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406 / 421 sbc builds
Anyone care to share their build experiences with a 400 small block chevy? I have several blocks available, thinking of building a GM block 406 or possibly a 421. Wonder who has built one and if they'd do it again. Would build similar to a couple 383 builds that I have done. 10:1 give or take a few points. Aluminum heads, probably IK-200 Brodix. RPM air gap with a holley 780 vac. secondary I rebuilt for marine use last year. I have looked at several cams from Howards that I like. The 110525-12 is a 112 LSA with 227 IN / 233 EX and .560 lift with 1.5 rockers, or the 110255-12s is a 231 IN / 235 EX and .541 lift with 1.6 rockers. Trying to build it for 55-5600 rpm and cruising around 38-4000 rpm. The 2nd cam is really close to the comp XFI 280 cam. Just curious how many sbc 400's are out there and how you built them / what you'd do different?
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Not many use them in boats. What kind of exhaust do you have? |
Exhaust is GLM manifolds with reworked TRS risers that are extra height and inner pipe extended back 10” from std. cast risers. Side exit thru Hull, can extend the inner further but will involve turning the inner pipe 90 degrees inside the outer elbow. The cams I’m looking at I think I’ll be okay on reversion as is. Just don’t know if water introduced at that point will keep me from reaching rpm potential of the motor. |
I did a 406 years ago that went in a Donzi but it was old school and not a real high horsepower deal. It held up very well. I have done several 421's over the years for cars but not for marine use. I don't think I would consider one without an aftermarket block. Probably would say the same for the 406 now days.
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The 421 I have in muscle car is a torque monster. Originally built a 383 years ago and not even a comparison to the 421 with brodix heads and single plane brodix intake. In fact I have to swap out 411 to 355 gears. I'm just waiting usable power. It is an endurance solid roller circle track cam. The car is well maintained however gets driven extremely hard and has held up great with no issues. Recently went back to turbo 400 cause I couldn't keep the 350's together. I wouldnt change a thing on this build for marine use other than marine spec's. |
Saw a mild 421 on the dyno not long ago, 520 hp, 510 torque, the owner said it was all leftover parts.
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Originally Posted by BillK
(Post 4664677)
I did a 406 years ago that went in a Donzi but it was old school and not a real high horsepower deal. It held up very well. I have done several 421's over the years for cars but not for marine use. I don't think I would consider one without an aftermarket block. Probably would say the same for the 406 now days.
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I have a SHP 4.125x3.875 (414). Cam spec similar to your second. 460hp@5500 with 8.4:1 comp, 615 with 6lbs boost.
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I built a 406 four years ago that has been trouble free since I got the EFI dialed in. My 22 foot bow rider is not really a performance hull so the nine mph top speed increase that I gained over the stock 350mpi is probably less than what other hulls would have picked up. Even with six people on the boat, it will pop out of the water pretty quickly at 22 to 24% throttle position. With three people I only need 20% and can tow tubers at only 16% throttle. The Holley system has a digital dashboard that shows throttle position. I remember looking at the first cam that you had named before I picked the next step below. (Howard’s 180325-12) I felt that it better matched my conservative 9.25:1 compression. I wish that I would have gone with a half point higher. I can’t get away from the habit of buying good fuel for it. I strongly agree with the other recommendations above to start with an aftermarket block. There are too many inconsistencies in the factory blocks. Here is a link to the build thread that I wrote several years back. https://www.offshoreonly.com/forums/...efi-build.html Pee Wee |
Originally Posted by Pee Wee
(Post 4666722)
I built a 406 four years ago that has been trouble free since I got the EFI dialed in. My 22 foot bow rider is not really a performance hull so the nine mph top speed increase that I gained over the stock 350mpi is probably less than what other hulls would have picked up. Even with six people on the boat, it will pop out of the water pretty quickly at 22 to 24% throttle position. With three people I only need 20% and can tow tubers at only 16% throttle. The Holley system has a digital dashboard that shows throttle position. I remember looking at the first cam that you had named before I picked the next step below. (Howard’s 180325-12) I felt that it better matched my conservative 9.25:1 compression. I wish that I would have gone with a half point higher. I can’t get away from the habit of buying good fuel for it. I strongly agree with the other recommendations above to start with an aftermarket block. There are too many inconsistencies in the factory blocks. Here is a link to the build thread that I wrote several years back. https://www.offshoreonly.com/forums/...efi-build.html Pee Wee Pee Wee, Where does that cam top out in your 406? Did someone recommend to you to split your rocker ratio? I would think with a 406 and that cams duration split you would benefit from 1.6 rockers on each. Thank you for sharing your build info, much appreciated. |
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