I've been contemplating getting a low base circle cam for my 383 to prevent the rods from hitting the camshaft. I've heard that there are some disadvantages to pushrod length when you do this. Does anybody know anyghing about this??


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Riccioli Performance Motorworks

--RPM Racing--

-Currently building first ever engine. 383 with Super Ram plenum and runners, Edelbrock lower intake, Bosch 24# injectors, either AFR or Trick Flow heads, MSD ignition.
-Will be installing Baer Brakes
-Will be installing full Spohn suspension in rear
-Art Carr 700R4 trans??

 

Generally you need to lengthen the pushrods by about half the amount that the base circle of the cam is made smaller than stock; i.e., the sum of ½ the cam base circle diameter, plus the pushrod length, should remain constant. That's assuming that all else in the whole system is already perfectly selected, and all you're doing is swapping the cam. However, it's a good idea to select a pushrod length that results in the best possible valve train geometry, no matter what the details of the modifications to the various parts; the length given above is really only a convenient starting point for checking. Also keep in mind that pushrods only come in .050" increments which may or may not lead to exactly matching the formula above.

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"So many Mustangs, so little time..."

 

Paul,

As RB says, the pushrods would be longer - significantly longer. The increase in length typically is not matched with an increase in wall thickness, making the longer pushrods inherently weaker and more prone to bending.

You will also lose some flexibility in cam choices unless you're willing to go to a custom-ground cam.

I'd recommend sticking with the stock base circle and carefully inspect pistons #2 and #6 for rod bolt interference as well as the other 6 cylinders). A quick touch with ye old Dremel will clearance the bolts with no significant loss in strength. It is not enough weight to upset a street balance on your reciprocating assembly either.

It's SOP on stroker motors where the builder did not opt for the rounded-shoulder 4340 rods.

Good luck.

BOR

 

Do only the heads of the bolts have to be ground down, or also the side of the rod? If it's only the bolt then I'll do it. If not, Comp said that I would need longer pushrods, but the cam would be the same price so I might go that route. Let me know please.

------------------
Riccioli Performance Motorworks

--RPM Racing--

-Currently building first ever engine. 383 with Super Ram plenum and runners, Edelbrock lower intake, Bosch 24# injectors, either AFR or Trick Flow heads, MSD ignition.
-Will be installing Baer Brakes
-Will be installing full Spohn suspension in rear
-Art Carr 700R4 trans??

 

That depends on the individual rods. Most of the time, the bolt is all you have to touch; but with some rods, there's enough casting out past the bolt head that it will interefere too.

What I would suggest you do is mock the whole thing up and test it. Put the crank and cam in the block; get the rods put on the pistons; then one (or 2 if you need to get both of them on the journal to line them up exactly where they will be) at a time, put them on the crank, and check the clearance for each one. You don't need the rings installed for this. Since you will only be grinding on the rod you shouldn't have to worry about metal shavings in the blcok, if you clean up each rod after you get it fit. Then after they are all done, if they required very different amounts of grinding, grind them all down by about as much as the one that required the most, to restore their balance. Like BOR said, even that might not be necessary.

ARP rod bolts for 400 have a bevel cut into their shoulder to avoid this problem, and consequently often don't need any work. If you don't have your rods prepped yet you might want to use those.

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"So many Mustangs, so little time..."

 

I think because of their design the cap screw I beam rods do not have a clearance problem.

 

so whats teh advantage of going to a cam w/a smaller base circle? i see the disadvantages, but why, or how can it help?

 

more lift?

 

The only advantage I know of is preventing clearance problems.

------------------
Riccioli Performance Motorworks

--RPM Racing--

-Currently building first ever engine. 383 with Super Ram plenum and runners, Edelbrock lower intake, Bosch 24# injectors, either AFR or Trick Flow heads, MSD ignition.
-Will be installing Baer Brakes
-Will be installing full Spohn suspension in rear
-Art Carr 700R4 trans??

 

Paul,

As always, RB has the definitive answer. You cannot substitute a mockup assembly - this way you will know EXACTLY where clearancing is in order.

Also, again, depending on which brand you chose for rods, you may have NO clearancing to do - most of the 4340 H-beam manufacturers got wise quickly, and saw that a pre-clearanced rod would sell better.

The smaller base circle cam has its' centerline located in the exact same spot as stock, but since the diameter of the core is smaller, the lobes do not protrude as much as a stock unit.

Good luck,

BOR

 

Unfortunately due to my college student budget I had to stick with stock rods. I'm guessing this means I'll have quite a bit of clearance problems? They are the 5.75 inch rods.


------------------
Riccioli Performance Motorworks

--RPM Racing--

-Currently building first ever engine. 383 with Super Ram plenum and runners, Edelbrock lower intake, Bosch 24# injectors, either AFR or Trick Flow heads, MSD ignition.
-Will be installing Baer Brakes
-Will be installing full Spohn suspension in rear
-Art Carr 700R4 trans??


***** Flyby: (n) When you let off the throttle after beating a ***** Racer by a few car lengths and he then blows by you in an attempt to convince you he really won the race.

 

RB83L69
Why would the ARP 400 bolts have this bevel and not the ones for a 305-350? Don't most people use 5.75 rods on a 383 buildup? I mean I know you can also use 6" rods and I believe another size, but I thought 5.75 was the most common.

------------------
Riccioli Performance Motorworks

--RPM Racing--

-Currently building first ever engine. 383 with Super Ram plenum and runners, Edelbrock lower intake, Bosch 24# injectors, either AFR or Trick Flow heads, MSD ignition.
-Will be installing Baer Brakes
-Will be installing full Spohn suspension in rear
-Art Carr 700R4 trans??


***** Flyby: (n) When you let off the throttle after beating a ***** Racer by a few car lengths and he then blows by you in an attempt to convince you he really won the race.

 

That's a good question Paul, I don't have the answer to that one, other than that they charge more for the ones with the special feature...

Even a stock 400 will sometimes have clearance problems, especially if it's been align-bored. I've been racking my brain trying to remember what a stock 400 rod bolt looks like; seems to me it's different from a 3½" stroke rod, like the factory was trying to prevent problems in that same area.

I couldn't say what "most" people use. For years, most of the people I knew that built 383s used stock 400 rods and 350 pistons; but in the last 6 or 8 years, with the cheap availability of aftermarket cranks and rods and off-the-shelf pistons with non-stock pin heights, the universe of choices is alot wider. That's a good thing of course, but it also means there's a whole lot more combinations, so things aren't quite as cut and dried as they once were, as far as the details.

With the stock rods there's no real guarantee about clearancing; some of them have a big chunk of forging flash at the parting line, some have very little. You may find that you only have a couple that have extra material, since the factory doesn't match them for that.

That brings up a good point - you no doubt have noticed that new car engines rarely have balance problems, yet we in the field have to work extra hard to get that right. Ever wondered how the factory balances motors? Well, they don't. Instead, as rods come off the production line, they are checked for length, big end weight, small end weight, etc., and put into an n-dimensional array of bins; when a bin gets 8 rods in it, they build a motor with them. So they don't actually do any "balancing" as such, at least not the way we think of it.

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"So many Mustangs, so little time..."