From reading here on the forum it appears that head porting is worth the time and effort to gain some 'free' horsepower. As a reference I have the book guides, "How to Hot Rod Small Block Chevy's" and "Engine Blueprinting". Concerning "pocket porting", does that mean to clean up and blend the factory tooling sharp edge marks inside the valve bowls and smooth out the 'short side' radius .. ? .. It appears the 'long side' radius also gets some blending treatment.

The SB Chevy book also shows the port runners being ground straight so as to be tangent to the bowl radius. Also shown is to grind away the port wall next to the push rod holes to widen the port entry. Should these things be done to a street headed motor .. ? .. And, the valve guide bosses that protrude into the valve bowls; should these bosses be contoured, as shown in some of the pics, for a street driven motor, also .. ? .. Along with port matching the head intake runners to the gasket, should the intake manifold port runners also be port matched to the gasket .. ? .. Should the exhaust port be ground in the valve bowl area, or just port match to the gasket only .. ? ..

My heads are the 1.94/1.50 valve size. Would increasing the exhaust valve size from 1.50 to 1.60 gain some efficiency .. ? .. Duke mentioned about the LT-1 cam's longer exhaust duration helping to scavenge the cylinder better but would a 1.60 size valve "help" more.. ? .. Would valve shrouding be a problem as has been said with installing 2.02 size intake valves in these heads. ? .. I plan to use stainless steel valves ..

I've read where it's recommended that the hardened exhaust valve seats be installed in the early 'non lead' type heads, but that the intake valve is ok with it's soft seat, as is, because it runs a lot cooler. I would just as soon as not install those hardened seats, but now I'm wondering again .. The idea I have in my head is to use the LT-1 cam rocker arm valve lash as a kind of 'checker' to see if the valves might be receding, over time. If so, then go ahead do the valve seats .. ? .. Or, is that too 'Bubba' .. :D

mrg

 

Following the pocket porting guidelines will pay dividends, but don't mess with the valve guide bosses. The most important operation is to blend from the valve seat to the bowl to eliminate the ridge that is formed by the tool that rough forms the valve pocket. Open up the port inlet to match the manifold on both sides, and remove any rough casing flash in the port interior, but little work needs to be done on them for a street engine. It's best to be conservative so you don't remove too much material and leave a thin spot or actually break through to the water jacket.

Mark the heads with dykem blue and then place them on the block and scribe a line around the bore. Use this as a guide to eliminate any overhang over the edge of the bore.

The LT-1 has very soft action on closing, and if you keep the valves properly adjusted there will be no seat recession. The 1.94/1.5 valve combination is more durable and won't crack between the valves as 2.02/1.6 valves are prone to due. Increasing the exhaust vavle size to 1.6 will help slightly, but not in a big way. It depends what other parts and procedures you have to tradeoff relative to your budget.

The F-M Speed pro S2323 (1.5") and S2469 (1.6") are both 21-4N stainless (21 percent chromium, 4 percent nickel) - a very high quality material that will be more that adequate. You don't need stainless on the inlet side. It's total overkill because the inlet valve does not get that hot. The V2351N 1.94" valve is 8440 alloy steel and is more than adequate for the application. It is fully machined with a hardened tip and chrome plated stems as are the two exhaust valves mentioned above.

Make sure you use quality valve guides that are compatible with chromed valve stems and a good valve seal system.

Duke

 

David Vizard wrote a whole book on SBC heads. good reading :thumbs:

 

Don't do it. It's not necessary unless you plan on continous high output usage as in towing, road racing, etc. Even then the intake valve runs much cooler than the exhaust side. Today's heads are build with hardened seats but vintage heads will hold up if the valve trains are set up correctly and especially if the engines see limited use. My builder didn't believe in cutting into a "cherry set of double hump" heads to install seats. In his view it wasn't neccesary, for the reasons stated above and could actually risk damaging the heads if not pressed in correctly. He did install SS valves on both intake and exhaust, "overkill" or not.

As Duke mentined, the LT-1 has a gentle action. Both Duke and JohnZ highly recommend this cam as the best replacement for early solid cam profiles. I am changing my 097 Duntov in my 327/340 to the LT-1. It will be a better cam for my lowered compression ratio, (9.5-1) vs the standard 11.25-1 that my engine was built with originally. That's my concession to lower octane fuel and drivability. The LT-1 engine had 9.5-1 compression in it's final year of production. JohnZ says you gain 20% more usable low end torque and no give up on the top end.

On porting, I had mine mildly ported and polished but left if to a pro, my builder, to have it done.

You could practice on a unusable set of heads to learn and then do it yourself but I wasn't inclined to try it myself. It really didn't cost that much. You have to decide what your time is worth and don't forget you need the equipment too.

Don't change the valve size, the 1.94/1.50's are plently big enough. You didn't say which engine option you have, but I'm guessing 327? If you are re-building a vintage (especially an original motor), engine, it's better to be conservative and not try to re-engineer what Chevy originally intended. You want to maintain as much of the original character of the motor as possible. A couple more important dont's, don't overbore and don't deck the block and don't over cam it. Stick with Chevy valvetrain components for dependability and long life.


[Modified by Solid327, 7:35 PM 10/9/2003]

 

Duke .. Thanks for your input. I think I will heed what you suggest and just clean up the bowl and short side radius' in the valve pocket areas along with port matching to the gasket. A bit of reasonable performance gain without getting too carried away with the grinding operation sounds good to me, as I've never done this before. As for scribing the combustion chamber area in the head and grinding away material up to the scribe line, will this significatly lower the compression ratio so that the heads will need resurfacing to gain it back .. ? .. I have flat top +.030 TRW forged pistons installed in the engine.

Matt - I think I ran across that book at the local library, of all places .. Thanks .

Solid327 .. Yes, my engine is a 327. I had in mind to change out only the 1.5 exhaust valves for the 1.6, but I think now I'll just leave it as is as you mentioned. I'd rather have the durability more so than an extra few horsepower . I plan to use the stock GM valve train components too, this time around. The valve lifters in my engine now are severly concave due to excessive valve spring pressure. JohnZ said his '69 Z/28 still revs to 6500 without any problem; on stock valve train components with 85# valve seat pressure. I figure the softer operating valve spring pressure might help to keep the valves from receding, too, in the older 'non lead' heads. I guess I could just throw on a set of modern day heads and be done with it but I like the look too much of those 'double hump' vintage heads. Heck, I like anything vintage .. ! ..

I have never had a solid lifter engine before so the LT-1 setup should be interesting. Lopey cams have that "sound" but I think I can live without that overly rich idle mixture smell. Solid cams have a pretty cool tappet sound of their own. I just haven't heard it in a long time .. Thanks Solid327 ..
mrg

 

There should not be a dramatic increase in chamber size - just a few cc. Most the the overhang will be on the exhaust side. Just taper it back to the edge of the bore. Remove as little material as possible.

Duke

 

Thanks for clarifying, Duke .. I hope to get started sometime next week ..

mrg

 

Small block heads have come a long way.
Might want to look around if you are serious about HP.

 

396 .. From what I've read the newer generation heads flow really well, as is, right out of the box. I considered going with World, Brodix, or GM heads. I would guess any of these heads really shine in the high rpm environment. For my use I'm more interested in mid range performance and figure my old 'double humpers' should work ok given the size of the motor. If I had more cubes under the hood, I think I would have gone with the newer style heads.
mrg