I just got my E31 head back from the Z Doctor. The head has an Isky Stage II cam with .480 lift. The head was not shaved other than clean up. We put 260 Z exhaust valves; 35mm vs 33mm (1.38 in. vs. 1.30 in.). Intakes are 1mm oversize (43mm vs. 42mm ; 1.69 in. vs. 1.65 in.) Is there any mods that have to be done to the block for clearance reasons, or should I be OK as is? I know eyebrowing the block will let it breathe, but is it necessary for clearance?

I believe that an L24 block needs to have clearance notches in the bores for larger intake valves, unless it has been bored at least 1 mm (0.040"). No personal experience though, so I can't swear to any of this.

It all depends on the bore size, valve lift, and valve size. It should be checked to give you peace of mind, and to possibly save some damage. If you feel good about your die grinder skills, you can do it yourself. It also has a side benifit of smoothing the flow within the cly./chamber. Two important notes: DO NOT grind down below the top ring! Stay above by about 1mm or so. Place a used head gasket on the block, and scribe the block, around the inside of the gasket. Then very carefully grind the block as in the pics below. The pics are of an "O" ringed L24 block.

Phred

...dumb question follows....

I'm not an engine builder, so set me straight. What good are larger valves if you haven't enlarged the intake and exhaust runners in the head? If those don't flow more air, what good does the bigger valve do?

Thanks from the peanut gallery

xray,

Head flow is controlled by the restrictions inherent in it's design. In most cases, the restriction to flow is from the valve. Meaning, the stock port is capable of flowing more air than the valve will permit. Initially, by increasing the area ( valve size ) of the valve head, and valve seat, the head will pump more air. Increasing the size of the ports is the last step in optimizing head flow. If ports get too big, and this happens all the time, the velocity of the flowing air will decrease, causing a decrease in low/mid range performance. Big ports help top end performance, not bottom end, which is what street cars need. The first step for street performance, is eliminating as much restriction as posible by the use of slightly larger valves. Experience is needed here, as too big of a valve can cause flow to be disrupted by not allowing enough space in the combustion chamber for air to flow around that big valve. To realize the good effects of a larger valve, different seat angles should be used to allow the air to smoothly flow over the seat area. As you see it can get quite involved. Properly tuned, an engine that pumps more air can produce more HP.

Phred.....The block is an L24 bored .040 over (+ 1mm) The intake is 1mm oversized which means the valve is actually hanging out .020 more than stock on the block side of the valve. The pistons are flat top with valve reliefs. Valve lift is .480 inches. Of course I'll turn it over by hand to check clearance, but do you think that I would have to notch the block for clearance purposes?

All the heads/blocks I've done were full race engines, so I don't have any particular info on your setup. My guess is you won't have to do anything. If you choose not to check it, to verify it's not hitting, Just leave the spark plugs out, and turn it over by hand. But you need a small amount of clearance, and just turning it over won't tell you that. To verify, do a pre-assembly with one int. and one ex. rocker installed with only the inner valve spring installed. Set cam timing, run the valves to max lift, and push to open the valve further. The valve will open a little further before it crashes into the block. How much more it opens will tell you the clearance. Safe minimum distance is .060 Int., and .090 Ex.

Phred

I do a little machining but am not an engine builder like Phred.

Just curious....

Couldn't you take a set of calipers and measure the valve width and compare it to bore width? Of course there would need to be a little fudge room for inconsistencies in the casting or bore spacing. Don't people use some sort of clay for testing clearances also?

Curious,

Jim

Phred......That makes sense.....The block was bored out .040 and the intake valves are 1mm (.040 in. ) oversize so it would seem that the extra overhang of the valve would be offset by the overbore from stock....but the valve is coming down at an angle and that probably makes a difference. I think I'll go ahead and eyebrow just for breathing if nothing else.I always turn my engines over by hand also ( experience is a good teacher). Guy

xray,

Head flow is controlled by the restrictions inherent in it's design. In most cases, the restriction to flow is from the valve. Meaning, the stock port is capable of flowing more air than the valve will permit. Initially, by increasing the area ( valve size ) of the valve head, and valve seat, the head will pump more air. Increasing the size of the ports is the last step in optimizing head flow. If ports get too big, and this happens all the time, the velocity of the flowing air will decrease, causing a decrease in low/mid range performance. Big ports help top end performance, not bottom end, which is what street cars need. The first step for street performance, is eliminating as much restriction as posible by the use of slightly larger valves. Experience is needed here, as too big of a valve can cause flow to be disrupted by not allowing enough space in the combustion chamber for air to flow around that big valve. To realize the good effects of a larger valve, different seat angles should be used to allow the air to smoothly flow over the seat area. As you see it can get quite involved. Properly tuned, an engine that pumps more air can produce more HP.

What you say here makes good sense. However, can we attack this idea from a different angle?

Replacing the valves in the head is a fairly involved procedure. Great for increasing the top end power potential of the engine but at what cost to the lower rpm torque band? If the head was to stay on the engine, would a change in cam profile be more cost effective in trying to achieve the desired flow through the heads and improve the top end horsepower rather than larger valves?

I know enlarging the valves seems like the logical way to reduce aiflow restriction into the cylinders but if the new, larger valves were the 'right size' valves in the first place, wouldn't the factory have already installed them with all the necessary unshrouding to best effect? Apart from cost, what specific criteria do the OEM engineers use for determining 'optimal' cylinder head valve sizes in your opinion?

As you've seen many times, ports are often too big or made that way by the owners wish to have bigger ports. The velocity of the intake charge drops and the engine becomes less responsive to the throttle at lower rpm's. What if a cylinder head was installed on an engine that had smaller ports than the original head, with correspondingly smaller valves that weren't at all shrouded and the characteristic airflow into the cylinders were governed more directly by the profile of the camshaft rather than the size of the ports or valves? Do you see where I'm heading with this? For a percentage of the drivers in this forum, most of their driving is done at sedate suburban speeds, ie less than 4K rpm. Smaller ports promote faster velocities in the intake so that improved cylinder filling is the result. Cam changes could be viewed as a preferred tuning method which wouldn't be as involved (or costly) as changing the valves, valve seats or hogging out the ports. There would also be flexability in the direction in which the owner could 'tune' their engines. The owner could run a small street cam or a much larger performance cam all on the same head! (with the appropriate attendance to things like valve springs, retainers, piston notching, suitable compression ratio's etc etc etc). Wow, wouldn't that be a revelation!

So where would you find cylinder heads for an L series with smaller than original ports? I'm lucky that I have found several L20A cylinder heads that fit the above idea perfectly. I'm just toying with the idea and was looking for input from anyone else who may have thoughts on this. I was thinking of installing one of these heads on an L28 with a stock bottom end and a 2mm HG to boost compression to around 10:1 on 98 octane pump fuel. The intake ports are 30mm's and 1.5 inch intake valves and 1.3inch exhaust valves with liners. Torque below 4K rpm is my top priority!

Your thoughts?

PS Diseazd, I hope you don't consider this to be a thread jacking, I just was looking for input into my own thoughts and this comment by Phred struck a chord with me...!:embarrass:

OZCONNECTION........Not at all....I'm trying all kinds of combos myself. I'm building my third engine....the L28 has a stage III cam, head shaved and shimmed .080. An L24, E 31 with a stage II with 1mm oversized intake and an L26 exhaust (bored .040 over) and finally the L24,E31 with stock valves and a stage I cam ( bored .020 over). What I've found out is I thought I wanted low end, but the top end is awfully fun too !!!!! The stage I should be a low end car....Stay in touch on your findings. the L series engine never ceases to amaze me. Guy

Remember that there are tolerances in the head, valves, etc. If you are at almost minimum clearance, fit tolerances in the head-to-block and valve diameters could produce a "nicking" situation. (Been there, done that).

I always prefer to have at least "some" room for tolerances / clearance in moving mechanical systems, especially for a street application.

Dumb q - eyebrowing is recommended when putting larger valves (say either in E31 / E88 or using an N42 head) in a head on the L24. What about simply shaving the head to increase compression?

Assume I took an E88 head, shaved / shimmed .080", hotter cam - do I need to notch the block? I am thinking no, but shimming is only done to the cam towers - wouldn't the shaving effectively make the valves .080" closer to the block / pistons?