Exhaust Cam Timing Adjustment

[tom_v]

Hi, i have a R33 with the usual minor mods (Exhaust, pod, cooler) and have tuned the exhaust cam timing on a dyno to get the best air/fuel mix and power.

Recently i changed the timing belt and even though i marked where the exhaust cam adjustment was, after putting it back together i notice it is running rich and using a lot of fuel. Also lacking that little bit more power i was able to get previously.

I have searched the forums and couldnt find any other previous post regarding the adjusting of the exhaust cam.

What i would like to no is, which way does the cam timing adjust need to be turned to retard or advance this timing. Also which will be leaner, retarding or advancing.

Thanks

[mad082]

altering the cam timing shouldn't have any effect on fuel mixture as all it is doing is determining then the cams are opening the valves (if you are talking about adjusting the actual cams). if you are talking about adjusting the ignition timing via the CAS (crank angle sensor) then that isn't adjusting the cams, just when the spark plugs make the fuel go bang, but again shouldn't really alter the fuel mixtures unless you are getting detonation and the ecu is backing off the timing and richening up the mixtures a bit.

maybe your power loss is due to the hot weather as that can affect it quite a bit if you get 1 dyno run done on a cold day and the next done on a stinking hot day.

[tom_v]

It must be called something different to what i was told so i pointed it out on this picture below.

Previously when i ran it on the dyno, we adjusted this and managed to get 6rwkw more and and more stable and better air/fuel mix. I would just like the crack the bolts and move it slightly one way to hopefully make it a touch leaner if i can.

Can anyone tell me which way is retarding and which is advancing?

[blind_elk]

That's the CAS - Crank Angle Sensor - and is used to set ignition timing. The only relationship to the exhaust cam is that the exhaust cam drives the CAS.

You really shouldn't be fiddling that because you risk setting hi-load hi-rev timing that will induce detonation. Just set it so your timing at idle is 15 deg BTDC, and leave it alone after that.

As mad082 said, you may have lost a bit of power due to the hot weather.

[tom_v]

Thankyou heaps for the info, that makes so much more sense now, caus even driving in 3rd just slowly accelerating at about 55km it has a slight flat spot like when the coils fail. Ill give that a go and see how it turns out

[mosquitocoils]

Hey tom, how'd you go with your timing? I'm gonna have to check and adjust mine soon

[08yannch]

the big problem with adjusting timing is first working out how to use the timing light because they are made 4 cars with leads this can be overcome by putting a extension piece between cylinder 1 coil and spark plug. 2 u wont know how the car will react under boost and u may get detonation unless running on a dyno adjust 1° and give the car some bean in 1st to redline if all is good then u can go 1° more, if ur car starts to make any funny sounds then go back 1° also u will be able to tweak the timing more on high octane fuel and also when u test ur car try to do it at night as the air is cooler.

i would not tweak the timing any more than 13° unless running water injection if so go all the way to 9°

*note plz dont hold me responsible if any thing goes wrong

[mad082]

um, i'm pretty sure the stock timing is 15 degrees, and if you are going to 13 degrees you are actually retarding the timing and you will lose power and if you get pinging from retarding the engine you have some serious engine issues, or don't know what detonation is. secondly, often detonation isn't audible over the sound of a screaming engine until you are getting a hell of a lot of it. on some cars just the sound of the injectors or noisey lifters can sound like detonation.

also running the car in 1st gear to check for detonation isn't the best as in first gear the car will rev up quickly and not get as much load on it as in higher gears. the higher the load the more chance of detonation. so you would be best of checking for detonation in 4th gear, or whatever gear givs you a 1:1 ratio (for example 4 spd autos it's usually 3rd gear)

also working out how to use the timing light isn't a big problem. it's a small problem for cars with coil packs and as you said, very easy to overcome.

[08yannch]

ok i have pulled out the nissan book and it says that a RB25det should be at 15° +/- 1° at 650 rpm +/- 50 rpm. this is the same 4 rb25det and rb25de so im guessing that the ecu will automatic retard the ignition.

im pretty shore that the closer to 0° the more compression u will have = more power right hmm but at the risk of detonation

the orange line = 0° and each white line is = to 5°

i said 1st because u wont be speeding so that makes testing on the street a lot more safer it would be better to test in a higher gear but u dont want to get done doing +120km/h in a 60 zone

[mad082]

timing works the other way. the earlier the spark the more bang you get, to an extent. the reason being that all the fuel doesn't ignite the moment the spark fires. this takes time. if you ignite the fuel at 0 degreesthe piston will already be falling away by the time the fuel starts to ignite and therefor will lose most of it's power as even though it is expanding as it fires, it is now expending at the same time as the area it has to expand in is also getting bigger. also it means the some of the fuel will be moving away from the flames and take much longer to get burntup, or possibly not get burnt at all.

if you make the timing fire sooner (advance, so make the number higher) you end up increasing the pressure in the cylinder as you have more fuel getting burnt before maximum compression. and as we all know, hot air is less dense than cold air, so once it is fully compressed then you have a higer pressure and it will push the piston back down with more force. where detonation/pinging comes into this is that by having more fuel burnt, and more gases in the chamber there is more heat. and the more you compress something the hotter it gets (hence why we have intercoolers), you get higher cylinder temps and engine temps. then once the engine is on its next cycle and fresh fuel/air comes in, there is more heat being radiated out of the cylinder walls, and as the piston starts compressing the air and fuel, this extra heat can cause the fuel to ignite prior to the spark being fired (hence the term pre-ignition) or can just make the fuel burn unevenly.

pre-ignition is most obvious in an older car not running fuel injection. if you drive it hard when the timing is a bit out or it is a bit lean and get the cylinder temps too high then stop and turn the car off it may continue to run. this is because when you turn the car off only the spark stops, but the carby supplies fuel based off vaccum and while the engine is turning it is creating vaccum so it continues to pump in fuel and any hot spots in the cylinders will make the fuel ignite and the car will continue to run until it cools down enough to not ignite the fuel anymore, or until you put the car in gear and stall it. the missus old datsun stanza used to do this occasionally on hot days if you drove it hard.

that is my understanding of timing. may not be 100% correct, but i think it's pretty close.

[mosquitocoils]

Mad, thats a great explanation, thanks.

Can someone explain what kind of extension to use between the coilpack and spark plug to hook a timing light up to? Just any kind of wire with 3 prongs, or something more sophisticated?

[blind_elk]

Any old spark plug lead will do the job.

[08yannch]

i used a extension piece from a socket set and some electrical tape it was very dodge but it worked

ok mad

i have looked around and u are right about ignition timing i was wrong sorry if i confused any body

its just if u think about it like i was if the spark plug is firing b4 TDS wont the engine spin backwards. "every body ignore this statement as it could be confusing"

[mad082]

i know what you mean about thinking about shouldn't the engine spin backwards. if all the fuel combusted the exact instant that the spark fired then that would be the case, but since it takes time ignite and burn then it doesn't cause the engine to spin backwards.

[MrRx7s3]

Can I ask a silly question, which way turning the CAS retards and advances, is clockwise retard and anticlockwise advance or the otherway around, thanks.

[Ben C34]

Clockwise is retard. Never go full retard.

[MrRx7s3]

Just wanna retard it a little to pass my emissions test lol, ok so I did well haha lol.

[Ben C34]

That won't help emissions.

[MrRx7s3]

It should help lower the NOX level which is the only one I'm failing at.

[blind_elk]

Maybe you need a new catalytic converter.