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Lots of threads 'why recurve.' Read up.
When the EGR system is functional, it allows a small amount of exhaust gas to be sucked back into the engine while driving. The EGR is off at idle.
Injecting some exhaust into the intake dilutes the percentage of oxygen that gets sucked into the combustion chamber since exhaust gas is devoid of oxygen (for the most part).
An air fuel mixture that contains less oxygen burns slower and cooler. The goal of EGR is to get the combustion temperature below 1200°F (by reducing the %O2) to minimize the creation of Nitric Oxide (NOx) which is a nasty pollutant.
Since the EGR diluted charge burns slower, the spark needs to happen earlier (more advanced) to ensure efficient combustion.
The USA distributors on the FJ60s were all "curved" (calibrated) at the factory to match the combustion speed of an EGR mixture. The "curve" refers to the timing of the spark at different RPMs. If the timing advance vs rpms is graphed and the dots connected, the resulting representation is referred to as the "curve".
When a distributor gets "recurved" for non EGR use, it's being modified by changing the weights & springs inside it to advance the spark more slowly or to less of a degree for non EGR use.
When the EGR system is disconnected, the air fuel ratio contains more oxygen which will allow the charge to burn faster (and hotter). If the stock distributor is used without EGR, it's spark advance "curve" will be too advanced and the engine will pre-ignite/ping/detonate/knock at certain RPMS.
Injecting some exhaust into the intake dilutes the percentage of oxygen that gets sucked into the combustion chamber since exhaust gas is devoid of oxygen (for the most part).
An air fuel mixture that contains less oxygen burns slower and cooler. The goal of EGR is to get the combustion temperature below 1200°F (by reducing the %O2) to minimize the creation of Nitric Oxide (NOx) which is a nasty pollutant.
Since the EGR diluted charge burns slower, the spark needs to happen earlier (more advanced) to ensure efficient combustion.
The USA distributors on the FJ60s were all "curved" (calibrated) at the factory to match the combustion speed of an EGR mixture. The "curve" refers to the timing of the spark at different RPMs. If the timing advance vs rpms is graphed and the dots connected, the resulting representation is referred to as the "curve".
When a distributor gets "recurved" for non EGR use, it's being modified by changing the weights & springs inside it to advance the spark more slowly or to less of a degree for non EGR use.
When the EGR system is disconnected, the air fuel ratio contains more oxygen which will allow the charge to burn faster (and hotter). If the stock distributor is used without EGR, it's spark advance "curve" will be too advanced and the engine will pre-ignite/ping/detonate/knock at certain RPMS.
I would like to add this to FAQ. Thanks OS.
Done!!
Done!!
Last edited:
X2 about adding to FAQ
So is there a good guide on doing this curve?
It's my understanding more advance helps with efficiency at altitude, is that accurate?
If the vehicle is to be operated mostly at high altitude, say above 6000', would the stock curve be a better choice on a totally de-smogged 2F?
Would this configuration remain effective at low altitudes if premium gas was used?
If the vehicle is to be operated mostly at high altitude, say above 6000', would the stock curve be a better choice on a totally de-smogged 2F?
Would this configuration remain effective at low altitudes if premium gas was used?
Keeping the stock curve and changing your fuel with altitude isn't going to be a great solution. Without EGR, the problem isn't that the stock curve is too much advance at all engine speeds, but rather that there's too much at some RPMs and too little at others. That means if you adjust the timing for one part of the range, it'll run okay in there but poorly in elsewhere; you either get pinging under some conditions, or you have to set it so it won't ping at all, which means it'll be running without enough advance through most of the RPM range, and only 'just right' in a couple of particular spots. Result? Smelly exhaust, less-than-ideal MPG/power/torque. It'll run, though (mine has been desmogged with a stock distributor curve for 7 years now), just not ideally.
Now, re: altitude, I've found that my setup runs about the same (reliably 'okay') from sea level up to ~7000 feet, and the timing not being advanced enough up at 7000 probably has a lot less effect on the running than there only being ~78% as much air at that altitude (since it doesn't have any sort of turbo to help with that).
I'm totally de-smogged, '85 FJ60, headers, JimC recurve, Trollhole carb w/small jets. My house is at 7000'. I go up from there. Highighest I've been is a bit over 13,000' and it was sluggish but ran ok. Took it down to Phoenix last week, felt like someone stealthily dropped a turbo in it.
Awesome explanation I've always wondered as well. Thanks.
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Being not such an old thread I'v decided to post my question here. Is there any way to tell if my dizzy has been recurved? The truck was de-smoged by the PO who did a good job, but I have no idea if he did this or not, and I definitely have some issues with pinging and power loss at certain RPS's (though only after I'v been on the highway for a while, so I don't know...).
Thanks!
Clark
Thanks!
Clark
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I can run 87 octane fine unless I'm at medium throttle for an extended time (think flat interstate), then it starts pinging and eventually loosing power. If I floor it or back off, she runs just fine. Premium gas does help this phenomenon, but only lessens the symptoms, not a true cure. I'v been fighting it from a fuel delivery standpoint, but then got to thinking perhaps it's the dizzy?
Clark
Clark
An original (non recurved) distributor would cause those EXACT same problems if the EGR valve was non functional (or missing).
If you move the inner vacuum hose on the distributor to the outer smaller diaphragm and test drive it and the engine runs a lot better-- my guess is that the distributor hasn't been recurved.
There's one sure way to find out though-- send it off the Jim C. He's the one who recurves these and he'd be able to tell.
Or if you have the PO's name, maybe he could look it up in his records??
If you move the inner vacuum hose on the distributor to the outer smaller diaphragm and test drive it and the engine runs a lot better-- my guess is that the distributor hasn't been recurved.
There's one sure way to find out though-- send it off the Jim C. He's the one who recurves these and he'd be able to tell.
Or if you have the PO's name, maybe he could look it up in his records??
@distributorguy, Jeff can recurve but no vac adv pot rebuilds. @FJ60Cam i ‘think’ was going to have him put a non oem vac adv single diaphragm on his. I’m curious myself if that went well?
That wasn’t me, but sounds interesting!
Darn, sorry! I’ll find the thread and the name of who told him.
I'd like to know that information as well. My pig, Bucky, has a bad primary vac pot. Right now it's just running mechanical advance with a little boost from the secondary diaphragm. I'd like to source a new vac pot one way or another.