Dart Big M 9.8 short deck

[Velocity Vector]

The guy is telling me the block has a 4.5 bore and the rod length is 6.585 on a Dart Big M 9.8 short deck. I was thinking you had to have a 10.2 tall deck to run that long a rod.

Can this be true?

 

[Chris]

Bore has nothing to do with it. Piston does to a degree, but eventually the pin will end up too high. The importany measurement you're missing here is stroke. Run a 3" stroke and you can use a 6"+ rod.

 

[DAREDEVIL]

Bore has nothing to do with it. Piston does to a degree, but eventually the pin will end up too high. The importany measurement you're missing here is stroke. Run a 3" stroke and you can use a 6"+ rod.

Chris your right again, mine has 5.6xx rods !!!!!

 

[DAREDEVIL]

The guy is telling me the block has a 4.5 bore and the rod length is 6.585 on a Dart Big M 9.8 short deck. I was thinking you had to have a 10.2 tall deck to run that long a rod.

Can this be true?

1.65 rod ratio . @ 4800 rpm u would be safe.

 

[MERPerformance]

9.800 deck, 4.00 stroke, 6.585 rod, if that is the case the compression height would be 1.200 figuring the piston is -.015 below deck. Shortest compreseion height for shelf stock pistons 1.270. If it's a custom piston, it can be less.

 

[BUIZILLA]

I've had many 454 4.312+250 arm, 496-500's, with 6.535 rods.. turned them mid 7,000s..

edit..

 

[Velocity Vector]

He did say custom pistons so maybe so. Thaks guys

 

[Chris]

On the short deck you're always dealing with a less than ideal rod angle and when you lengthen the rod, you start dealing with a short piston with the pin intersecting the oil ring.

 

[MERPerformance]

The 6.585 lenght is odd, but not if Merc did it. It's not an standard rod lenght for off the shelf.

 

[Velocity Vector]

I know without measuring it is hard to tell but here are photos of the rods and Diamond pistons Said it was a 9.5:1 compression, supposed to be de tuned super cat engines but not sure if the super cat race engines were 9.8 or 10.2 decks I have heard they were tall but not certain...These are defiantly Dart Big M 9.8's

 

[DAREDEVIL]

I know without measuring it is hard to tell but here are photos of the rods and Diamond pistons Said it was a 9.5:1 compression, supposed to be de tuned super cat engines but not sure if the super cat race engines were 9.8 or 10.2 decks I have heard they were tall but not certain...These are defiantly Dart Big M 9.8's

I CAN ASURE U 1000% that supercat engines are and where all 10.200 TALL DECKS !!!!!!!

They where also drysump !!!

What i would say happend on these, is somebody changed blocks from 10.200 to 9.800 and ofcoarse had to use custom pistons , because otherwise the pistons would come out to far with the rod combo.

I have a copy of a supercat as a short deck, i use a shelf piston but had to change rod length to do so !!!
I still have the supercat pistons here as well , so if i get a chance i will take some pics and mesurements for ya....just to show the differents.

 

[Velocity Vector]

They were dry sump but not 10.2

 

[Tyler Crockett]

9.800 deck, 4.00 stroke, 6.585 rod, if that is the case the compression height would be 1.200 figuring the piston is -.015 below deck. Shortest compreseion height for shelf stock pistons 1.270. If it's a custom piston, it can be less.

Diamond has a shelf stock piston with a 1.245 CD for a 9.800 deck 4.000 stroke and a 6.535 rod.

 

[DAREDEVIL]

They were dry sump but not 10.2

Then they where never real supercats from merc. Do u have an other pic of those ??? on this one its hard to tell if they are /where tall decks.

 

[Velocity Vector]

 

[DAREDEVIL]

I SEE A TALLDECK ( 10.200 ) on this pic, if u zoom in and look at the distance between the water crossover uper bolt and the head.

On a short deck its right at the edge , here it looks like its 1/4 away. If my eyes don't fool me ?! LOL

STILL HARD TO TELL ,FROM THE PIC.
If not, then they are not true supercat engines.

 

[PatriYacht]

Mercury super cat motors are large bore, short stroke. I forget what the exact numbers are but they are about 3.8 stroke. I remember Brian Bunch was looking for a couple of these cranks a year ago, never did find any. That makes it a lot easier to get the long rod in there. I wouldn't try to use that long of a rod in anything more than a 4.00 stroke unless it's in a tall deck. The piston gets too short.

 

[PatriYacht]

It sounds like this engine has some supercat parts in it but is not a complete supercat engine.

 

[Velocity Vector]

When I put the two side by side it does look like big pic is a tall deck and the small is short deck what do you guys see?

 

[Chris]

I suppose the question would be "what's a detuned supercat engine?".

The supercat engine is a high-compression, high RPM piece. To detune it you're going to take cam and compression out of it. So now you have an engine that's a pig because the ports in the heads are too big, so you swap them out. And you pull the dry-sump out and put an internal pump in it. So now you have a short stroke engine of modest displacement that doesn't have enough stroke to make strong midrange torque... And most of the "supercat" has been removed.

The other thing I'd be concerned about is the cranks. That's an item with a finite life expectancy in an engine that turns that fast. hard to tell if it's on it's first hour or it's last.