Taijutsu

I've noticed the sharp edges on the valve pockets in some pistons. I would think that all sharp edges in the combustion chamber would be smoothed to some degree to avoid hot spots? Where am I wrong?

R

REELAV8R

I agree. Just don't go overboard and change the shape of things unless you know what you are doing.

Taijutsu

I would think that breaking the sharp edges at 45* would make a difference?
This might make the difference in knock and preignition?
Not to mention allowing full advance.

R

zwede

You want to round any sharp edges. Your goal is to create a surface where carbon does not stick, so think polishing.

Of course you need to stay away from the valve seats. If you slip and damage the seat...

Taijutsu

I've always wondered what the difference would be between two identical engines.

One comes off the assembly line.
The other gets the full treatment.
No cam change, just fine tuning and machining.
Balance and blueprint, deck the block.
Pocket port the heads, nothing extensive.

Anyone seen this done?

R

zwede

I did something along those lines years ago with a '96 Z28 (LT1 engine). I ported the heads myself. A bit more than a pocket port, I took the intake runners from 155 CC to 175 CC. But I kept the stock cam, and all emissions stuff was left intact. It picked up 35 rwhp.

Taijutsu

I like to think of that as Free Horsepower!
It does cost time and money,
but it doesn't cost low/mid/top end power.
Engine runs cooler and gets better mpg.

I'm thinking about stk 350 w/AFR 180s and a 224/224 HR cam.
425/425 like a good 383 seeems reasonable?

Comments?

R

zwede

I don't like to make engine dyno predictions as you can make an engine dyno show whatever you want. Like the "How to make a 500 hp junkyard 350!!!!" articles you see in the magazines. Then that "500hp" engine makes 250 at the wheels once it's in a car.

But it sounds like a decent combo. I'd probably bump exhaust duration 5-10 degrees if you're running street exhaust (mufflers).

Taijutsu

Don't forget the 1.7 RRs and an Air Gap to get the air in.
Get the scr up to 10.5 or more, should pull hard.

7t9l82

1.7 rockers in a small block? I think the 35 h.p. figure for some head work is realistic.try unschrouding between the valve and chamber a little too and around the guide boss. If you go with aftermarket heads they can often take a little touch up too, but you better know what your doing.

Taijutsu

Check out what they did to this stocker.

http://www.dragzine.com/tech-stories...-stock-racing/

TheSkunkWorks

CMIIW, but I believe the above is referring to unshrouding the intake in the area indicated in red...

Flyinace3

Light unshrouding around valves and matched to bore

REELAV8R

That looks pretty good to me. On my heads I shaved off 030" of material which acted similar to unshrouding. Could have used some more but I left it. I also polished out all that pebbly textured surface from casting. I have never heard my engine detonate and I have tried as much as 38 degrees total timing. Running at 34* right now, seems to perform the best there. Also run 85 octane on 9.9 CR. Mix it to about 86 sometimes 87 on hot days just to be safe. 8.25 DCR.

Flyinace3

Decked to 62cc, 9.3:1 compression 36 degrees timing on 93 octane and haven't experienced detonation . My cranking compression is 210 psi.

BOOT77

Some effort cleaning up and rounding the chamber is good, just don't remove too much material and lower your compression. My advice on home porting is don't get greedy and just clean up and enhance.

hugie82

many get carried away and do more harm than good. Opening the area up around the intake with big valves seems to wake things up a bit. Removing burrs and casting marks helps a tiny bit. Port matching your intake will help a lot if it's off to begin with.

TheSkunkWorks

Yep, BTDT. Back in the day I ruined the first set of heads I attempted to port at home. Good thing they were a cheapo iron set for a FSB. Since, I've always left any serious head work to a pro with a SuperFlow.

Port matching tip: Despite what the interweb or TV "experts" may advise, done properly this does NOT involve hogging both the head and manifold out all the way to the gasket. There's a good chance you could hurt flow by introducing a wide section area in an otherwise well designed port tapper, and create an undesirable pressure pulse. Match to the existing port profile in the head. Should there be an abrupt intrusion into the flow path at the head, seek professional help. My $.02 on that.

REELAV8R

I don't believe that port matching is always as beneficial as some think. If the intake port is larger than the head port then yes you probably want to remedy that.
However it the intake port is smaller than the head port then that provides some anti reversion advantages. This may allow for greater overlap periods, longer duration and later intake valve closing with fewer of the undesirable characteristics typical of those moves.
I did not port match my intake to the head. Intake port was slightly smaller than the head ports.

7t9l82

I've seen port matching ruin the flow, and more often than you think.