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Direct vs Gear Drive Longevity?

G

Gbenzx

Well-known member
This keeps coming up abt how hard direct drive is on an engine.

We're looking at 2 different Airboats in Airboat Trader that are same year models with very close hours. One is a 350 with gearbox & has had to undergo rather deep surgery in same hours that caddy has ran w/o gearbox & no probs with what appears tobe same type of prop. The caddy also is pushing a heavier fiberglass boat.

Other than abuse is there something we're missing here? Short block? Long block?

Thanx, Gben

Btw, It's mentioned in another thread that one could only use 2 blade prop on a direct drive engine? :(
 
Gbenzx, I have a 472 caddy rig that I bought from a guy last September. I ended up having to get a new hull because I didn't check it out good enough before I bought it. The motor seems to be great, fires up in one turn dead cold. I was also told no three bladed props for direct drive, not enough power without gear reduction. I do want to try a composite blade though. I would think that the lower rpm that a direct drive pushes would mess the motor up, but I guess wrong after reading your thread.( I do know a guy whos blown his 9th 350 motor with gear reduction but he's also pushing 700 horses out of them!) I have heard alot of good things about the caddy motors and that makes me feel better about mine. I have to admint that I can't wait to get my hands on a big block motor with a reduction unit one of these years.
 
Gben, two factors are at work here:

Number one is main bearing size. The Cadillac has bigger journals than the SBC. Bigger journals can handle more load. Look at a crank from a tractor engine. A 45 HP 4 cylinder has about a 3" main bearing diameter. They are made to last! SBC has 2.45" main journals and 2.1" rods.

Number two is that with a gear box you are turning more RPM all the time. RPM greatly affects lifespan. Additionally, with the extra torque available with the box, one is more likely to "stand on it" and over-rev

The key is that you drive sensibly and don't over-rev your engine.

Given the same price, I'd go with the gear box Chevy over the direct. It's really a better way to go from a longevity standpoint if you remember your RPM limits and a lot more neat parts are available for the Chevy. Plus they will be available long after the last Caddy block has been bored to the max.
 
BTW, What kind of prop is on the Chevy boat and what's the ratio of the box?
 
I got a cad , but Red is rite,small block - gear box is the way to go .You can walk in any parts store and get what you want .And remember gear boxs almost doubles your bang for the buck .There nothing wrong with cads i got a 16 ft boat and i needed that extra boom .I love mine its thinks its a 572 which it almost is.I went to the next step had a thunder moment :D
 
Wade,

We're looking for a machine now & check it out is big words to us.
And no 3rd blade on smaller direct drive blocks is probly what they meant.

Red,

Yep, I understand abt main journal durability & gearbox for more rpm, torque & sensible. The payoff then would be the jump in power at a small loss in longevity. More load via the box leverage & less fuel flow?
No don't have the prop size yet but it's wide wood & says it's 2.5 to 1. All three boats we're considering at the moment(16, 16 & 18) suprisingly have wood props & two are gear box drive.

Fatboy,

Gearbox doubles your bang for the buck is very clear indeed.
Also the Thunder Moment is clear indeed. :)

Has anyone owned both gearbox & direct drives in same big blocks like Caddy, 454, 455 Olds, etc for lengthy time & care to reflect upon it?

Thanx Guys, Gben

Well don't know how the icon above got there but should read 16, 16 & 18.
 
I have built a lot of motors utilizing a 2.68 gear box and have yet to have a customers motor come apart. I have broken my stuff but that’s me testing the limits. The journal Dia. Is a fact but it’s not what controls longevity. Hell Nextel Cup motors make 900Hp for hours on end and they run a 1.900 Honda diameter journal yes a Honda journal. The main issue with a gear box airboat motor is the oiling system. Take any drag race motor big block or small block Chevy that runs let’s say 10 sec then put a stock oil pan on it the motor will blow up and run out of oil within ten seconds. I have seen this a hundred times at high RPM’s the oil takes much longer to gravity feed back to the oil pan than it does to get pumped under pressure to the top of the motor. When turning 5,000RPM and up, a stock oil pan that only holds 5 quarts will run out of oil in under 15 seconds. A direct drive motor can turn 3,000RPM's for a week and will not starve for oil. I see all these so called 500, 600 and even 700 horse power motors with stock 5 quart oil pans or a cheap deep Mororso pan. They all spin bearings and blame it on things from the machinist to which way the wind was blowing. I have been installing 10 to 12 quart oil pans on every airboat motor I build. Well I didn’t on my own personal boat because there is no need to turn 7,200RPM for more than 15 Seconds. I usually set a prop with a 2.68 box to turn 5,800 but I have been racing so I pitched it to turn around 7,500RPM. Well I let a buddy drive my boat and he held it wide open and turned around 7,200RPM for over a mile. I’m still amazed he didn’t flip my boat. I guess he had to show everybody how fast he or the boat could go. Well you guessed it my stock 5 quart oil pan will not hold enough oil to sustain a motor at 7,200RPM very long so it ran out of oil during the second mile long run on they way back. All my bearings are black and the crank is blued and the oil pump still works so I know it ran out of oil. I know not to turn them kind of RPM’s for a long period of time but he didn’t. So I must either not let people drive my boat that are going to run it wide open for a long period of time or fabricate a 10 quart or larger pan for it. Well I chose to do both. My point is if your motor has a gear box and the capability to turn 5000RPM plus don’t do it for very long unless you have installed a large aftermarket oil pan. I always preach this and still got bit by it myself.
 
I just done almost the same thing ran mine for a long time at 4000 or 4500 and looked at the oil gage and it was at zero the the motor started tapping now its time to build a new one with a deep oil pan. But it still runs shut it off for 30 seconds re fired it and had 40 psi and went to the ramp.
 
Gbenzx,Check it out will be what I'll do from now on. Guess I trust people to much. I personally would not have sold the one I got without feeling guilty. But I'm learning alot about fixing Them. It will be right when and if I sell this one! Thanks again. :oops: WADE
 
Thanx Thunder,

If I didn't learn nothing else today.......
Believe tho that me & whoever buddy was would be into a little heart to heart prayer meeting immediately if not sooner!
Best do some very close exams & a little oil lab if we decide on a used gas boat.

But Guys how many men that sell motors would be man enough to tell us this stuff? Wish he sold diesels but if any of our gang needs a gasser you can bet your sweet keel where I'll send em. :)

Will the real Cracker Jim please stand up :(

CC are you Chris, I mean Chris you're CC? But still looking at used ones & not finding much above the hulls tobe amazed with. Our engine will be so heavy that all 5 souls including me will have to set on a cushion in the bottom of the boat. :)

Thanx Guys, Gben
 
Hey Gbenzx when buying a used motor there are two main things I recommend. 1st and foremost do a leak down not a compression test but a leak down test, this tells you everything about the rings, valves, valve guides, piston and bore condition. Then remove the oil filter and cut it open people say this is extreme but I have had friends and customers ask me to look at used motors for them. Twice when I went to cut open the oil filter I had the seller look me in the eye and say forget it. Well you know both motors were hurt. The oil filter is the only way to inspect the condition of the bearings and crank, lifters, cam ect. If you find a bran new oil filter on a used motor it sometime means the motor had low oil pressure and they installed new oil and filter to bring the pressure up or there may be problems in the bottom end. Every motor I build I request the oil filter be changed after break-in so I can inspect it. The oil filter can tell you everything. Thanks for the complement Gbenzx ten years ago I was very concerned about what people said about my motors then I realized the truly great engine builders I know never commented on other engine builders because they know what their doing and what can happen. I always tell any potential customer beware of any engine builder that bad mouths another for your business run like hell because he don’t know what he is doing or hasn’t built that many motors. 90% of the time when somebody trash talks an engine builder I ask them a tolerance that any builder should know something trivial like crank end play tolerances or lifter preload and they usually don’t know it. When they don’t know simple tolerances I really don’t care what they think. Plus the fact on my own stuff I really push the envelope far beyond what I would ever do in a customers motor. They only way to find limits is to push them and on my personal stuff I do my testing and beet the snaught out of it. I could build a 500HP motor for my boat and it will live till the earth ends but I have already done a hundred times.
 
Wade,

They are folks out there that simply don't know.
As recent as yesterday near Orlando I pulled the dipsticks on a Cat Tractor & found the wrong smell in a power shift transmission. When I told the guy there was no sense in wasting time cranking it up he was speechless. Well it works ok he said when I told him he had abt 25-35 hours use left before abt $10k was needed to fixit. I really believe this is just one more example where people just don't maintain their machines.

They can be funny too.
Looking at a 16ft Classic Airboat last nite on ebay at Hollywood the guy says it has a ZZ3 motor & The Rubber Pad on the bottom of the boat :(

Red,
There is a couple very nice Powerstroke Diesels coming across ebay right now that are a lot more fuel friendly than some of the gassers I know.
Just knew you'd want to know. :D

Barhop,
Been years sense we did leak tests on aircraft & would like to hear it also from someone that knows.

Thanx Guys, Gben
 
A leak down test is a set of gauges with a pressure regulator, most tool trucks sell them and so does Jeg’s. You screw a hose into the spark plug hole just like a compression tester then you hook up an air house to the tester. On most motors when the piston is at bottom dead center both valves are closed but on some radical cam motors you may have to remove the valve train due to overlap in the cam. Anyway you set the pressure regulator so the first gauge reads a 100PSI of air pressure then what ever your second gauge reads is you percentage of seal. If your second gauge reads 95PSI then you have 5% leak down that’s normal for a car motor anything over 10% is getting worn out accept for aircraft motors their ring seal is horrendous most aircraft motors I checked are 15% to 20% even fresh rebuilt motors read 12% to 15%. Leak down isn’t as crucial at higher RPM’s but it’s very crucial for throttle response and torque. When you do this test if you hear air leaking out of the header your exhaust valve isn’t sealing good or if you hear air coming out the intake you know your intake valve isn’t sealing well and if air leaking out of the crank case vent you know the motor has a bad piston or rings. You can tell the exact condition of the motor and also narrow down exactly what component is causing the problem I never buy a car or motor without doing a leak down test.
 
Thanx Guys,

Latest boat builder visits resulted in more of same ole story; Buy what we want to sell you or get outa dodge!

What they want to sell me is $60k plus big ones. Not an Airboat but best described as a little Air Ship 24-25ft long & something like 9.5ft wide.
When I said I didn't want a tour boat they said it was not but thats what was needed to hold up the diesel motor that THEY wanted to use. :(

Man come to us & get it just like YOU want it. Not at this place so one more off the list.

I really think he thought I was lying when I answered the question abt hp & tq of the Navistar. It is 250hp & 680tq at 2300rpm. I too would probly need it repeated if I was used to say 450hp & 450tq at 5000rpm for 15-20 years. The prop people too, say what?

If I say 250hp their brain box spits out something that keeps them from hearing when I go on to say 680tq @ 2300rpm. They must think, man this guy is waay outside the box. Reckon who told him that stuff. :(

Just need a prop that matches the continous power tq of 680 @ 2300rpm.
You mean their calculator won't work at less than 5000rpm?
Is anyone versed abt what SAE 2 is? How abt 2.88 throwout bearing, would that help the cause? Would like to know if such a prop will fit the shaft from someone that is not trying to sell me something else.

Thanx, Gben

BTW We're learning even if we can't afford a ship.
 
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