why not use a psru and get even more torque?

 

Psru?

Are you direct driving the prop or is there a belt reduction? I'd love to see pictures of that boat!

Direct drive. I have a few friends building 4.3's and running them on boats. A little friendly competition to see who makes what with their given setup.

I am staying direct drive because the fuel economy is much much better and to keep the weight down. My hull was cut down by whoever was racing it originally. I am using more for pleasure and cruising. My goal is to be able to cruise 50+ and have a max speed of 75-85mph.

Of the boats together right now, we have a couple of late 80's 4.3's that are bone stock direct drive which actually do pretty well considering, one friend has a 2014 all aluminum 4.3 with a 1.6:1 reduction and kept the electrionics and fuel injection... He boogies. And the craziest build going on is a balance shaft 4.3 with a 4.71 weilend blower and nitrious on direct deive. That is still in the works so it will be a min before he is done.

I am looking to be somewhere in the middle. If I can have a fast cruise, plenty of response and power, and the ability to spray nitrous if I was going to race, i would be thrilled.

 

Are you direct driving the prop or is there a belt reduction? I'd love to see pictures of that boat!

 

psru propeller speed reduction unit
you will be most efficient if you run your engine at the best "BSFC"
BSFC= brake specific fuel consumption
at this rpm the engine will be producing the most power(usually torque) compared to the least amount of fuel per horse power produced (usually at W.O.T.)

If you tune the engine to make peak hp at or around 4400-4800 rpm (stock ish)then your best BSFC will likely be around 34-3600 rpm. With a 3:2 psru the prop will be turning 22-2400 rpm which is a good speed to keep the tips from approaching super sonic. Of course a constand speed prop would allow you to keep the engine in the best power band a lot better than a fixed prop.Running a low rpm,hogh compression direct drive will have tuning issues to say the least and way more likely to have bearing problems as the thrust bearing is not all that big in a 4.3 liter chevy. The prop swinging at a lower rpm will be a lot quieter too

big prop lower rpm is better than small prop at higher rpm.If you want even better low speed manners consider a 3 blade constant speed prop with more pitch.

also a 4.3 will make more hp and torque with a 4000 rpm use able power range than 2900 rpm. The psru will add torque through gear reduction. The higher rpm engine making more power will use more fuel BECAUSE,,,it makes more power.It will be a lot faster and quieter.If you dont want the stock fuel injection then use a 4bbl

 

I am staying away from any reduction units on this boat. My last boat had a LS3 with a 2.3:1 reduction and a 80" 3 blade composite prop.

1.68:1 is going to be the closest I get to 3:2 without using a custom built reduction unit and already have one on a 4.3 on another boat.

Honestly, i was told that it is impossible to make a direct drive 4.3 boat go fast... I am here to prove that statement wrong.

I am not worried about the thrust load on the bearings. We have a few 4.3 engines on boats as direct drive applications. They have hundreds of hours on them, they have been run hard and they are still running great with zero oil leaks or issues. Not to mention, a junkyard engine is like 2-300 bucks. I am long overdue for some R&D

Now, this will be the first time I dig into higher compression pistons and start messing with valves. My choices in the OP is from theory and reading about developing torque. From what I understand about CR, just cause I use 12:1 pistons does not mean I will end up with a DCR of 12:1. Depending on how the cam works and a plethora of other factors, my dynamic compression ration will be determined. So, the idea was 12:1 allows for greater compression and easier exhaust stroke. Why is higher compression harder to tune?

As far as the heads are concerned, everyone hates the swirl ports heads becuase they do not flow... Well, they have a hard time breathing over 4000rpm. BUT, they make great torque from idle to 4000 rpm when compared to aftermarket heads. Lets be real, nobody is making performance parts for turning under 4000rpm. Lol. This is an odd build i know. The Swirl Port heads seem like a great place to start since they work as designed in my rpm range. the idea here is to take advantage of the already low rpm intake ports of these heads and build around them. Would there be any clear advantage to a bigger exhaust valve and porting the exhaust side of the head?

With the carb, Every 4bbl I have used on direct drive applications eventually have the secondaries get stuck open. I would like to stay away from 4bbl and remain with a 2bbl. I was thinking about the 500 holley because it has always done well on 4.3 boats. But, would I make more torque with a smaller cfm? I had a 500caddy a few years back that i believe had a 470cfm webber on it but cant quite remember.

My theoretical formula is this:

Bring compression up, make as high intake velocity as possible at lower rpm and allow the swirl heads to do their thing, allow the exhaust to breathe easier to keep from losing torque. But, the right combination to acheve this is what I am after. Or, to be schooled and learn the right equation for what I want to do.

 

the 4.3 will make more power at 3500-4000 rpm than 2900.
If you do not use a constant speed prop you give up a wider range of rpm you can use the engine,shorter life for the engine(you dont care),The careful and maybe expensive construction of acomposite blade that can absorb all of the power from the engine and lose the ability to run the throttle at wide open over a wider rpm range. This all loses some of the available power available. It will end up being more of choosing the better prop rather than the best engine combo.

The engine can be built on two ideas,using the best parts combo for the rpm range being used and saving on parasitic power losses,,,all at a cost.

your proposed build is more about the prop. Im not a fan of using high cr and a carb. what about computer controlled ignition,,,

 

I agree completely. 3500-4000 would be the optimal rpm range but to stay direct drive would require the use of a thin multi-blade prop cut down to keep from reaching too high of prop tip speed. I can use a warp drive propeller and get very close to .9 mach and they will stay together. I despise these props because they are very, very noisy.

Just about every other propeller manufacturer that make propellers for airboats has a max rpm of 3000 rpm or less. Again, I completely agree it is about the correct prop for the application. I can work with my dealer to try a couple on the engine after it is built. I am trying to build the 4.3 to make as much power as possible up to 3500 rpm and then match a prop to it at that point.

I understand that i am asking for the 4.3 to do something that it is not designed for and that I would be getting optimal performance at higher RPMs. there is no argument there.

Why not high cr on a carb engine? I will not use a computer controlled ignition because as I understand it, it would require being sent out and programmed if I make significant modifications along the way... It is much easier to test and tune. more fun too.

What would be the highest cr you would go with on pump gas and why? Since I am going to have a custom cam ground for the application, wouldn't most of the typical problems of running high compression be avoided? Can I have the company grind the cam to operate optimally with whatever piston/head/valve/rocker arm combination? Or should I start with them and have them tell me what combination would be best?

I am not expecting to make record numbers here for 4.3 chevy engines. from the dyno sheets I have found on factory 4.3 chevy engines, they normally peak torque around 3200 rpm then fall off above 4000. Hence my interest in the factory swirl port heads. What I am looking for is small gains here that accumulate to extra torque and hp from idle to 3500rpm. If I could manage 200hp and/or 300+ ft/lb at 3500rpm, I would be pretty damn impressed imp:

 

I was thinking 170 hp as being realistic. The continental I.O.240 makes around 125 hp in the lower rpm range so using a 4.3 I can see another 60 plus hp. You will have to build the engine with perfect deck heigh/gasket combo to use higher cr. I would go 10:1 or probably less if using straight gasoline. combustion chamber design is important. The best guy here is probably "oldbogie" for whats available in 4.3 combustion chamber designs available.

the hp saving ideas are expensive for the little return you will get,,,IE: balancing the engine,oil scraper,dual"parallel" remote oil filter,,,etc

 

Nobody is going to be able to grind you a low rpm pump gas cam to fit with 12-1 compression and pump or marina fuel....it just isn't going to happen, it's like finding a unicorn, or a pot of gold at the end of a rainbow....it doesn't exist.

Low rpm power mean short cam timing events to trap a lot of intake mixture, couple that short cam timing event with 12-1 static compression and your dynamic compression ratio is far from being pump fuel compatible.

!0-1 to maybe 10.5-1 is going to be about the best you can do, considering you're out over water where air is cooler and higher humidity, and the fact your engine is out in the open air and engine water temps are easy to maintain.

 

Okay, so if we are looking at 10-10.5:1 CR, is there any point to pistons? Or would it be better to make the compression by decking the block or milling the head?

What is that process all about?

 

you have lots of answers about the engine

i have been thinking of something for a while. building windmills for a friends house up in california city where it blows like 50 miles an hour ever afternoon..

perhaps.. this idea might work for you.. mounting your prop or the prop mounting spindle on a one automotive hub and bearing assembly.. perhaps bolting it to the scatershield opening for the normal manual transmission. using one of the adapters used to direct drive a powerglide without a converter.

some kind of splined shaft to drive the splines on the inside of the hub and bearing assembly. heck.. use half a CV shaft with a CV joint thru the hub and bearing assembly. splined shaft to slip fit into the modifed powerglide drive tube. it it need to be a back to back installation?? using two bearings for stability?? cars and trucks exert a lot of side loading on these bearings. so it should not be a problem

its only a thought.

why .. because it moves all the thrust away from the crankshaft.

will the 3500 prop speed exceed the design spec of the hub and bearing assemblies. perhaps. again.. this is just to save your thrust bearing. and do it on the cheep.. design spec for those might be 1000 RPMs.. as most tires are 500 to 700 turns per mile. again just thoughts.

 

Low,any more thoughts?

 

You seem well versed in 4.3s, but I'll mention it for others who may read this. There were many running changes made to the 4.3 over the years, and its important to be cognizant of that when mixing and matching parts.

There were some aluminum heads made for the 4.3; but they're hens teeth and Im not sure given your RPM requirements if they flow more air down low than the swirlies you have. The benefit would be a few less pounds, and the ability to run a bit more compression.

Have you thought about a supercharger? An SC off a 3800 series or similar ( Nissan Frontier?) might be interesting. You'd have to mount it off to the side and plumb the air into a box around the carb.

Im with Vinnie, you're definitely shooting yourself in the foot with the direct-drive setup, but I understand "The Road Less Traveled."
You might want to get a wide-band O2 sensor and put a gauge in your dash cluster; if you're going to push the compression to the edge on the water with marina gas...this is going to be an expensive experiment.

 

Chevy 4.3L 262ci V-6 - Engine Build

 

You guys are awesome man. Thank you for the feedback.

My knowledge on the comes from a friend of mine who has built a few 4.3's and a bunch of reading and searching for info on 4.3's.

I am expecting a call back from holley and comp cams. I am looking for holley to get me info on the best carb to use for the application and comp to let me know what the best cam (probably grind the factory cam) and rocker arm combo would be. I was told 1.6 would help a lot but I have never messed with the ratios like that before.

So, going direct drive on 4.3's (sbc's in general) is not uncommon with airboats. We talk a lot of **** amongst each other and all of the boats with 4.3's on them right now are flat bottom aluminum boats. Some direct drive and one with a reduction unit. The flat bottom boats have much more drag than what Iam putting together. For sake of illustration, flat bottom boats will have no less than 25 sq/ft of surface area in contact with the water over a 6x5 foot area. I will have no more than 5 sq/ft of surface area in contact over a 10x4 foot area. I have a step boat. So, I will have 2 very small contact surfaces 7' away from each other versus one very large contact surface. That in addition to my hull and rigging being drastically lighter (glass vs alum)

I would like to put a reduction unit down the road but right now i want to get the boat together and whomp some *** with a dd4.3 just cause they say it wont outrun them. Being that it seems running a (basically) stock 4.3 and freeing up some power with shaving heads and/or porting the exhaust is me looking for that slight edge.

I like the O2 sensor idea. May make tuning easier too.

Do you guys know how much I can/should shave the heads?

 

Port your heads (if you're going to) and then "CC" them with the valves you plan to use. Then you can accurately calculate the amount to deck the heads.

 

Right now, it does not make sense to change the size of the valve. I am just going to open up and smooth out the exhaust side of the head.

So, I am assuming that I need to identify which heads I have and how many cc's they are and then, determine how much to deck the heads?

 

your heads probably flow around 130 cfm on the intake side,why bother porting the exhaust? If your heads are out of breath around 36-3800 rpm thats more than enough considering you wont rev past 3k

 

You guys are awesome man. Thank you for the feedback.

My knowledge on the comes from a friend of mine who has built a few 4.3's and a bunch of reading and searching for info on 4.3's.

I am expecting a call back from holley and comp cams. I am looking for holley to get me info on the best carb to use for the application and comp to let me know what the best cam (probably grind the factory cam) and rocker arm combo would be. I was told 1.6 would help a lot but I have never messed with the ratios like that before.

So, going direct drive on 4.3's (sbc's in general) is not uncommon with airboats. We talk a lot of **** amongst each other and all of the boats with 4.3's on them right now are flat bottom aluminum boats. Some direct drive and one with a reduction unit. The flat bottom boats have much more drag than what Iam putting together. For sake of illustration, flat bottom boats will have no less than 25 sq/ft of surface area in contact with the water over a 6x5 foot area. I will have no more than 5 sq/ft of surface area in contact over a 10x4 foot area. I have a step boat. So, I will have 2 very small contact surfaces 7' away from each other versus one very large contact surface. That in addition to my hull and rigging being drastically lighter (glass vs alum)

I would like to put a reduction unit down the road but right now i want to get the boat together and whomp some *** with a dd4.3 just cause they say it wont outrun them. Being that it seems running a (basically) stock 4.3 and freeing up some power with shaving heads and/or porting the exhaust is me looking for that slight edge.

I like the O2 sensor idea. May make tuning easier too.

Do you guys know how much I can/should shave the heads?

 

Comp said:

Rpm range 1100-4000
.206/.206 and .500/.500 with 110 seperation on lobes
Roller rockers
Springs and retainers
Oe style hyd. roller lifters

Holley said
350 cfm 2bbl performance carb w/ no choke


So, it looks like it would be best to deck the heads and leave the ports alone.

 

I am going to try to turn 3400-3600rpm

I can shorten up on my prop blade length and get some more rpms.

That should let me get into a little more power right up to where the swirl port heads are about to run out of breath.

Back to decking the heads, whats the most I can take before screwing everything up?

 

ANNNNNNND...

NITROUS!

I believe that a 2bbl carb is rated at 4" of H20... when compared to a 4bbl at 2".

What does this mean when using nitrous? I have never used nitrous on anything. what changes would have to be made to accommodate say 100hp shot of nitrous?

 

you need the ring gap to be 2-4 thou wider. You will be using a 4 bbl intake(I prefer a single plane for NO2). You will just blow off the prop unless you go constant speed for obvious reasons. A 150 hp prop will just slip when you add "60"hp shot.
If you want a "100" shot then open ring gaps .004 to .006. You will need a 300 hp 4.3 that revs to keep the engine from scattering through the bottom of the boat.You will need to pull about 4º out of the timing.

are you using a counter shaft 4.3?

 

By counter shaft do you mean balance shaft?

Ring gap = piston ring gap?

If I Was going to run nitrous, i would re-pitch the prop to allow for additional power. Getting the right prop would not be too difficult. I have a few I can try out. Biggest thing with the props is to keep the tip speed under .92 mach at WOT

 

balance shaft, yes
piston ring gap,yes
If you repitch the prop to match the nitrous power,how will it perform with out NO2 being introduced into the manifold when cruising? The engine wont have enough power to rev very high?

If you just want to beat your buddies cheap?
home port the heads
good intake
headers
cam that makes power to 5200 rpm
"buy" a psru for home built air planes with 9:4 ratio (gilmer belt style is reasonable cost)
you should be making 200 hp maybe more
200+ pounds of torque times 2 because of 9:4 psru

Im sure you are aware that props are very efficient from 2200 to 2600 rpm. I know I keep saying prop,,,

that is the answer
look through "kit plane" magazine for suppliers of reasonable priced PSRUs

200 hp with 400 pounds of torque is gonna destroy 150 hp with 180 pounds of torque.

you can stroke the 4.3 but we are back to cubic dollars.
I cannot think of a cheaper engine than the 4.3?

The o-290 GPUs are less powerful (145 hp)
diesel is too heavy and or expensive,,,
GM made a 5 liter V-6 but it was a dog.

sorry,Im out of ideas atm, but it is still a fun puzzle

 

OOPS. Missed the air boat description in first post :/