What would you build if limited to 372ci - with boost as an option

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71GSSDemon

71GSSDemon

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So, would there be an advantage to destroking a big block, like 400 to take advantage of the bore size, or just use a 360 or 360mag. The 400 could use Max Wedge ports. Does the same opinion hold true when considering boost?

This is for my Bonneville effort. The class I would rather be in would have a limit of 372ci
 
Depends on how much total HP you are looking for. You made need an aftermarket block. Most guys have gone with the cough, cough LS motors for boosted applications as they hold up so well.
 
LO23M8B said:
Depends on how much total HP you are looking for. You made need an aftermarket block. Most guys have gone with the cough, cough LS motors for boosted applications as they hold up so well.
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I will need as much HP as i can get out of the combo in question. Not going LS or anything like that, despite much easier and cheaper.

Just wondering if there would be advantage to having BB heads and architecture vs a SM of equal displacement.
 
A 4.362 bore (.020 over if you can even get that size any more in a decent piston) and a 3.100 stroke will get you 371 inches as I assume the sanctioning body for Bonneville rounds UP displacement once you go .001 over you go up.

The above math says the displacement is 370.6072 so it would round up to 372 inches. Anything other than 3 zeros after the decimal point makes the round up.

All that being said, the 400 piston is so long it’s nuts. The decks on the block are far too tall for the stock 383 stroke let alone a 3.100 stroke.

If you have to go bigger than a .020 over bore you have to reduce the stroke even more.

Sounds like a bad combo to me.
 
If you go with a small block a great option would be picking up on a used W9/R3 motor. They are at 358 cubies and are lower compression and have sturdy blocks. I've seen them from about 12k -15k.
 
Newbomb Turk said:
A 4.362 bore (.020 over if you can even get that size any more in a decent piston) and a 3.100 stroke will get you 371 inches as I assume the sanctioning body for Bonneville rounds UP displacement once you go .001 over you go up.

The above math says the displacement is 370.6072 so it would round up to 372 inches. Anything other than 3 zeros after the decimal point makes the round up.

All that being said, the 400 piston is so long it’s nuts. The decks on the block are far too tall for the stock 383 stroke let alone a 3.100 stroke.

If you have to go bigger than a .020 over bore you have to reduce the stroke even more.

Sounds like a bad combo to me.
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Their max displacement for this class is 372.99.

This is the kind of info I am looking for!
 
How much total vehicle weight are you pushing without a motor?
 
A56 said:
How much total vehicle weight are you pushing without a motor?
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Total weight will likely be in the 5K range, but Bonneville, weight is your friend, high torque is not. There is always a traction issue regardless of speed.
 
71GSSDemon said:
Total weight will likely be in the 5K range, but Bonneville, weight is your friend, high torque is not. There is always a traction issue regardless of speed
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If someone would build a short deck B block you could do what you want and smoke beeotches with it.

You and I would probably be the only ones buying it though lol
 
LO23M8B said:
If you go with a small block a great option would be picking up on a used W9/R3 motor. They are at 358 cubies and are lower compression and have sturdy blocks. I've seen them from about 12k -15k.
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I would love an R motor, but this was merely about advantages or disadvantages of acquiring the displacement in different ways.
 
Newbomb Turk said:
lol you know I hate math but I’ll have to run the numbers.

Pulling the numbers out of my arse I’d say a 9.000 deck would be about as tall as you’d want it.
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That's shorter than a small block, that would be unreasonable lol, nearly 1 inch shorter.
The stroke you mentioned 3.100 with a 7.100 rod bbc journals and a 1.320 ch piston would work with a standard deck height block with .010 to deck square.
Fairly lightweight and rev to the moon, put some -1 or bigger heads with a tunnel ram, nitrous and 2 hair dryers.
 
MOPARMAGA said:
That's shorter than a small block, that would be unreasonable lol, nearly 1 inch shorter.
The stroke you mentioned 3.100 with a 7.100 rod bbc journals and a 1.320 ch piston would work with a standard deck height block with .010 to deck square.
Fairly lightweight and rev to the moon, put some -1 or bigger heads with a tunnel ram, nitrous and 2 hair dryers.
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This is interesting. I was looking at my 383 and run twin 72mm turbos, but under 372 would be a better class. No nitrous as that goes into a Fuel class.
 
MOPARMAGA said:
That's shorter than a small block, that would be unreasonable lol, nearly 1 inch shorter.
The stroke you mentioned 3.100 with a 7.100 rod bbc journals and a 1.320 ch piston would work with a standard deck height block with .010 to deck square.
Fairly lightweight and rev to the moon, put some -1 or bigger heads with a tunnel ram, nitrous and 2 hair dryers.
Click to expand...


You’d have a rod to stroke ratio of 2.29 and if you ran a car with over 270 advertised duration you’d never get the valve notched deep enough.

You could make the piston thicker at the valve pockets but you’re just making the piston heavier than it needs to be.

If it’s an unlimited induction class I’d shoot for a R/S ratio of 1.60-1.65.

If it’s an induction limited class then I’d be lookin at a R/S ratio around 1.80-1.85ish.

That’s IF I could pick the deck height.

That would be a 5.000-5.525 rod for unlimited induction and 5.600-5.75 rod for limited induction.

And whatever deck height I would need I set the pin height at a reasonable position so I don’t compromise ring seal.

My guess of a 9 inch deck may be a bit high for that length of connecting rod.
 
I recommended that platform, because it has everything you need allready designed in. Probably pick up a used one cheaper than building a lowdeck BBM
 
Newbomb Turk said:
You’d have a rod to stroke ratio of 2.29 and if you ran a car with over 270 advertised duration you’d never get the valve notched deep enough.

You could make the piston thicker at the valve pockets but you’re just making the piston heavier than it needs to be.

If it’s an unlimited induction class I’d shoot for a R/S ratio of 1.60-1.65.

If it’s an induction limited class then I’d be lookin at a R/S ratio around 1.80-1.85ish.

That’s IF I could pick the deck height.

That would be a 5.000-5.525 rod for unlimited induction and 5.600-5.75 rod for limited induction.

And whatever deck height I would need I set the pin height at a reasonable position so I don’t compromise ring seal.

My guess of a 9 inch deck may be a bit high for that length of connecting rod.
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Those are all unavailable options though. There is another rod length that is a tad shorter which would make more p/v and since he wants boost that might make it better. I'm not sure if a turbo cam would need to have that much duration
 
doug 371 said:
60 over 361 b engine...Indy top end
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Exactly what I was thinking cheap, got a decent bore to stroke ratio should be able to spin it fairly high if needed to, lots of high flowing head options, even port out stocker can get you pretty far, probable custom pistons the only main negative. 371.95 cid 4.185 x 3.38
 
MOPARMAGA said:
Those are all unavailable options though. There is another rod length that is a tad shorter which would make more p/v and since he wants boost that might make it better. I'm not sure if a turbo cam would need to have that much duration
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True but the R/S ratio is still too high.

I was giving examples of how I’d build it. And that’s the math that comes out.

Which BTW is pretty close to the deck heights in Pro Stock. IIRC they are in the 9.200 range with about a 3.6xx stroke.
 
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