I am new to the air boat scene. I have a 12'7, hull already polly'd with all the rigging and a 2 part prop 72" whirlwind stump puller. I don't have a motor yet. My question is this. I have read about gear reduction and that seems to be the best way to go but I bought this air boat to fix up and sell. I want to get a 318 dodge motor because I am going to make this boat a General Lee. I have read that my 2 part prop can only get to about 3000 rpms. and without a gear reduction, I wont be able to pull much power. What is the best way to make more power to the prop with an engine like this. I was also thinking of a 383 or a 440. Any help would be appreciated. I will be building boats with gear reduction in the future I just cant seem to find a good used one plus im only going to sell this boat for about 6 grand. I paid 1300 for it.
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Direct Drive
- Thread starter Beau
- Start date
SWAMPHUNTER45
Well-known member
Welcome to Southern Airboat!
I think you are going to find that an engine with a lot of low end torque will be the best choice for a direct drive selection.
We don't see a lot of Chrysler products in my neck of the woods so you and the Lil General will sure get a lot of attention.
Good luck with the build.
I think you are going to find that an engine with a lot of low end torque will be the best choice for a direct drive selection.
We don't see a lot of Chrysler products in my neck of the woods so you and the Lil General will sure get a lot of attention.
Good luck with the build.
Slidin Gator
Well-known member
You are right on max speed (more like ~2,800 RPM actually). You are looking to make 400-600 ft-lbf of torque to push that prop, which is a tall order for anything automotive in direct drive. However, once you put on a gear box you are aiming for a larger prop turning a slower speed to maximize thrust.
Chances are you have a boat built and rigged for a direct drive, so unless you want to tear it all apart and start over, that is probably the best route to a resell. 12' 7" is pretty small for a gear box boat and it is probably too narrow for the torque roll.
Research torque curves vs. weight for the engines you have in mind and see how they compare. I say forget about the 318, maybe the 383, but probably the 440 and then some. An inline 6 is a good option to, it's all about the low RPM torque, any performance above 3,000 RPM is un-usable.
Do you know what engine was on it with the rigging you have? Was it automotive or an Aviation motor?
Chances are you have a boat built and rigged for a direct drive, so unless you want to tear it all apart and start over, that is probably the best route to a resell. 12' 7" is pretty small for a gear box boat and it is probably too narrow for the torque roll.
Research torque curves vs. weight for the engines you have in mind and see how they compare. I say forget about the 318, maybe the 383, but probably the 440 and then some. An inline 6 is a good option to, it's all about the low RPM torque, any performance above 3,000 RPM is un-usable.
Do you know what engine was on it with the rigging you have? Was it automotive or an Aviation motor?
John Fenner
Well-known member
Well, I did a small bit of research, I would suggest a 360 LA "not Magnum" a set of early 273 heads and intake for a 4v 390 Holley, plus use either a cam for the later 273 "hydraulic lifter, earlier 273 was solid" or send cam to be ground for peak torque at 3000.
emmitt Kelly
Well-known member
Question: when considering the Chrysler v8 engines...I may be wrong...I am thinking the starter bolts to the bell housing ...if so..that is going to be a little tricky hangin a starter...no doubt a good machinist could work something out with the gear reduction but direct drive..maybe so too...anyway...just wondering
I run a 302 small block Ford DD and I took the mid plate between the block and bell, traced it out on 1/4" steel plate. Flame cut it with a torch free hand rough cut and used a carbide burr to take it to it's final dimensions. Starter bolts up perfect, no grinding while turning. Just takes some time and patience. Getting ready to build a 351 Windsor stroked to 408 to replace the 302. Should double my power, hopefully!
SWAMPHUNTER45
Well-known member
A 408 Windsor that should be a neat little stroker. What you gonna use for the cam?
Haven't decided yet. Thinking a Comp Cams Xtreme Energy XE250H. Short duration with a pretty decent lift, but gonna consult with the techs at Comp to see if there is a better option because of the displacement. Should weigh in close to the 302 with iron heads by the time I bolt on aluminum set.
flcracker9
Well-known member
hdsadey said:
A 408 with the right cam and heads, should run great on a DD. My Son runs the same hull as you (with poly), with a DD 385" SBC, 70" Q prop, and pretty much goes where he wants to, runs the hill darn good. Don't go too big on the heads, keep the intake runners on the small side to help keep max torque down low in the RPMs. I believe the 351W is even a little lighter than the SBC? Keep us posted on the build, if my Son's 385 takes a dump, he has a 351W based SBF he would like to put on it next.
kwanjangnihm
Moderator
http://www.simpsonqd.com/hdsadey said:
Thanks Cracker. The 351 weighs in at 510 with iron heads and intake, so the websites say. Estimating about 60 lb savings with aluminum heads and intake. SBC supposedly 575 all iron. Looking to do 180cc or intake runners. Block going in the oven next week and then I'll strap it in the Rottler to bore it .030. Cast Eagle 4.000 crank, SIR 6.200 rods and dished forged pistons are the plan. I run a FiTech injection now so tuning will be easy. Excited, tired of getting stuck with the 5.0.
Thanks kwanj I will check them out. Thought about buying one of them Ebay low buck hanging scales and tryin myself but.....
Thanks kwanj I will check them out. Thought about buying one of them Ebay low buck hanging scales and tryin myself but.....
OneBFC
Well-known member
solid advice from Kwan to use Dave Simpson. His methods and means are solid and he has been doing it for a very long time.
Pretty much can get anything from Dave. It's a one stop shop with everything done on premises from scratch. His boat designs are very leading edge and unique. You won't be disappointed if you work with him.
Let us know what the numbers turn out to be once you get them.
Good luck!
Pretty much can get anything from Dave. It's a one stop shop with everything done on premises from scratch. His boat designs are very leading edge and unique. You won't be disappointed if you work with him.
Let us know what the numbers turn out to be once you get them.
Good luck!
SWAMPHUNTER45
Well-known member
Great advice all around flctacker9 is spot on with picking the heads. You do not want a big set of ported high flow cfm heads on a DD application. A set geared to street use with slightly larger valves would be a good choice.
You want low lift flow and velocity. Picking the right cam will be a huge factor in how it all comes together. I would expect a nice gain in torque if done right. Probably in the 30% range adding compression, stroke and cam and heads.
What intake do you plan to use on it ?
You want low lift flow and velocity. Picking the right cam will be a huge factor in how it all comes together. I would expect a nice gain in torque if done right. Probably in the 30% range adding compression, stroke and cam and heads.
What intake do you plan to use on it ?
Edelbrock Performer. Has an idle to 5500 rpm range. None of the other manufacturers seem to have a dual plane intake designed for low RPM for the 351. Offenhauser, Weiand, and Holley all start off at 1500. Professional Products is offshore junk so not going there! I run a Weiand Stealth on the 302 which says idle to 6000, but for the 351 they claim 1500 as a staring point. I need every bit of advantage I can get. Now the question I have been wrestling with is with the increase in CID and stroke, would it not benefit from a slightly taller manifold? The dyno is probably the only way to answer that I guess.
Deano
Well-known member
I believe you're probably correct. May not need a Dyno for confirmation.
There is a discernible difference on a Caddy with a one inch spacer.
When you thrust test it, try it with and without a spacer and you will know.
I had a Windsor in my offshore boat and it liked one, admittedly not quite as rpm limited up top.
It cruised about 2800 which was were my concern was. Boat was to big to haul ass.
Anyway, just a thought . . .
There is a discernible difference on a Caddy with a one inch spacer.
When you thrust test it, try it with and without a spacer and you will know.
I had a Windsor in my offshore boat and it liked one, admittedly not quite as rpm limited up top.
It cruised about 2800 which was were my concern was. Boat was to big to haul ass.
Anyway, just a thought . . .
Slidin Gator
Well-known member
Just to veer as far off topic as possible!
I had a 1980 Bronco with a 302. After a rework the motor made great low end torque, but the truck was light (see pic, interior was stripped to 2 seats). The best part was sitting at a side street entrance to a main road with heavy traffic. A quick tap of the throttle and then the brake would make the nose surge and pull back, just enough to make on-coming traffic hit the brakes and give me an opening to go!
I suggest you put the spacer in to start with and plan on it. Porting the heads is fine, but only at the intake/exhaust transition and leave the runners a bit rough. You want turbulent flow through the intake to maximize mixing, rough it up if needed. Polishing can make the flow laminar at lower RPM flow, preventing mixing and good burn. Swamp's input on a low lift cam and larger valves is also good advice. None of the stuff that Comp etc. sells is optimum for the 800-3000 RPM range of interest here, they are all tuned to run higher (as noted), even the typical "RV" cams come in at 1,500 as you have found.
Honestly, the most typical starting point for a good DD auto motor is a non-emissions factory cam (or copy), cam timing, higher compression and attention to intake conditions. Stroking is a definite plus, that's more torque arm. Exhaust system is also of interest, but it generally boils down to mufflers for a bit of back pressure but you can't make the exhaust tubes long enough, so forget it.
I had a 1980 Bronco with a 302. After a rework the motor made great low end torque, but the truck was light (see pic, interior was stripped to 2 seats). The best part was sitting at a side street entrance to a main road with heavy traffic. A quick tap of the throttle and then the brake would make the nose surge and pull back, just enough to make on-coming traffic hit the brakes and give me an opening to go!
I suggest you put the spacer in to start with and plan on it. Porting the heads is fine, but only at the intake/exhaust transition and leave the runners a bit rough. You want turbulent flow through the intake to maximize mixing, rough it up if needed. Polishing can make the flow laminar at lower RPM flow, preventing mixing and good burn. Swamp's input on a low lift cam and larger valves is also good advice. None of the stuff that Comp etc. sells is optimum for the 800-3000 RPM range of interest here, they are all tuned to run higher (as noted), even the typical "RV" cams come in at 1,500 as you have found.
Honestly, the most typical starting point for a good DD auto motor is a non-emissions factory cam (or copy), cam timing, higher compression and attention to intake conditions. Stroking is a definite plus, that's more torque arm. Exhaust system is also of interest, but it generally boils down to mufflers for a bit of back pressure but you can't make the exhaust tubes long enough, so forget it.
Thanks for the input gator. Do you have any suggestions for better cam companies? I've looked through Howards, Lunati, Crane, Eglin, even thought about calling Bullet to see if they might have a custom grind. Was planning on port matching only, Edelbrock makes a set of E-street heads in 170cc intake runners with 1.90 or 2.02 valves that would compliment the Performer intake. Cam will be installed advanced, most likely 4 degrees. Looking for stump pulling power.
Slidin Gator
Well-known member
My specific knowledge of Ford stuff ended at that truck, just restarted with a 2017 6.7, but totally different stuff.
What I am suggesting is that you jump into the way back machine and find some late 60's parts, or modern copies. The original versions of that motor were designed for load hauling, low torque applications. They changed the heads and tune around 1970 for higher RPM performance and then the mid 70's oil embargo and emissions issues gave us nothing but crap. If you read the hot rod mags you will find that everyone throws the late 60's stuff away to "Hot Rod" the motor. For a direct drive application, knowing nothing else, you should read the Hot Rod advice on opposite day, do the exact opposite.
Without going the custom grind route, I am guessing (seriously guessing) you will not find the cam you really want new. Ebay is where I would go, there has to be someone trying to unload that 69 old school cam so they can put in the Extreme Energy stuff. I mean who wants a cam that runs out of wind at 3,500 :!:
On the subject of heads, note the comment about higher compression. Generally the easiest way to do this is by using heads from the same block, but smaller bore. There are plenty of high compression 302 heads out there. With 0 research on the subject, the 302 is where I would start. Again, I would check ebay for someone upgrading for higher rpm/flow. Port match them and run a burr grinder around the runners to rough them up as best you can. If you are ready to spend aluminum head money that is ok, but assume it is all for weight savings other than larger valves and make sure the volume is correct for good compression (10+-1). For a direct drive, compression vs. valve size, compression give the most bang for buck. You can get small chamber volume in cast iron off ebay etc.
Add in a 69' stock low lift cam, a 500 cfm max carb and mechanical advance all in way early and you are there. Build it for premium and tune accordingly.
That's the best advice I can give without research, use with grains of salt.
What I am suggesting is that you jump into the way back machine and find some late 60's parts, or modern copies. The original versions of that motor were designed for load hauling, low torque applications. They changed the heads and tune around 1970 for higher RPM performance and then the mid 70's oil embargo and emissions issues gave us nothing but crap. If you read the hot rod mags you will find that everyone throws the late 60's stuff away to "Hot Rod" the motor. For a direct drive application, knowing nothing else, you should read the Hot Rod advice on opposite day, do the exact opposite.
Without going the custom grind route, I am guessing (seriously guessing) you will not find the cam you really want new. Ebay is where I would go, there has to be someone trying to unload that 69 old school cam so they can put in the Extreme Energy stuff. I mean who wants a cam that runs out of wind at 3,500 :!:
On the subject of heads, note the comment about higher compression. Generally the easiest way to do this is by using heads from the same block, but smaller bore. There are plenty of high compression 302 heads out there. With 0 research on the subject, the 302 is where I would start. Again, I would check ebay for someone upgrading for higher rpm/flow. Port match them and run a burr grinder around the runners to rough them up as best you can. If you are ready to spend aluminum head money that is ok, but assume it is all for weight savings other than larger valves and make sure the volume is correct for good compression (10+-1). For a direct drive, compression vs. valve size, compression give the most bang for buck. You can get small chamber volume in cast iron off ebay etc.
Add in a 69' stock low lift cam, a 500 cfm max carb and mechanical advance all in way early and you are there. Build it for premium and tune accordingly.
That's the best advice I can give without research, use with grains of salt.
Gotcha. I was just in Crower's website and they have a hydraulic flat tappet that makes peak power at 3500. Redlines at 4500. Looks like that may be a viable option. Compression shouldn't be an issue, most aluminum heads are small chambered. And yes my thought process is for weight savings only, trying to keep the power to weight ratio up as far as I can. I can afford to throw some extra money at it since I work in an engine machine shop so all the labor is free. Plus not having to spend for a gearbox helps to offset.