18), to the base makes my total advance 8 degrees more. I guess as long as it does not detonate, I should be okay. I could always modify the timing advance table at high rpm which would be high boost so it is not so great. I hate to do this as I will loose power at high rpm. Fred |
Reguarding timing:
Total advance is the most important number. I like to have all the advance in by 2500 rpm on quick accellerating motors Initial advance is usually less than 20 degrees or starting the motor can be troublesome. Some motors need high rpm spark retard, especially under boost. |
Fred-
Can't you change your total timing independently of your initial timing? You should be able to - if you can't then perhaps your distributor could be sent out to a shop specializing in doing this..... Jeb is on the right track as to why your initial needs to be different. Something you must consider is one of the various kinds of timing retard boxes available from companies like MSD that will save your engine when the boost kicks in and your enemy is too much advance! I believe they are all programmable so that you can decide how much retard you want at what RPM.... I hope this helps. Pat |
Pat,
Yes, I can change the total timing independent of all other variables. I only set the base at 10 so the computer knows were it started. I had this years State safety inspection completed Friday so now I can modify the timing curve and see how the motor runs on the street. It just confused me why there was such a big difference with the timing when nothing was changed but the heads. Thanks, Fred |
Fred - are you using the EEC-IV Ford computer, or some other aftermarket type unit? If you're using the EEC-IV, I'll ask the obvious question --- did you pull the "spout" connector in the distributor wiring when setting your base timing?
If you're using the EEC-IV w/ standard advance curves, with 18° initial, then you've got about 34° total advance. That's probably 8-10° too much for a normal, safe, supercharged setup! I don't know how much the intercooler helps, but I'd be real scared at 18° initial unless you're running a boost-retard setup (MSD, Crane, etc.). How much boost are you making? I run about 14° initial, and then use the boost-retard feature of my Crane Hi6TRC ignition to pull out about 1/2° - 1° per pound of boost (depending on octane, temp, etc.). So, at my peak of 7 PSI, I'm between 23° and 26.5° total. |
Wade,
Yes I am running the EEC-IV with an added programmable card. The card allows me to modify most of the computer parameters. I am currently running 34 degrees total at 2500 rpm, 32 degrees at 3500 rpm and 30 degrees at 5000 rpm. I do not use a boost retard box. The SPOUT jumper was removed when setting the base timing. I had the car disassembled to save it in my divorce. I may have installed the wrong timing pointer. I am also going to dig out my piston stop and degree wheel to so I know where TDC is for sure. The supercharger has a 14psi head. My boost gage is connected to the intake manifold and reads 15 psi at 6000 rpm and was pegged when I over reved to about 7000rpm. With this much boost and the large advance, everything I read would indicate detonation. The sensor did not detect any detonation. I run 93 octance gas and boost it to 97 octane with an octane booster. Something just does not seem correct. I know my motor is tired and maybe the blow by combined with the long duration cam is limiting my peak pressure in the cylinder. Fred |
I would start w/ the piston stop/degree wheel idea to verify the timing pointer is correct or the balancer hasn't spun (assuming stock rubber mounted ring).
I'm very surprised you're not detonating. Might be a combination of alot of things that's saving you. Aluminum heads, high octane fuel, timing pulled out at high rpms, possible low compression, cam. Just curious, what cam are you running? I'm building a .030-over 302 for mine w/ TFS Twisted Wedge heads and will be using the Trick-Flow Stage 1 cam. |
Wade,
I am running the Ford Motorsport F303 cam. I am not sure if I would recommend it. I was after the low end torqure Ford stated it would generate. It does not generate a lot of low end torque. The motor however generates huge amounts of power from the 3000 to 6800 rpm range. I am currently working on a 347 stroker motor to get the low end torque. Not that I need it but you can never have enough torque and horsepower. Fred |
Secondary problem resolution
On the related subject of emission testing in PA. From your post, it sounded like you reset your timing curve in order to pass the PA emission test, since you live within the Pittsburgh metro area. If memory serves correctly, the PA emission laws exempt you from the test if you drive the car less than 5000 miles per year. Once you get the timing set, you shouldn't have to redo it unless you exceed 5000 miles per year. It doesn't matter the year of the car, either. The wife's previous 1991 MR2 was exempted because she didn't put more than that mileage on her car annually during the last three years we lived in PA (1998-2000). The exemption was specifically put into the law to exempt the kit car and street rod guys, but was worded to include all street vehicles. Unless the laws have changed in the past two years, you should be able to use this to avoid a lot of hassle in the future. You still have to take the car to the check station and pay the fee, but the operator should be able to verify that your mileage is less than the requirement from the odometer reading and will sticker the car without actually running the test.
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Chopper,
I am in the state of PA. The cobra is emission exempt as it is titled as a 64 Ford. I still believe timing is a little to far advanced and I am going to start adjusting the timing. My exhaust temperatures are high and so is the cooling system. Best thing about the computer control is I can drive around with my laptop pluged in and change the timing at each stop light. I will keep you posted. Fred |
High exhaust temps are indicative of not enough advance.
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