70 dart swinger slant turbo build and mild restoration
Cool thanks the reason I was asking about the port size is to know whether or not I can fill the port on the flange with weld a little or just leave the flange the same size I'm gonna port the head later I have alot of experience porting mostly just port match and clean up with diesel stuff but it should make a difference even with the small valves.
I spent my long, and basically-uneventful life, playing with naturally aspirated engines and trying on several occasions, to squeeze the last iota of power out of whatever it was I was working on. I was racing NHRA, and was deeply entrenched in the protocol that preached that sweating the details was the path to glory and more power. I saw combustion chambers and ports that had been the recipients of hundreds of hours of polishing and tender loving care from owners who spent ALL of their spare time seeing to it that the output of their projects never suffered from a lack of attention. That was my education.
It went on for years and years...
Then, I discovered forced induction. For my money, the rules herein are different, at least, in some ways.
Having relied on air flow that comes from ambient pressure (14.7 psi?) for many, many, years, all of a sudden, the game had changed; no longer is one atmosphere ALL that's responsible for filling the cylinders, using all the tricks of the trade, such as matched ports, smooth port-walls, multi-angle valve jobs, tuned intake-lengths, smooth radai bends in the manifolding and research on ram-tuning, being of ultimate-importance...
YOU CAN BLOW IT IN! :D
Simply changing (increasing) the boost-level can do wonders for performance! DUH!!!!
We used to make incremental changes in camshaft positionong (advance/retard,) to gain a couple of hundredths of a second in e.t., but simply turning up the boost (within reason,) can give you performance increases that will blow your socks off.
It is a different mindset, but comes with its own group of caveats.
For instance...
I never worried about detonation on my naturally-aspirated motors.... if the thing was a little lean, or the timing a bit too "fast," I'd just hear it PING" and adust things, accordingly.
That attitude will get you a broken motor with a turbocharger, in short-order. You must be
ever vigilant against detonation in all cases, and try your best to avoid it.
Religious adherence to the monitoring of air-fuel ratios is the first line of defense against ruining your motor through detonation resulting from too-lean of a mixture, under boost. Too much spark advance is almost as important. Fuel selection is critical, as sufficient octane for the circumstances is crucial. Having built the right compression ratio into the basic rotating infrastructure is also, highly-important. Riding herd on boost-levels is another important discipline to be followed constantly. Boost creep has destroyed many an engine...
So, the things to watch out for are quite different from those in our naturally-aspirated efforts, and some things that used to be very important, don't seem to be as much-so, with this new protocol.
For instance, the critical importance of flow as regards port-matching, may not have such a big effect, if you're running 20 pounds of boost. With higher (20-pounds and up) boost-levels, the air/gas mixture WILL get into the cylinders, because the turbo has created enough pressure to overcome the slight mis-match at the port-opening, regardless.
I think cylinder-filling is an area wherein forced induction has eased the fine points of flow-enhancement for us.
It's kind of like swatting flies, I think... a fly that is the recipient of a blow from a six-pound sledge-hammer, will be just as dead as one who got hit with a 12-pounder...
That's a poor metaphor, but the crux of the matter is, I think, boost can cover a mulititude of "sins." Mismatched (to a small-degree) ports is one of them.
Another is cam-selection, I think. Naturally-aspirated motors are pretty finicky when it comes to cam specs because they have to rely on ambient air-pressure for cylinder-filling, and it usually takes a cam that works well at high rpm to produce the needed power. This well may require a valvetrain that works well at 7,000-plus rpm. A lot of science and r & d needs to go into making that work well. Titanium retainers, stronger, thick-walled pushrods, lightweight valves, beehive springs and roller-rockers may be required.
Turbocharged slants. conversely, run out of breath at about 5,500 rpm and don't work well with much overlap, so the cam specs are mild and forgiving. A stock cam should work very well, in a low-boost application. High-tech valvetrains just are not needed for most turbo apps. Stock valvetrain pieces are usually all that's required.
The requirements for monitoring and maintaining a turbo motor are very different from a high horsepower naturally-aspirated motor, and, with lots of trade-offs.
You pays your money and, you takes your choice! :banghead:
