Mondial8/QV WSM 281/83 as Eric mentioned: Fig 5, page D4 = euro i-2V (MED 802A) Fig 6 page D5 = US i-2V (MED 801A)
In reading the Advance curves published in the Mondial WS manual for USA 2 Valve cars, one sees that the max advance is 40 degrees at 280-490 mmHg and 20 degress advance above 490 mmHg. Several questions come to mind. Is the mmHg limits given a function of the mechanical attributes of the dissy, or has Ferrari found that anything above 490 mmHg to be not relevant to the car's performance? Also, if not limited by the mechanics of the dissy, did Ferrari develope this curve for performance, fuel economy, or just US DOT? All this leads to the following: Has anyone created and verified the best performance based Advance curve matrix for the 2 valve 308 taking in consideration RPMs and kPA (mmHg)? One more thing: has anyone actually measured and recorded the manifold vacuum through the entire RPM range? It would be interesting to see just how much vacuum is created.
The intake manifold vacuum can't really reach 490 mm Hg unless it's decelerating with the throttle plate(s) closed (i.e., no engine load).
I've done that on the dyno, would have to dig up the records. altitude, temperature, engine, fuel etc.. many factors at play. I've done some very aggressive maps and the engines respond well to the advance at the upper end, some have +40. the other thing to note is that the WSM mmHg is the delta from atmosphere. so the 490 is gauge 260mmHg or 34.6 kPa. WOT would be the other delta end of -75 near atmosphere. the best I've seen at idle is 44 kPa on a new rebuild. spikes lower than that depending on throttle response and deceleration.
If 1 atm = 101.3kPa = 760mmHg, then a vacuum of 490 mmHg would equal 270mmHg absolute or 35.99kPa ((760-490) x 0.1333). (Please verify my math). However, the lightbulb moment was you statement "WSM mmHg is the delta from atmosphere". It should have been obvious to me. And, just to be sure I'm on the same page, kPa denotes APSOLUTE pressure in this situation - right? It's been 10 years since I plotted out the advance curve for my QV, and perhaps I knew all this then. But I will redo my chart just to verify.. Thanks. BTW, please post your charts for the 2 valve if you can. -Paul
I'll get the 2v ones up soon, real busy with pressing projects at the moment. here's the QV one I dug up recently, posted it in another thread but don't recall where. Image Unavailable, Please Login
Here's a link to a thread from 2005 when I first started playing with EFI and ecu's on my 308. http://www.ferrarichat.com/forum/showthread.php?t=75528&highlight=advance+curve The 2V curves are in an excel spreadsheet in post #8 but I had Ferrari's definition of manifold pressure/vacum backwards ... meaning what I thought was WOT was actually full vacum. I found that a stock motor with low compression will handle a lot of advance on street gas. Because I had the curves "reversed", I was running 42 deg at WOT and the stock motor didn't mind it one bit. My high compression motor didn't like it though. It was after installing that motor that I realized the curves were reversed. On that motor (over 12:1 compression) I run 32 WOT with street gas and 36 WOT with race gas or av gas mixed in. A lot of guys don't even run premium fuel in their 308's with the stock digiplex. Running premium fuel with more aggressive timing curves really wakes up the 2Vi cars I've found. BTW that is the euro curve I posted ... I don't think a person should worry about copying the US curve if they're looking for performance. I recall someone posting a noticeable gain when they swapped the U.S. digiplex for euro units, but for the cost of them it was easier for me to just use and aftermarket ECU that was capable of reading the stock triggers and firing the stock coils. It worked fantastic for a couple of years but finally I wanted to throw away all the plug extenders, caps, and wires, so I installed the coil on plug units which I'm still running to this day. cheers
this thread is getting interesting now folks. Scott, I think you mentioned when we spoke that Nick's motor is running some 40 degrees of timing with it's massive compression isn't it? Sean, I believe the timing myself and Nick are able to run with our compression is down to piston design. Your high comp pistons have a massive dome if I remember right which squeeze right into the combustion chamber creating better 'quench'. Myself and certainly Nick are running pistons which are pretty much flat as a pancake hence they have poor quench in that massive domed combustion chamber thus poor combustion efficiency thus the ability to dial in more timing. I can run up to 40 degrees (I won't try over that!) by 5k rpm, can't wait to get it on the dyno. I dyno'd my motor last with 38 degrees total timing and fixed the a/f ratio and made 272bhp with 225lbs torque at the wheels. I'm sure I have a bit more poke in it now with 40 degrees although now I think I need to up the main just size a taste, it's amazing where all this power is coming from.
Yeah John but keep this in mind. I'm running way more static compression than you or Nick and I'm running the stock CIS cams right now to boot. I think I am actually a tad over 12.5:1 static. I could pull my dome volume numbers up if you're bored today . When I first built this motor I had some .406" 242 duration (@.05" lift) custom cams but they only lasted 180 hours and went flat. That was back when ZDDP was first being blamed for everything so of course the cam guys blamed my oil . When I had those cams I was at 36 full advance on street gas FWIW. My 'busa throttle body car is at 36 full advance right now and it just is not pulling up high like I thought. I think I will try more timing in that car now I've got some miles on 'er. Took a fellow chatter for a ride in 'er the other day . That car has the euro carb cams (.350" lift) and the compression is only like 10.5:1. In fact, it might be less 'cause I had to re-cut all the valve pockets in the pistons 'cause they were wrong, but I got a free set of pistons out of that deal . I think I have an extra I cut ... I would like to know the actual dome volume. Come to think of it maybe I did measure it and I just need to dig it out ... that was a few years ago now also. cheers edit: John ... I don't know if I have it backwards or what, but I thought the domes were "bad" for combuster chamber efficiency ... ??? I really need a dyno .
there's lots of technical design around the quench, too little and you get detonation and bad swirl issues, too big and detonation risk increases and chamber evacuation drops as there is less pressure. it's a narrow window too, .060"~.035". the 2v motor is a true hemi head design and the dome can help with evacuation, shrouding and pocket design are also key too. the 4v head is a pent roof with large quench pads which really help with efficiency of the mix and flame front. as a result the static compression can get really high on larger bores and flat pistons, increase the stroke it goes higher. 79mm stroke and 89mm bore on the 4v with flat pistons and a .065" gasket and a .015" out of deck nets a 14.8:1 !! timing is also affected by piston velocity and dwell, if one can do chain calcs in trig on scalene angles and a bit of differential calculus you can spreed sheet the build and pull a wealth of info from pressures and acceleration etc.. the good dyno sym programs do this internally for you, but they're not exactly cheap. the advanced timing on Nicks motor was slowly tickled out on the dyno in controlled runs, however Nicks motor is so highly modified from stock that's it's an apples and oranges comparison. from a mechanical perspective the theoretical best ignition timing would be 0* TDC, however due to pressure dynamics, temperature and torque angle that's not going to happen. knowing the cylinder pressure at 'X' and given Cr then with some thermodynamic calcs you can figure the air volume, density and temperature. then you'll be able to figure the fuel needed, and where this occurs at crank angle 'X' then targets the timing advance or retard for a given angular frequency. acceleration rate of change is a bit more complicated. another reason why the rod/stroke ratio can be critical, but is also highly specific to that engine, a one size fits all isn't good practice as cylinder head design and cam events effect the CFD (computational flow dynamics) of a given layout.
I think it more has to do with bad domes are bad for combustion efficiency. You running little timing on pump gas even with your hotter cams tells me your dome was just right as your efficiency did not require so much timing for an efficient burn. More space left open in a combustion chamber makes for poor efficiency hence why stock engines with their tiny dome and sunken deck clearance can run such high timing, their dynamic obviously helps a ton too. I'm running a bit over .010 over deck and have a basically perfectly flat piston (dome is maybe 1-1.5cc) and I can run gobs of timing for whatever reason (which I believe is down to all that empty 'space' in the chamber), and that's at around 10.7:1 static and 9.2:1 dynamic. CUP cars shoot for .020 quench IIRC, I shot for .040 and ended up just under that. CUP motors have that funky wedge dome which basically matches the shape and side of the combustion chamber just right and makes for a tight squish all the way around, and I remember on turbo blue fuel I ran 35-36 degrees total on 13:1 compression with somewhere around 9.5:1 dynamic compression and really tight LSAs. The flipside is with 911 motors which have to run twin plug setups due to not having a true hemi head, and their piston designs really look funky at times, but I believe with 10.5:1 and twin plugging and a decent S spec cam the most they can run safely is around 31-32 degrees. The twin plugs makes the burn more efficient thus not requiring the same amount of timing. I wish I knew more about thermodynamics and how to really dial this stuff in before I have to spend hours on a dyno.
