any advantage to "boring/polishing" inside stock

[Anonymous]

just wondering if it would be worth boring out the inside of a stock manifold on my 200. i will go to headers but like to experiment on stuff like that. my thought was to use a dremmel to clean and bore the inside of the manifold. anyone tried it?

 

[Anonymous]

I would imagine that if you were to match port the exhaust manifold you could pick up a few ponies. Remove the manifold, take a new manifold gasket, or make a template from poster board, transfer the exhaust ports from the head to the manifold and then remove the excess material. Let us know if you feel the difference.

 

[CZLN6]

Howdy Pongo:

There is a small advantage to be had by knocking down casting flashing inside the log and ports. These are sharp ridges left over from the sand casting process. Boring, or enlarging the inside of the log is very labor intensive with marginal returns. If you goal is to increase the volume of the intake track it is simpler and easier to acquire a later model head with a larger intake already in place.

More volume in the log acts like a plenum to each port and may have an effect on higher rpm performance.

If you spent your time and effort rounding right angle corners in the intake you'd be better off than trying to enlarge the volume. There are two ways to get more air/fuel into the engine. Increase flow velocity and increased flow volume. Both work to varying extent and are somewhat interdependent. For example- at low rpm high velocity and low volume works pretty well, at high rpm high velocity and low volume runs out of breath. On the other hand, at low rpm a low velocity/ high volume would be sluggish, Higher rpm would increase velocity and high volume would be better. Bottom line- our log heads can benefit from increases in both!! Cleaning up flashing and rounding right angle corners does both.

And remember that the most important area to flow is the inch above and below the valve seat. If you don't have this area right, modifications to the inside of the log will be very marginally helpful.

If you decide to go at the inerds of your log with you dremel, take before- and-after measures of the volume. just so you know.

Enjoy the journey.

Adios, David

 

[Anonymous]

thanks for the info!

 

[Anonymous]

how much cfm can you flow through a late model 250 head?

 

[CZLN6]

Howdy All;

That is a very difficult question to answer. There are many variables. If your are talking carb cfm, consider this; A stock 200 engine with an OEM 200/250 head (say a '78, for example) will run out of breath at around 4 grand. Simple swapping an RBS (rated at 215 cfm) in place of the stock YF (rated at 187 cfm) will likely turn more. So it may be safe to say that a stock late model head will flow more than the stock Carter YF one barrel can offer.

Other variables would include exhaust system and condition, ignition system and condition, initial spark timing, Cam timing, gearing, vehicle weight, engine condition, and probably others.

We know that on my brothers modified D8 head; using an Autolite 2V 1.08 which is rated at 287 cfm, mounted directly to the log. We swapped the carb for an Autolite 1.21 rated at 351 cfm and saw a significant improvement in performance. Since we can't find an Autolite 1.33 the next step is a Holley 500 cfm 2V.

Someone else on this forum swapped a Holley 350 for a 500 and saw a significant increase, and that was with a stock cam.

Clearly, a log head, well tuned, on a balanced engine package is capable of flowing considerably more than a 1V is capable of.

Another scenario- when the 170 was introduced in '61 the head was a recommended swap onto the earlier 144s, It had a slightly larger intake volume and a slightly larger intake valve. It supposedly had a 30% improvement over the 144 head. Can those small improvements make that much difference? If so, then a late model head should be significantly better too.

Adios, David