little 4.3 that could...25p Prop or get a 1.47 ratio drive?

[kpforce1]

Update. I came across a an aggressive stainless Solas 14x25p 4 blade prop new for $100. I put it on a d took the boat out loaded to around 3700lb on fairly rough water today. Topped out at 54mph at close to 4700 RPM. With just me, no extra weight and less than a half tank i am confident it would turn 4900+RPM. I am impressed with this little 4.3 lol.

 

[achris]

Update. I came across a an aggressive stainless Solas 14x25p 4 blade prop new for $100. I put it on a d took the boat out loaded to around 3700lb on fairly rough water today. Topped out at 54mph at close to 4700 RPM. With just me, no extra weight and less than a half tank i am confident it would turn 4900+RPM. I am impressed with this little 4.3 lol.

Even stock, my 4.3MPI pushes my 20 cabin at 42 knots. They are one of the most underrated engines around. For their size they deliver a great punch.

Here's the prop numbers for your figures. A 13% slip is right (my slip calculation takes into account the cupping, of adding 1" to the 'standard' size. Which is why it may sometimes not line up with some of the online calculators. And I don't know of any props that aren't cupped these days.) If you are getting 320hp, I'd be very concerned about reversion.

 

[kpforce1]

Even stock, my 4.3MPI pushes my 20 cabin at 42 knots. They are one of the most underrated engines around. For their size they deliver a great punch.

Here's the prop numbers for your figures. A 13% slip is right (my slip calculation takes into account the cupping, of adding 1" to the 'standard' size. Which is why it may sometimes not line up with some of the online calculators. And I don't know of any props that aren't cupped these days.) If you are getting 320hp, I'd be very concerned about reversion.

I added the stainless valves,and additional 3" riser spacers as a precaution for reversion.. i took everything apart last season to inspect for reversion and there was not a single sign that water was making it down past the riser. The cam i have is aggressive but not anymore than the comp xm270. I am installing a wideband O2 for tuning soon as i am leaving some power on the table. i cannot seem to get the carb dialed in 100%.

 

[achris]

Adding the risers to the exhaust does absolutely nothing to combat reversion. Reversion occurs right at the end of the elbow, so you could put 2 metre risers on, and if the engine's going to pull water in, it'll still do it.

Good luck with the carb. Don't go too lean (err of the side of rich). Marine engines have a tendency to eat pistons when run lean, more so than a car engine. Where do you plan on setting the O[SUP]2[/SUP] sensors?

Chris......

 

[kpforce1]

Adding the risers to the exhaust does absolutely nothing to combat reversion. Reversion occurs right at the end of the elbow, so you could put 2 metre risers on, and if the engine's going to pull water in, it'll still do it.

Good luck with the carb. Don't go too lean (err of the side of rich). Marine engines have a tendency to eat pistons when run lean, more so than a car engine. Where do you plan on setting the O[SUP]2[/SUP] sensors?

Chris......

I guess I am not following. How would increasing the distance from the exhaust ports on the head to where the water enters the exhaust stream not provide a small buffer against reversion vs a shorter riser? It also raises the elbow's higher off of the water line.

O2 is going in the port side elbow as close to the spacer as I can. This is where most people are putting them with success. I will only be using it for tuning. I will keep the AF ratio on the fat side. My pistons are a little more forgiving that stock but I will still proceed with caution.

 

[Lou C]

Reversion is not the same as water running up the exhaust from a quick stop. It is caused by a vacuum developing in the exhaust system that can actually pull the water just exiting the elbow back in the exhaust outlet of the exhaust elbow. So because of that it does not matter if you add taller risers. In fact VP added check valves to the elbows to prevent vacuum from developing in the exhaust system when they went to the Vortec 4.3. They allow extra air to enter the exhaust to “break” the development of vacuum in the exhaust.

 

[Scott Danforth]

you may want to look in your manifolds. at that cam spec, you are experiencing reversion at idle unless you have a thru-hull exhaust and mix outside the transom. your duration is greater than 270 which is the limit with stock wet exhaust, even with a 3" riser.

 

[alldodge]

I guess I am not following. How would increasing the distance from the exhaust ports on the head to where the water enters the exhaust stream not provide a small buffer against reversion vs a shorter riser? It also raises the elbow's higher off of the water line.

There is just a short distance in the exhaust elbow when water and the exhaust start to mix

If a taller riser is installed, all that means is water needs to be pumped higher before it reaches the elbow. The distance in the elbow has not changed, only the height. So if there is to much vacuum being produced, the water still get drawn back in it just has farther to fall once in

 

[kpforce1]

Reversion is not the same as water running up the exhaust from a quick stop. It is caused by a vacuum developing in the exhaust system that can actually pull the water just exiting the elbow back in the exhaust outlet of the exhaust elbow. So because of that it does not matter if you add taller risers. In fact VP added check valves to the elbows to prevent vacuum from developing in the exhaust system when they went to the Vortec 4.3. They allow extra air to enter the exhaust to “break” the development of vacuum in the exhaust.

I am aware of the difference between the two, I just pointed it out in case someone thought I hadn't considered it. I have checked the manifolds, elbows and heads for signs of reversion and there were NO signs of reversion. The longer the physical length/distance from the water injection point to the ports on the actual head could help prevent reversion of water back into the exhaust port due to water having to travel further down. The longer the distance, the more likely the next exhaust pulse will interrupt reversion coming back into the manifold and "spit" it out.

you may want to look in your manifolds. at that cam spec, you are experiencing reversion at idle unless you have a thru-hull exhaust and mix outside the transom. your duration is greater than 270 which is the limit with stock wet exhaust, even with a 3" riser.

ADV duration is not really the greatest thing to go by when looking at a cam considering manufactures measure it differently... especially when deciding what duration is deemed a max to prevent reversion. Duration at .050", the actual cam lobe profile (ramp rate etc) coupled with IN/EX lobe overlap duration etc. is what matters. My cam specs at .050" are about as much as one would want to go.

Seems like I recall some people that have built a 4.3. Names I am recalling are Tail_gunner, Dsmmace, or Bondo?

 

[Scott Danforth]

what is the cam overlap?

 

[kpforce1]

what is the cam overlap?

The overlap for my cam should be around 60*.

 

[Scott Danforth]

most marine cams are around 0 to maybe 4 degrees total overlap.

for example, here is my cam in the BBC

and it has 2 degrees total overlap and i still had reversion at idle and i wasnt supposed to. I also have some really good high-speed footage of the exhaust pulses pulling water back into the elbows at idle.

 

[kpforce1]

most marine cams are around 0 to maybe 4 degrees total overlap.

for example, here is my cam in the BBC

and it has 2 degrees total overlap and i still had reversion at idle and i wasnt supposed to. I also have some really good high-speed footage of the exhaust pulses pulling water back into the elbows at idle.

All i know is that when I pulled the Thumper manifolds and risers off, there were no signs that water was making it past the "ramp" that prevents water coming down the elbow. The exhaust ports, valves, plugs and chambers showed no signs of water reversion. This was after i did low idle testing for 10 min at 575 RPM (lower than I usually run at idle).

 

[kpforce1]

Last update on the prop trials. I found a killer deal on a stainless 4 blade 25p Solas prop and pulled the trigger as it was on the "big" end of the prop spectrum. It does 54mph (GPS) with 4 people, gear and 3/4 tank turning ~4750 RPM and trimmed out. To my surprise, hole shot did not suffer that much albeit the 21p 4 blade aluminum prop I have is much more of a neck breaker from an idle take off lol.

With the way the motor is built, a prop putting me in the 5100-5300 RPM range would be the "max effort" choice for all out speed. I think a really good 23p 3 blade stainless would be the ticket but It is hard to argue with the cruising performance i get with the 25p Solas though haha. I did get a 21p cleaver style 3 blade stainless prop for giggles but haven't tried it yet.