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Registered
Join Date: Dec 2015
Location: Dallas, Texas
Posts: 698
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Timing for higher compression engine
I'm close to the first start on my recently turbo'ed 83 911. I am using 78-79 930 CIS with a Bosch -044 fuel pump, Kokeln intercooler, and .5 bar wastegate spring. Also headers/muffler if that's of any consequence to timing.
I still have my factory 83 SC US spec distributor, but have an MSD BTM connected. There's a wealth of information on ideal timing for stock/modified 930 engines, but given my higher static compression (9.3:1) and lower boost level, I'm not sure what my best starting point might be with the boost retard. Factory timing specs (with vacuum advance disconnected) for my N/A engine are 5 degrees BTDC at idle and 19-23 degrees BTDC at 6000 RPM. Am I thinking correctly that I can disregard the vacuum advance for this exercise since it would presumably be completely negated I'm under full boost/full throttle? I'm having a little trouble understanding the factory charts, but I think full vacuum advance should add another 3.5 degrees - so total factory advance - with VA and initial timing - would be in the 31.5 degree neighborhood (5 initial, 23 disconnected vacuum, 3.5 VA). I can pull out up to 3 degrees per PSI with the MSD, so at full retard and max boost I'd be down roughly 21 degrees at full boost (.5 bar or a little over 7 PSI). This seems overly conservative, to say the least, but I wanted to check whether anyone might have any experience/thoughts on the matter. I was thinking 2 degrees per PSI might still be a good/conservative starting point, or possibly 1.5. Last edited by sigchuck; 07-22-2022 at 09:17 AM.. |
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07-22-2022, 08:59 AM
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Interested in what other members offer, as well, but will give you my thoughts. I am also in somewhat similar scenarios as I tune a 3.3, however with 8:1 compression and higher boost threshold.
I think that below boost levels/in the area a normally aspirated engine would run, in your case you can stick with a standard SC map for starters. I am assuming your compression is standard for that year SC engine ... so am thinking the turbo effect will be zero below the range where you build boost. Once you start to build boost, I believe at that point you will want to introduce timing retard, more fuel, etc. up to your rpm and boost ceilings. There are numerous threads in here with timing maps from dyno work and other general timing conversations - which may give you a loose framework to work with for you SC based powerplant. Then get to a dyno with an expert who can help you arrive at your engine's best safe max settings. I hope this helps.
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Emery 1988 930 coupe - Silver Metallic TurboKraft 3.3L 8:1 CR, SuperSC Cams, GT35R, B&B Headers, TK intercooler, Tial WG, ARP, tecGT based phased sequential EFI & ignition, Wevo shifter/coupler, ... 
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07-24-2022, 06:28 AM
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Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 4,184
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Quote:
I hope you're not using a 3DLZ with that, LOL.... I'd suggest that, with a spring that soft, you'll want to use a hardware boost controller to route boost pressure to the wastegate diaphragm to keep it the valve shut until it reaches desired/set pressure. Avoid the temptation to put the controller in the cabin, so you can't dick with it while driving along... When I went EFI with boost control, I thought "what's the point of running a stiff spring? Do it all in boost control". This was a Big Mistake, as it turns out. The only guarantee you'll have with a 0.5 bar spring is that it'll be COMPLETELY open by 0.5 bar. Which means it'll start to open long before that. And because 0.5 bar is such as soft spring and the valve is quite large, probably considerably before that. With a stiffer (0.8 bar) spring (because I have yet to take my own advice and get around to fitting the 4-port MAC valve to route boost to the diaphragm to hold it closedl), the difference is that even when quite leisurely running up the RPM in the lower gears, I get loud dump valve noises when I shift up. Personally, I'm not a fan of the chuffing noise. But it was very noticeable - because this rarely happened with the 0.5 bar spring fitted (had to be trying much, much harder, which I rarely do in the lower gears, with N/A gearing). Oh yeh - and boost builds a lot faster too. Which makes sense; there was effectively a large boost leak before... Quote:
Although we like to think of vacuum advance as being all gone as soon as the manifold pressure goes positive, that's not actually what the factory graphs show. *shrug*. Quote:
I'd focus on the dynamic timing @ 4000 RPM, if I were you. 930 uses 26 +/ 4 degrees BTDC for US 930 types, and my RoW 930/60 was spec'd at 29 degrees BTDC. Pretty sure. Probably with the same tolerance. I can't imagine the SC advance springs behaving significantly differently (the early 930 advance chart shows mechanical advance actually dropping off after 3000 RPM), but you might want to run it up to the 6000 RPM spec one time and check to see what's going on, Just Because... Quote:
Also, the 5 degrees you're adding in from idle - IMO, that's included in the 23 dynamic timing figure. So 23 + whatever the vacuum pot adds gives you is your off-boost timing. (You could do worse than check this total advance figure with a timing light after setting it, I think). But it looks like you should be right around 26.5 degrees BTDC off-boost, dropping back to 23 BTDC under full boost. Which is a lot more retarded off-boost than I'd be happy with, but is apparently stock timing for your motor. So it should be fine (although if you have boost retard you might want to experiment with extra off-boost timing; they seem to respond well to it). I think it's been observed and generally agreed that the 930 vacuum pot adds 8-12 degrees to the timing imparted by the mechanical advance @ 4000 RPM, with vacuum plugged. They vary... My factory manual shows the "Vacuum Retard Curve" for a '77 as being 9-11 degrees, which seems to be in the ballpark. Quote:
Rightly or wrongly, I don't vary ignition timing for dynamic compression (boost). Doesn't seem to matter for me. With CIS, I ran my 930/60 with 8:! & SC cams with a RoW dizzy set to 34 degrees @ 4000 RPM (vacuum disconnected), and used the Safeguard to bleed off 0.5 degrees of timing per 1 PSI of boost, starting at 0 PSI manifold (0.8 bar ~= 12.5 PSI). It never knocked under boost, only climbing hills off-boost under 3300 RPM (which, as it happens, it also did with bone-stock, by-the-book, factory timing, with either a US or RoW dizzy). However, this extra advance did translate to vastly improved response to throttle in the mid-range, which greatly enhanced the driving fun. If you check the dynamic compression chart:  9.5:1 @ 0.5 bar looks to be roughly the same dynamic compression as 8:1 @ 0.8 bar, so that doesn't seem wildly off, IMHO.
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'77 S with '78 930 power and a few other things. |
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07-24-2022, 02:51 PM
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