stickmantijuana

iTrader: (1)

I was curious to know the approximate ft-lb gained per degree of timing advance near MBT for single turbo FD rx7's. My engine is 20b with gt4202r, so I will assume mine is approximately 50% more. I tried 20b forum, but not many solid data points available there. This FD subforum gets many more views, so I figured can't hurt to try asking here...

knowing some concrete single turbo FD MBT data points would help me gain some confidence while approaching my own MBT when tuning on dyno. it would give me some sensible limits as well... that's why I ask. thanks in advance!

arghx

iTrader: (3)

If it's "near MBT" , not a whole lot. 1-2 lb ft.

stickmantijuana

iTrader: (1)

thanks for the reply. wow, that seems really low. are you running NA? I had rx8 renesis with tiny greddy turbo and its MBT was around 5 ft-lb. arghx, this is nothing you don't know, just to clarify for the forum members, MBT isn't the point you're making the maximum torque. It's the point at which your last torque gain per degree of timing advance was more than your torque gain with another timing advance. for example,

timing advance | peak torque
+10 | 200 ft-lb
+11 | 205 ft-lb (change = +5 ft-lb)
+12 | 213 ft-lb (change = +8 ft-lb)
+13 | 222 ft-lb (change = +9 ft-lb)
+14 | 232 ft-lb (change = +10 ft-lb)
+15 | 240 ft-lb (change = +8 ft-lb)
+16 | 245 ft-lb (change = +5 ft-lb)
+17 | 249 ft-lb (change = +4 ft-lb)
+18 | 251 ft-lb (change = +2 ft-lb)
+19 | 251 ft-lb (change = +0 ft-lb)
+20 | 250 ft-lb (change = -1 ft-lb)

timing advance at MBT here is +14 degrees, not +19 degrees. this means you should set your timing at +14 as long as there is no knock.

arghx

iTrader: (3)

Renesis engine has a different combustion chamber and different exhaust ports. I'm not saying that adding a degree of spark on the older rotaries always means 1 or 2 lb ft. But just as you put in your chart, you need to do spark sweep to study the sensitivity.

The further off you are, the more you will pick up from adding timing. But When you get in there on a dyno and you know there's a bunch of money riding on the decisions you make in the moment, it's often the most prudent to try adding a degree or two at most. Then ask yourself hmmm... Was that worth it, considering there's no knock sensor retard?

Spark is a small **** to turn to increase output especially considering the risk. AFR is a little better, but you need to leave a margin to account for changes in weather and driving conditions, even if your compensation tables are well tuned.

Boost, porting, and turbo sizing are where the biggest impacts come from.

j9fd3s

iTrader: (3)

i would be really careful about trying to apply theory from another engine to your engine.

we do a lot of suspension testing, and we have this nice spreadsheet that, when the cars values are input, it tells you how its going to behave. except it doesn't.

we've found we need to try a few setups, and calibrate the spreadsheet, once you do that though, the spreadsheet does become predictive, for that chassis.

so the theory will probably apply, but how much and what, it can't tell you.

we did use a mustang dyno, and it lets you hold an rpm and change the load, so we did do MBT timing up to whatever rpm it was (3500-4k). we actually did have results similar to what Arghx suggests, its an NA miata on E85 and changing timing makes almost no difference in power, to change power we need to change airflow.

grab a lane

strap it to a dyno and start conservatively increasing timing. once you start losing power or the gains start diminishing you will have found your peak. at that point back it off a few degrees and call it a day. there is definitely a point of no return.

my recommendation is to do this a couple times in different conditions to make sure your results are consistent and sustainable.