rayaapp2 Posted November 28, 2012 Share Posted November 28, 2012 So Im working on a 73 240Z with a L28 and an automatic transmission. The L28(N42 dished pistons) has an E31 head, and the compression is low enough that it should be fine on pump gas. Interesting enough the car had a L24 with flat tops and the same top end being fed by the same stock SU carbs and had the same exact issue. So Im down to Fuel and timing. IF I pull timing back to 5* BTDC both engines still have heavy ping during loading in all 3 gears. I pulled the timing back on both to TDC and ran 91 octane(oh and the gas tank was boiled out between fuel change so not even a hint of 87 was in the tank for mixed octane). They both still had the same issue. Im not getting any funny AFR's indicating lean issues with the wideband hooked up now on the L28(In fact Im getting into the 13.8:1 range under heavy loads in top 2 gears which seems good on SU's with an L28). So Im suspecting a distributor timing issue. Did the 240Z's come with a retarding distributor when equipped with an automatic? The distributor that was on both engines had "12" weights(and are identical to some of the early('71ish) L24 distributors I have) and for giggles I disconnected the distributor advance, but Im really starting to suspect that I need a distributor capable of pulling timing back for the automatic transmission. I really dont feel this is a matter of high compression though I know some would jump straight to that. I would expect this kind of ping on SU's with something closer to 10:1. Thanks for your thoughts on this. Ray Quote Link to comment Share on other sites More sharing options...
motomanmike Posted November 28, 2012 Share Posted November 28, 2012 (edited) The L24 240z had a dual points distributor when equipped with an automatic transmission. I'm not sure what the timing was set at but I know it was a different distributor. I'm still running the dual point on my L24 powered Z. I get better power out of it than I did with the pertronix so i switched back to the old school points distributor. Edited November 28, 2012 by motomanmike Quote Link to comment Share on other sites More sharing options...
Leon Posted November 28, 2012 Share Posted November 28, 2012 It could be something as simple as a poorly-seated distributor cap (been there, done that!). Was the same ignition system used on both engines? If so, I suspect something is up with your spark. Quote Link to comment Share on other sites More sharing options...
skirkland1980 Posted November 29, 2012 Share Posted November 29, 2012 13.8 sounds a little lean. What spark plugs are you using? Like a bpr7es? Quote Link to comment Share on other sites More sharing options...
rayaapp2 Posted November 29, 2012 Author Share Posted November 29, 2012 The L24 240z had a dual points distributor when equipped with an automatic transmission. I'm not sure what the timing was set at but I know it was a different distributor. I'm still running the dual point on my L24 powered Z. I get better power out of it than I did with the pertronix so i switched back to the old school points distributor. That's right! I had forgotten. And the points were setup so that one set is for retard if memory serves me! Toyota used the dual diaphragm vacuum units! That doesnt bode well for me though. I wonder what the 280ZX used with an automatic transmission. I may have to use something like that so we can stay away from points(this isnt my car or Id just find one of those distributors stick points back on it). It could be something as simple as a poorly-seated distributor cap (been there, done that!). Was the same ignition system used on both engines? If so, I suspect something is up with your spark. Indeed it is the same distributor, wires, plugs, coil, etc used on both engines. The distributor cap has been on and off several times. Now Ive changed the whole distributor out and transferred the pertronix pickup to the new distributor and it still has the same issue. So it has to be the ignition timing coupled with the automatic transmission. If it were AFR Id have seen it on the wideband. I wish I could hook up a mobile 5 gas on the car. 13.8 sounds a little lean. What spark plugs are you using? Like a bpr7es? Put a wide band on any L24, L26, or L28 with stockish SU carbs... 13.8:1 is fantastic! Most are lucky to see 15 or even 16:1. If you can get it richer than 15:1 you have compromised your idle AFR so bad it will likely foul the plugs at idle esp on the larger L26 or L28 engines. Quote Link to comment Share on other sites More sharing options...
Leon Posted November 29, 2012 Share Posted November 29, 2012 My distributor cap seating issue had to do with the cap not physically being able to seat on a 240Z distributor. From memory, the distributor had a little plateau sticking out of it but the cap didn't have a corresponding notch. This was a new cap that was bought at an auto parts store. The car had pinging issues no matter how rich I ran it. I only noticed the problem after removing the distributor and seeing the gap between it and the cap. With the distributor mounted, it was almost imperceptible, unless you really looked for it. Just sharing my experience, YMMV! Quote Link to comment Share on other sites More sharing options...
rayaapp2 Posted November 30, 2012 Author Share Posted November 30, 2012 My distributor cap seating issue had to do with the cap not physically being able to seat on a 240Z distributor. From memory, the distributor had a little plateau sticking out of it but the cap didn't have a corresponding notch. This was a new cap that was bought at an auto parts store. The car had pinging issues no matter how rich I ran it. I only noticed the problem after removing the distributor and seeing the gap between it and the cap. With the distributor mounted, it was almost imperceptible, unless you really looked for it. Just sharing my experience, YMMV! Thanks for the tip! I will have to look and see if that could be an issue. Quote Link to comment Share on other sites More sharing options...
Tony D Posted November 30, 2012 Share Posted November 30, 2012 Put a wide band on any L24, L26, or L28 with stockish SU carbs... 13.8:1 is fantastic! Most are lucky to see 15 or even 16:1. If you can get it richer than 15:1 you have compromised your idle AFR so bad it will likely foul the plugs at idle esp on the larger L26 or L28 engines. "Yes, now again guys, tell me why the Flat-Tops with separate, independent idle circuit from the main needle are boat anchors again!" Ray, my L26 with an L28 N47 on it pings like crazy as well. I've taken to buying the Turbo 108 octane booster (off road formulation, not catalyst safe...) and put half the big bottle in per tank when empty. That's usually the right amount or the 10-12 gallons I normally put in. Car is rattle-free afterwards. Same with 100 octane VP. This engine has 150# across the board in compression (The Blue Turd.) The CA formulation is not carb friendly. The same gas runs knock-free (even 87!) in my L28 stock EFI 2+2... That really is the last experiment for The Turd... Put in EFI changing nothing else and see what it does, since its an FI Head it's a relatively quick swap...I just don't have the time! Quote Link to comment Share on other sites More sharing options...
rayaapp2 Posted November 30, 2012 Author Share Posted November 30, 2012 "Yes, now again guys, tell me why the Flat-Tops with separate, independent idle circuit from the main needle are boat anchors again!" Because I cant find good float components dammit! Oh and have you ever tried to salvage floats from those carbs? JEEZ what was the engineer thinking when they designed that pin with no way of getting at it from both sides? Guess they never considered where these carbs would be almost 40 years later. The owner of the car has stated he would be okay with putting premium in the tank, but that doesnt sit well with me. My L28 runs 87 with SU carbs and an N42 head and it does not suffer from knock at all even with a little more advance. strange. Quote Link to comment Share on other sites More sharing options...
Tony D Posted November 30, 2012 Share Posted November 30, 2012 My L28/N42 as well! The engine in the Fairlady has 185 across the board, you would think... Quote Link to comment Share on other sites More sharing options...
Leon Posted November 30, 2012 Share Posted November 30, 2012 Hell, my L24 with triples runs just fine on 89 (haven't tried 87) with 36deg of advance, even heat soaked on a dyno... Quote Link to comment Share on other sites More sharing options...
rayaapp2 Posted December 1, 2012 Author Share Posted December 1, 2012 (edited) The original motor had .009" more thickness on the head and was a stock L24 flat top engine with the same problem. My other L24's with the same setups do not ping. Ive been reading this and thinking: There is absolutely nothing at all wrong with that head. It has the same ports as the later N42, already has the larger exhaust valve, and the only difference between that chamber and an N42 is the spark plug boss. One benifit of that head is that it's already as unshrouded as you can get, for an unmodified head. Also...everyone hates on them, so they're cheap! If you are looking to control detonation with a cam, then run a longer exhaust duration, longer intake duration, and LOOK AT THE INTAKE VALVE CLOSING TIMING. Close the valve a little late and it'll bleed compression off at lower RPM. It'll actually help cylinder filling at higher RPM, but the main benifit in this case is lower dynamic compression. This exact mechanic is why I can run Kentucky 87 octane on a dished bottom end with an open-chamber N42 that has been shaved to provide 9.7:1 compression, and STILL run a full 36 degrees of timing with no tendency to ping or knock. Just set the valve closing event to bleed off some of that compression from idle to about 3500 RPM, and you can avoid a LOT of detonation. I wish I had an adjustable cam gear now just so I could experiment. Im going to also try and stick some colder plugs in it and cross my fingers they dont foul at idle. Edited December 2, 2012 by rayaapp2 Quote Link to comment Share on other sites More sharing options...
Tony D Posted December 2, 2012 Share Posted December 2, 2012 Mine starts to ping above 3500... Quote Link to comment Share on other sites More sharing options...
rayaapp2 Posted December 2, 2012 Author Share Posted December 2, 2012 Isnt the blue turd a manual Tony? I have a hard time believing simply going with EFI will make the difference on this setup. I set the SU's so that they fouled the plugs at idle and got a nice fat AFR under load and still got the ping. I set the idle at 2K rpm for this test! It ran while pouring black smoke from the exhaust at 1100rpm before I brought the idle up. The automatic transmission loading is a big part of my problem I feel. Im going to try and track down a Maxima automatic distributor. I think that may help me out as well. This setup will ping under 3500rpm in 2nd gear up a hill. If my foot is more than half way into the pedal it it pings and I loose torque. I found a set of brand new BRP7ES plugs and Im going to put those in today after I lean the mixture out a little. Im also going to try and retard the cam and see if I dont loose all the power. I set the cam at TDC with a degree wheel and dial indicator. Id venture a guess that most stock setups have the cam slightly retarded. I have a brand new timing assembly in this engine so its very tight. My only worry is that the stock cam adjustments will be too much and I will loose power before I correct the ping. I assume youve tried most of what Im thinking about trying on this setup. Colder plugs, retarding cam, less mechanical advance, and richer mixture? Everything is telling me that this cam is just plain wrong for this setup with an automatic transmission and that I would be in much better shape with either a manual transmission or a different cam. Im thinking the timing curve change is only a band aid even though in theory it would fix the problem. The dual points was just how Nissan decided to address the issue of loading with their stock setup. Just my theory at the moment. I think more duration and overlap would correct the issue with this transmission loading. Im not good with cams though. Quote Link to comment Share on other sites More sharing options...
Xnke Posted December 2, 2012 Share Posted December 2, 2012 More duration and lift. Overlap is a red herring! Intake valve closing is the event you need to pay attention to, to stave off detonation. You can retard the cam one degree by slipping a tooth and then moving to the #3 position; thats -9*, plus the 8* from the #3 hole...or you can slip a tooth then add back four degrees by going to the #2 hole. Quote Link to comment Share on other sites More sharing options...
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