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View Full Version : How much HP can a Velvet Drive 71c?


dpcsar
08-10-2011, 01:25 AM
Does anyone know how much HP a 71c velvet drive can handle? I have checked velvetdrive.com and the only thing it shows is a graph, which tops out at 360 HP.

I am curious as I am rebuilding my engine and the HP is going to much higher than the stock 240 HP. ('89 Prostar)

j.mccreight@hotmail.com
08-10-2011, 02:07 AM
All you can go off is what they rate it at, unless you trial and error, if you get the 72 series I think those are rated over 400 hp.

http://www.simplicity-marine.com/velvetdrive-borgwarner.htm

j.mccreight@hotmail.com
08-10-2011, 02:19 AM
If you are doing the standard upgrade modifications, heads and intake to get around 300 hp it will hold up, if your going to be over 400 hp I don't think the 71c will last very long.

89BigBlock
08-10-2011, 11:52 AM
Upgrade! Nice. 300hp is fun :D

dpcsar
08-10-2011, 04:57 PM
It is looking more like I am going to be over 400 HP. The engine is rebuilt as a 393 stroker. I didn't plan on doing the top end at this time, which would have put the engine at ~350 HP. The top end is getting redone now because I don't want to have custom push rods (standard ones are not working). And by the time I do custom push rods I might as well bite the bullet and get heads with adjustable rockers. Overall, it is getting a mellow cam, for some economy, but will be in the 450-500 hp range. :S... Guess I should start looking for a 72c to replace the 71c. It is going to out sooner with the extra stress.

j.mccreight@hotmail.com
08-10-2011, 05:55 PM
You do relies just because you have more horsepower your boat is not going to go faster, it's just going to get there faster. It just might pickup 1 or 2 mph because it can breath a little better hence the max rpm might increase by a couple hundred rpms but not anymore than that.

03 35th Anniversary
08-10-2011, 06:46 PM
You do relies just because you have more horsepower your boat is not going to go faster, it's just going to get there faster. It just might pickup 1 or 2 mph because it can breath a little better hence the max rpm might increase by a couple hundred rpms but not anymore than that.

If he does not do anything with the prop then more than likely you will be correct.

dpcsar
08-12-2011, 05:53 PM
I know it will not get there faster, unless I re prop it. :)

The motor was a "why not" type thing. I bought the boat with a broken block. So my friend that I am trading work for said, "why not make it a stroker"... So I did. The stroker crank only cost $250 more. All other parts had to be bought anyway. Looking back, if I was just trying to save money, I would have just rebuild to a stock motor.

j.mccreight@hotmail.com
08-13-2011, 10:55 AM
72C

http://www.halemarine.com/index.php?main_page=index&cPath=5_10

brat
08-13-2011, 12:13 PM
500 HP outta make for a smooth idle......

TRBenj
08-15-2011, 05:30 PM
The Velvet Drives are pretty stout. According to my tranny guy, you can put all the hp you want through them... its when you start trying to turn a bigger prop that will stress the transmission.

Of course, with more cubes at your disposal, you will want to go with a bigger prop- but I doubt you will want to upsize much past 13x14 (assuming you have a 1:1), even with a 393. Thats an appropriate size for a big block 454. The 71C should handle that just fine.

Im skeptical of the "450-500hp" claim if youre going with a "mellow" cam. There are lots of heads out there with adjustable valvetrain, and not all are created equal. If you know what youre doing, strokers can make big power. Then again, if you dont know what youre doing, its easy to build a 250-300hp motor that falls on its face over 4500 RPM. Hopefully you have matched your components together properly!

If the combo comes together nicely, and you prop it correctly, then you will be looking at much improved acceleration AND top end. Whoever says you wont gain top speed it out to lunch. I can point you towards a few Correct Crafts flirting with the 60mph mark if you need some proof!

j.mccreight@hotmail.com
08-15-2011, 09:29 PM
So your saying in a 1:1 direct drive with a small block stroker with a few extra hp the engine will able to pull 6k rpms in the water under load with all the drag these boats create the angle of the prop pushing the bow down it will get close to 60 mph? It would take some serious hp and rpms to overcome all this. In a direct drive rpm is relative to mph and you would need to change to a larger prop to go faster. Im not saying it can't be done its just going to take a race quality engine and a boat load of money to do it.

I think however with all the standard upgrades that people do ie pheads, intakes, carb work, ei its very possible to get close to 50 mph with the correct prop, mostly because the engine is breathing easier and able to turn 300-500 rpms more under a load meaning an increase in a couple mph which I did say he will get, just because you have more cubic inches doesn't mean more hp. We will have to wait and see how more speed the op gets out of his new combination.

My boss has a 35ft offshore motion cat race boat that has twin turbine engines that each have 1000 hp and he barely gets 105 mph out of it, drag is a big problem in boats.

TRBenj
08-17-2011, 02:02 PM
So your saying in a 1:1 direct drive with a small block stroker with a few extra hp the engine will able to pull 6k rpms in the water under load with all the drag these boats create the angle of the prop pushing the bow down it will get close to 60 mph?
Im not sure I ever said just "a few extra hp" but the rest is correct.

It would take some serious hp and rpms to overcome all this.
Yes it does!

In a direct drive rpm is relative to mph and you would need to change to a larger prop to go faster.
Maybe, maybe not. If adding cubic inches (stroker), then you get more torque at any given RPM, so propping up (more pitch) may be necessary. When modifying the top end components ONLY, most of the time Ive found that the stock prop size tends to work pretty well until you get past the 1hp/ci range, and then going to a larger prop may be beneficial. Of course, every engine and hull combination is different- its all a game of matching WOT RPM's to the motor's powerband.

Im not saying it can't be done its just going to take a race quality engine and a boat load of money to do it.
"Race Quality" is a term Im unfamiliar with, but I do agree that making these boats fast is not necessarily an inexpensive undertaking.

I think however with all the standard upgrades that people do ie pheads, intakes, carb work, ei its very possible to get close to 50 mph with the correct prop, mostly because the engine is breathing easier and able to turn 300-500 rpms more under a load
Youre absolutely correct. The common GT40p/Performer (or RPM) intake/Cam Research cam top end upgrade on a Ford 351w makes for a fantastic ski boat motor. Big time improvements in holeshot, midrange, and 400-600 RPM gain up top with the same prop... which roughly translates to upper 40's for top end in a 19' ski boat. I built this combo about 5 years ago as a "holdover" until I built a proper stroker... and its performed so well that I plan to keep running it until it breaks. (Ive found better hulls to flirt with 60mph anyways!)

just because you have more cubic inches doesn't mean more hp. We will have to wait and see how more speed the op gets out of his new combination.
I think you'd have a tough time not gaining any hp if you grow displacement over 10%. But, like I said, a group of mismatched components could result in a 280hp engine that falls on its face above 4000 RPM. On the other hand, a well matched combo might make something like 500hp at 5500 RPM. I would say that a 400hp @5000 RPM combo should be pretty straightforward to build- and would result in a real nice ski boat motor.

My boss has a 35ft offshore motion cat race boat that has twin turbine engines that each have 1000 hp and he barely gets 105 mph out of it, drag is a big problem in boats.
Dont I know it! Add enough hp though, and you can overcome the effects of drag, to an extent.

Jorski
08-17-2011, 02:11 PM
It's been done before...

For reference:

***2008 MALIBU CORVETTE LIMITED EDTION SPORT V with Corvette Z06 package and LS7 Engine pumping out 505hp..THIS IS The #2 OF 25 BUILT WITH THE Z06 ENGINE.This exclusive malibu Corvette Sport-V limited edition is the fastest inboard on the water with a top speed of over 60mph,.... Windows that roll. ***

http://www.powerboatlistings.com/cgi-bin/listings/db.cgi?db=default&uid=default&manufacturer=MALIBU&view_records=1&sb=date&so=descend

TRBenj
08-17-2011, 05:34 PM
It makes sense that the Corvette is topping 50mph since its a v-drive. All the weight in the rear should really help get the hull out of the water... direct drives tend to speed up a bit with extra weight in the back seat, bringing the nose up. If wakeboard boats werent designed to displace as much water as possible, and hold as much weight as possible (and are propped accordingly) then they would surely be faster than their direct drive counterparts... and in some cases, they still are.

My comments above were made with the smaller (18-20') direct drives in mind... though v-drives should, in theory, have more potential- at least when it comes to top end speed. Holeshot would surely suffer, though.

LYNRDSKYNRD
08-17-2011, 05:41 PM
****Thread Jack****
How fast should a stock '87-'89 PS190 w/351 run on top end?

j.mccreight@hotmail.com
08-17-2011, 06:29 PM
45 according to manual

LYNRDSKYNRD
08-17-2011, 06:32 PM
Thanks

***end of thread jack***

dpcsar
08-22-2011, 09:10 PM
The Velvet Drives are pretty stout. According to my tranny guy, you can put all the hp you want through them... its when you start trying to turn a bigger prop that will stress the transmission.

Of course, with more cubes at your disposal, you will want to go with a bigger prop- but I doubt you will want to upsize much past 13x14 (assuming you have a 1:1), even with a 393. Thats an appropriate size for a big block 454. The 71C should handle that just fine.

Im skeptical of the "450-500hp" claim if youre going with a "mellow" cam. There are lots of heads out there with adjustable valvetrain, and not all are created equal. If you know what youre doing, strokers can make big power. Then again, if you dont know what youre doing, its easy to build a 250-300hp motor that falls on its face over 4500 RPM. Hopefully you have matched your components together properly!

If the combo comes together nicely, and you prop it correctly, then you will be looking at much improved acceleration AND top end. Whoever says you wont gain top speed it out to lunch. I can point you towards a few Correct Crafts flirting with the 60mph mark if you need some proof!

Thanks for the information. It is a 1:1. Currently I have two props. a 3 and 4 blade both are 13x13. They need repairs, and the repair guy says that he can pitch them to 13x14. I can't find anything with more pitch in a 13. I am not sure a 14 will fit, but will look into it.

I am not doing the rebuilding, it is being professionally done. The guys is a friend, so we are trading work. :) Anyway, I was shooting for about 350 HP. Just do the bottom really strong now with all the upgrades in the bottom to make it last longer. Then the top go added to the list because it is a pain with out adjustable rockers, so getting that done also. Anyway, everything is matched and we paid a little more for the better parts. He describes it as a drag engine minus the cam for the 6-7000 rpm top end. The only problem I may have is the Holley 4160 not having enough CFM. Stock is 600 CFM, and the motor may need the 750 CFM version. :( I am not sure if it can be re-jetted to the 750 or if there is more to it. I am finding that out while I am waiting for the correct heads and intake to come in.

Really hope to get it on the water this year, but with all the little things not matching up, it is hard to say. Parts are still 1-2 weeks out. The heads have to go on and then we will have to have the push rods measure for correct length and then order them.

I will let you know how great it is :D

TRBenj
08-23-2011, 05:27 PM
I would not repitch a prop to add an entire inch. Thats probably the quickest way to turn a prop into a inefficient piece of scrap metal. There should be plenty of choices for LH props in 13x13 and larger sizes. I dont know model numbers though, sorry- my boats all spin righties.

A "drag engine minus the cam" sounds like a lousy ski boat motor to me. Hopefully your friend has a clue, but I wouldnt think so based on a comment like that. Shooting for 350hp is an extremely conservative goal- I sure couldnt imagine spending money on a full 351w stroker rebuild for a hp level like that.

You cant rejet a carb for "more CFM". Carbs are rated for how much air they can ingest (measured in Cubic Feet per Minute), not how much fuel they flow (which is what the jets do). A 600cfm carb would feed a 350hp engine just fine. A 750cfm would be way overkill. You may benefit from something like a 650cfm, but only if youre going to be spinning it above 4500... which doesnt sound like the plan.

DooSPX
08-23-2011, 05:39 PM
I agree with TRBenj as usual. Listen to the this man.

dpcsar
08-23-2011, 07:32 PM
I would not repitch a prop to add an entire inch. Thats probably the quickest way to turn a prop into a inefficient piece of scrap metal. There should be plenty of choices for LH props in 13x13 and larger sizes. I dont know model numbers though, sorry- my boats all spin righties.

A "drag engine minus the cam" sounds like a lousy ski boat motor to me. Hopefully your friend has a clue, but I wouldnt think so based on a comment like that. Shooting for 350hp is an extremely conservative goal- I sure couldnt imagine spending money on a full 351w stroker rebuild for a hp level like that.

You cant rejet a carb for "more CFM". Carbs are rated for how much air they can ingest (measured in Cubic Feet per Minute), not how much fuel they flow (which is what the jets do). A 600cfm carb would feed a 350hp engine just fine. A 750cfm would be way overkill. You may benefit from something like a 650cfm, but only if youre going to be spinning it above 4500... which doesnt sound like the plan.


The re-pitching and rebuild of the prop will be done at a prop shop. I think it is a good idea as a new prop is ~$300+ while the rebuild is much less ~$150. I can do one of the two I have and see what happens and then decide what to do with the other. Both need the rebuild, they both are dinged up pretty good. I would like to turn a higher pitch as the engine should have more than enough torque to keep a decent hole shot. As long as I am not always going ball out, I should get a little better GPH and not spend so much time at the pump. ;)

Overall I should have 450-500 HP in the stroker (mild). He is a cam guy and spun the cam to be mild and provide a lot of low end torque with a max rpm of 5500-6000. A drag cam uses more RPM, which is not good in a boat. The guy has been turning cams for years, and for all different motors. I didn't want a racing cam because I don't need that much HP. I am used to outboard motors and know that a 19' boat needs 140+ HP with a 19 pitch prop to pull a 200+ lbs solemn skier out of the water without dragging them for a long time. So anything over 200 HP is good for me. 350 HP was easy to make on the rebuild without going crazy in parts. 350 is a stroker with stock top end. Since the stock top wasn't adjustable, we ended up doing the top. I don't need the 450+ HP. All I wanted was a good working engine that was going to last for years. All the upgrades to get it to last longer (rollers every where) with the stroker and better adjustment to dial it in are creating the extra HP. We went for the best parts for the money, ones that were better, like forged pistons rather then cast.

Thanks for the information about the carb. I take it I may need a new carb with higher CFM. My friend told me the motor may starve out at higher RPM. :(

j.mccreight@hotmail.com
08-23-2011, 07:43 PM
Spend top of the line money for the engine and nothing on a prop. How will all that hp propel your boat, sounds assbackwards to me. since you metioned drag racing imo kinda like having a pro-stock running on a set tires for the prius. Btw what are you going to do with the transmission?

dpcsar
08-23-2011, 08:14 PM
Spend top of the line money for the engine and nothing on a prop. How will all that hp propel your boat, sounds assbackwards to me. since you metioned drag racing imo kinda like having a pro-stock running on a set tires for the prius.

The prop needs to be matched to the HP. I can't play with the pitch until I have the motor finished in the boat. From what I know, you want the WOT (wide open throttle) to be under you red line (<6000). If your WOT is allowing the motor to reach the red line then you can go higher pitch and get more speed out of the boat. What you loose is hole shot with higher pitch. So the pitch of the prop really depends on what you want for the HP/torque you have. I am looking for a balance. I want a hole shot that will pull a 250+ lbs solemn skier out of the water, but isn't sucking gas like crazy. Increasing pitch allows a lower RPM when cruising.

When I know what prop gives me what I like, then a new one is on the list of things to buy. :) I might be able to borrow one or trade for the right pitch. I have been told by the prop guy that it is trial and error. ;)