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Rick in Miami Member
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I continue to be puzzled by the usually recommended total advance @ 2500 RPM of 27* for our cars. The JH Lucas distributor has an 11* mechanical advance (22* crankshaft advance) that when added to an 8*-12* static advance comes to 30*-34* total advance. Adding to this puzzle even more is the Lotus Eclat distributor I recently picked up with a 14* mechanical advance. I have seen both 27* & 32* recommended as total advance all in @ 2500 RPM. Which is correct? Am I leaving some performance on the table with the advance limited to 27*? Thanks ~ Rick |
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redracer Member
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We have almost always used 30 degrees BTDC for the full advance and usually the vacuum retard capsule has been plugged off or removed(don't care what the idle advance is)./ The American V-8 engines with their advance in the 35 degree range is too high. You may wish to call Richard reyman at West Coast Cylinder Heads ; he has done planty of dyno work for this. |
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Esprit2 Member
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Rick, The Lotus distributor with 14.0° dist/ 28.0° crank degrees of mechanical advance is from late Federal emissions engines (Eclat, Elite & Esprit S2). The important thing to note is that it's a very slow advance curve, and the 28.0° isn't all in until 6500 rpm. The J-H distributor is 17.0° Maximum @ 2500 rpm. At that same rpm, that Lotus distributor's mechanical advance is only 13.33°. With your Lotus distributor, you can dial in more static advance to improve the total advance at 2500 rpm, but you can't ignore the fact that the mechanical advance keeps increasing right up to redline. There's such a thing as too much total advance, regardless of rpm. Jeff at JAE once told me that anything more than 36-37° Total was excessive and could actually hurt performance. Having said that, I have two Eclats and one Esprit S2, all of which have that Federal emissions distributor. I autocrossed all of those cars in various degrees of mostly stock to hotrod, and I used a Static advance of 12-16 BTDC. I don't have easy access to a dyno, but significantly lower lap times with 16° BTDC indicated there was still a power-benefit to be had (16° Static + 28° Mech = 44° Total at near redline). The day I first bumped the static timing up from stock to 16° BTDC, one of my competitors asked me where I hid the supercharger. However, the engine felt really edgie on the street with 16° Static, and I didn't like it. So for street driving, I'd set the static to 12-14° Static. The engine ran well, didn't 'ping' or display any poor running symptoms, and it performed much better than stock. I should probably add that the cams were also re-timed to 110° MOP (both cams - 1st thing I did after buying each car), and the carbs were tweaked. Initially, the stock Z-S carbs' adjustable needles were adjusted for best running instead of emissions lean. Later, they were replaced by Dellorto 45E carbs with 36mm chokes. Later the cams were replaced with a pair or 104s, and later with a pair of DS-2 (a step beyond 104). But always with that same stock distributor. IMHO, a better solution would be a faster advance curve. Something similar to the J-H & Lotus non-emissions distributors, with 16-18° mechanical advance all in by 2500 rpm. The Vacuum advance is really RETARD, and it's an emissions feature intended to reduce oxides of nitrogen on closed-throttle over-run and at idle. It's not functional once the throttle is opened beyond idle. As a side effect, running with Vac Retard results in less total advance at closed throttle for a more benign idle. However, that can also make the engine more prone to run-on after the ignition is switched off. Without Vac. Retard, the engine will have more total advance at closed throttle, so the idle will be more edgie/ nervous. The upside is that with more advance at idle, the engine is more prone to stop cleanly when the ignition is switched off. J-H Distrib Mechanical ... 17.0° Max @ 2500 rpm Static Adv. .... 10.0° BTDC ... 8.0° BTDC ... 12.0° BTDC ... 2.0° BTDC. Total Adv ...... 27.0° .......... 25.0° ............ 29.0° .......... 17.0° (lame) Total Spec ............ Stromberg ... Stromberg ..... Dellorto ....... Dellorto .................... Pre-4803 ..... 4803-onward.. Pre-18900 ... 19800-onward J-H Distrib -- Federal Emissions Mechanical ... 18.0° Max @ 3250 rpm (13.85° @ 2500 rpm) Static Adv. ..... 8.0° BTDC ..... 5.0° BTDC Total Adv. ..... 26.0° Total .... 23.0° Total Carb Type ..... Stromberg .... Stromberg Market ......... Federal ......... Euro & Australia Lotus Distrib -- Euro & Early Federal Emissions Mechanical ... 16.0° Max @ 3000 rpm (13.33° @ 2500 rpm) Static Adv. .... 12.0° BTDC ... 8.0° BTDC ... 16.0° BTDC ... 9.0° BTDC ... 10.0° BTDC Total Adv. ..... 28.0° .......... 24.0° ............ 32.0° ......... 25.0° ............ 26.0° Total Spec. Number 1 ................ 2 .................. 3 ............... 5 & 9 ............ 10 Carb Type ..... Dellorto ....... Stromberg ..... Dellorto ...... Dellorto ......... Dellorto Lotus Distrib -- Federal Emissions Mechanical ... 28.0° Max @ 6500 rpm (13.33° @ 2500 rpm) Static Adv. ..... 8.0° BTDC ... 10.0° BTDC ... 16.0° BTDC ... 9.0° BTDC ... 10.0° BTDC Total Adv. ..... 36.0° ........... 38.0° ............ 32.0° .......... 25.0° ........... 26.0° Total Spec. Number 4 & 6 ........... 7 & 8 ............. 3 ................ 5 & 9 ........... 10 Later, Tim Engel Last edited on 03-22-2018 09:09 pm by Esprit2 |
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Rick in Miami Member
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Thanks Bruce& Tim, I ask because I’ve been running an increased static advance of 14*-16* on my 11* JH 25D distributor with vac retard disconnected. Carbs are Weber 40 DCOE 2 with 34 chokes. Cams are set to 110-110. Springs allow all in @ 2500 and I’ve installed a collar on the spring post to limit the total mechanical advance to 28*-30*. This is the part that confuses me: J-H Distrib Mechanical ... 17.0° Max @ 2500 rpm Static Adv. .... 10.0° BTDC ... 8.0° BTDC ... 12.0° BTDC ... 2.0° BTDC. Total Adv ...... 27.0° .......... 25.0° ............ 29.0° .......... 17.0° (lame) Total Spec ............ Stromberg ... Stromberg ..... Dellorto ....... Dellorto .................... Pre-4803 ..... 4803-onward.. Pre-18900 ... 19800-onward How can the Max all in Mechanical be 17.0* @ 2500 RPM if the distributor allows 22* crank advance? Is there is a benefit to be gained by going beyond 30* or would I be venturing into dangerous territory? Is 32*-34* safe? 36*? Regarding the Eclat distributor, if I ever do use it, I will modify it similarly – but simply out of curiosity -even though the Eclat dist doesn’t get full advance until 6500, the 14* mech advance would still allow more than 28* total advance with static set to anything greater than TDC. Rick |
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Distributorguy Member
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The 11* cam doesn't really give 11 degrees unless you install the shorter style advance weights. None of them are very precise as built. As for the "stock" advance curves, they were +/- 10% at best, and they were designed around running a lower base timing setting with more mechanical advance to help overcome the timing retard which causes the exhaust gasses to heat up. I'd also argue that there's power to be gained if you draw the all-in timing to come in at a higher rpm. I've built a few thousand of these distributors at WAY different specs than factory, with exceptional dyno results. 32 degrees total is very safe in most cases, unless your cylinder pressures are extremely high (think compression test values over 200 psi) and an aftermarket cam setup. Any advance that takes place over 4000 rpm is a waste and potentially dangerous. Completely unnecessary. No engine running DCOEs or similar should have vacuum advance. |
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Rick in Miami Member
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Thank you for your experienced input and for joining this forum. |
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