I'm planning on building a 383 stroker for my Camaro. With the increased stroke and a cam with around .5000 lift am i going to have a problem with piston to valve clearence? What about if i run 6" rods?

Which way are you going to stroke it. most common is 350 block + .030" 400 crank, 350 rods(5.7"). With flat top pistons and open chamber heads (70 cc+) you can safely run up to .525" lift. I would stay away from 6" rods as it tends to push the pistons into cyl. walls too hard...which shortens the life of the motor.

Why are the 383's run with larger chamber heads? Arnet 64 cc heads recommended for 383's?

BCoop,

With all due deference, Emmet has it backwards. All other things being equal the engine with the longer rods will have less side wall thrusting, not more. Longer rods also cause the piston to dwell at TDC and BDC a touch longer than shorter rods. At TDC this is good because this keeps the combustion space smaller for a longer period of time which tends to extract a bit more power from each combustion event. The slowing of the piston near BDC and its slightly increased dwell time there is thought to inhibit cylinder filling on the intake stroke which is not good for power (alas, no free lunch). Clever engine designers will try to mitigate this weakness by using an intake manifold which keeps intake charge velocity high.

If you are going with 6 inch rods you will need pistons that are designed with this rod length in mind or they will stick out of the block, which does not promote longevity!! The wrist pin is relocated higher in the piston to make up for the extra rod length.

Concerning the combustion chamber size issue: you can run whatever combustion chamber size you want to, but bear in mind that the compression ratio will be higher than on a 350 engine, assuming you used the same sized chamber on each engine. The 383 has 9.4% more cylinder volume at BDC than a 350 and if that extra volume is squeeze into the same space the compression ratio will go up accordingly. The good news is that engines with longer rods are also a bit less prone to pinging.

Concerning hitting the pistons with the valves: I would ask your cam grinder that question and then do not trust his answer. In other words, mock assemble the engine and use some clay to check. Better to find out while the engine is still on the stand than later.


[This message has been edited by MikeM79 (edited 02-28-2000).]

Hey Mike,

As I was reading your post a thought occurred to me. If two engines (short rod and long rod) are spinning at the same RPM and the long rod version is spending more time at the bottom and top of the stroke then the piston stroke acceleration will be greater for the long rod engine. The time has to be made up somewhwere right?

Doesn't this means that the long rod engine would be more stressed than the other one?

Kevin,

Indeed, you are correct. The piston attached to the longer rod will be moving faster "in the middle" of the stroke to make up for lost time at TDC and BDC. I don't have a concrete answer concerning your point concerning the stress on the rod. What you say sounds right, but whether or this increased stress would be enough to make a real difference I just don't know.

Kevin,

You assumptions may seem correct, however, as we all know, nothing is quite that simple.

I guess first off we should distinguish between VELOCITY and ACCELERATION. I liken it to a drag car. At the start, the car has a high acceleration, but a low velocity. At the traps the cars velocity is very high, but the acceleration is low.

The key with a long rod motor, and the reason they are favored in the racing community is that the piston VELOCITY is greater at MAX VALVE LIFT than a short rod motor. This allows the piston the create more "suction" if you will at the point it is most beneficial, max valve lift.

Anyway, the piston DOES dwell at TDC and BDC longer, but the short rod motor will actually achieve a greater VELOCITY during the stroke than the long rod motor. The difference is, as I stated earlier, WHEN the piston achieves this velocity. The short rod motor achieves its max velocity WELL before the max valve lift point.

Acceleration is another story altogether. The maximum ACCELERATION, and therefore the maximum stress, occurs when the piston is at TDC, because the piston has to be decelerated, stopped, and then reaccelerated in the opposite direction. The long rod motor dwells at TDC longer, therefore it only makes sense that the short rod motor decelerates and accelerates from this point at a higher rate. Therefore, the short rod motor has a higher stress in the rod at this point.

Just to give you some numbers, I have mocked up an engine on an excel spreadsheet I designed:

3.75 stroke at 6000rpm

5.70 rod:
Max Acceleration = 81,973 fps^2
Max Velocity = 6,204 fpm
Velocity at Max Lift = 5,207 fpm

6.00 rod:
Max Acceleration = 80,958 fps^2
Max Velocity = 6,173 fpm
Velocity at Max Lift = 5,235 fpm

Both engines acheived a maximum piston velocity at about 73 degrees ATDC, however the long rod motor was able to MAINTAIN a higher velocity up to max valve lift point, whereas the short rod piston velocity was decreasing rapidly.

This brings me to your point. Since the long rod motor dwells at TDC and BDC longer, it MUST travel between these two point quicker at a given RPM. That does NOT mean, however, that it's MAXIMUM velocity is greater, it just means that the AVERAGE velocity must be greater. It's is like a low, wide torque band versus a high, narrow torque band. Anyway, I hope this hasn't confuse you. If it has, I will try to clear up any questions.

As how this has any bearing on valve to piston clearance, the longer rod dwells at TDC longer, and therefore REDUCES piston to valve clearance. The key is, though, we are only talking about a couple of thousandths of an inch.

Take care.


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69 SSRS Frame-off Resto
81 Z-28 377ci Drag Car

Now I have a headache!!!

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Tim
67 ss/rs, MODIFIED
http://www.geocities.com/sixt7ssrs/
IM - sixtsevnss

69SSRS,

Super post!! Thanks for the info, you have helped me understand the issue better than I did before.

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- 69 SS350 Daytona Yellow
- 79 Z28 tons of mods