Cubic inches and boost

Okay fellas it's time to find a rotating assembly for the Hellcat block. This engine will see boost between 12 to 18 lbs, so it will make some power. ( I hope, anyway lol )

I understand that a boosted engine does not need to be too big cubic inch wise and I figure I have two choices from the Molnar catalog:


3.593 stroke with a rod ratio of 1.7:1 making 377 cubic inches

3.795 stroke with a rod ratio of 1.61:1 making 398 cubic inches


Overlap on the smaller crank would make it stronger as far as I can see, but what do you guys think? Either combination is going to use the " Turbo" rod which is stronger.

Use what you can still use a 6.20 Rod with IMO. Either of those in that case. The 3.58 could even do a stock Rod length or even jump to a 6.30.

I calculated these with 6.125 rod. Any reason 6.2 is better? Just trying to understand. Thanks

Less is more IMO.
Mopar brought the 6.4 down to 6.2 for the hellcat for a reason and I don't believe it's emissions related.
Although I'm new to the Gen 3 stuff but the mains are probably gonna start bouncing with a bigger stroke.

Longer rod= less Rod angle= lower thrust loading of cylinders which can also let the block take more power.

Thank you gents. Looks like I will be building a 377.

I'd go for the smaller displacement

Anybody know the deck height for the Hellcat?

I would call a couple of piston manufacturers and see what they say about the needed ring pack and pin height required for the boost and cam you want to run. The more boost and the bigger the cam, the thicker the piston, etc. Ditto on the rods and crank. Sonic check the block. Then I would try to bend the ears of a couple of guys pushing these motors hard with boost to see just where the problems come up an how to build the motor to take it. You never know where this project will end up years down the road. Speed is addictive, plan for more boost/hp/cam now inside your budget. I look back at my last drag motor build and know that had I done it differently, I could be pushing 1100 hp instead of 880 hp for a few thou more. Good luck with it.

Good advice, I need to get some more info. Thanks!

Decks on Gen III Hemis are all 6.239". Would love to see a 6.5" version.

Goodness gracious, would love to see how those 6.300 long rods that you recommended would work in such a short deck height lol
But I thought deck heights were as follows:
5.7L & 6.1L=9.252
6.4L & Hellcat=9.284

No replacement for displacement, I'd go the longer stroke. It's more torque for the same $$. I have a boosted 6.1

Yeah I was still in rod length land lol! They're not all the same at 9.239". I guess the 5.7 is 9.25" and 6.4/6.2 is 9.284". I figured people swapping 6.4 and 6.1 intakes back and forth, they were the same. Learn something new everyday.

If I use the pistons I currently own, coupled with a 6.200 rod and a 3.795 crank, I will have 398 Cubes and the pistons will be in the hole .0225

Any opinions on this? Should the block be decked for a zero deck engine?

This will be using hellcat heads and I am shooting for a compression ratio of 9.5:1

If I did my math right they will be 9.53:1

Depends on if it's a Procharger, Magnuson or turbo. That's fine for a Magnuson but I would want more with a turbo. Like 10 to 1. My Procharger will break up a little in the upper RPM with the stock compression of the 6.1 but only in good air. I tuned most of it out with a plug change but the predator only gives you very little adjustment. This is with the 10PSI pully

Let's say it's a Hellcat Supercharger...


11.6 lbs boost but could go for more. I would prefer to over build the short block since boost can be addictive

That's a Magnuson style, I need to ask what fuel and what's the designated intent. If it's pump gas and mostly street you should be fine.

My turbo big block is 12 to 1 but it's race gas. C-16. It has no detonation at 30PSI

This will be pump 93. 75% street, 25% strip

I think your in the ball park, what EFI system?

Either Holley Dominator or EFI source Hellcat system

You should have no issues with the Holley, I have no experience with the other so can't comment.

Nice build

Thanks. Piece by piece brother! The MS3 system is not out yet but it is being worked on.

I helped put a MS on a turbo Miata, was impressed with it. I run FAST on my Turbo car and N/A Valiant. My Challenger is stock computer with Diablo Predator and not impressed at all with that setup

Cool cars! I will probably go with the MS3 as long as the price is right, which it more than likely will be. This will be my forst step into fuel injection.

Hope I helped a little.
The Challenger is for sale. I got bored with it and don't want to take the drive ability out of it with a,stroker and such. The wife can get in it now and drive it to the grocery store lol.

I like your build, your going to have a fun ride. Is this motor going in the Hornet?

NOT a Magnuson style...The factory supercharger and the Magnuson are completely different from a design perspective. Magnuson uses roots style rotors from Eaton (same as an Edelbrock supercharger) and is far less efficient than the IHI screw compressor that comes factory.

OP:
Why aren't you going with a factory longblock?? There are guys running a nitrous plate setup just aft of the throttle body with a smaller pulley and Diablo tuner running 9's in a factory weighted (~4500#) hellcat! You don't really need any more stroke; just add boost. The factory stuff is pretty dang stout and for the price of a longblock, you might be wasting money going a different route.

You did, Thanks a lot! The car is a 68 Charger. Pure twin h power for the Hudson!

I bought a bare block before the short blocks were available. I also have a bunch of different Gen III internal parts and figure I could get something to work.

Here is what I have:

New factory 6.4 crank

New Factory Hellcat rods and Pistons

Manley 300m 6.125 2" journal connecting rods

Manley 4.09 bore platinum series pistons

Good stuff but it's kind of like legos that don't match lol.

Figure whatever way I go I will have to buy something.

I think with the Manley pistons and rods you already have, might as well go with the Manley 4.05 crank. Dont know your comp height on the pistons but if they're the 1.09" that combo will work to but compression will be higher.

Compression height is 1.206

Ok, sell what you have except for the pistons and do the 398" deal with the 6.20 rods. Would be a killer,solid combo. That is exactly what I would build, except my broke a$$ would be using a 5.7 block.

I have been thinking the same thing. Anyone know if the manley pistons are good enough quality for boost?

My son is getting ready to start his 6.1 build. It will be boosted not sure if spray or turbo but leaning more turbo. FI w/Megasquirt. What do you think the power limits are with the stock crank using aftermarket rods and pistons? thx matt

From what I have read, srt cranks are good to 1000hp. The factory oiling is what does them in at high rpm.

Is this correctable or is it inherent?

I believe it is a cross drilled crank, better off to get a Molnar crankshaft if you are going to turn it up high.

6500 RPM would be my limit on a stock crank.

Anyone ever try plugging the cross drilled hole?

R.

Not that I know of...

I was always told that boost motors need swept volume (stroke) for most power. Hellcat was set-up for overbuilt-durability, you'll see guys putting stroker cranks in them soon enough though. I'd be investigating the 400" boosted BES 3rd gens over the stock stuff

That applies for both NA and boosted because you are getting more displacement. The problem, as stated previously by another user, is the fact that you run into higher rod angles with the lower rod ratio you create. That, along with the higher cylinder pressures, can cause large side loading on the pistons and cylinder walls. For 5.7 and 6.1 blocks (which are already notorious for cracking because of thin cylinder walls) this should be taken seriously. On top of that, you reduce dell time and increase piston speed with a stroker...

knyech1 check you pm's.

There are guys on other forums running TT 6.4L in the 1200hp+ range with stock cranks. Can you say breaking the tires loose at 80mph?
I would look into the pistons a little before you use them; call Manley and ask them a couple questions. Your crown thickness needs to be thick (at least 0.230") to accommodate the higher cylinder pressures and the first ring needs to be about 0.280" min from the top of the piston so it doesn't get fried. Wrist pin is often overlooked in high hp boosted applications so you might need a thicker walled pin; the pins take quite a beating in there. Learn from others' experience.

Thanks. I will be calling Manley to find out. After talking to Tom Molnar he recommended CP Bullet series pistons.

NA is different - its not ever going to get the same volumetric efficiency as a forced air engine. Engine is just an air pump - more in, more out = more power.

The bigger the engine, the bigger the supercharger/turbo needs to be to make the same boost. In a boost application like this, a 420"+ engine wont make much more power than a 390"+ engine. Why does Goss ONLY have 404" when he could easily be using 440" engines? Its been said A LONG time ago in Gen III land that for NA build as big as you can afford, high boost builds seem to take ALOT more power with a crankshaft 3.80 or smaller and a rod 6.20 or longer because of block strength when you get over the 1000hp mark.

I hear you, and that's why I'd LOVE to know the specs on his engine (bore/stroke/heads). BTW he's having blocks made in the UK that can handle the power and he's selling them. The word of that needs to get out.

It is, the wet blocks are $5500. For me a stock 6.1 block does just fine, but if these can hang at 2500hp, that is a whole new engine market for these radial guys.

$5500 is stout but that can come down with volume. With BGE 6.4L over the counter heads flowing 340 cfm out of the box there is zero reason anymore to bother with antique blocks and heads IMO

Yep I agree, I have a set of Apaches under my bench that will eventually replace my CNC'd 6.1's currently on top.

So how is NA different?? Larger swept volume = more cubes = more air space = more power so I'm not sure what you mean when you say they are different. Boosted engine obviously have a better volumetric efficiency but that wasn't the topic of conversation.

+1
Exactly what I am saying about higher rod ratios. These engines don't like high cylinder side loads. Hellcat and 6.4 are stronger, but at a certain power level you need to be thinking about things like rod ratio because of stress issues.

Again, be careful because Apache and 6.1 heads are made of 316 (unlike the hellcat heads that are made of a more durable 356-T6) and are known to flex and lift under high cylinder pressures.

My current engine is NA and staying that way. Just would like to get it up around the 700hp range on pump gas.

Bada$$. Very possible with these new engines. Can't wait to get my junk done