Is it risky to run low timing at high rpm for some time?

[Lazeum]

The question is in the title. I have a total timing at high rpm at 28º so far.

It could sound stupid but I guess it could make sense in some cases, including mine (I hope!!! )

 

- My Story -

I had knock for some time (actually since I bough it over 1 year ago but I run the car very little so far - 6 months winter did not help) and decided to fix it. It occured only at low rpm (3k / 3.5k rpm) at WOT.

 

FYI, my setup is : L28, flat top pistons, mild cam, N42, 3 DCOE, headers, 6-2-1 2.5" exhaust, Mallory Unilite distributor, Mallory Hyfire 6AL.

 

I decided to back off timing a little to eliminate detonation but I went far enough to have very erratic idle with missings. So I went back a little to run ok. the car was still detonating, the idle timing came at 10/11º.

Advance timing set at the factory was untouched: +24º

 

From my readings, total timing at idle should be more at 14-16 and at 34-36 at high rpm. Knowing my car is detonating, I had to play with the distributor timing (probably also with the curve since there was no detonation after 3500rpm).

 

Then this weekend, I setup the advance at 18º (to have 14+18 = 34º).

I did not touch anything anymore so far; idle timing is still 10º.

 

The car runs fine with no detonation (!!!!) but does not seem to pull as strong as before (since it is only for some time, it is fine with me).

 

 

Then the reason why I would like to run like this for some weeks:

I want to record with dyno curves the tuning progess (with an inboard sofware I need to tweak first); my Webers are not tuned very well and I want to observe improvements and progress.

 

What do you, guys, think? Can something bad happen to my engine running this way?

[z-ya]

Running less advance will not hurt your motor, but detonation will! Check your compression, you may have a head gasket that is failing.

 

I run basically the same engine in my track car and recently blew the head gasket. I am assuming because of detonation I couldn't hear when on the track. I have a problem with fuel pressure dropping after a number of track sessions. This was causing it to lean out and detonate.

 

Did you check your full advance with a timing light? Full advance should happen above 3500RPM or so. I suspect it is in the 28-30 degree range with your setup.

 

Pete

[JMortensen]

The bad thing that will happen from not running enough timing is that you'll be making less power and the engine won't want to accelerate, especially at higher rpms. Some EI units like Electromotive DIS actually use timing retard as a rev limiter. Having experienced that one in person, hitting the rev limiter (which I think took timing from mid 30's to 0 in this case) felt like hitting a brick wall.

 

What would be interesting is dyno plots with the timing backed off and then advanced to where it makes most power. In general, if you can hit that point at which the most power is made with the lowest octane fuel that prevents detonation, that's going to give you the best power overall. Although I don't have a feeling for exactly how much power you're losing out on, I would say that it's going to be a decent amount.

 

I'd rather have 9.5:1 compression and be able to run optimum timing on the fuel that I want to use than have 10.5:1 and have to reduce the timing to prevent detonation.

[Lazeum]

First of all, thanks guys for your answers. They make me feel better

 

I've checked my timing before the advance timing tweak. I was at 10º at 1k rpm, 25º at 2k rpm and 34º at 3k rpm.

I'm using a regular timing light which is not so accurate... A friend of mine is having one with timing correction. I plan to use it as soon as I can.

 

I've checked my AFR as well with a wideband, I'm on the (too) rich side (10 to 12.0 AFR) but before to mess up with the carbs or do anything else with the engine, the detonation had to go.

 

The engine is also freshly rebuilt, it has less than 2000 miles on it.

 

I'd rather have 9.5:1 compression and be able to run optimum timing on the fuel that I want to use than have 10.5:1 and have to reduce the timing to prevent detonation.

I fully agree with you, it does not make sense otherwise.

 

Finally, I'm not going to leave the engine the way it is. I'm running 93, there's no reason I could not run optimum timing.

With no detonation, I'll be able to go little by little to the optimal performance of the setup.

[z-ya]

We dyno tuned the track car in August, and we played around with timing a lot. We targeted 13:1 AFR, and tried to tune for max power. The goal was to have a little safety margin.

 

25deg - 151HP

28deg - 168HP

30deg - 179HP (slight detonation)

 

Here is the dyno plot at 28deg:

 

 

 

This is with a mild cam (460lift/280dur), and no head work.

[1 fast z]

With less timing, the motor WILL run hotter, and ALOT hotter. What it is, is that the charge is a couple hundred degrees hotter when the ex valve is open, and heats the head up, and thus the water.

[Lazeum]

25deg - 151HP

28deg - 168HP

30deg - 179HP (slight detonation)

 

This is with a mild cam (460lift/280dur), and no head work.

 

What kind of timing is it? Total or advance timing? Since it could be both, I'm confused...

 

1 Fast Z, it is something I did not think about. I did not run the car long enough to see any unusual high temperature (I drove the car for 15 miles with this setup). I'll definitely check the temp next time I run the car.

Being on the very rich side with AFR probably reduces heat a little also, I guess.

 

Regarding timing, I was able in the past to see big improvements with my previous car (Mitsubishi Evolution), I gained 20-30 whp with timing adjustements vs. stock settings.

[z-ya]

What kind of timing is it? Total or advance timing? Since it could be both, I'm confused...

 

Yes, timing over 3500RPM, WOT (no vacuum), full mechanical advance.

 

Pete

[Pyro]

reduce the amount of advance and use 24 initial and 32 total. (4 degrees in the distributor = 8 at the crank).

 

you could also install a bigger cam. this will allow more total timing due to lower cylinder pressure from a later intake valve closing point. a cam with 280 to 290 degrees would be ideal for your compression ratio and use about 36 degrees total.

[Lazeum]

reduce the amount of advance and use 24 initial and 32 total. (4 degrees in the distributor = 8 at the crank).

 

You would run 24 initial timing? That's 14° over what I have today Am I that far off?

 

Regarding my cam, I have a schneider cam with a duration of 274 if I remember well.

 

For the distributor, I do understand there are 6 sparks per rotation on the distributor vs. 3 ignitions on the engine. Distributor turns 2x lower than the engine

 

z-ya, what was your initial timing? I guess you were having quite a lot of timing. could initial timing be low because of vacuum advance?

[z-ya]

I don't really look at what the idle timing is with or without the vacuum advance connected. I don't have any control over the advance curve of the distributor I am using, so it doesn't matter what the idle timing is. What is important is the timing at high loads and RPMs, this is where I have had detonation problems.

 

This is where a dyno comes in handy. You can put a full load on the engine, and watch the timing as the RPMs increase. Once the mechanical advance is done advancing (somewhere over 3000RPM), the timing is steady at 28 deg. I prefer to set the timing this way because it is really all that matters, especially in a track car with not a lot of power (your either on the throttle full, or not at all).

 

Pete

[Lazeum]

Thanks all for the explanation

 

I'll ramp up timing on my car soon and see what I can get out of it.

Hopefully I'll be able to get dyno curve before and after.

[ZR8ED]

Lazeum.

 

Could you tell me your jetting/ chokes(venturis) and carb size 40's-45's etc. Could you also tell me a bit about your engine specs? compression ratio, cyl pressures..are they even? cam, etc..

 

You mention that your webers are not tuned correctly, and that you are playing with timing to combat pinging (detonation) Are you experienced with webers? or do you have someone helping you? You are only about 4 hours drive from me, and I may be able to help you out. Webers on our z's work very well with approx 20 degrees initial and between 34-38 degrees full advance at WOT. If you can't hit those numbers without ping, then I would be looking first at the carbs (assuming the rest of the engine is in a decent state of tune)

 

There is a lot more information required to help you out, but this is a good start.

 

PM me if you want to talk about more details, but I can go through some of the basics here on the forum.

 

Scott.

[Pyro]

You would run 24 initial timing? That's 14° over what I have today Am I that far off?

 

Regarding my cam, I have a schneider cam with a duration of 274 if I remember well.

 

For the distributor, I do understand there are 6 sparks per rotation on the distributor vs. 3 ignitions on the engine. Distributor turns 2x lower than the engine

 

z-ya, what was your initial timing? I guess you were having quite a lot of timing. could initial timing be low because of vacuum advance?

 

longer durations cams need more initial timing. But you would need to reduce the advance amount in the distributor so you don't over advance at the top end. If you want throttle response off idle, then initial timing is very important.

[Lazeum]

longer durations cams need more initial timing. But you would need to reduce the advance amount in the distributor so you don't over advance at the top end. If you want throttle response off idle, then initial timing is very important.

 

I am definitely looking for good response off idle. I'm more looking for a fat torque curve with decent power than for a very high HP number.

 

Scott,

Your proposal is very nice but I'm no longer in Michigan (I'm in France, Europe). It is a little over 4h

I'll send you a PM about my setup.

[josh817]

I just about needed this help too! I have a California dizzy that is sitting half way on the little plate thing, but its reading 35º total advance! California models should be running less I thought! 25ish at idle but my idle won't go lower than 1200RPM so I'm dropping it down or just living with it. I can't imagine 36-38 though... What size are your carbs? I bet you're running lean if they're 40's with small venturis like I probably am. :/I'm sure a crappy timing light will screw with our brains all month... so I'd get on that quickly..

 

 

Still though, I'm betting you're running lean, way lean perhaps. With good fuel, good ignition, crappy timing light, and questionable carbs, the only 2 things I can think of is your light is screwed or the carbs need working. :/ We both need to sit down and write all of our jet sizes and venturi sizes are. Do some calculations and hope for the best... cause its expensive.

[Lazeum]

Hey Josh,

Actually, I'm running very rich.

 

As far as my full carbs setup, I've got:

3x 40 DCOE

Main 130

Air correction 180

Emulsion tube F11

Venturi's 32mm (to be confirmed)

Accelerator 50

Idle jet 50F9

 

see link below for some AFR during a regular drive

AFR log

 

the timing light I am using is a no advance Craftman unit.

To setup the advance timing, I've used gages to setup the Unilite unit. I did not check with the light total timing.

 

btw, I've order the tuning kit from Mallory with different set of springs, I'll see what I can do with that