I see a lot of you LT1 guys building 3.875 stroke small block chevys (winds up being about a 396 on a .030 bore).

I was thinking of running a scat 9000 3.875 stroke crankshaft, eagle 6" rods and a set of SRP pistons.

What is involved in this as far as clearance between the rotating assembly and the block/internals? Do you need to run a small base circle camshaft with the 6" eagle H beam ESP rods? Anything else I am forgetting?

Seth

 

You may need to run a smaller base circle depending on what rods you use or you will need to notch the rods a little. It's not that hard but like Bling said check everything twice as you go or you will end up re-assembling over and over! We have test cams for weird rods and strokes but you can usually tell by how thick the rod is up near the top of the rod bolts.

 

Yes, a lot of clearancing will be needed, to clear the h-rods. and a small base circle cam will be needed unless you want to notch the rods, I wouldnt.

 

Eagle H beam rods + 3.875 stroke = water jackets...

Do a 383 instead if you want H beam rods.

Trust me on this one.
Mike

 

This is a 3.875 stroke with Eagle H-beams on a road race engine that makes 465 at the wheels and is still running. It wasn't that bad.

 

What kind of compression height are you looking at with 6" rods?

I'm assembling my 383 with 6" conrods and I thought that was bad (1.120" compression height).

I know it's been done plenty of times before, but I'm curious about piston durability and block prep for these 396's.

Even with a full-deck height of 9.025", your compression height would be a maximum of ~1.090 (if piston was 0.020" out of the hole). Even worse, cut to 9.005" with a 0.005" deck height (my 383's likely route), the compression height would need to be 1.075".

Doesn't seem like much when compared to a LT1's stock piston thats ~1.560". Looking through the Wiseco catelog, I'm using the smallest flat-top they make (for the shelf at least).

Oh cool... looks like SRP makes a bunch of 1" compression height flat-tops, and even some 1.050" as well. Most look to be rated for 400hp, but others are pretty mean looking.

Any experienced engine builders have advise for us "short-compression" guys?

 

CH is 1.0625 @ 9.000 deck height.

 

Just get pistons with a "button" to support the oil ring in the pin bore. I don't think the height of the ring lands is any different in these pistons regardless of the compression height. OTOH, the top ring land on a "blower" piston is quite thick and there might be some issues with a long rod if that were what you had in mind.

Rich Krause

 

As Rich said the oil rail needs to be supported by a button or a oil support rail in any of these low compression height setups.

The problem that you can run into is that on a blown motor the long stroke 396 will give you some piston problems when running a 6" long rod. You really have to go down to a 5.85 or 5.7 rod but that's going to hurt your side loading on your piston taking away engine life.

Even a 383 is going to have some side loading problems with a 5.7" rod, on tear down you will see signs of scuffing on the skirts.

When you get into NA short compression height setups you can run some short CH pistons. 1.000" is about as low as you can go realistically. Even a 355 can run a 1.000" to 1.025" with a 6.250" rod. On 396's a 6" rod gives you issues, if you don't cut the deck as much just square it up you can run a higher compression height piston, all though it's marginally taller.

The nice thing about 383's is the common 1.125" compression height when running a 6" rod.

Bret

 

How big of a problem is the side loading issue on long stroke s/b. Reason Im asing is b/c this is ultmately going to be my next setup. Is there anything to prevent this on a 383/396 blown stroker motor?.

 

Not one I'd worry about now or ever. Chevy's factory 400 CID version used even shorter 5.565 rods and was fine. With a 6 inch rod it's actually almost as good as a regular 350.

 

From what I have seen on teardowns indicates otherwise. The long rod is definately going to help the motor out with that issue though. I'm still a big fan of 383's vs. 396's and this is one of the reasons. Cost is mostly the other.

Bret

 

Most blower motors don't last long enough to worry too much about the side loading issue

But as Bret said, for an NA motor a longer rod has a number of advantages. I use 5.7" rods for a couple of reasons on my blower + N2O setup. But for an NA 383, I'd go with a 6" rod.

Rich Krause

 

I'm with stroker there...
It might be cool to have a 396, but I question the VALUE of a 396 vs. a 383.

On a strong street motor the 396 might have ~20hp advantage over the 383, but I question the value & cost effectiveness of chasing that 20hp in a street motor.

 

It all depends on what you are doing but there is not any huge difference between the 396 and 383 at all. You are talking about .0625 lower in the bore. With a 6 inch rod it works great and with the 5.850 it even works great. That's what Lingenfelter liked to use on his SC 396s and 420s. Rod side loading issues are what people that can't build engines use as an excuse for bad machine work usually. I'm NOT talking about Bret in particular though.

We've had SBC SRPs that went through the whole Hot Rod Power Tour in second gear with a blown out 700R4 with only a 5.700 rod and they spent two weeks cruising at 4500 rpm. They hung way out the bottom compared to a 6 inch rod. We put a steel crank and solid roller in now and the skirts on that deal looked brand new. Most shops skirts always look like crap because they don't prepare anything right and then they blame parts or rod ratio.