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Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

General engine tech -- Drag Racing to Circle Track

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rustbucket79
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Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

Post by rustbucket79 » Thu May 14, 2020 12:23 am

My low 10 second bracket engine will eventually need a refresh and I was kicking around the idea of changing from my 3.75 crank to a 3.875. Crank and rods were purchased in 2006 so they don't owe me anything. Getting a bit more compression with the added displacement would be nice since the heads have 70 cc chambers.

Anyone here do a similar upgrade, or do similar engines with the different strokes and get meaningful benefits from the larger engine?

At 1.62 hp/cu in (my current setup) adding 15 inches seems like wasted effort, but perhaps there is more to it. [-X :^o

lefty o
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Re: Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

Post by lefty o » Thu May 14, 2020 10:38 pm

more cubes is never a bad thing. wont see a large gain from the inches themselves, but upping the compression along with it, heck its only money.

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Re: Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

Post by dannobee » Fri May 15, 2020 2:10 am

Just doing the math, compression is worth about 2% per point. The cu in difference would be worth about 22 lb ft of torque, all else considered.

I'll let you decide if it's worth it.

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Re: Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

Post by RevTheory » Fri May 15, 2020 6:22 am

Assuming you turn the same rpm, will your heads be able to handle the increase? Cam change? Will you have piston-to-valve clearance problems now?

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Re: Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

Post by CamKing » Fri May 15, 2020 2:12 pm

If you leave the cam the same, you will see very little gain in HP, but it will make peak TQ and HP sooner(about 200rpm).
If the heads are the restriction now, that'll also keep it from making any more HP, and it'll nose over sooner.
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Re: Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

Post by rustbucket79 » Fri May 15, 2020 9:48 pm

CamKing wrote:
Fri May 15, 2020 2:12 pm
If you leave the cam the same, you will see very little gain in HP, but it will make peak TQ and HP sooner(about 200rpm).
If the heads are the restriction now, that'll also keep it from making any more HP, and it'll nose over sooner.
I still have your cam recommendation from August of last year, changed nothing for this season other than a broken oil pump pickup. :shock: :shock: we'll have to see what happens this year before making the decision.

Currently trap at 7100/7200, engine peaks at 7600 (what I was pulling to on the dyno, might have peaked higher or at least wouldn't have fallen off abruptly) and have been shifting at 7800 or so, so lowering the powerband would be welcomed. If I change the cubes a cam change will certainly be part of program.

Current ancient Shelf Wisecos had plenty of clearance to deal with running the current cam from 4 degrees advanced to 4 retarded, next set would be Wiseco lil quick 16 pistons as a minimum, customs if one of the manufacturers has a purpose built piston for the large 70 cc chambers with more dome volume.

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Re: Anyone do a 406 to 421 SBC comparison? Results? Worth the change?

Post by bob460 » Sun May 17, 2020 10:30 pm

rustbucket79 wrote:
Fri May 15, 2020 9:48 pm
CamKing wrote:
Fri May 15, 2020 2:12 pm
If you leave the cam the same, you will see very little gain in HP, but it will make peak TQ and HP sooner(about 200rpm).
If the heads are the restriction now, that'll also keep it from making any more HP, and it'll nose over sooner.
I still have your cam recommendation from August of last year, changed nothing for this season other than a broken oil pump pickup. :shock: :shock: we'll have to see what happens this year before making the decision.

Currently trap at 7100/7200, engine peaks at 7600 (what I was pulling to on the dyno, might have peaked higher or at least wouldn't have fallen off abruptly) and have been shifting at 7800 or so, so lowering the powerband would be welcomed. If I change the cubes a cam change will certainly be part of program.

Current ancient Shelf Wisecos had plenty of clearance to deal with running the current cam from 4 degrees advanced to 4 retarded, next set would be Wiseco lil quick 16 pistons as a minimum, customs if one of the manufacturers has a purpose built piston for the large 70 cc chambers with more dome volume.
Nice upgrade for you 420ci 864hp 640tq.....4150 carb.......7500rpm........Higgins heads.....Australia

https://scontent.fmel4-1.fna.fbcdn.net/ ... e=5EE78AB3

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