Hey guys, first time Ive posted on Camaro Z28.
Ill list what I have currently and then Ill ask some questions for you engine builders out there.

1989 Iroc-Z
All forged 383 (4340 - 6" rods - Wiseco flat top pistons)
Mini-Ram II intake- Accel DFI
AFR 210 heads Competition ported.
1.75" full length hooker super comps - 4" mufflex exhaust.
Art carr 700r4 w/ 3000 stall 9.5" converter.
Ford 9" w/ 3.70 gears.
Cam - Hydraulic roller custom ground 231@ .050 .561 lift w/ 1.6
rockers (single pattern cam)

Im going to go to a larger cam - and probably switch to the Holley SR intake. My pistons are .005" in the hole and have the -5cc valve reliefs- 65 cc comb. chambers - I should be around 10.7: 1 compr.

1. At what point would I have to worry about piston to valve clearance? I want to go to another Hyd. Roller around 242/246 with .615/.610 lift with the 1.6 rr. Ive seen many motors with these heads and flattop pistons with cams this large.
I built this engine - and it runs great - I feel the cam is just not matched well to these larger port heads. Ive been getting 6800 rpm with this setup ( 360lb springs / and a rev kit).
I can upgrade the engine myself, but If the piston/valve clearance is a real concern at this point.. Id rather bring it in to a shop here and have them perform the measurements.

2. (not as long winded)
I currently get 370hp and 405ft/lbs at the rear wheels.
Would this new cam I want to try get me closer to 425hp at the rear. Thats what I am shooting for.


3. Would shaving the heads to get to around 11:1 be that much better than just keeping the 10.7:1? A shop here recommended I do that - I know with alot larger cam Id bleed off some compr. but again is it worth it to pay to correct the intake and shave the heads for 3/10s of a point?

Thanks for any help you guys can give me!

Jerry

 

I'd be worried about piston to valve clearance with the cam/rocker setup you have right now. It's possible some valves may already be slightly bent. Flat top pistons with the tiny valve reliefs start getting close at .500 - .550 lift range but it all depends on the valve timing ground into the cam. It has nothing to do with how much clearance there is when the piston is at TDC because the valves should be closed at that time.

The only way to know for sure is to use some modeling clay in the valve reliefs. You'll need a pair of solid lifters because you can't check it properly with hydraulic lifters. Lash the rockers to zero and bar the engine over 2 full revolutions. Pull the head and see how deep the impressions go into the clay.

Recommended minimum clearance is .100" on the intake and .125" on the exhaust. Aluminum rods increase that by .030". Any closer and you risk smaking a piston into the valve.

If you don't get the required clearance, the pistons will need to be flycut or you'll need new pistons with deeper pockets.

 

Thanks for the info,

The engine currenly runs just fine, and when I first put it together
I had the engine builder at the shop I was working at check the piston to valve clearance. There were no issues with this cam.
(they said the pockets on my flat tops were large)
Unfortunately I didnt keep those numbers - and now Id have to pay to have someone do that. With the solid lifer Id also have to use another pushrod that would be the correct length too. So, I guess Ill be bringing it in to the shop for the test. Thanks again!

 

I started responding to this post and then realized I could type out about 17 paragraphs to cover all the bases. It's complicated.

Let me lay at least TWO things on you that are critical above all else....

1. Piston to valve contact happens during the OVERLAP period between the exhaust and intake strokes where the piston is near TDC and both valve are open. NOT at peak valve lift. Peak valve lift has damend little to do with whether the piston will smack the valves.

2. There is simply no substitute for checking. Too many variables to say for sure from a list of specs.

One last thing- "adequate clearance" does not mean .001" at the closest point. .080" intake and .100" exhaust is about as close as you ever want the valves to come to the pistons at their closest point on a motor that's gonna get beat on some and expect it to live a long life.

 

Good point Damon,

Not many people stop and think about what the piston is doing
on the intake stroke - it's moving downward.

During the overlap, the piston is rounding out an exhaust stroke
and meeting up with two hanging valves that want to create a
recipe for scavenging.

I believe it was you that made me realize that a couple of years ago!

As for flat top pistons, I'm a little confused as to what he has in
this setup. Does a flat top piston contain valve relief? That would
be considered a dish type, or negative dome? Maybe I'm getting
caught up in the "Internet Buzz Word Game", but a flat top was
always explained as it reads, no valve relief or cut-away.

One bit of info I was taught to torque the heads and setup the
valve lash as it would be used in full flight (zero lash with solid)
when checking clearance.

 

the LS1 has true flat tops with no valve reliefs, but a piston that is flat other than the valve reliefs is still considered a flattop.


get some cheap modeling clay and put it on the piston, bolt the head down and turn the engine over and you'll find out if you have any problems

 

Need to check radial clearance too (.050 or better)... it's not just about the amount of clay squished under the valve. I prefer to use check springs but you surely don't want to use a spongy clay like Play-Do. Hard to get accurate measurements when the stuff is swelling after compression.

-Mindgame

 

Radial Clearance?

That would be valve to valve? As in, "get yourself some canted heads
if you want to run that much lift?"

Close, or way off?

The valves would never touch side by side...would they? Hmmm...

 

No, "radial" just refers to clearance between the perimeter of the valve and the edge of the valve pocket. Could really pose a problem if the head is not located properly (cylinder #1 may be a ok with #7 being a problem) or the valve pockets aren't the same piston to piston.

Gotta check everything.

-Mindgame

 

If I'm understanding this correctly, radial clearance is the amount
of "space" around the valve as it sits in the piston relief?

If the head is skewed slightly, the valve may clip the edge of the
relief?

 

Yes sir, now consider another variable... piston rock and you can see why you might need to check clearance there. Especially for those running short pistons with tight ring packs.

-Mindgame

 

I was already thinking that (piston slamming around in the bore)
as I saw the 0.050" clearance rule.

BTW - Reading some of Widmer's articles. Crazy dude.

Back to the topic then...

I would guess that as the engine wears and cylinders wear,
the piston clearance will get loose and these tolerances should
be checked again (for street guys using wild cams).

What is the periodic check for such clearances? Would someone
running a wild street motor want to tear down every 10, 000 miles
or is that being ****?

I normally pull my heads every couple of years and check lash
once a season (hydraulic cam). I had an engine blow recently
and now more than ever, I'd like to keep tabs on the routine PM.

Any tips for periodic checks?

 

It is always a good idea to go through your engine as the build progresses and check every possible clearance to make sure it is adequate or not excessive and record this data for your records, especially to help you assess wear when you tear down the engine.

It is pretty common among the manufacturers to build plenty of extra clearance into any of their parts that are built for street use or even their shelf parts. You'll mainly be checking for minimum clearance to make sure there are no crazy problems but usually on a street engine you will have way more than enough. When you get into custom pistons, cam, heads, and such then it becomes much more common to see valve reliefs that need machining and p to v vertical clearance gets pushed to the limit when you are trying to push the envelope. In general people building street engines don't want the hassle or expense of all the extra fitment and machine work and the manufacturers know this.

 

Yes, you're making a mountain out of a mole hill here, AFAIC. Sure, the more clearance/wear there is in the bore, the more the piston can rock, but you're talking 'peanuts' here, guys. It's not like std. pistons were put in a 30 over block here.

I use both methods, when mocking up a short block. I use checking springs as MG noted, along with an indicator, for the valve/piston (depth) clearance, and use the clay to make sure the radial clearance is adequate.

 

After checking with a degree wheel and a dial indacator you can use clay to verify the results.Press the clay in the relief and smooth it down with something streight making sure it is stuck everywhere. Put a drop of oil on the clay and the valve to keep it from sticking the clay to the valve and pulling it out of the relief and ruining your check.Take a razor and cut the clay in different spots and measure the thickness with the butt end of dial calliper's.Measure it in several places and visual the radial. I have run less than 100 exhaust and 80 intake but not on a street engine.Remember the piston is chasing the valve's home.If you need more clearance you can do it without tearing the engine down.ISKY has cutter's you rent and you take all the valves out of one head and cut the reliefs in the car.All instructions come with the cutters.If you are running example.-- 2.100 valve get a cutter for 2.150.Its easy if you get everything real clean in the hole (no oil) cut them one cyl at a time, re set up your cutter in the next cyl. and vacuum and blow that cyl clean of shavings beside the piston above the top ring and go to the next one.Don't be afraid to wipe and wipe and wipe. They run with a reverseable hand drill.