Scats Drop In 383 – Simple build, simple budget
By
David Vizard
Just check through some of our posts and you will see that most of our readers are, figuratively speaking, up to speed on the importance of torque. Torque is a function of the amount of air an engine can, in one working cycle, (two revolutions for a 4 cycle engine) inhale, trap within the cylinder and then utilize. This can all be covered under the topic headings of a high volumetric efficiency (or supercharger boost), CR, trapping efficiency and cubic inches. Increasing any one of these elements will mean more torque and the element we are going to look at here is the last on that list – cubic inches.
A popular move for at least the last 15 or so years has been to install a 3.75 inch stroker crank in place of the stock 3.48 inch stroke in a 350 Chevy. The result, with a 30 overbore is, as most of us know, a displacement of 383 cubes (that’s 6272 cc for you metric guys). For most rod/crank/piston combinations there was usually a need to grind on something for the rotating assembly to clear the block and cam.
I know in the early days when we used to use stock 5.7 inch rods there could, with some blocks, be an issue with grinding sufficient clearance as doing so took you into the water jacket.
Here is the Scat series 9000 cast steel crank and stroker kit going into Robert McDonalds 383 truck motor build. It’s a working truck and needs heavy duty parts plus a whole lot more torque than the stock 89 350 will deliver!
Also there was an issue with rod length. Many hot rodders built a combination that utilized a 400 Chevy rod of 5.56 inch center length. Although it mostly cleared everything it made for a bad rod/stroke ratio. Although going the 5.7 inch rod was way better it was realized by most engine builders that a 6 inch rod would be better yet. To get the 5.7 factory rods (or most any through bolt style rod) to clear it was necessary to grind the through bolt shoulder and this, over time, would deform. Sooner or later this bolt in any kind of big output situation would fail. What was needed here was a stout cap bolt style rod with a low shoulder that would clear everything.
When Scat introduced their Pro Comp 4340 stroker rods (Part # 2-ICR****-7/17 where the asterisks represent the length in thousands) they virtually eliminated any 383 stroker build hassles. Using the new rod with either the low budget Scat cast 9000 series cast crank, or the moderately priced entry level forged crank, building a 383 was now no more complex than building a stock 350.
Because block castings vary so much there is no guarantee that the rods will always clear the block - but in most cases they do. If you are building using a roller cam block (87 on up) then the chance of everything clearing has, based on the few builds we have done at GFN and a few of the shops local to us, got to be 95% or better.
This is an example of KB’s recently introduced budget forged heavy duty piston. If it is as stout as the race pistons we anticipate it will be a long life item.
As of now we have two 383’s in late model blocks going through the GFN shop. One Robert McDonald is building as a low buck but tough high torque replacement for the 350 in his 89 Silverado extended cab Chevy truck. The other Dusty and I are doing as a high output, injected motor with a cam spec that stops just shy of being all out race. The target is - can we make such an engine streetable?
From this shot you can see that even at it’s closes approach there is plenty of clearance between the Scat rod bolt and the block even when a 3.75 inch stroke crank is used.
Cast Kit.
Let’s look first at the 383 Robert is doing. The recipient of this engine will be a real working truck that not only has to pull a race car trailer around but also big heavy farm equipment. A woosey motor is not going to cut the mustard here. As proof we offer the stock 350 engine which that year was among the most pathetic Chevrolet offered at a dubious 190 hp.
First the choice of crank. Cost is a major factor here and although the Scat crank is a bargain price it none-the-less is well and truly up to the job of handling this demanding work horse application. The 9000 series cranks are made of cast steel as apposed to the factory cast iron and are about 20% stronger than stock. I am sure that some one has broken one of these cranks but since first using them 10 years ago I never have. And don’t think for one moment they get an easy life on our dyno. Some of those cranks have endured 580 hp for pull after pull! (and by the way since we are on the subject of breaking parts do you know what the definition of the ‘typical racer’ is? Answer – someone who can screw up an anvil with a rubber mallet!)
The rods for this Scat 3.75 inch Stroker kit were the 6 inch long 2-ICR6000-7/16 stroker items mentioned earlier. In fact these rods are an essential ingredient if the rotating assembly is to be a ‘drop-in’ deal.
Pistons for this setup were to be the least expensive KB forged pistons as seen nearby. Although the cast hypereutectic pistons are stout pieces we wanted to check out this new KB forging. Our initial reaction on this piston is it looks good - without being overly heavy. This will make balancing of the assembly a little easier.
The plan was to use this crank/rods and piston combination with one of Professional Products external balance dampers.
Forged Kit
Here is the Mahle piston Scat forged crank 383 stroker kit being used for our high output 383.
Our crank was internally balanced at Scat. This entailed one slug of heavy metal in the rear counter weight (arrowed). After that only a minor amount of drilling was needed.
For the motor Dusty and I are doing we wanted something that could turn to 7800 and hang together For this we chose to go with Scats basic forged 3.75 stroke crank. This standard weight crank, made from 4340, features a nitride hardened surface and hollow big end journals for easier balancing. The pistons we planed on using were from Mahle. This will be about the sixth set we have used in the last year and they continue to impress us. As for a damper the plan is to use a BHJ unit as per those used on many cup car applications. Of course ours won’t be exactly like a cup car one as it will be tuned to suit a 3.75 stroke and heavier parts than one used for a 3.2 inch, or so, stroke engine.
In our case we ordered the crank and rods from Scat and supplied the piston assembly weights so they can balance the rest of the rotating assembly before shipping. This costs more but they do a nice job and it’s the way I would recommend you buy your crank kit. There is not so much counterweight drilling as is usually seen when done by your local machine shop. If there is any real amount to come off because of the low weight of the piston/rod combo chosen the crank goes in a lathe and has most of the excess metal turned off. This makes for an overall lighter crank and it looks a whole lot better than one with a dozen holes in it.
Here is the rotating assembly installed in our block. We have the high rpm 383 cubes – now it’s time to address the needs of the rest of this project.