the modern car tuners have it made. Being able to tune and monitor things live, tons of safety parameters to keep things in check when knock shows itself. Its easier for them to keep the engine running at peak performance and efficiency without melting things.

a carbed old big block, not so much. That being said, i dont buy into the old rhetoric of make it fat, yank timing , and hope for the best.

in one of my research books, they illustrate a graph of flame speed vs lambda. Peak flame speed occurs around 13.2 afr, while max power is 12.5. Its easy to see how detonation can be induced when flame speed is at its fastest. Its also easy to understand, how being on the rich side of max power, with slightly retarded spark , would net the safest setup, with a minimal power loss %.

example, afr in the 11's in boost, with 32* of spark, is prob alot more detonation resistant, than afr at 13, and 32* of spark, and also make more power. Of course if you experience knock at 13afr and 32*, you can retard spark to make it stop. However, the power loss would be greater theoretically.

of course, if youre running a bbc with a single carb , the afrs from cyl to cyl vary by a mile, so you really gotta tune around the leanest/hottest cylinder. https://cimg4.ibsrv.net/gimg/www.off...d2aef94d1d.jpg

The numbers on the bottom are Lambda why do they have the extra zero should it be 0,85 would be 12.5 afr? Just wondering thanks.

not sure, but they do have the lambda conversion chart on the following page. Pretty good book, mostly about airflow dynamics

The numbers shown are FUEL AIR ratio.....not AIR FUEL ratio.....reciprocals of one another

Like lambda vs. equivalence ratio

What is the squish clearance on your motor? If you are running a tight squish (.035-.045") then you can run more boost and timing before detonation than you can if you are running an open squish (over .055"). This, and intake charge temperature are the 2 main things to focus on when trying to raise your boost level and power output. Since your motor is assembled, you can't change the squish clearance - but you can change timing, boost, fuel ratio, and cooling parameters (block cooling, exhaust cooling, and intake charge cooling).

mc

Not sure on that...I have .005 piston to deck clearance, .041 mls head gasket, stock stroke, dart iron eagle heads?? is that enough info?? does that mean the .005 plus the .041 head gasket so .046 squish??

when i mentioned controlling cylinder pressures earlier in this thread, i was mostly saying that in regards to timing.

There really is only one correct time to ignite the spark plug. Finding that number, is the hard part. You can have an engine that makes a very high peak cylinder pressure , but doesnt make the most power. The way to accomplish that is with incorrect timing. You want the spark to ignite at the proper time, not so that it makes peak cyl pressure, but so that combustion takes place at the right time, and puts the most leverage on the crankshaft. Too much timing is negative work, reduced power, and dangerous. Too little timing, is wasted efficiency, and in a boat can melt things.

was watching a webinar the other day from the hp academy. They had an import car on the dyno. They did steady state pulls, focusing on one area of the fuel and timing maps at a time. They ran an afr of 13.0, 12.5, and 12.0. There was very little power difference.

timing, they adjusted live while in steady state on the dyno. It was easy to see the torque rise as timing was increased , and once they crossed the line, the torque began to fall off again. I dont remember the exact number of degrees, but its irrelevant since every engine is different.

im sure the crew at mercury racing tunes in a fairly similar manner. I doubt they are doing 300-600 rpm per second sweep pulls on their modern efi engines in order to get a tune.

Gasoline seems to make its best power in the 12-13 range. Going a tad richer than 12, doesnt seem to kill power much, at least from what ive seen on supercharged stuff. I think id rather stay on the richer side of max power, on a boat engine. Timing wise, for me, its not the peak timing that worries me nearly as much as the low to midrange area.

heck, even the carb engines from merc with the thunderbolt 5 used to add timing while at a cruise state, sensing for rpm increase. What is was actually looking for was to see if the engine was making more power with the extra advance, if it did, it stayed there. If it didnt, it backed it down. Sort of a similar concept. Without pressure transducers and fancy monitoring eqiipment installed, finding the optimal point of ignition is impossible. Thats why most use the MBT method, which imo is still better than just guessing at numbers of what one might think is good on paper

squish? On a forced induction engine? Many forced induction pistons dont even have quench pads these days.

For the less informed could you elaborate on the "MBT method"??

Thanks!!

its really something that needs to be done on a dyno in a controlled enviroment.

Lets assume you are at 2500rpm. The piston is coming up the bore, and you light the spark at 18*btdc. The combustion event, may take place at say, 12* atdc , using all that energy from the explosion to force the piston down the bore, which then puts leverage on the crank. Thats what makes torque.

now, lets assume again you are at 2500rpm, but timing is at 34* btdc. Now combustion event may take place when the piston is at 0*, or even 2*btdc. This wants to force the piston backwards for a split second, resulting in torque loss, or even detonation.

and finally, at 2500, you may have timing at 4* btdc. Now Combustion event takes place at maybe 30* atdc, when the piston is further down the bore on its downstroke. Now you again have lost torque, because you wasted a bunch of energy that would have bee used to exert leverage on the crank. Also, with the retarded timing, the combustion mixture may not have fully burned , and then that gets pushed right out the exhaust valve. Raising the exhaust gas temperature.

Contrary to popular belief, you arent looking for max cylinder pressure. Your looking for max pressure over a longer time. You can make a very high cyl psi for a brief second with lots of timing, that wont do anything good for power, or safety. a dyno will tell you alot about how close you are to achieving your goal of getting the spark right. The problem is though, very fast sweep pulls most shops do, dont really give enough time for things to stabilize and gather data accurately when tuning for that. Also, things change with load as well as rpm. So what the engine wants at part throttle for timing, isnt the same as it would want at full throttle.

without spending some time on a dyno, my advice to start with, would be to follow mercurys timing tables for their carb supercharged engines for a baseline. Stay away from locked timing , or excessively retarded timing. Get a wideband installed, get your air fuel ratio in line, run a good ignition that will allow for a colder spark plug to be ran without fouling issues, keep the engine cool temp wise, and enjoy.

Sounds good that's the gameplan...thanks for the help!!