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Squish Test
- Thread starter Boyd12
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New build, top and bottom.
91 octane is a given (additional fuel cost per year is about $120).
"Raising compression on a 951 that isn't a fresh complete rebuild with quality parts is a recipe for disaster." Agreed.
The performance gains from previous engines have been significant! Not much difference in top speed, but it gets there a lot quicker! Just changing from the air box to Pro K was a big difference. There is increased water ingestion risk, but I am diligent about keeping water out of the hull as much as possible.
91 octane is a given (additional fuel cost per year is about $120).
"Raising compression on a 951 that isn't a fresh complete rebuild with quality parts is a recipe for disaster." Agreed.
The performance gains from previous engines have been significant! Not much difference in top speed, but it gets there a lot quicker! Just changing from the air box to Pro K was a big difference. There is increased water ingestion risk, but I am diligent about keeping water out of the hull as much as possible.
You really need to talk to someone that knows tuning a 951 if you are going to remove the stock airbox. The carbs are lean from the get go and the loss of vacuum signal from an aftermarket air cleaner makes it even worse.
From Group K...
"Flame Arrestors – While the installation of aftermarket flame arrestors has been a very popular modification for many recreational pwc’s, we do not use or recommend them on our Swift Kit or Y2K Sleeper Kit. The technical reasoning for this choice is related entirely to the stock carbs used on the 951 engines. The carbs used on all 951s are the first generation of a carburetor that is designed as much for emissions friendliness as it is for performance. Among the most notable design features that make this difference, is the significantly increased distance between the butterfly and the “booster venturi” style fuel atomizer (aka “bombsight” atomizer). We cannot claim to know all the effects of this design departure, however we know a lot about one particular effect called “fuel delivery signal”.
In short, signal is the amount of vacuum within the inlet tract that helps to draw fuel from the metering circuits in the carburetor. The air restriction of the stock flame arrestor serves greatly to keep that vacuum very high. This high vacuum actually helps the quickness of throttle response at all engine speeds. When a free breathing flame arrestor is installed, this vacuum (aka signal) is reduced. The reduced signal means a slightly leaner fuel mixture through the entire range. This is normally not a dangerous issue for most pwc engines. However the emissions-conscious stock 951 carbs have an exceptionally weak signal at ¾ throttle opening. This means that a more open arrestor will cause the 70 – 80% throttle range become “a lot” leaner than the rest of the range. There is no jetting that can solve this problem (we tried). The “only” effective solution is to leave the stock flame arrestor (with the bolt on air horns) intact. Despite the restrictive appearance of the stock arrestor, it can easily pass enough air volume for excellent performance up to the 7140 rev limiter. The stock arrestor has received an undeserved reputation as “choking” the 951 into a bad rich condition. In truth, the rich condition is a correctable jetting issue, not an air access issue."
From Group K...
"Flame Arrestors – While the installation of aftermarket flame arrestors has been a very popular modification for many recreational pwc’s, we do not use or recommend them on our Swift Kit or Y2K Sleeper Kit. The technical reasoning for this choice is related entirely to the stock carbs used on the 951 engines. The carbs used on all 951s are the first generation of a carburetor that is designed as much for emissions friendliness as it is for performance. Among the most notable design features that make this difference, is the significantly increased distance between the butterfly and the “booster venturi” style fuel atomizer (aka “bombsight” atomizer). We cannot claim to know all the effects of this design departure, however we know a lot about one particular effect called “fuel delivery signal”.
In short, signal is the amount of vacuum within the inlet tract that helps to draw fuel from the metering circuits in the carburetor. The air restriction of the stock flame arrestor serves greatly to keep that vacuum very high. This high vacuum actually helps the quickness of throttle response at all engine speeds. When a free breathing flame arrestor is installed, this vacuum (aka signal) is reduced. The reduced signal means a slightly leaner fuel mixture through the entire range. This is normally not a dangerous issue for most pwc engines. However the emissions-conscious stock 951 carbs have an exceptionally weak signal at ¾ throttle opening. This means that a more open arrestor will cause the 70 – 80% throttle range become “a lot” leaner than the rest of the range. There is no jetting that can solve this problem (we tried). The “only” effective solution is to leave the stock flame arrestor (with the bolt on air horns) intact. Despite the restrictive appearance of the stock arrestor, it can easily pass enough air volume for excellent performance up to the 7140 rev limiter. The stock arrestor has received an undeserved reputation as “choking” the 951 into a bad rich condition. In truth, the rich condition is a correctable jetting issue, not an air access issue."
Thank you for your input!
About twenty years ago or so when I got my first SD 951, I was reading about performance parts. One website (R&D?) said removing the air box causes a lean condition with a flat spot at 4-5,000 rpm. I read what Group K wrote and came to the conclusion that only one of them was right. So, I removed the air box and went to the lake. R&D? was correct. It had a flat spot and was very lean. It also ran a LOT better. Removing the airbox alone made a noticeable difference.
I put the R&D flame arrestor, intake manifold, reed stuffers in and re-jetted it. It ran VERY well, with no flat spots, lean conditions etc. Top speed was about 64.5 mph per the GPS. I had to be very careful about water in the hull.
Also, I am diligent about maintenance. I check plugs often, clean RAVE valves, inspect pistons/skirts at the same time and check the balance shaft oil once per season. I get many hours out of the engines.
About twenty years ago or so when I got my first SD 951, I was reading about performance parts. One website (R&D?) said removing the air box causes a lean condition with a flat spot at 4-5,000 rpm. I read what Group K wrote and came to the conclusion that only one of them was right. So, I removed the air box and went to the lake. R&D? was correct. It had a flat spot and was very lean. It also ran a LOT better. Removing the airbox alone made a noticeable difference.
I put the R&D flame arrestor, intake manifold, reed stuffers in and re-jetted it. It ran VERY well, with no flat spots, lean conditions etc. Top speed was about 64.5 mph per the GPS. I had to be very careful about water in the hull.
Also, I am diligent about maintenance. I check plugs often, clean RAVE valves, inspect pistons/skirts at the same time and check the balance shaft oil once per season. I get many hours out of the engines.
How the hell do you have all this knowledge
I dont think he ever sleeps! Reads/works constantly!
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