L28 engine, 10 to 1 compression, using a D612-53 distributor from an early L24 engine, static timing 5*

Pertronix ignition, mild Crane cam.

Can not advance the timing past 10* without ping, vacuum advance disconnected because of triple carbs.

I have tried 94 octane gas with colder plugs but still getting some ping.

I thought my next move was to pull the head and install a thicker head gasket to bring down the compression a bit but my local mechanic suggested that a cam with a longer duration might be worth a try before pulling the head. Is this a real option?

This is the cam I'm running right now,

Yes a longer duration cam can help for sure

Have a link to your build - L 28 specs

Curve specs on dizzy?

When does the pinging happen- rpm?

Edited November 3, 201311 yr by madkaw

Got rid of my pinging with a recurved distributor.

Have a link to your build - L 28 specs

Curve specs on dizzy?

When does the pinging happen- rpm?

F54 block, N42 head, port and polish, port matched,

Crane mild cam grind on a Nissan blank cam,

Flat top pistons, not sure which ones,

Cylinder bored 0.020" over, 10:1 compression,

The timing curve reference I'm using is this great chart put together by Walter Moore, scroll down to post #19

http://www.classiczcars.com/forums/electrical-s30/34192-280zx-distributor-advance-curve-reference.html

Ping happens under hard acceleration in 3rd 4th and 5th from about 3500 rpm and up. I thought the triples had solved the problem and they did help but after I got use to the louder induction sound I realized most of the pre detonation was still there

I can loan you a cam to experiment with if you want.

I'de be doing some serious testing to ensure your timing is where you think it is..

Chris, Im with zKars on this. I would like to know what the dizzy curve really is before changing cams.

You can map it with a timing light and plot the curve on graph paper or MS excel along with the one from Walter Moores chart and compare them.

It could be something as simple as weak springs in the dizzy advance and if it is not that then you can more on.

Chas

You should have full advance before 3500 RPM. 24 + 10 = 34 total. Are you using projected tip plugs? I've read that because they start the flame closer to the center of the chamber it's like adding 4 degrees of advance. That would put you at 38 total. I think that worn weights and the pins they ride on can let the advance move further than stock. A timing light would tell you what you are getting, but 24 + 5 is still 29 total. The pros would say you should get some dyno tuning to find what you really need. Do you feel like there's more power to be had, or are you looking for cheaper gas?

Also, have you measured cylinder pressure? We all talk about CR but it's really the cylinder pressure that matters. That's why the longer duration cam can help, it lowers static cylinder pressure. It will give you a better idea of your starting point for cam changes or head work.

Edited November 3, 201311 yr by Zed Head

You can get a digital "dial-back" timing light at NAPA, and I think I saw one at an Auto-Zone once. It is a really great tool for verifying the dynamic spark timing.

That's a good point. It's difficult to measure advance when it's off the marks. I've estimated mine by setting static to zero and revving the engine. Still have to eyeball/estimate the final few degrees. I regret not spending the extra few dollars for a dial-back light.

Uh boy... this is reminding me of the nightmares I went through on this thread,http://www.classiczcars.com/forums/help-me/49271-engine-will-only-run-above-4000-rpm-carbs-timing.html

The long and short of that thread was once I threw away the HEI and switched to the old 240z distributor(D612-53) with Pertronix ignition, I got the engine running pretty well. But there is obviously still some timing/ignition issues that haven't been solved.

I have a good quality timing light now with an advance dial on it so I'll try to do a graph of the timing mark as I take the revs up.

Zed, the spark plugs are NGK BPR7ES, the threaded portion is 23mm long the same as the BPR6ES.

The compression numbers that I took in June are #1 cyl 215lbs #2 195lbs #3 210lbs #4 220lbs #5 215lbs #6 200lbs

I know these numbers seem high but they consistent with a compression test I did 2 months earlier in April.