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Small block RPM
I purchased a 29 non step fountain with twin 396 small blocks in it. Currently has 23 mirage plus on it. Previous owner claimed 48mph at 3200rpm. Gps 81mph. Rev limiters set at 6200. I only ran the boat on muffs and then winterized.
Reading other posts it seems everyone is spinning the stroker motors to 6k. This seems way too high to me. My main goal is midrange cruising. Would jumping to a 25 mirage plus be too much strain at 3200? My goal would be a 50mph cruise with best efficiency. I have the dyno sheets on the motors. Stevehttps://cimg1.ibsrv.net/gimg/www.off...3e7b1b145a.jpg |
If you have the dyno sheets, where is peak torque, would it make sense to drive it the way it is before making changes on assumptions.
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What is your max rpm at top speed. I run the dog sh.t out of a well built stroked 421 sbc and pulls over 7,400 but shift at 7,000 for years with a solid roller. No issues what so ever. As mentioned above I would try and get dyno sheets to prop appropriately but that rpm wouldn't scare me a bit with good internals/valvetrain, drives etc. worse thing you can do in my opinion is over prop and load the engines at WOT. Opinions vary in this however I've always like to prop for a couple hundred rpm under peak hp. Propping near peak torque not good. Free the engine up and roll. Usually 900 to 1,500 above peak torque. Varies on build. |
Btw nice looking set up. |
Here is the sheet from one of the engines. Lookinig at this, how would I determine where the best cruise rpm is at?
Thanks getrdunn, it was spotless till I winterized it. have to clean it up in the spring when I can get the hose in the engine compartment.https://cimg2.ibsrv.net/gimg/www.off...98dd768906.jpg |
Based on what getrdunn is saying I should be propping for closer to 5000.
The engines have Callies rotating assemblies, Inconel valves, stainless rockers, and dart pro 1 aluminum heads. |
That M+ 25 is a big wheel from the 23. I’d maybe try having the current props tweaked for a little more pitch. |
As of now it is screaming for AirGap RPM intakes. I'd love to see the build sheet,thanks.
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Based on the dyno sheet, you should prop it for around 5400rpms WOT with a rev limiter around 5600rpms.
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Thanks Griff, thats what I was looking for. Now I can run it and see where my top rpm is at and start adding prop to bring it down. I am guessing I will end up with 26p Bravo 1.
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Originally Posted by Katanna
(Post 4662866)
Thanks Griff, thats what I was looking for. Now I can run it and see where my top rpm is at and start adding prop to bring it down. I am guessing I will end up with 26p Bravo 1.
Switching from a 3 blade to a 4 blade prop is a different animal. Too many people throw props on without understanding the characteristics of blades and geometry. Getting a "better cruise" speed out of a 4 blade might totally change the characteristics of the boat. My Panther loves 3 blade Mirages, runs good with 4 blade Spinellis, really doesn't handle good in rough water with Bravo 1's and is a death trap with hydromotives. Get use to the handling and characteristics of the boat before making prop changes. See how it carries the bow, how much stern lift there is, does it chine walk or porpoise. Try to borrow props before purchasing. Each prop has it's advantages, I know, at one time I was prop poor with 8 sets on the wall... JMO And my 37 AT likes Bravo 1's |
It would be faster if you prop it to turn more rpm than peak power imo |
Good builds. Rookie has some very good points and can help and save you a lot in the long run. Maybe ck the fountain section out and start asking some questions in regards to your boat, builds, etc. |
Those things need more cam, it`s done by 5200. Spin it to 6K ! :D
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Those are some great engines! I have new 427cid stroker small blocks. Dyno max HP at 652.3 at 6200 rpm. Max torque is 610.2 at 5000. Haven't had in the water yet so we'll see who that translates. It is making 580.9 hp at that same 5000.
I've had a small block 327 cid in a Chevy II all my life and these things like to spin! Looking forward to watching this one! Tom |
I don’t know fountains but the Mitage + seems unusual as they’re very much for bow lift and I believe most fountains need stern lift, but that may be different with the small blocks. As Rookie said, find others with similar set ups as props can make or break a boats performance. |
It should be pretty balanced with the small blocks and aluminum heads. I have a set of new 25 Mirage+ and will start there. It has 23s on it now. My old Magnum sport ran very good with 280 small blocks spinning 23s through TRS. When I compare this to my current boat it just seems like a no brainer requiring a bigger wheel with more horsepower, no transmissions and Bravos.
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I think you'll be surprised how hard the 25 m+ is to spin
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Originally Posted by ttrm007
(Post 4663004)
Those are some great engines! I have new 427cid stroker small blocks. Dyno max HP at 652.3 at 6200 rpm. Max torque is 610.2 at 5000. Haven't had in the water yet so we'll see who that translates. It is making 580.9 hp at that same 5000.
I've had a small block 327 cid in a Chevy II all my life and these things like to spin! Looking forward to watching this one! Tom |
The cams have been mentioned a couple times so here is the card. Pretty mild.https://cimg3.ibsrv.net/gimg/www.off...850cecea5a.jpg
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Wow.
Tow truck cams.:wink: Have I mentioned Air Gap RPM intakes yet ? :) |
Originally Posted by Katanna
(Post 4663626)
The cams have been mentioned a couple times so here is the card. Pretty mild.https://cimg3.ibsrv.net/gimg/www.off...850cecea5a.jpg
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Originally Posted by SB
(Post 4663645)
Wow.
Tow truck cams.:wink: Have I mentioned Air Gap RPM intakes yet ? :) Power curve looks good. Those good flowing heads helped that baby cam to make power into the low 5000 range. There are always ways to make more power. If your happy, fill it up and enjoy!! Motors look good BTW!! |
Originally Posted by SB
(Post 4662849)
As of now it is screaming for AirGap RPM intakes. I'd love to see the build sheet,thanks.
Originally Posted by SB
(Post 4663645)
Have I mentioned Air Gap RPM intakes yet ? :)
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Originally Posted by Rookie
(Post 4663788)
If I find 2 Air Gap RPM's on the cheap, I'll buy them just to test back to back in a boat.
Same setup rebuilt to a 385ci, AFR 195R, 234/240 cam. Portmatched rpm AirGap to AFR 1205 gasket. Loved 6000+ rpms... IMO.... Under 6000rpms an RPM Air Gap is your friend... |
Originally Posted by Rookie
(Post 4663788)
If I find 2 Air Gap RPM's on the cheap, I'll buy them just to test back to back in a boat.
I recommend vic jr on many 502's where others recommend air gap rpms's. One engine is not another. |
I'm just curious how your going to like the 29 ftn in rough choppy water in comparison to the 27 magnum you previously owned. Btw what rpm did the 23's run at WOT. If in line with your peak hp I don't think your going to like the response in the boat. I've been down that road numerous times. More than likely your going to find it to be sluggish in comparison to the 23's unless your WOT rpm is way above 5,300 which I doubt very much. It's all about what you like to do and taylor to your needs. BUT.... I'd bet anything with the right cam and a little head work you could spin your 25's about 5,500 or the 23's 5,900 or so and see some nice speed gains. The difference btwn the 23's and 25's is probably close 400 rpm plus or minus. Probably could even be done without head work. BTW where did I miss what intakes your using now. I will look for cam in my 421 sbc. It's the king of cams for that build. Unbelievable torque. |
The Dart intakes are on the build parts sheet listed on page one under additional parts. It currently has 6200 rev limiters in it. With two in the boat low fuel I am able to hit the limiter. This is why I questioned where the motor should run based on the dyno sheets, thats beyound my expertise. I know from propping stock 350 mags you should be 46-4800. Sure I could prop down and spin 5000+ but thats not the happy area for the motor. Having more power and torque, I just feel like i should be spinning a bigger wheel. You guys have given me the info needed. When its time for freshening, I may bump the cams up. Currently has 30 hours on the build.
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