Info Wanted - SCCA 2.96" Stroke Engine

[71GSSDemon]

Does anyone know what the combo was for the Mopar destroked small block for SCCA racing? 340 block, 2.96" stroke crank, T/A heads, but what rod, length and piston/compression height? Mostly looking for information on replicating the rotating assembly.

 

[TT5.9mag]

I’m following this one.

 

[TT5.9mag]

https://www.hotrod.com/features/gurneyized-trans-am-mopars-april-1970-982-714-66-1/

 

[TT5.9mag]

February 1972 hot rod magazine has an article following the build of a mopar 305. I took some pics of the article, hopefully the text is readable.

 

[NC Engine Builder]

Some of that info is a bit obsolete now, you know your bore and stroke, so it's a custom piston, use whatever length rod you want, probably something in the 6.250 range. A short deck R3 block would be ideal, shorter piston height, less weight

 

[273]

Probably had stock length rods, doesn't mean you have too, if you used around a 6.30" rod pretty much can go with stock height pistons.

 

[TT5.9mag]

The rods they used in the hot rod article were “modified 383 rods, 6.360 in length”

 

[Newbomb Turk]

That’s one reason why it was hard to be competitive in limited displacement classes.

The decks are way too tall. A 9.00 deck is too tall for that 3.00 or under stroke.

The R/S ratio gets way too high, you have to spread the LSA so you can get enough p/v without .750 inch valve pockets and the piston is too top heavy no matter what.

It’s amazing the 340 did as well as it did in Pro Stock.

By that time they were using X blocks and milling .500ish off the decks and IIRC the stroke was around 3.25 inches depending on bore sizing.

A 6.36 rod and 2.96 stoke is a 2.15 r/s ratio.

That’s hard to do.

 

[TT5.9mag]

That’s one reason why it was hard to be competitive in limited displacement classes.

The decks are way too tall. A 9.00 deck is too tall for that 3.00 or under stroke.

The R/S ratio gets way too high, you have to spread the LSA so you can get enough p/v without .750 inch valve pockets and the piston is too top heavy no matter what.

It’s amazing the 340 did as well as it did in Pro Stock.

By that time they were using X blocks and milling .500ish off the decks and IIRC the stroke was around 3.25 inches depending on bore sizing.

A 6.36 rod and 2.96 stoke is a 2.15 r/s ratio.

That’s hard to do.

How much material is left after you cut .500 off the deck of an X block?

 

[71GSSDemon]

The rods they used in the hot rod article were “modified 383 rods, 6.360 in length”

So, to reduce the big end diameter from 2.5" to 2.25" did they just face off the split line and rebore?

 

[TT5.9mag]

So, to reduce the big end diameter from 2.5" to 2.25" did they just face off the split line and rebore?

It doesn’t specify but we can assume that’s how they did it. They do mention Carillo having a rod but no mention of specs on that one either.

 

[Newbomb Turk]

So, to reduce the big end diameter from 2.5" to 2.25" did they just face off the split line and rebore?

My assumption was they welded the crank. But that’s a wild assumption guess.

I’m not sure you could cut enough off the cap and rod to get a 1/4 inch off diameter.

If you did the rod would end up significantly shorter with no way to make it longer again.

 

[273]

Maybe they started with rods before any machining was started, might have enough material to allow them get down to size.

 

[Newbomb Turk]

Maybe they started with rods before any machining was started, might have enough material to allow them get down to size.

That’s certainly possible and plausible.

 

[NC Engine Builder]

Or they could have misunderstood

 

[71GSSDemon]

Maybe they started with rods before any machining was started, might have enough material to allow them get down to size.

This makes the most sense. Being the manufacturer, it would be easy to spec some bored at a different size for a special project. We do stuff like that all the time here at work.

 

[71GSSDemon]

February 1972 hot rod magazine has an article following the build of a mopar 305. I took some pics of the article, hopefully the text is readable.
View attachment 1716391350View attachment 1716391349View attachment 1716391352
View attachment 1716391351

Mike, do you have a link by chance? I can't read it

 

[71GSSDemon]

Mike, do you have a link by chance? I can't read it

I did find this...
https://www.hotrod.com/features/blacks-mini-mopar-magic-april-1971-982-818-42-1/

 

[71GSSDemon]

This at least shows how Carillo did it..

 

[273]

I would assume now days you'd turn the crank to a chev size and use one of there aftermarket rods.

 

[71GSSDemon]

I would assume now days you'd turn the crank to a chev size and use one of there aftermarket rods.

LS rods in 6.3 are readily available if the crank is turned to 2.1 and 4" bore pistons. Now just need to get the needed compression height to make it all stack up.

HMMMMMM

 

[Locomotion]

I've seen articles that say Keith Black fabricated the 2.96" cranks from 4340 billet steel, that Mopar sold forged versions and there is a 3rd source that was said to be a crank manufacturer, but I can't recall the name. Regardless of the source, they were said to have 8 bolt crank flanges.

Same variety with rods - B-engine, Carillo & Mopar "Hemi footprint" rods.

Maybe @rumblefish360 can confirm one of the sources.

 

[71GSSDemon]

I've seen articles that say Keith Black fabricated the 2.96" cranks from 4340 billet steel, that Mopar sold forged versions and there is a 3rd source that was said to be a crank manufacturer, but I can't recall the name. Regardless of the source, they were said to have 8 bolt crank flanges.

Same variety with rods - B-engine, Carillo & Mopar "Hemi footprint" rods.

Maybe @rumblefish360 can confirm one of the sources.

Moldex is willing and able to make the 2.96 crankshaft with a 2 or 2.1" con rod big end sized ground on. $5500 with a 20 week lead time

 

[Locomotion]

Moldex is willing and able to make the 2.96 crankshaft with a 2 or 2.1" con rod big end sized ground on. $5500 with a 20 week lead time

Maybe there's an old-timer or family member there that you can ask if they made cranks that ended up in the Trans Am Mopars back then.

 

[71GSSDemon]

Maybe there's an old-timer or family member there that you can ask if they made cranks that ended up in the Trans Am Mopars back then.

I believe Moldex was one of the companies making them. Although this post was for a displacement class racing at Bonneville, the cost is leaning more towards an R5P7 as that would be very close by the time all is added up and they MPH of the records isn't cost effective most likely. Great to have the info though!