SC V6 or MPI V8?

About inboard or outboard motors.

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Denon Osterman
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Post by Denon Osterman »

somewhere, i read that 6-71 means something like it blows 71 CI of air per revolution into 6 cylinders(units, etc are probs wrong)...but other then that i don't really know, so i couldn;t tell you. not sure what the available range of boost is...i'd think between 5-20 PSI? we were only considering having it mildly blown(so assuming my range is correct, 5-10PSI), nothing too extreme...dad needs to be able to use/drive the boat and feel comfortable doing it, and reliability is of course key. It has to be able to run on either 87, 89, or 91 octane gas...i know if the compression is too high at TDC before the spark the fuel will combust("knock") if its not high enough octane, but surely you can super charge an engine and still have it run on pump gas. when i said "crate engine", i used the term to distinguish between a fully dressed up TKS merc type engine, and one thats built from the ground up using specific parts...so its more like a "crate block", if you will. I imagine that whoever ends up building the engine will know what pistons/crank/everything else to use?

Nova SS
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Post by Nova SS »

Denon Osterman wrote:somewhere, i read that 6-71 means something like it blows 71 CI of air per revolution into 6 cylinders(units, etc are probs wrong)...but other then that i don't really know, so i couldn;t tell you. not sure what the available range of boost is...i'd think between 5-20 PSI? we were only considering having it mildly blown(so assuming my range is correct, 5-10PSI), nothing too extreme...dad needs to be able to use/drive the boat and feel comfortable doing it, and reliability is of course key. It has to be able to run on either 87, 89, or 91 octane gas...i know if the compression is too high at TDC before the spark the fuel will combust("knock") if its not high enough octane, but surely you can super charge an engine and still have it run on pump gas. when i said "crate engine", i used the term to distinguish between a fully dressed up TKS merc type engine, and one thats built from the ground up using specific parts...so its more like a "crate block", if you will. I imagine that whoever ends up building the engine will know what pistons/crank/everything else to use?
The amount of boost you will get out of it will be dependent upon the pulleys you use. The blower can be under driven (turn slower then crankshaft speed and over-driven turns faster then crankshaft speed.)

I was assuming you were buying an off the self motor not getting one built for this specific purpose. That's cool they should be able to select the correct parts to make this thing live and run on cheap(relatively speaking) fuel. Don't be surprised as you might end up with 8-10K (could be more)invested in a durable blower motor.

I have an article on building a 500hp 87 octane fuel motor that might interest you. Its not super charged however. But I suspect it could be built for less BUT it may not be as mild mannered as a 500hp super charged motor. The article is car specific but you would really just need to change a few internal clearances and it would be good to go for a marine motor.

But with your typical supercharger you can expect HP gains in the area of 50%. Sooo if you start with a 350HP build and add a supercharger to it you could resonably expect 500+ hp out of the motor. Thats pretty cool

Denon Osterman
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Post by Denon Osterman »

Duane,

8-10k isn't that bad...lord knows a new 350 merc is more then that(well, close). did some more research on blowers...which would be best for a marine application, a roots or lysholm twin screw? is the 6-71 the right size?

That article would be very interesting...good to know that supercharged engines are mild mannered though! whats the difference between car/marine engines?

50%?! that's ridiculous! we'd probably be starting with a lower HP build, if that's the case, but as you said, that's pretty cool. but surely that 50% gain must wreak havoc with the internals, cooling, etc?

Denon

upspirate

Post by upspirate »

Don't forget that to build a blower motor,you need to "beef" up a lot of stuff if you want the motor to live.


They usually start with a comp ratio of 8 or 8.5 to 1,different pistons forged crank,different rods,rod bolts,pistons,block machining(o-ringed block etc).

Usually when I see a blower on a V/6 in a street rod application,it's a 471.

Most jets from back in the 1960-70's ran big blocks & ran usually around 55-70 mph.

alot easier to just start out with a big block!!

Nova SS
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Post by Nova SS »

well a forged rotating assembly isnt a bad idea no mater what your building so to me that part is a wash. I'm not sure if you need to "O" ring the block for lower boost levels but that is something you would want to look into. Getting a performance balance on the motor is a must as well but then again that would be for any performance build soagain that part is a wash. YA depending on boost anywhere from 7.5 to 8.5 for your static compression ratio would be a good place to start.

If it were mine I'd use either a 6-71 or an 8-71 but of course I believe you would have to use differet pully ratios to keep the same level of boost. BUT its hard to beat the cool factor of a SBC with a 8-71 sitting on top. Couple that with a pair of holleys or injection of some sort and the wow factor is there.

The BBC is definately a beefier place to start but it can be done with a SBC. Now the reason I said that the blower motor would be mild mannered compaired to an equale HP normally aspirated motor is you dont have to go as wild with the camshaft, etc with the Blower motor as the supercharging will still alow it to make goobs of power.

I'm surprised that caber has not piped in on this. Jets and performance motors are right up his alley.

BTW I forgot to add. Marine blocks due to using cool lake/river water for cooling do not heat up like car motors do(read that as they dont expand as much). BUT the pistons in a marine engine heat up at least as much if not more then they do in a car. SO normally to compensate for the lack of expansion in the block you will increase piston to cylinder wall clearance in a marine motor.

here is a web page that shows some of the various piston to wall clearances based on application.

http://kb-silvolite.com/clearance_pop.php

Nova SS
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Post by Nova SS »

here is the article I was talking about. Being able to run a performance motor on cheap swill is a real advantage. The cheaper it is to run the more your apt to use it :wink:

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Nova SS
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Post by Nova SS »

And here is one on how to get 401 cubes out of a 350 block. BTW if you start with a 400 SBC, use the same stroke crank, and bore it .060" over you will have 427 cubes or with a .030" overbore you will be in the 421 cube range.

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rasorinc
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Post by rasorinc »

see my post under this heading re: Crusader Price list. These are the finest engines you can buy and I know they can be had for less, Stan
PS I might be able to save you some real money and there is nothing at all in it for me. Just a favor.
Wood lasts generations.

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Dave Grason
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Post by Dave Grason »

Awsome Duane, it's posts like these that are making this forum better and better. Thank you for those articles.
Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

Nova SS
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Post by Nova SS »

just trying to help out.

My dream motor would be a combo of the two articles. It would be a 427 SBC that make 500 ponies and runs on cheap swill

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Caber-Feidh
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Post by Caber-Feidh »

kens wrote:Just get a MPI Big Block and be done.
http://www.michiganmotorz.com/Marine+En ... cations%29
Going to agree.

Jets need massive hulking gobs of torque in the 3.5-4.5K range. Small engines just don't do it.

I have seen nodular iron cranks hold up as good as anything, cast may be getting a little too weak. Blower drives raise all kinds of heck with the front bearing. an 8-71 takes over 200 HP to puff 15PSI into a 502. All that power is trying to lift the crank into the top bearing. The lifespan is, well, short! Mine will need a new set of crank bearings at least once a season. Normally they are through the babbitt, and down to yellow metal by then.

O ring anything over a peak of 8 PSI at the port, and O-ring anything with an off-idle WOT boost over 4. The low RPM/high boost is a killer for head gaskets.

A 8-71 would just be under driven to turn 1/2 the speed of a 4-71 for the same boost. Best looking setup is the 6v-71 on the SBC, if that matters. (the splayed bolts ae allot nicer to keep torqued too) a 6-71 on a 6-cyl would require an offset distributor...it's just too long.

Don't underrate the little blowers, There is a relic from the '70's on my lake, with a little B&M 144 CID blower on a clapped out 350 from a camaro his kid wrecked. Allot faster than i would have expected. Marinization and blower-prep was a can of bright yellow kryon and a fuel pump. A stock targetmaster motor and a little Holley (b&M quit building them I think) make a great pair, nothing else is needed for reasonable boost levels. Plus they are quiet off boost, no gilmer-whine... good for proper relations with the local constabulary.
http://www.holley.com/types/(Marine)%20 ... Series.asp


It's not only the block but also the heads that need to be built loose. General rule is .004" over in the piston-cyl clearance, and .006" over in the valve guides. It's not so important on the intake, but the exhaust is critical in a huffed engine. The valve temps run well above design spec. It is a result of the added charge, and the little bit of overlap allowing the pressurized fresh charge time to escape, and burn in the port. The valves like to fail about the time you get on the gas hard after a nice slow cruise. (read end of no-wake zone) Man, you just haven't experienced all life has to offer until you stomp the throttle to the floor, hit the giggle-gas, and have a valve impact a piston, bend over and hold the intake open. Now, that nice beautiful scoop doubles as a pretty respectable flame thrower. I smell burning hair again just thinking about it. Engine banging like it's full of hammers, and trying to pat your beard out just destroys the calm.

I have been frozen to the floor... and couldn't get up!

Denon Osterman
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Post by Denon Osterman »

Caber,

how much torque qualifies as "massive hulking gobs"?

is the BB really essential(1200 pounds..yikes!), or will something like this:

http://mercurymarine.com/engines/inboar ... _specs.php

do the job? reading the description makes the motor seem like its allll about torque. strangly, no actual number given. will ask at everyones favorite exhibition exhibit this weekend(the boatshow! yaayy!!!) what the actual figure is.

i think at this point its safe to count super charging out...just because i dont want my boat having any chance of turning into a flame thrower.

the only concern with the big block, besides finding some way to convince my dad, is the extra 400 or so lbs(or so mercury claims...still cant find weight on the michigan motors site!). not only will it affect performance, but more importantly, i'd feel a lot safer leaving the water(and then returning, of course) with only 800 pounds trying to snap my stringers in half, as opposed to 1200...muskoka can be a wavy lake!

Denon

Denon Osterman
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Post by Denon Osterman »

better yet(maybe),

http://www.michiganmotorz.com/Marine+En ... rndrive%29

492 ft lb doesnt sound too bad...

seems to beat the merc in HP, weird how that happens consistently across thier entire product line, especially since the "tow sports" has an extra .2 of a litre helping it out.

Denon

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Dave Grason
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Post by Dave Grason »

Caber-Feidh wrote: The valves like to fail about the time you get on the gas hard after a nice slow cruise. (read end of no-wake zone) Man, you just haven't experienced all life has to offer until you stomp the throttle to the floor, hit the giggle-gas, and have a valve impact a piston, bend over and hold the intake open. Now, that nice beautiful scoop doubles as a pretty respectable flame thrower. I smell burning hair again just thinking about it. Engine banging like it's full of hammers, and trying to pat your beard out just destroys the calm.

I have been frozen to the floor... and couldn't get up!
I enjoyed this paragraph. No, I can't say that I've experienced it but I could sure picture it as I read it. Funny stuff. :wink:

Caber, you sure talk in metaphors. Sorry to be a little slow on the uptake, but could you explain: "I have been frozen to the floor... and couldn't get up!?" Is it anything like ROTFLMAO?
Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

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Caber-Feidh
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Post by Caber-Feidh »

Small block torque curves start higher, are peaky, and in the case of most high HP engines, are "lightswitch" on/off. It comes down to a simple case of physics. In very rudimentary terms, Low end torque (low being the 3.2-3.8K range) is a simple measurement of how much fuel you are burning, and how long the force is being applied to the piston. In both cases the big block dominates any small block. End of story. HP is a measure of how much fuel you can burn fast. High RPM engines are the bane of impellers. Even with a stuffed suction piece, and an inducer the cavitation will be very destructive at 5.5K unless the rest of the hull and pump are well setup, and you are easy on that low speed stomping (what jets are best at!) When you see a $1900.00 custom impeller completely destroyed by cavitation erosion that get's painful!

Well setup a jet pump is a pure power sink, unlike a prop, where you can play with blade-count, pitch, cup and rake to get an engine past a flat spot, when you hit a flat spot in the power curve with a jet, rpm increase stops there. The only way past it is giggle-gas, or a softer impeller cut. If small blocks worked well, then we would see them in at least a few muscle-jets. But we don't. There is something in that fact. A prime example was the olds 455, in 68-70 they had an engine rated at 360 hp, and 500 ft/lbs. You don't ever see any SB engine with a torque number well over the HP. That is hulking gobs of torque.

Any small block can be built, blown, and fogged into high numbers, but a marine engine has to turn out those high numbers for an extended period, not 10 seconds down a drag strip. There is a BIG difference. Besides, you can find a chev crate 502 ready to run for less than you are going to put into an unreliable small block. Some people's grenades have pins, others have life jackets!

Man, it's been so darned cold I couldn't make it out to my office to come visit the forums for a few days. We are talking silly-stupid cold! -44 and a nice fresh 30 mph of wind. Ya, waiting for the global warming! I hope it gets here soon!

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