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What RPM should I be at WOT?

Old 07-08-2020, 10:58 PM
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Default What RPM should I be at WOT?

5.7 Vortec
Quadrajet Carb
RPM Airgap Intake
XM270 Cam 218/224 - 112LSA
Corsa Captain Call and factory style manifolds/risers
21 Prop
Alpha 1 Gen 1 Drive
86 Scarab 1 - 21.5 ft boat.

Prior to rebuilding the carb the boat wouldn't have enough fuel to rev to 5K. Post rebuild I'm seeing 5K relatively easily but nervous to push beyond that. The cam says 1200-5200RPM range so I'm figuring I'm near the top of the power it will make but still curious to mash it. Will it live?

Does the thunderbolt ignition system have a rev limiter? Also, any guesses on top speed with this setup?

Thanks!
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Old 07-09-2020, 12:36 AM
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The cam is spec'd at 1500-5500 rpms. https://www.summitracing.com/parts/cca-k12-417-8

There should not be any issue spinning to 5200-5400rpms.
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Old 07-09-2020, 06:17 AM
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There is no rev limiter with the Tbolt IV ignition.

Griff is correct with rpm range, however, experimenting with props may changevthis a bit.

Small 20ft hoat I tune on/work on with stock 5.7 shortblock, Comp XFI 218/224 , Brodix 180 heads, Eddie Marine Exhaust, will go 65 mph stock Alpha, 68mph stock Alpha with Hydromotive nose cone, 23’ Stiletto prop. 5400 rpm or so. Best of 72.8 with Alpha SS drive and labbed 21” Cleaver justi over 6k. Obviously needs larger prop with this drive but it get’s there. This engine has plenty of valvespring though.

About 20 gals fuel and just driver.

5.7 small blocks, even when modded don’t like larger diameter props like Mirage/Mirage +’s/Bravo’s etc. They do much better with smaller diameter props like Turbo’s , Merc Laser II, some Hydromotive’s, and etc.

Anyway, you might end up flipping between 21” and 23”’s like this boat depending on type of ride planned fir day and water conditions.

Last edited by SB; 07-09-2020 at 06:44 AM.
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Old 07-09-2020, 06:33 AM
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Originally Posted by 1986Scarab1 View Post
5.7 Vortec
Quadrajet Carb
RPM Airgap Intake
XM270 Cam 218/224 - 112LSA
Corsa Captain Call and factory style manifolds/risers
21 Prop
Alpha 1 Gen 1 Drive
86 Scarab 1 - 21.5 ft boat.

Prior to rebuilding the carb the boat wouldn't have enough fuel to rev to 5K. Post rebuild I'm seeing 5K relatively easily but nervous to push beyond that. The cam says 1200-5200RPM range so I'm figuring I'm near the top of the power it will make but still curious to mash it. Will it live?

Does the thunderbolt ignition system have a rev limiter? Also, any guesses on top speed with this setup?

Thanks!
2 or 4 bolt mains?
The Vortec HD version 350 uses a 4-Bolt main block, upgraded exhaust valves and cylinder heads forheavy duty applications.
You can take the serial numbers of the block and usuallt look on the chart to see but some say either...
the 4 bolt would hold up better over 5k rpms. I like to prop most small blocks at or below 5k for longevity.
the ignition control module rpm limiter but im not sure what rpm. Maybe some one can chime in with that info.
​​​​​​​there are aftermarket add on rpm limiters...
good lick
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Old 07-09-2020, 06:53 AM
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Originally Posted by oconnor marine View Post
the ignition control module rpm limiter but im not sure what rpm. Maybe some one can chime in with that info.
I did chime in with the info. Lol. I’ll be more specific There isn’t one on the Tbolt IV used on black motors. There is an added on ‘over speed’ module on the Merc racing blue motors that use the Tbolt IV ignition.
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Old 07-09-2020, 12:36 PM
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Originally Posted by oconnor marine View Post
2 or 4 bolt mains?
The Vortec HD version 350 uses a 4-Bolt main block, upgraded exhaust valves and cylinder heads forheavy duty applications.
You can take the serial numbers of the block and usuallt look on the chart to see but some say either...
the 4 bolt would hold up better over 5k rpms. I like to prop most small blocks at or below 5k for longevity.
the ignition control module rpm limiter but im not sure what rpm. Maybe some one can chime in with that info.
there are aftermarket add on rpm limiters...
good lick
There is NO rev limiter on a TB IV. The only engines with TB IV's that had rev limiters were Merc Racing/Blue engines and it was an add on component.

TB V's do have a rev limiter built in to the module and that rev limit varies per application.
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Old 07-09-2020, 01:25 PM
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7500 always sounds good
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Old 07-10-2020, 02:31 PM
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Prop for the higher end of the rpm range or slightly over so you have good throttle response and the trim factor. Lazy props/rpms suck as far as I’m concerned.
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Old 07-13-2020, 11:27 PM
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Originally Posted by SB View Post
There is no rev limiter with the Tbolt IV ignition.

Griff is correct with rpm range, however, experimenting with props may changevthis a bit.

Small 20ft hoat I tune on/work on with stock 5.7 shortblock, Comp XFI 218/224 , Brodix 180 heads, Eddie Marine Exhaust, will go 65 mph stock Alpha, 68mph stock Alpha with Hydromotive nose cone, 23’ Stiletto prop. 5400 rpm or so. Best of 72.8 with Alpha SS drive and labbed 21” Cleaver justi over 6k. Obviously needs larger prop with this drive but it get’s there. This engine has plenty of valvespring though.

About 20 gals fuel and just driver.

5.7 small blocks, even when modded don’t like larger diameter props like Mirage/Mirage +’s/Bravo’s etc. They do much better with smaller diameter props like Turbo’s , Merc Laser II, some Hydromotive’s, and etc.

Anyway, you might end up flipping between 21” and 23”’s like this boat depending on type of ride planned fir day and water conditions.
Thanks to all for the feedback. I think the prop may be worth experimenting with at this point. I really don't know much about them, should I ditch the mirage 21 pitch prop for a different brand as quoted? Should I also look at adjusting the pitch as well?

Thanks!
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Old 07-14-2020, 01:35 AM
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Originally Posted by 1986Scarab1 View Post
Thanks to all for the feedback. I think the prop may be worth experimenting with at this point. I really don't know much about them, should I ditch the mirage 21 pitch prop for a different brand as quoted? Should I also look at adjusting the pitch as well?

Thanks!
First thing is run what you have to get some baseline numbers
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