Tinkering on the 4.3l this winter...

[Pmt133]

If that is an engine out of a Car/Truck, it is not going to be much use in a boat, until some Changes are made to it.

Marine grind cam, ~9:1 compression on a 115 lsa and peaks around 5000 rpm... it was supposed to go in a boat and ended up in the vette. Does 300+ lb-ft from 1900 rpm onward. Only thing I don't remember about it is if I left the marine headgasket in it or not which I'd be checking/changing anyway. Core plugs and the peripheral parts are all set up for marine application and good to go.... long story.

I also came from a 3.0 powered boat its a nice upgrade.

I think most people get tired of the trough hulls pretty quick. Captains Call is probably the better bet.

Yeah. I'd probably have decent mufflers on it too. I'm not that guy. But the choice is nice.

Looking back, it impresses me how well the 3.0l did push this thing around.

 

[Scott06]

Marine grind cam, ~9:1 compression on a 115 lsa and peaks around 5000 rpm... it was supposed to go in a boat and ended up in the vette. Does 300+ lb-ft from 1900 rpm onward. Only thing I don't remember about it is if I left the marine headgasket in it or not which I'd be checking/changing anyway. Core plugs and the peripheral parts are all set up for marine application and good to go.... long story.


Yeah. I'd probably have decent mufflers on it too. I'm not that guy. But the choice is nice.

Looking back, it impresses me how well the 3.0l did push this thing around.

I enjoyed my 3.0 powered boat as well. With the right prop could do ok skiing and was cheap to operate and maintain. Now that the family is bigger the boat is as well.

What year vette? My other ride is a 65 GTO, just waiting for the builder to finish a new engine for it.

 

[Pmt133]

I enjoyed my 3.0 powered boat as well. With the right prop could do ok skiing and was cheap to operate and maintain. Now that the family is bigger the boat is as well.

What year vette? My other ride is a 65 GTO, just waiting for the builder to finish a new engine for it.

84. Nothing special. My uncle built a 67 firebird. We had to have the 400 redone a few years ago after he passed Sadly. Beautiful car those GTOs.

 

[Scott06]

84. Nothing special. My uncle built a 67 firebird. We had to have the 400 redone a few years ago after he passed Sadly. Beautiful car those GTOs.

its a 41 yr old car... hopefully in good shape.

 

[Pmt133]

its a 41 yr old car... hopefully in good shape.

It's a driver. Paint was resprayed in the early 90s. Interior is in good shape, just went through the suspension etc. It's just a go cart lol

 

[Pmt133]

So I figured I will bring this one back. I got to the initial testing with the 4 barrel and intake swap last year. I still need to hone in the jetting some but it was at a serviceable place. The testing was done on similar weather days, about 80F light chop. I abandoned all other modifications. If I am going to do any of that, I am starting with a non balance shaft block.

I have months of data on the 2 barrel and 42 MPH gps was about all it would ever do. This was with my old mirage an running ~4700 RPM. I then traded one of my other props for a 21p Mirage. (Why the mirage? it seems to run the best on this hull based on the dozens of props I've tested) with the 2 barrel it pulled RPM down to 43-4400, bottom of the range and speed at (drumroll) about 42 MPH.

Swapped the 4 barrel on and jetted it up and am consistently getting 4700 RPM with the same prop and base timing for the thunderbolt V. I confirmed timing before testing the 21 on the 2 barrel too so it was a fair a/b. At 4700 I see 46 MPH GPS. so a gain of 4 mph with nothing more than a carb swap. I feel that is a worth while investment and the 4 barrel runs and drives way better than the 2 ever did. Smoother idle, response is better... I am happy. I think with pulling a little jet out up top (it is very rich right now) another mph may be possible while staying safe.

Just guesstimating, it would seem like 25 horsepower gain isn't an unreasonable estimate based on mph gains. I will hopefully get out on it in the coming weeks and get the numbers finalized. I suspect my summer speeds with humidity will drop slightly too.

 

[Scott06]

So I figured I will bring this one back. I got to the initial testing with the 4 barrel and intake swap last year. I still need to hone in the jetting some but it was at a serviceable place. The testing was done on similar weather days, about 80F light chop. I abandoned all other modifications. If I am going to do any of that, I am starting with a non balance shaft block.

I have months of data on the 2 barrel and 42 MPH gps was about all it would ever do. This was with my old mirage an running ~4700 RPM. I then traded one of my other props for a 21p Mirage. (Why the mirage? it seems to run the best on this hull based on the dozens of props I've tested) with the 2 barrel it pulled RPM down to 43-4400, bottom of the range and speed at (drumroll) about 42 MPH.

Swapped the 4 barrel on and jetted it up and am consistently getting 4700 RPM with the same prop and base timing for the thunderbolt V. I confirmed timing before testing the 21 on the 2 barrel too so it was a fair a/b. At 4700 I see 46 MPH GPS. so a gain of 4 mph with nothing more than a carb swap. I feel that is a worth while investment and the 4 barrel runs and drives way better than the 2 ever did. Smoother idle, response is better... I am happy. I think with pulling a little jet out up top (it is very rich right now) another mph may be possible while staying safe.

Just guesstimating, it would seem like 25 horsepower gain isn't an unreasonable estimate based on mph gains. I will hopefully get out on it in the coming weeks and get the numbers finalized. I suspect my summer speeds with humidity will drop slightly too.

25 would make sense as the 4.3 2 bbl is rated at 190 hp, the reman 4.3 with 4 bbl webber eddy is rated 225. I had heard somewhere on here that it takes 10 hp per 1 mph but can only assume the hull has something to say about it.

I found the midrange throttle response on the 4 bbl to be way better than the 2 bbl. Not sure if it is due to 4 smaller throttle bores vs 2 larger ones.

I also found a similar top speed wall with my boat (47-48 mph) change props moved the rpms one way or another but speed was the same.

Id say it worked out well in your case to do the upgrade.

 

[Pmt133]

25 would make sense as the 4.3 2 bbl is rated at 190 hp, the reman 4.3 with 4 bbl webber eddy is rated 225. I had heard somewhere on here that it takes 10 hp per 1 mph but can only assume the hull has something to say about it.

I found the midrange throttle response on the 4 bbl to be way better than the 2 bbl. Not sure if it is due to 4 smaller throttle bores vs 2 larger ones.

I also found a similar top speed wall with my boat (47-48 mph) change props moved the rpms one way or another but speed was the same.

Id say it worked out well in your case to do the upgrade.

The horsepower per mph also changes the faster you go... figure drag.

Oddly, the alpha 4v was rated 225. Per service manual 18 mine would actually be 210 HP with the 4bbl. Using one of those online calculations I cone up that 215 hp is about right. Obviously some wiggle room on those but I've bench raced cars against those calculations too and they're typically spot on if you have good performance data to start with.

Either way, 3-4 mph for ~$400 is an incredible ROI.

 

[Pmt133]

So I found another 2 MPH testing today. I have on and off had a doel fin on the drive. With the old prop I preferred it as it cleaned up some porpoising you would get under some circumstances. It would shave 1-2 mph up top as well. I tried this outing the new prop (same 21 I have been running for the clean A/B between 2 and 4 barrel) and it exhibited no issues like the old one did. So it stayed off. I hit the limiter with a full tank and just me and got a clean 48 mph not on the limiter... so it has some more in it.

There is a decent bit of throttle travel left just off the limiter. I half wonder what a 23p would do.

Kidding aside, the 4 mph gain stands and I am closing out this saga. I will also note, from 3200 rpm on, 200 rpm is 2 mph. slip calculates out to around 8% from there to wide open. Thing is locked in hard. Thank you all for following along.

 

[Pmt133]

One other thing I've tested somewhat un scientifically... I've run the same route now a few times with both the 2bbl and 4bbl and it would appear that the 4bbl at cruise is a good bit more fuel efficient. Figured out I'm burning something to the tune of 5 gallons per hour and averaging nearly 4 mpg.

How is this possible? The 4 barrel, propped to the same wide open rpm as the 2 barrel but with a higher pitch prop to keep rpm in range (4700 typical load with the 2 barrel combo, 4800 the 4) my cruise speed went from 28 mph at 3200 rpm to 31.5 mph at 3200 rpm. A massive improvement. The hull always felt the best between 29 and 33 mph and woth the 4bbl you are out of the secondaries maintaining that speed. The 2 barrel to maintain those speeds was running 3400 to 3700 rpm, burning more fuel in the process.

Same rpm for same rpm, I went from 3.4 to 4 mpg and at cruise speed shaved off about a gallon per hour of fuel use. Granted I don't have a fuel flow meter and just check consumption by topping off after each outing... but the difference is noticeable per my receipts. It's 4-6 gallons per outing less than I was using and doing my usual running. So overall I'm very happy with the results.

Obviously I could've slowed down fuel burn by lowering rpms on the 2bbl but you don't make up the .5 mpg difference by doing that as you are still burning about the same fuel per hour just at a lower speed. I think it may be slightly more due to load of the higher putch at the same rpm but the 4bbl is tuned much better over its power band because of the ability to supplement with secondaries that I think rpm for rpm it's a wash.

 

[nola mike]

I guess maybe more efficient air flow through the smaller ports? Your numbers are about where mine are at using a 21p turning point 4 blade. I hit 46-48 mph @ 4600 rpm. I don't get much except for a cool sound when the secondaries open. I'm rich when the secondaries open but haven't felt like messing with it. I rarely run WOT though. It does feel effortless mid range compared to the 3.7. 30-35 mph easy if 1 or 10 on board. I do have a flat spot when I hit it which is annoying for water sports...I may pull the carb and rejet in the off season and replace my O2 sensor.

 

[Pmt133]

It's pretty snappy everywhere honestly. Again, metering through 4 jets when the secondary is open plus having a metering rod for cruise is probably where the improvement comes from vs having just the 2 jets to cover the entire operating range. I was actually able to watch the AFR rise slightly just before the secondaries opened which tells me that everything is jetted and timed properly.

Honestly it is like a completely different engine and I am in love lol.

I blow it out at wide open at least once an outing. 4 miles down the river to end the trip and let it sing the song of my people. Set a new personal best of 48.5 mph though it isn't bouncing on the limiter anymore. Half wonder if the recent rain swung the brackish concentration just enough to notice that as nothing else changed.

 

[nola mike]

Didn't realize you were using an AFR. I tuned mine with a vacuum gauge and AFR which was cool. My AFR rose a bit as well and I think I had the springs kick in around 3500 rpm. AFR was right around 14, but dropped to like 10 when secondaries open. I think I can get some hp if I rejet but #542 on my list of projects

 

[Pmt133]

I originally went off plug reads which indicated lean... coincidentally I did have a vacuum leak as well which probably skewed the plug reads. Currently I pulled the AFR meter out as I, at my jetting, was seeing 12.5-13.5 pretty much everywhere I would run which I would say seems safe. I had a seperate tuning thread somewhere but basically I ended up at the stock merc 4bbl jetting. Again being a later stock rebuild this makes sense. Current plug reads indicate the engine is happy all around with what it's being fed. Could take a bit more timing but I'm not pushing that one.

Regardless of all that... nearly 50 mph in a 19' cuddy cabin with a 4.3 is impressive. Dads little 17 bow rider with an I6 would do that... and it needed a pad and nose cone with pickup to achieve it.