slate blue

O.K, I didn't take these pics, they are of a Cosworth intake port, notice the shape is fairly similar to the S4 port in the shape of the port, however the port in the pic is more of a downdraft style. Normally raising the roof and possibly filling the floor to maintain your velocity. You must measure the CSA as too big a port will not have much or any ramming effect.

The CDs I sent you Tuomo will give you lots of info on this type of stuff, It also goes into making your own flow bench with all the software you need to get meaningful results. Cheap too.

Keeping the intake straight is a big help as the air is quite heavy and it doesn't like to turn. In a Nascar engine at high revs, it is like it is being turbo charged with 7 psi boost, those Cosworth heads had a volumetric efficiency of 145%

As to heads losing flow when bigger valves are fitted, that sometimes has to do with the discharge co-efficient or D/C, it is the air losing too much velocity as gets to the throat. That is the throat is too large, or it might also be larger valves shrouding the bore. Depending on which heads and which bore size. Maybe the short side was not treated correctly, so many things it could be. It would be interesting to know the valve sizes.

Greg

ptuomov

Other than Variocam obviously and the ITBs, what's would you say is the main difference between the two heads that you've built? Are the heads ported identically, similarly, or completely differently?

ptuomov

This is the second Cosworth head out of the two that I've seen which has the divider that splits the port knife edged. I thought one was "supposed to" bullnose it, not sharpen it. Live and learn.

ptuomov

Here's an inlet port velocity calculator that might interest some of you: http://www.rbracing-rsr.com/machcalc.html

The calculator computes an index of air speed in the intake port. If the index is above 0.6, I would interpret it to say that a bigger valve and/or more lift is needed. The calculator says "The mach index for maximum volumetric efficiency is .6 . Beyond .6 the volumetric efficiency falls off. As the mach index rises beyond .6 the volumetric efficiency can be increased by later inlet valve closings (60 to 90 degrees ABDC)." See the page for full explanation.

For S4 at the factory redline:
"Your bore size is 3.94 inches with a stroke of 3.11 inches and has 2 inlet valves with a diameter of 1.457 inches. Running a valve lift of .354 inches at 6800 RPM, the inlet valve mach index is 0.508."

For S4 at 7300 rpm:
"Your bore size is 3.94 inches with a stroke of 3.11 inches and has 2 inlet valves with a diameter of 1.457 inches. Running a valve lift of .354 inches at 7300 RPM, the inlet valve mach index is 0.546."

For GT at the factory redline:
"Your bore size is 3.94 inches with a stroke of 3.11 inches and has 2 inlet valves with a diameter of 1.457 inches. Running a valve lift of .393 inches at 6800 RPM, the inlet valve mach index is 0.458."

For GT at 7300 rpm:
"Your bore size is 3.94 inches with a stroke of 3.11 inches and has 2 inlet valves with a diameter of 1.457 inches. Running a valve lift of .393 inches at 7300 RPM, the inlet valve mach index is 0.491 ."

Seems like the inlet valve size is not an obvious limiting factor for 5.0L motors.

Of course, this is just a web calculator in which I punched some numbers, so don't believe what you read and caveat emptor. I might have even misunderstood the whole thing, wouldn't really surprise me.

ptuomov

Another interesting calculator, to me anyway: http://www.rbracing-rsr.com/comprAdvHD.htm

"To see what the effect of bore, stroke, rod length, cam timing, compression ratio, boost pressure and altitude is on your dynamic compression ratio, simply enter the seven variables and the calculator will display the results in a new pop-up window. This will give you an idea of what happens before you assemble your engine and also how you should plan your motor for your target altitude whether it is for racing or day to day operation."

So I punched the numbers in for a four configurations, S4 without boost and then S4 with 7 psi, 14 psi, and 21 psi:

S4, no boost:
"Your engine summary is as follows: Bore 3.94 inches, stroke 3.11 inches, rod c-c length 5.906 inches, with a static compression ratio of 9.4 :1. Your camshaft specifications call for an inlet valve closing of 36 degrees ABDC (after bottom dead center). Your chamber volume is 73.97 cc's. With this camshaft your dynamic, or effective stroke is 2.88 inches. Your dynamic compression ratio is 8.78 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 179.26 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 0 PSI is 8.78 :1.

S4, 7 psi boost:
"Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 7 PSI is 12.96 :1." This I believe is safe.

S4, 14 psi boost:
"Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 14 PSI is 17.14 :1." Engine is probably right on the edge on pump gas.

S4, 21 psi boost:
"Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 21 PSI is 21.32 :1.". Engine almost certainly detonates on pump gas.

Ok, so with stock pistons the 14 psi and 21 psi boosts lead to too high dynamic compression ratios. (I guess someone with more experience would have not needed a calculator to figure that out.)

Let's see if put in 8.3:1 low compression pistons. I picked 8.3:1 because that is what Subaru wrx sti with the same basic dimensions and a turbocharger uses:

S4, 14 psi boost, 8.3:1 pistons:
"Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 14 PSI is 15.15 :1." Probably safe.

S4, 21 psi boost, 8.3:1 pistons:
"Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 21 PSI is 18.85 :1." Past the edge, but not by much.

Again, caveat emptor. Might not even know how to use, let alone interpret the thing.

BC

High compression and high boost works just fine when you use ethanol.

slate blue

By Ptuomov The reason that was relayed to me was the reason for the knife edge is space concern, no room for a bull nose. The size of the port is almost identical in relation to the CSA. However what you need to remember is that different designed ports will require different CSAs. The better the design the less the CSA.

Greg

slate blue

Sterling what were your flow rates? I know what your shop means about flow figures and they are not the be all and end all. CSA and velocities are important as well, wet flow is good too however from what I have heard it tends to be more important to 2V engines. They have more issues as the 4V engine are just fundementaly better.

On another front, Sterling instead of dropping the Variocam, why don't you have the cams regrind for greater lift duration instead of ditching this project? How much lift have you presently got. I am trying to come up with a way to get at least 12.7 mm lift. It is only a side project as my 2 V project proceeds slowly. If you are going to change cams you really need to know the flow rates are at various lifts before you choose a cam.

Greg

ptuomov

True. E85's not very convenient here, though.

Also, the dynamic compression ratio doesn't really tell the whole story in the charge is cooled post turbo. Above, I was fudging a bit making intercooler corrections by hand. Without intercooling, the engine would not live at those dynamic compression ratios with pump gas.

ptuomov

So what you are saying is that the port should have as straight of a shot as possible and nearly constant cross-sectional area from the valve throat to all the way up the runners? I say nearly constant, because I assume that you'd want to taper it by a couple of degrees maybe. This would keep the mixture velocity constant (or slightly accelerating if counting the small taper), allowing for low drag, high velocity, and good flow from a relatively small cross-sectional area. Did I understand this correctly?

ptuomov

I have been reading in a couple of places that the model year 89+ heads are better than 87-88 heads. Is this true and if it is true then why?

BC

They have thicker bosses around the bolt heads. If boost is run, and alot of it, usually one would want the "thicker heads"

Rob Edwards

The thicker bosses means you need 19mm longer head bolts- 18 of the 928 101 231 02 and 2 of the 928 101 233 02, so add $250+ to your parts list just for the head bolts.

ptuomov

>>BrendanC: 'They have thicker bosses around the bolt heads. If boost is run, and alot of it, usually one would want the "thicker heads" '

Why would one want thicker heads? I know that's a basic question, but bear with me. Do the '87 heads flex? If they flex, are the '89 heads thicker everywhere and help eliminate this flexing?

Has anyone ever found the limits of the '87 heads? Has anyone from the supercharger crowd really gotten them to flex (without first blowing a head gasket and softening the whole head)?

>>Rob Edwards: 'The thicker bosses means you need 19mm longer head bolts- 18 of the 928 101 231 02 and 2 of the 928 101 233 02, so add $250+ to your parts list just for the head bolts.'

If I am going with head studs anyway, there's no marginal cost, right?

BC

There have been reports of cracked heads in the region specifically re-engineered by Porsche for the 89 heads. Call it extra insurance.