CNC Porting- The Best?

[Bob Eyres]

Since the advent of computer aided porting has happened since the introduction of the LT5, I would have expected that in recent years programs to have been developed that greatly improved the breathing of our engines. Programs that would be far superior and repeatable over the hit or miss results of porting by hand. Yet it seems as though many LT5's still get ported manually. Why is this?

You would think that, over time, ideal programs would have been developed for relatively stock engines, as well as moderate, and big displacement LT5's. Programs that would perfectly match the 16 separate runners in ways far superior to hand grinding. Not to mention all the other operations applied to the heads and other parts in the air stream.

Can someone school me as to why this seems to still be such a "black art" rather than an easily predictable upgrade?

[Hog]

Because it takes cubic dollars to develop an effective CNC program with only a handful of prospective buyers. Anyone can make a CNC program, but engineering one that produces desired results, let alone 2 or 3 different levels. As with anything, junk in, junk out.

I disagree that hand porting is hit or miss. An experienced "chip maker" can produce repeatable results and is also the basis for many CNC programs.

Notice how everyone and their Brother sells a CNC ported head for the GEN 3/4 and soon GEN 5 heads. Thats because there are literally millions of these engine in circulation with many more possible customers. Heck even GM does CNC ported heads now.

[XfireZ51]

Bob,

The problem is that components like the plenum, IHs and heads are not as consistent as would be needed for CNC. Castings vary and runner walls can have pockets or be thin in different places. Even experienced tuners that have ported
many intakes and heads will tell you that even they run into unforeseen issues at times. Particularly if you are attempting to take porting to more extreme levels. CNC would be used to give you a start, but in the end, manual is how you finish it off. I'll let other more experienced players give you their opinion.

[Locobob]

You'd have to do a lot of testing and development, it would never be cost effective given the small production numbers. Hand porting can be very consistent with attention to detail and careful measuring.

[Paul Workman]

Well, IIRC, Pete had his heads digitized, and some number of heads out there have been cut - at least initially - using his CNC program. In fact, I believe I saw some pricing for CNC'ing the heads; pricing that ranged depending on whether he had to pull the heads off your motor, or not.

With regard to the variances in castings dimensions (aka "core shift"), it does exist. In fact, I believe it was after cutting Bob Gillig's 427 LT5 runners with the CNC map that some of that core shifting became evident. As I recall, he had to repair some seepage resulting from the program grazing the oil side of one of the head runners.

I'll leave it to Pete to put the light on this...

But, with regard to hand grinding vs. CNC: What comes first? Hand grinding or CNC mapping? And, is there just one and only one pattern to optimize the LT5, or is there room for improvements? Pete's CNC sets the bar pretty high, but I wouldn't go so far as to say it can't be improved upon. JMO.

Paul.

[rkreigh]

generally heads are hand ported, flowed, digitized, and then the CNC program can be generated

the modern technology makes this much easier. a digitizer pen is used to trace the port and put together a 3d cad image which can be refined and adjusted all you want to test with

but since there aren't a whole lot of people stepping up in line waiting to get their heads ported AT ALL we're durn lucky Pete took the time to capture a CNC of his handiwork

we were "debating" LS vs LT5 cyl head flow. while we clearly have improved flow quality and velocity 330 cfm is a starting point for a good head

we seem to be "flow limited" to around 750 hp with the heads

you can go up in displacement, but not see much additional HP due to the flow limitation

I think it's time for a billet head, using an improved chain drive, with a better port design

nelson has done it, and with the modern tooling the cost of this debauchery will eventually come down

my old crusty hands (like phrogs) are salavating as we see more ZR-1s getting parted out and the lt5 parts hitting the pipeline

"experimental" builds become more affordable

I'm doing LS motors right now (because I can) not by choice

one day I hope to boostamuv on a stock block LT5 on E85 and see if I can get it to live at 800 with stock internals

my bet is it will do just fine, factory cams and all

looking for a decent price on a crate motor or good used one (high miles ok too, just want good cyl sealing!!)

we are doing this with 4.8 LS iron block motors and finding the limits are quite high for the 2004 and up motors with the better pistons and ls2 rods

and many people are literally throwing these engines away because they are so plentiful I have 3 of them in the garage right now if anyone needs one. a few sets of forged internals, ect...

I'm really thinking of pushing a stock short block with studs and wider ring gaps to 800 hp which is nothing new to these guys and seeing what happens!!

[tpepmeie]

Originally Posted by rkreigh

we were "debating" LS vs LT5 cyl head flow. while we clearly have improved flow quality and velocity 330 cfm is a starting point for a good head

we seem to be "flow limited" to around 750 hp with the heads

you can go up in displacement, but not see much additional HP due to the flow limitation

I think it's time for a billet head, using an improved chain drive, with a better port design

nelson has done it, and with the modern tooling the cost of this debauchery will eventually come down

Ron,
I'll be pushing the limit upwards to (hopefully) 800 this time. Bigger valves and 400 cfm should do it easily. When you get to those levels, the camshafts present more of a constraint then the headflow--the duration required to efficiently fill the cylinder at 7000 rpm with anything above a 3.75 stroke exceeds 300 degrees seat-to-seat, and you need to lift the valve proportionately higher with larger valves. Most people don't want to run something like that around the street. Stage 3's are something like 282 degrees seat-to-seat; my current cams are 297.

As far as the heads go, the main limitation is the valve spacing and port angles. You can't get more than 42mm inlet valves in there--period. The intake port angles are too low to use the really high airspeeds you see with the best 4-valve heads. Still good, but not ideal.

We'll see what 427 version 2.0 makes next year.

Todd

[Harvie]

The bottom end strength of the lt5 sleeved blocks and the sizes of their crankshaft journals are the large limiting factors along with the induction systems multiple sharp turns in air low.

Over 750HP power requires a bigger steel crank with large dia full radiused crank journals to last. Then blocks would then have to be fitted with a strong steel girdle and studded.

[tpepmeie]

Originally Posted by Harvie

The bottom end strength of the lt5 sleeved blocks and the sizes of their crankshaft journals are the large limiting factors along with the induction systems multiple sharp turns in air low.

Over 750HP power requires a bigger steel crank with large dia full radiused crank journals to last. Then blocks would then have to be fitted with a strong steel girdle and studded.

Well, I was assuming at the power levels we're talking about a new crank was a given. Which journals specifically do you think need to be "bigger"?

The LT5 bottom end has been proven to handle >1000 in boosted applications. None of us are building 24hr endurance engines, so sustained high rpm, high load conditions are rare anyway. Now that we have studs, the bottom end isn't a worry.

For NA power, the limiting factors are the cylinder head geometry, and the user's tolerance for extra large camshafts. Always has been, always will be.

[FU]

IMO FWIW any thing above 3.80 stroke offers little ROI.