SB

iTrader: (1)

I bet a pad would do it. The 21 Velocity was one of the faster small boats around even with stock power.

BTW: I did a top end and exhaust on a 20ft boat with 500+hr motor. Did not touch the shortblock. Went from 53/54 stock other than EMI exhaust after playing with many props and etc to a best of 68mph with redoing the top end. Still, stock Alpha. No SS or anything.

I'm mentioning this because you would be starting from a higher MPH and thus you'd probably break 70 with your boat by just doing a top end too.

Something to think about.

tommymonza

I have a 15 year old almost 0 hour 355 built by a known offshore builder here in town. Nothing to crazy and older stock heads with 202s and a shave and some port work. Can't remember the cam but it is .50 lift with a ton of duration so i bought some CMIs years ago too. I am thinking its probably 325 with a stock intake and rejetted Quadrajet- so 65 horse over the stock 260.



I was hoping to see 70 with a 2 1/2 inch raised X and different prop with this motor so an overall gain of 12 mph

The pad is something to do just for an experiment since the boat will be upside down anyways, and I have been thinking about a multi stepped pad for years, actually 25 years long before they were the rage.

What hull were you running and what did you do for cam , heads , intake and carb to get those gains?

Any idea what kinda power you think it made and was the compression all good before you did the upper?

I have kinda lurked on your postings because you are one of the few sbc guys on here.

chrisf695

iTrader: (3)

Just my .02

I have a 89' Powerplay Integra... same hull design as yours I believe. Integra is probably a little heavier considering is a open bow.... But best Ive ever had was 75.4 gps. Has a Alpha SS with a 383 490hp sbc using a 23p mirage plus. But most of the time I get 72-74, the 75 was with without the drive shower and speedo/depth finder handing off the back. At that speed its a handful but one hell of a thrill otherwise. I can cruise all day long at 47-52 at 3300-3850rpm. Ill be trying a 25p this week and might be sending out my 23p to get labbed and get another 2-3mph hopefully.

SB

iTrader: (1)

Ditch the stock quadrajet intake unless it's one of the Bowtie Highrise cast iron Intake.

The list of mods I did is below.
You could run Vortec or RHS or Engine Quest cast iron heads (170-180cc) / Comp XM270HR / and Performer RPM and probably be within 10hp or so of this engine. I used some different from the norm parts just because that's what I do. LOL.

Note: Prices have changed since I installed these.
Note #2: Did not dyno. No reason on a motor like this. Good guess would be 350HP.
Note #3: Did not do compression test. Boat ran as strong as it did when it was new when pulled to do top end.
Note#4: Engine was '91 5.7. Block had provisions for OEM hydraulic roller, but was outfitted from GM/Merc with flat tappet cam.

Carb: BG 650 Vac Sec Marine Sea Demon #1282010VEM (74P/84S 6.5PV/softest Yellow vac secondary spring)
Intake: Weiand #8501 Stealth Air Strike / Was Hard Anodized Locally
Intake Gasket: Fel-pro #1205
Intake Bolts: ARP #434-2101 (stainless 12pt)
Cyl Heads:
Brodix IK180 64cc with w/ Comp >918= beehive springs, retainers, locks, ID locators
Rocker Studs: ARP #134-7101
Guideplates : Comp Cams #4808-8
Cyl Head Bolts: ARP #134-3701 (70 ft/lbs)
Exhaust Gasket: Fel-pro #1404
Head Gasket: Fel-pro #17030 // Head Gasket, Stainless Steel, Composition
Type, Marine, Chevy, 283/ 307/ 327/ 350, Each

Pushrods: Comp Cams #7955-16 Hi-Tech 5/16@ 7.400@
Rockers: Scorpion 1.6 Roller 3/8@ stud
Camshaft: Comp Cams XFI HR #08-466-8 / 268XFI HR-13
268,276 at .006@ / 218, 224 at .050@ / .570@, .565@ with 1.6 / 113LSA / 109ICL
Lifters: GMPP Hyd roller lifter kit (includes dogbones, spider, lifters)
#12371042 List $269.00 Purchase $215.20
Timing Chain: GMPP #12371043 List $60.00 Purchase $48.00
(includes cam both style retainer plates)
T-Cover gasket set Felpro TCS45121
Valve Covers: Proform #141-751 // Tall Black w/red Chevrolet // Baffled
Hold Downs: Proform #141-757 // Black w/red
Fuel Pump: Holley #712-327-11 /// 110GPH , 6.5-8psi, 3/8@ npt inlet/outlet, marine
Oil Filter: Napa Gold (WIX) #1069
Oil Filter Mount /Bypass Eliminator: Canton #22-570
Spark Plugs: NGK BKR7E-11 (V Power) Pre-Gapped at .043@ (1.1mm)

phragle

iTrader: (3)

Imnot a boat designer, nor do I play one on TV. These are just logical thoughts and if I am wrong please correct me.

A step works by by aeration. As the water moves from the step aft it creats an area of low pressure which allows air to be introduced. The pad could definitly yeild benefits it set up correctly, raising the X can help. THe shingle steps in the pad... without an air source to fill the low pressure zone, one would think would cause sticktion and actually be slightly detrimental.

payuppsucker

Tommy, I've read several of your posts here as well as on a blow boat forum. You obviously have more knowledge and hands on time concerning hulls and designs as 95% of the people on here as evidenced by this thread, myself being in the 95%. So what's the purpose of this thread, seriously? Don't take this the wrong way because I'm not trying to be an azz, just being realistic.

tommymonza

Nuthin taken the wrong way. Just tossin around an idea I've have had for 30 years and never acted on. plus something to talk about

The boat needs a regel on the bottom and a transom and stringers anyways., so just thinking while I am there what I can do to improve the hulls efficiency.that's all.

tommymonza

Got me thinking Phragle.

What if the front of the pad had an Aeration tube feeding it from deck level with an adjustable valve.

tommymonza

Great build and thanks for the specifics., Hope you didn't have to type all of that LOL

Just don't have 3 grand more to put towards the motor right now.If I could spend the 1500 I have to spend for materials for my stringer/transom / bottom regel I would buy some new ARP heads or what you suggested and a new intake to start.

But I have to work with what I have for a next o nuthin budget for now but I appreciate your details in your engine build for future mods.

payuppsucker

Well let me know if you need a project, I'll bring my angler cc down for stringers and a transom. It's all torn out, just ready for the install.