blue thunder

What dynamic compression ratio does your calculation suggest with 9.5 and 5psi? Probably around 16:1 which is I guess what my two engines have survived 2 years very hard running on 93 octane and with hyper pistons You might want to double check your math or produce your formula and I will debug it for you.

Rookie

iTrader: (4)

From the earlier posts.

RHM253

I would look at how much ring end gap was measured during assembly If this engine is running 13:1 f/a at 6psi it would need a ton of end gap to servive. If nothing else it needs to be fattend up to probably 11:1 f/a. Just my opinion.

PatriYacht

JE has a series of -20 cc dished pistons that they say will produce a 8.4-1 comp. ratio. You have to do the math on your engine to be sure. With 6.5 boost and no intercooler, that's the way to go. Your fuel mixture still needs to be richer or you will continue to melt pistons. Get it tuned by a guy who understands boats. Cars are set up leaner because they are rarely at full throttle or even half throttle and only for a few seconds when they are. Your boat seems like it was given a car tune.

ezstriper

I would look at KB blower specific (or someones) pistons, try to get into the 7.5 comp range, do a plug check correctly and make sure it's rick enough, run no more than 26 deg timing, and if good fuel a problem water/alcohol injection is going to be needed as well....that should get you in the right direction..Rob

stevesxm

well... you are correct and i was being euphamistic. true dynamic compression ratio will always be less than static in a non blower motor due to the valve timing... which is why you always run more static with more agressive cams...

and looking at simply the dynamic compression ratio by that rigid definition in a blower configuration, the cr does increase by a degree more in line with your figures than mine...

and i should have been clearer but was writing more for the end conclusion. my number stems from an algorthym derived from a three dimensional nomogram that looks at dynamic cylinder pressures vs valve timing vs engine rpm and then gives me an answer in numbers that i find easy to understand and compare. this was created from emprical data gathered on the dyno over time using scientific grade instruments.

thats where my 17 + number comes from. in my world, and by my way of thinking for my decision making process for all the tuning parameters on the turbo motors i did, i would look at this motor in this configuration and think " i have to set the running parameters as if this thing has a 17 to 1 STATIC compression ratio... and in that way i could go to my look up chart and come up with fuel and ignition maps that would give me a place to start. i found it helpful to not melt the motors down and then start looking for the answers. not that that didn't occassionally happen. and i'm sure that your combination is just fine as you say. but when this guy publishes those numbers and implies that there aren't enough pistons currently in stock in the united states to keep him running for the next season , then i would suggest ( as i did) that he has a gross problem as opposed to some minor tuning issue and to me that sounds like ( obviously) nowhere near enough fuel and ( also obviously) dynamic combustion chamber cooling.

so the answer is to either reduce the thermal load ( less static compression) or decrease the boost or get enough fuel in there at a high enough octane to not melt the pistons.

you can say his combination is the same as yours as often as you want. but the bottom line is that his melts and yours doesn't... so his answer isn't yours.

and thank you for the kind offer but the bugs have worked themself out over time.

jeffswav

No matter what you do with your engine. Before I would run the boat on the water I would have a wideband O2 gage installed.

niceguy

You know the whole thing about 9.5:1 comp with boost and running 93 octane.... Not trying to bash, but there are VERY few people and I mean VERY few people who can run this combo and not melt stuff to the ground everytime.

PatriYacht

I don't think you can run 9.5 comp. and 6.5 boost without an intercooler and better gas, say a 50/50 mix of 93 and 110 racing gas. I think we're all being polite and trying to help and not to bash. That's the way it should be.

Panther

The old set-up on my engines were 9:1 with 5 lbs of boost, no intercooler, and they lived but they were set up very fat, in the 11's. When we re-did them we brought the comp. ratio down to 8.3:1 and ran A/F ratio's in the low 12's.

I think 9:5 is high. It would be a good setup on low boost cause you could benefit in the low/mid range by having the extra compression and then the boost up high. The aluminum heads help a little also...

There's a reason Mercury goes with a very conservative 7.5:1 on their blower engines.