I'll direct my first one at RB83L69:
On the post (Too much cam) you stated that after .475 lift that the flow stops increasing. I'm sure that you meant with the stock unported head. What if it has been ported (bowls, runners and intake matched)? More lift or just add dur.?

Second RB, Vader and others have been stating go with screw-in studs. Now that I have the heads apart should I drop an end mill on there before tapping and spot face for the shouldered studs and if so how much should be milled away for proper height with a stock set up with roller rockers?

Third (I hope I'm not being annoying!);
The machine shop that tanked and magnafluxed the heads suggested also putting postive seal on the exh also. Is this a good idea instead of the umbrella's that were on there?

Thanks

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91 RS 5.0 TBI....LT4 cam....Edelbrock headers....3"Dynomax exhaust....5spd.... 3.08.....Ultimate tbi....afpr...ZR 255-50's...Koni's

 

I have a .480 lift at .050" cam in my L69. I did a porting job on it, bowls, port matching, and general cleanup. I also slightly milled the heads, mainly for flattening them out, but it has 10:1 compression now.

I also port matched the factory intake and put headers on it. The bottom end is stock rebuilt and there is no emissions equipment whatsoever.

This thing runs really strrong right up to 6 grand and (oops) sometimes beyond.

 

Kool.

Glad to know that it pulls well. I'm replacing my swirl-port heads with the 416's and am glad to know that 2 days of grinding will pay off.

 

The size of the throat area behind the valves, and various other parts of the ports, become the limiting factor to the flow once you get to somewhere around that lift. Porting definitely will help. Exactly how much will depend on the porting job. The biggest gains in porting are in the bowl area right behind the valves, the exhaust short side radius, and the part of the intake port next to the push rod holes. IMHO gasket matching is the least productive procedure on stock heads; that part of the port is already larger than most of the rest of it. It's only useful for smoothing the shape and ensuring that there's no weird "step" there when the intake is installed.

Keep in mind that when you see the lift spec for a cam, that's its maximum lift. The valve actually spends only a very short time at, or even very near, that lift. So there is some benefit to using a cam with a higher lift spec than the "port stall" flow would indicate, because then the whole system will spend more of its time "wide open", and flowing at the capacity of the castings instead of being limited by the valve opening.

The heads will need a fair amount of machine work to run a cam with more than .500" max lift however. Screw-in studs (preferably with guide plates) greatly improve the stability and reliability of the system. There are no valve springs with the stock outer diameter of 1.25", i.e. that will fit in the heads as they are, that I will trust at above .500" lift. Others may feel differently, but then again, I prefer to continue to have zero valve train issues in my motors. So the srping pockets would need to be cut to the next larger diameter (1.45") to accomodate better springs.

You can get more flow, that is a greater number of molecules, through the valves in one of 2 ways: either increase the flow capacity, or hold the valves open for longer. Porting and lift are ways of increasing the flow numbers. Duration is obviously how you allow more time for more molecules to get through. But on the intake side, as you increase the duration, you rapidly reach a point where the low-RPM performance and overall behavior of the engine will suffer drastically, so you have to pick that cam spec with more caution. In a L69 with the stock computer, about 220° of intake duration is the absolute most I would recommend, and the engine will probably run better in most cars with somewhat less than that especially if the exhaust side of things hasn't been opened up. You will get the best results if you use a cam with the steepest possible ramps, which places higher loads on all the valve train parts including studs and springs, and further reinforces the need for those parts to be improved over stock. Exhaust duration can be much greater than intake duration without causing a problem; Comp's Xtreme series, as well as cams from other mfrs. of similar design, have about 10° more on the E than the I, and IMHO it could be even greater than that without causing any problems in the L69 application.

It's not real critical how much material is taken off the stud bosses. You just need to get them low enough that there are enough threads for the rocker nut to screw far enough down and so that the tapered part of the stud is below the rocker trunnion. If you mill them too far the studs will hit the top of the exhaust ports which is a PITA. About .100"-.125" of milling is typical. Most of the better studs (ARP, Manley, Comp, etc.) are designed for using with guide plates so they have a long bottom section; without guide plates, they're just about certain to run into the ports.

I run the positive Teflon seals from Comp or Crane on all my heads, both I & E. This requires machine work too. You may find that by the time you add up the cost of all that machine work, that it doesn't make much sense to spend all that on stock heads; you will be approaching the cost of heads that already have that done to them and work better than your stock ones can ever be made to. So there's some economic decisions to make too.

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

 

Thanks for the info! As always great!

For the time being I will be running the stock LT4 cam with 1.6 Gold Cranes. I hope that all I have to do to run these late model self-centering rockers is open up the slot to guide the pushrods. I beleive that I read that in an earlier post; use only one system to quide the rockers. IE; self-guided and not the pushrods.

I have done alot to the bowls and the short side radius. Nothing to the ports at the gaskets. I wanted to straighten the port walls on the intakes but the walls look pretty thin next to the pushrods and was kind of leary of breaking through.

So far all that I have in these heads is $50 for the machine shop and the grinding time. I will return to the M.S. for the final valve grind and that's all unless I spring for the studs which I probably will since only the parts will have to be purchased.

FYI; there's not much work that I can't do and I hate haveing to have someone else do anything. IE; hot tanking and magnaflux valve grinding.

The cam might be changed later for now tuning the prom for this will be my 1st step.

Thanks for all the help!

 

By the way RB; the cam that you have in your 305 looks like a good future choice. Comp Cams 264HR. Int. 488 lift and 212 dur. Exh. 495 lift and 218 dur with 112 lc.

It would still not exceed the .500 lift problems as stated earlier with 1.5 rockers.

[This message has been edited by DM91RS (edited November 10, 2001).]