Questions about head work.

[Valvebounce]

Im real new at head work. In the past ive done nothing apart from get a shop to replace guides, seat inserts, and recut seats, and valves. And I've lapped valves, and shimmed up my valve springs to get the correct pressure.

But I now need to get my heads sorted out on my 318 before I proceed with assembling the block. (I'd hate to have the decks cut, and then find my heads had already been skimmed heaps)

So I've been reading lots of threads on this website. as well as browing the odd book. My heads are 675 castings. Someone already fit 1.88 inlets to them. (looks like a 1 angle valve job too) I grasp the idea of blanding the bowls, and just giving the casting inside the port a general tidy up. And I plan to port match to the manifold too. But thats about all I have the confidence to do.
in the SA design (Cartech) book "engine blueprinting" it says when blending the bowls it's important not to disturb the "venturi" beneath the valve seat where the bowl narrows. Is this refering to the bit where it "waists" down?
pics of my inlet

You can't quite see, but on one side of the valve opening there is quite a waist. on the top one, it's on the left side which is kinda in darkness. Should I hog this out? The way it looks to me, although a 1.88 valve has been fitted, I doubt it would have flowed any more then the 1.78 would have. I really really can't afford to stuff up these heads.


My other question is concerning fitting teflon valve guide seals. I have some comp cams seals that go with the valve springs to match my cam etc. My machinist (who works for free) reckons he can turn the guide down to accept those seals, without the need to buy the proper tool that comp sells. However, since I need to replace the guide as well, the tip where the seal goes will be so thin between the guide and the seal (Like 15 thou of iron left) that chances are the interferance pressure from the guide will cause the thin bit of iron left to crumble. Imagine a guide with a .500 OD, and the seal having an ID of .530. The remaining bit will fall to bits.
Is the cure to install the guild from the valve side, and leave the last .500 or so at it's original ID so as not to leave it thin and weak once the OD of the guide is reduced to take the seal.
Or does someone sell a guide with a .530 OD?


I hope that makes sence? (it's twenty to one in the morning, and I'm not feeling too sharp)

 

[godfatherofchry]

pic # two looks to be very nice.as far as valve seals if the gides are good just the chep mushroom typ works good.I have seen alot of the teflon ones make the valve gauld to the gide my self I dont run any seals the vale will move freely and if it does puff a little oil on start up I know there geting oil.Im shoure you will here difernt.I lerded this from some top notch engen bilders.hope this helps....Artie

 

[C130 Chief]

If that is a picture of your intake ports now, you are correct insofar as the larger valves provided approximately zero power gain. The area under the valve seat should be opened up and blended to the diameter of the seat (bowl blend). I'm unsure what the author in whatever book you are reading means by "venturi" within an intake port, but it sounds like he is confused. A venturi creates a differential in gas pressure by a restriction in flow. The area directly under a valve is the place within an intake port where you least want restriction.

A "wet runner" intake, as found in a carburetted engine prefers a steady and reasonably high velocity throughout the intake tract to keep the atomized fuel in suspension. Too large a port can cause gas velocity to slow or stagnate allowing fuel to condense out of suspension adversely affecting low RPM throttle response. A set of W2 heads on an otherwise stock 318 would likely produce only modest power gains, but could make the engine rather doggy at part throttle. A "dry runner" intake system, like the port injected Magnum 318 can tolerate a far larger port because the fuel is atomized near the end of the intake tract rather than at the beginning.

By this authors apparent reasoning, it would be beneficial to have a smaller restrictive point at the end of of a large port. In this model, gas velocity would certainly be highest at the valve, but it would be slower throughout the port upstream. I contend this is exactly the opposite of the ideal, giving all the power restriction of a small valve with the response liabilities of a large port. In my opinion a reasonably sized, constant cross section port with a large, less restrictive valve will be better.

Fitting 1.88 intake valves, with the associated port work, is a worthwhile upgrade to 318 heads. However 1.60 exhaust valves should also be fitted and similar attention should be payed to the exhaust ports. In fact the larger exhaust valve would be of more benefit if only one set of valves were to be upgraded.

Regarding your Teflon guide seals. Yes, the guides can be cut down to allow these to be fitted. A specific cutter is made for this task and is reasonably priced. Also, many aftermarket bronze guides are pre-machined for such seals. But honestly, the factory style Viton rubber "umbrella" type seals work well, cost less, fit under any reasonable spring and require no additional machining.

 

[moper]

I believe your auther is referring to the angle and throat directly under the seat angle that would normally be part of a 3 angle valve job. You do NOT want to enlarge the throat to the diameter of the seat angle cut. There needs to be an approach to that angle, normally created on a 5-angle valve job by the throat and top cuts. Those help the mixture turn without going turbulent and makes a big difference in low and mid-lift flow. Blending right to the seat angle sacrifices low and mid lift for high lift only, which is not a good sacrifice in most engines. For what you have here, I would not do any major enlargement other than to smooth the transition from the port roof around the guide boss and the throat. Leave about 1/4" of that taper above the seat angle untouched and remeber the trick is smooth transitions, not a step or low points as it changes shape. Less is more here. The last thing you want is a lot larger volume of space at the bowl and throat just ahead of the valve head.

 

[Valvebounce]

Thanks.
yes the ports in my pictures are what I'm starting with. (Untouched) I also have 1.6 in valves for the ex as well. Do you think it would be worth while to mark the port with marking out blue, and scribe a 1/4 line below the seat face with jenny calipers to give me a line to blend to? Unfortunately me crappy cell phone pics don't really show just how big a waist there is on the left side. Even leaving in that small part above the seat face, there is quite a bit of meat to remove.

As for the guides, does anyone have a part number for some with a .530 OD to match my seals? I perhaps neglected to mention that my camshaft choice requires dual springs, so I'm stuck with using the teflon seals. (Engines that puff on start up give me the screaming sh!ts)

 

[moper]

I would normally use liners, so the guides and spring seats would be trimmed at the same time, then the guide is drilled and the liner installed.

 

[Valvebounce]

I'm looking at using liners, but is there a liner with a .530 OD? (That the seal can run directly on) Because the liners I saw had a .500 OD, which means the iron part at the tip of the guide (where the seal goes) will be too thin. The seals only go on about .300in.

 

[moper]

Liners are not full guides. They use the existing guide... So you have to cut them for your seals. New guides will have the OD you want.