Jetaholic
12-16-2007, 04:51 PM
So I've been doing a lot of thinking/brainstorming....whatever you want to call it. Lots of questions have been posted and asked by me to jet pump greats such as Duane (HTP) and Tom (JBP) as well as Jim at Performance Jet in search of the ultimate tuning info. Figured I'd post what I've learned so far in hopes of putting other jetters into a different mindset on how they tune their engines.
One of the things I've seen firsthand with jet boaters are guys who tune the engine the same way you would tune an engine in a car. However, one must know that an automobile drivetrain and a jet pump are two completely different kinds of loads. A prop boat behaves very similar to a car...but not a jet.
Let's take for example a car. Let's say you're in a car at a dead stop. You stab it wide open throttle. The RPMs will only swing up to the stall speed of the torque converter until the car gets moving. The drivetrain unloads the engine as the car picks up forward speed, allowing the engine to spin faster. Once the engine reaches peak RPM, the transmission shifts, which loads the engine more, thereby dropping WOT RPM. The car goes faster...unloading the engine once again...allowing it to pick the RPMs back up.
Prop driven boats...same thing. A prop acts a lot like a torque converter. The prop size/blade pitch is going to govern the prop's "stall" speed. With the boat at a dead stop, you stab it WOT and the prop spins up to its stall RPM. As the boat gets moving and gains speed, the prop unloads, allowing the engine to pick up RPM.
Jet Pumps...totally different animal. Jet pumps do not unload as the boat gains forward speed like a prop does. From a dead stop, you stab the engine WOT, the pump spins up to the maximum RPM it will allow the motor to swing to. The size impeller and the available horsepower from the engine will determine what RPM this will be. Regardless of boat speed, that RPM does not change. The "stall speed' of a jet pump is the highest that pump is going to spin regardless of hull design/weight or forward speed.
A jet pump has no idea what it is mounted to or any idea that it is moving. As far as it's concerned, it's just pumping water..that's all it does. Hull design, boat weight, and forward speed are all irrelevant to max available RPM. Hull design/weight will however, determine the max available speed. When you're dealing with a jet boat, it's very similar in operation to a jet aircraft. A jet engine generates thrust just like a jet drive does. The design, weight, and drag coefficient of the aircraft and the thrust/weight ratio of the jet engines will determine max forward speed (thrust overcomes drag). When a jet is at a dead stop on the runway (not on a treadmill), the pilot goes to max RPM...that RPM does not change as the jet picks up forward speed down the runway. However, a jet engine uses air thrust whereas a jet drive uses water thrust.
Think of the jet drives as a "high stall torque converter". I'll use my truck as an example. My truck's torque converter stalls at 2,000 RPMs. This means that if I lock the brakes up on my truck, then stab the engine WOT, the RPM will only go up to 2,000 RPMs...it will not go any higher than that until the vehicle starts moving.
Assuming your max RPM is 5,500 RPMs, a jet drive is like having a 5,500 RPM stall converter. You stab it WOT, and the jet pump will only spin up to 5,500 RPMs...it will not go any higher than that. If it's mounted to a stationary object, it will just pump water. If it's mounted to an object that is free to move (such as a boat), the thrust generated by the pump will move the object. But the max RPM will stay the same regardless of what it's attached to.
Knowing this bit of information should make it easy to tune the engine to work this type of a load to the best of its ability.
Most carburetors are set up out of the box for a street car. If it's a marine carb, it's set up the same way since props act very similar to a street car. Very seldom will you find anything preset up for a jet boat.
Take for example the accelerator pump. The accelerator pump's purpose is to cover up the fact that from idle to part or wide open throttle, there is a sudden drop in vaccuum due to a massive inrush of air into the engine. This inrush of air must be met with the proper amount of fuel. However, due to the sudden drop in vaccuum, at that slow of an engine speed the engine cannot pull hard enough on the main jets to get them to supply fuel quick enough to match the sudden gulp of air the engine just swallowed. From the time the air enters the engine to the time it takes for the main jets to kick in and supply fuel, there is a "hole" in the fuel mixture curve, which will cause the engine to momentarily lean out, causing a bog or a hesitation.
When the engine is loaded to the point where it can only spin at a low RPM at WOT, it cannot pull on the main jets hard enough to get them to supply fuel quick enough to prevent that momentary 'lean-out'. Thus the engine requires a bigger pump shot. The greated the load on the engine, the more pump shot it will need to get going from a dead stop.
However, if the load on the engine allowed it to spin higher at WOT, say...5500RPMs...the engine will pull harder on the main jets at that high of a WOT RPM, allowing them to flow fuel much quicker, eliminating the need for a big pump shot. Most jet pumps max out at around the range of 5000-5500 RPMs with the engine at WOT, so it would make sense to dial down the pump shot on a jet boat engine since the jet pump doesn't impose much of a load on the engine until it gets to WOT.
One of the things I've seen firsthand with jet boaters are guys who tune the engine the same way you would tune an engine in a car. However, one must know that an automobile drivetrain and a jet pump are two completely different kinds of loads. A prop boat behaves very similar to a car...but not a jet.
Let's take for example a car. Let's say you're in a car at a dead stop. You stab it wide open throttle. The RPMs will only swing up to the stall speed of the torque converter until the car gets moving. The drivetrain unloads the engine as the car picks up forward speed, allowing the engine to spin faster. Once the engine reaches peak RPM, the transmission shifts, which loads the engine more, thereby dropping WOT RPM. The car goes faster...unloading the engine once again...allowing it to pick the RPMs back up.
Prop driven boats...same thing. A prop acts a lot like a torque converter. The prop size/blade pitch is going to govern the prop's "stall" speed. With the boat at a dead stop, you stab it WOT and the prop spins up to its stall RPM. As the boat gets moving and gains speed, the prop unloads, allowing the engine to pick up RPM.
Jet Pumps...totally different animal. Jet pumps do not unload as the boat gains forward speed like a prop does. From a dead stop, you stab the engine WOT, the pump spins up to the maximum RPM it will allow the motor to swing to. The size impeller and the available horsepower from the engine will determine what RPM this will be. Regardless of boat speed, that RPM does not change. The "stall speed' of a jet pump is the highest that pump is going to spin regardless of hull design/weight or forward speed.
A jet pump has no idea what it is mounted to or any idea that it is moving. As far as it's concerned, it's just pumping water..that's all it does. Hull design, boat weight, and forward speed are all irrelevant to max available RPM. Hull design/weight will however, determine the max available speed. When you're dealing with a jet boat, it's very similar in operation to a jet aircraft. A jet engine generates thrust just like a jet drive does. The design, weight, and drag coefficient of the aircraft and the thrust/weight ratio of the jet engines will determine max forward speed (thrust overcomes drag). When a jet is at a dead stop on the runway (not on a treadmill), the pilot goes to max RPM...that RPM does not change as the jet picks up forward speed down the runway. However, a jet engine uses air thrust whereas a jet drive uses water thrust.
Think of the jet drives as a "high stall torque converter". I'll use my truck as an example. My truck's torque converter stalls at 2,000 RPMs. This means that if I lock the brakes up on my truck, then stab the engine WOT, the RPM will only go up to 2,000 RPMs...it will not go any higher than that until the vehicle starts moving.
Assuming your max RPM is 5,500 RPMs, a jet drive is like having a 5,500 RPM stall converter. You stab it WOT, and the jet pump will only spin up to 5,500 RPMs...it will not go any higher than that. If it's mounted to a stationary object, it will just pump water. If it's mounted to an object that is free to move (such as a boat), the thrust generated by the pump will move the object. But the max RPM will stay the same regardless of what it's attached to.
Knowing this bit of information should make it easy to tune the engine to work this type of a load to the best of its ability.
Most carburetors are set up out of the box for a street car. If it's a marine carb, it's set up the same way since props act very similar to a street car. Very seldom will you find anything preset up for a jet boat.
Take for example the accelerator pump. The accelerator pump's purpose is to cover up the fact that from idle to part or wide open throttle, there is a sudden drop in vaccuum due to a massive inrush of air into the engine. This inrush of air must be met with the proper amount of fuel. However, due to the sudden drop in vaccuum, at that slow of an engine speed the engine cannot pull hard enough on the main jets to get them to supply fuel quick enough to match the sudden gulp of air the engine just swallowed. From the time the air enters the engine to the time it takes for the main jets to kick in and supply fuel, there is a "hole" in the fuel mixture curve, which will cause the engine to momentarily lean out, causing a bog or a hesitation.
When the engine is loaded to the point where it can only spin at a low RPM at WOT, it cannot pull on the main jets hard enough to get them to supply fuel quick enough to prevent that momentary 'lean-out'. Thus the engine requires a bigger pump shot. The greated the load on the engine, the more pump shot it will need to get going from a dead stop.
However, if the load on the engine allowed it to spin higher at WOT, say...5500RPMs...the engine will pull harder on the main jets at that high of a WOT RPM, allowing them to flow fuel much quicker, eliminating the need for a big pump shot. Most jet pumps max out at around the range of 5000-5500 RPMs with the engine at WOT, so it would make sense to dial down the pump shot on a jet boat engine since the jet pump doesn't impose much of a load on the engine until it gets to WOT.