02-17-2010, 02:01 PM #1
Detto Deaths of Big Boost 4-Techs
Given the stock RXP compresion ratio (cr) is at 8.4 to 1 and stock boost numbers are approx 8 lbs, this then gives a calculated ratio at that boost level of 13.1. This is an acceptable border for pump gas but when you look at a 16lb wheel you are now in the realms of 17.8 to 1. This is race gas only and good stuff at that.
Add a turbo to the mix and blow in 20lbs and your looking at 20.1 to 1, add an extra head gasket or some 8.0 to 1 pistons and you are still at 18.9 with the ecu doing double time pulling timing as a result of detto. I have two customer engines in my shop that have been running bigger boost numbers than stock and the results on the pistons and rings is notable. One has a piece off the piston edge after 80 hours running in the 85 mph range and the other less damage a smaller charger but still dead.
It has been noted over time that an engine with the latest and greatest sc wheel in it has a service life all going well of under 100 hours at optimum ( there will be some exceptions) with the engine leak down test being the first proof of a freshen up being required after the customer complaining of the engine breathing hard and or blowing oil. These signs are often blamed on other problems as we see in other threads. GIO and other buzz words are all still combustion chamber contents passing hammered out rings.
I wonder what comments some of the better tuners have on these numbers and look forward to the replys.
02-17-2010, 02:14 PM #2
Not necessarily disagreeing at all ... I do think big boost significantly impacts engine life on our skis, plus I think we sometimes get a bad batch of gas, etc. But I pasted the following from a previous post on this topic, just as one input on the subject ...
On our skis we run centrifugal SCs which run much cooler than roots SCs, especially at higher boost levels, plus we are running BIG water to air intercoolers, which has a huge impact on what kind of boost you can run. Check the info below as listed on Procharger website ... shows it is possible to run 15 - 18 lbs boost with a centrifugal SC and very good intercooling, on pump gas with our 8.5:1 engines.
Reliable High Performance
Clearly, the only type of performance that matters is reliable performance, and detonation is the single biggest threat to engine reliability. The boost range for reliable performance, without detonation, can be determined by looking at the type of supercharger technology being considered, and the compression ratio of the motor. With a lower compression ratio, an engine can safely handle more boost, everything else being equal. Similarly, if the temperature of the compressed air is lower, an engine will have a much higher detonation threshold (the point at which fuel ignites without a spark), and will be able to safely handle more boost. The chart below helps to illustrate how the overall efficiency of the entire supercharger system can be increased by both leading edge supercharger efficiency and the use of intercooling. Simply locate an engine's compression ratio at the bottom of the chart and trace upwards to determine the maximum reliable boost level. The amount of heat produced (adiabatic effiency) by each supercharging technology is what determines the boost limitation. While gear-driven centrifugal is clearly the superior supercharger technology, it is also clear that the biggest benefit comes from intercooling. These calculations assume moderate timing, 92 octane pump gas, and a good supply of fuel to the cylinders. As mentioned previously, detonation is the single biggest threat to engine reliability. It is heat and detonation that cause blown head gaskets and burned pistons, not boost. Achieving maximum performance from a given engine while avoiding detonation requires the right combination of intake air temperature, timing and fuel quality. For example, without intercooling a stock 5.0 with 9.5:1 compression ratio can only hold 5-6 psi of boost before detonation becomes a problem. The only way to safely run more than 6 psi of boost and still make a meaningful increase in power without an intercooler is by using racing fuel to avoid detonation.
02-17-2010, 02:18 PM #3
I dont doubt the detto but its is down the tune and people running crapy fuel, I know that you guys in the states struggle to get the better fuel so maybe this makes a diff. Also the type of charger you run, the centrifugal is one of the better ones for boost tempreture so add this to the good IC situation and you can run alot higher boost than what would be the norm on pump gas. R88
02-17-2010, 02:22 PM #4
Damn u Vern, got quicker fingers than me.
02-17-2010, 02:23 PM #5
Yup, I am certain detto is what was the reason for my most recent failure. I was running about 15Lbs of boost at 8550 RPM on a 50 degree day. Overall it was probably a combo of things (low oil, overheat, lean) but in the end I think the failure was mostley due to detto. I know of 2 others that same day that had the same problem as me with similar setups. Hopefully I wont have to run full race gas but was looking at running 25% race fuel and 75% 93 octane.
02-17-2010, 05:13 PM #6
So with a stock IC which line am I looking at on the graph?
02-17-2010, 05:24 PM #7
02-17-2010, 05:36 PM #8
I'm not worried at the moment, but I plan to go a 2rude setup in the future and am trying to do research for it
I've had a think and I may just get the Fizzle IC, so I've answered my own question there!
02-17-2010, 05:42 PM #9
- Join Date
- Nov 2006
- Delray Beach, Florida
The intercooler is a very VERY large part of the equasion. But the managment system is the missing link in most applications. People seem to think that becasue their LM1 AFR reading is 11.0 to 1 or lower.. that they are safe.. that is one small part of the puzzle.
Lets say you are running 15 lbs of boost and your AFR is 10.5 to 1 with a stock ECU with big injectors and a a RRPFR are you safe? I doubt it, not from what I have seen, becasue your EGT is probably 1650+++ at WOT, and if you are even coming close to the revlimiter, you are pulling timing and then what happens... EGT 1700+++ very fast.. How do you fix it? Well you can't with a stock or even a reprogrammed ecu, unless you can reprogram it right now.. If you could, how would you go about getting that EGT down?
Remember this.. the revlimiter is your enemy. You don't want to run right up to it like everyone does, but when tuning with a stock ECU you feel you "have to" How many readers here have burned an exhaust bell? Remember.. that heat came from your combustion chamber. A stainless bell, or cutting it off may solve the bell problem, but the heat is your enemy..
Dont add fuel... Dont add another gallon of race gas.. both wrong answers..
You could advance your cam timing, but you will loose power, and the range is slight, but.... this will get the combustion event done sooner, and subsequently drop egt some.
The real way to fix it is to add ignition advance. As you start throwing timing at it, the egt will plumet, and the AFR will rise. Now add fuel to settle your AFR back down, maybe add a little more timing... Balance... Keep the EGT under 1599F at WOT on a long run. Best part is you will be making more power. And get the revlimiter out of the way. Put it at least 1000 rpm away from the rpm # you are looking for. There is another reason for this, but it is a CC reason..
Most of these guys are burning down motors because they are not tuned right. They think they are, they did all they could do, but they are way off.
I have said it before, and I will say it again, our motors run high boost. 20psi is the practice day setting that my wife rides it around on. And a few of you guys from this board have been out and seen it run now. There is no idle or no wake time, every minute that the engine is running is full throttle 9000rpm around bouys. The race map is 25psi, with a 30psi overboost button on your left thumb. We run huge timing numbers 40+ degrees final. I run 11.5:1 on race days, and 10.9:1 on practice days. EGT never a degree over 1599F. Fuel is 50:50 mix 93 pump and VP C12 or Sunoco Supreme whichever I get that day.. At Worlds we ran MS109 and it was a mistake, I should have run MS103 as it burns quicker. our boat is in the water every single Saturday and burns 10 to 15 gallons. Static compression about 8.6:1 with no block guard, stock head bolts, stock head gasket with 1 layer removed.
The single best investment you can make is a stand alone managment system. You can spent the 3K now or spend it fixing your motor later. If you rec ride the boat, and only pull the trigger a few times a day, you may get it to last a while.. throw a dart???
The second investment you can make is in an intercooler that will keep your charge temp curve "board flat" at a few degrees above the temperature of the water you ride in. If you get a Managment system, you can log this temp and see the curve graphically compared to RPM, and Speed, etc...
These motors are tough.. it surprises me every time I pull the throttle, but you have to treat it right..
02-17-2010, 05:43 PM #10
Dont worry, Louis put the good info in above me lol. R88
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