a light coat of PPG DP90 to promote adhesion. Still planning on painting the block, I can't stand the look of bare oxidized mag. I would think that any rise in temperature due to the paint acting as an insulator will be small and not worth worrying about.
I'm the first to admit that we're probably splitting hairs with block paint. As for oxidized mag, I've never had a problem and I live in the northwest. The light film of oil on everything seems too keep the white fuzz at bay. I run mag Halibrands and a Ford roadster (real mag, not the colloquial 'mag'). I spray them with Gibbs-brand oil and they stay light gray--about like as-cast aluminum. Don't get the idea that I'm trying to talk you out of anything.
As for a surface finish, DP90LF makes a GREAT chassis/engine paint. As long as it's not exposed to UV it will stay nice and black for pretty much forever. All of my cars' inner fender wells and firewalls are DP90 and they're as black as the day I sprayed them. I'm pals with a PPG rep in Portland. He works in fleet and says there are a number of applications where the DP is considered a finish coating. Something to ponder (and it's cheap, too).
I'm about 99% sure theres no dish in the piston tops.
I asked John what the crown volume was and he said 4cc, so there is some sort of void (dish, bowl, whatever). I think it might be as simple as a bowl that mirrors the general chamber shape.
I don't want to be too specific as John at Aircooled wants the design guarded to keep copy cats from ripping off his product.
This isn't to knock John as I don't know him. In fact, by his presence he seems to be pretty knowledgeable. He's at least done some homework, so for that I commend him. But I don't understand what the deal is with the 'closely-guarded secret.' I mean if anyone wanted to know they'd just pony up the $400 for a set of slugs. That's cheap for R&D (receive and duplicate). Nothing will stop the copycats.
Anyway, there's nothing new under the sun. A lot of what passes for high-tech modern engine design dates back to Harry Ricardo's research with chamber designs that he published in
The Internal Combustion Engine. in 1922 and 1923. His subsequent books outlined the principles behind high-turbulence combustion-chamber design.
Basically Ricardo proved that a few simple principles (tight deck, small chambers among them) upset the fuel mixture as the piston compresses them. The turbulence that results helps propagate the flame. It also reduces the amount of ignition advance the chamber needs to light the fire during the combustion phase. And the side effect of this efficiency is greater detonation resistance. In John's marketing material he states that engines with 'squishy' pistons can take advantage of higher compression ratios. Well that's a sign of a more efficient chamber design (and remember, the piston is part of the chamber).
Now I don't want to reduce those pistons' design to just 'dished.' For all I know he may have a dish accompanied by a raised quench pad. That's entirely possible. In fact it's likely in light of the need for the modified cam timing.
Now are you sure that it's not a matter of the pistons raising the compression or that the pistons allow the engine to operate with more compression without detonating? I personally think it's the latter. But by what he told me those pistons had a crown volume, which indicates a dished area that's larger than any dome.
I guess a way to say it is that they add squish in areas other than the usual tight quench pad to piston clearance.
Yep, that sounds like it would be a quench pad.
I also wondered if the cars needing smaller pulleys were using thermostats and flaps when I read that. Seems that they weren't but I can't say for sure. Even running thermostat/flaps, if it runs cool enough to use a smaller pulley then I'd definitly run it that way. Slowing the fan down will require less hp to turn it; I'm all about small details that can pick up a couple extra ponies.
I'm all for thermostats. I don't understand why the air-cooled crowd thinks it's just fine to run an engine without them. Drag cars without water jackets or those with filled jackets aside, all performance water-cooled engines that I know of run thermostats. Without them the piston is always trying to expand in a cylinder that's always trying to contract. It blows me away when I see the scuffed skirts on peoples' pistons. For whatever reasons my engines don't suffer that (I chalk it up to favorable expansion rates that a thermostat allows). But forgive me; I know I'm preaching to the choir.
And I'm all for reducing frictional losses if the engine can live without the extra cooling air volume. I think people make a bit too much out of stock-sized pulleys. I mean they have their time and place, but look at the average VW dual-port 1600 engine. They have about .065" deck PLUS a .055" step in the head. That's a FAT deck! I mean you have to try and get a flame front across the whole bore! The surface area is INTENSE! That greater surface area combined with the extra time spent burning the mixture will make ANY engine run warm! It's almost surprising that those engines will run as cool as they do.
But when you run tight decks you confine the combustion surface area to the comparatively smaller chamber for the duration of the hottest part of the burn. So in that sense the fire in the hole has less of a chance to transfer heat to the head. Then there's also the ignition timing. Confining the burn to a smaller area also requires less lead timing. So you can fire the plug later. And let's face it, the later we can fire the plug the less time the combustion process has to transfer heat to the chamber. Lighting the fire real early in a bad chamber design increases the resistance on the piston as it's on its compression stroke--the engine basically fights itself! But the tighter chamber mitigates all of that.
And that brings me back to my main point: increasing the engine efficiency alleviates some of the burden on the cooling system. And a slower fan speed may sufficiently cool the engine if the cooling system doesn't have such a load. So I'm open to the idea that a properly designed engine doesn't need a huge pulley. It's not like we're going to climb hills at 35mph in fourth gear. Auto designers have to account for bad driving habits like that. That might be what justifies the large pulley.
Man, that was long winded!
Anyway, if you want to read up on a more concrete example of what serious builders are doing with squish, read the following. It's not even all that new, either.
John Kaase is an engine-building demigod of sorts. He's won Popular Hot Rodding's Engine Masters program in the past with engines he's built. I don't know if you are familiar with the program, but builders have to design engines that run on pump gas. They tally the dyno figures and calculate which engine made the most power across the entire range (rather than peak horsepower at one speed, for example). It's a great program that takes the whole engine into consideration rather than a few figures, and it's really opened a lot of peoples' minds to engine design.
Anyway, in '05 Kaase built a 510 Poncho that made 820hp and 767lb-ft torque--again, on pump gas. Among other things, he ran a 37cc chamber. Now even in VW circles a 37cc chamber is TINY. Forget about building a 2.3-liter engine--four cylinders of a 510 comes out to 4.2 liters. Now imagine a 4.2-liter boxer with 37cc chambers. Now imagine giving it .035" quench clearance (deck height).
He got away with that by using a piston with a fairly substantial crown volume that reflected the shape of the chamber (mirror dish). So the chamber was narrow so the flame could get across it fast; it was just deep, which is still easy for the flame to get across quickly enough. Even then the static CR worked out to 12.4:1 static CR. Now of course the huge duration on the cam bled off a bunch of pressure and got the effective CR to a normal level that could run on pump. But even then I can guarantee you that the effective CR is a lot higher on that engine than it would be in a 'normal' performance engine.
Now I fully understand that you're not building an engine to the same order as Kaase's, but a lot of the same principles that he used apply to what we should be doing in this neck of the industry. I mean there's nothing saying that our air-cooled engines can't run long and well on pump gas with 10:1 static CR. It just takes good chamber design, correct cam timing, and so forth.
Man, talk about making an already long-winded entry even longer!
If you want to read more about Kaase's engine, read the PHR article at
http://www.popularhotrodding.com/engine ... index.html