Boost Transition Flat Spot

[patkeefe]

Goran:
I sincerely appreciate your input on this.
Fist off, my turbo is what came with an old BAE kit. It is a Rajay, definitely old time turbo. What I have been able to determine, it is a T04B in "V" trim. It makes positive pressure in the 2200 RPM range, but boost really comes on at 3500. I have rebuilt the engine last year, with stock early SC pistons at 8.5:1 CR, and installed DC-15 camshafts from camgrinder (DR Camshafts). They are a bit more agressive at low end than SC cams, but not as much as 964 camshafts. I discussed the camshaft selection with John Doughtery (camgrinder), and we felt these would be a good choice. Just driving around town, etc, the engine operates just as though there were no turbo, and has all of the driveability of a stock SC. I also use stock SC heat exchangers, so I do have a bit of flow loss into the turbo. I suppose I could upgrade the exhaust to get better flow into the turbo, and faster spool, but I am cheap, and exhaust upgrades are quite expensive. I did port match exhaust manifolds, and all exhaust components, including the turbo mount flange, so I removed as much restriction as feasible.

What I refer to as an airbox is the factory airbox. It was made of some sort of thermoplastic. Standard CIS component from the factory for the n/a cars. It was never meant to be turbocharged, which is why there is such a difference in n/a and real 930 systems. Old one must have leaked, judging by the way the car has responded to the blocking of the popoff plate, and the new ss airbox. I think I have increased the area under my torque/HP curves substantially by the simple repair you guys have suggested.

I am in the minority of people who believe water injection is the greatest thing since sliced bread. My system is not some old windshield washer pump derivative. System has two different safety interlocks, plus electric solenoid valve and accumulator. I am possibly the only person on the planet who willingly injects water into his Porsche engine. However, I studied thermodynamics in college, and phase change is very effective in rmoving heat from the compressed air. I have designed a water/air heat exchanger for the car, but have yet to build it. I will take a look at the Saab intercooler you mentioned. Also, because I limit boost to relatively low level, I do not get such hot air after the compressor. Hottest air I saw was at the track, at 180F (I left my water injection off one run as a test). I would guess cars in the 1 bar range or above will see substantially higher temperatures. I use windshield washer bottle as the tank, which is about 2-1/4 gallons US. I can go two 25 minute run groups at the track before I empty the tank.

Also, I agree with you 100% about keeping AFR's above the 12.2 range. I really think most people will allow richer AFR on boost as insurance against detonation. I agree this is a waste of fuel; combustion chemistry says it is unnecessary. The nice thing about water injection is you achieve the exact same purpose as fuel over-enrichment.

I think next dyno run will be much better, and I will definitely let you know the results. I have calculated BMEP at over 194, which is not bad at all.

Pat

[rfng]

I would check for a vaccum leak around the injectors. My SC ran best with about 90-95 fuel pressure, and a 3.0 fuel mixture.

[RarlyL8]

Water/alcohol injection is very effective in cooling the charge and offsetting knock.
The reason I won't use it is compexity. It is one more maintenance item that I also view as a bandaid. It is optimal to have a system that requires no consumable fluids, electric motors, or added complexity. Air-to-water intercoolers suffer the same complexities as water injection.
An air-to-air intercooler is by far the least compex and the most maintenance free. There are several cars on this board with no-tail intercoolers that can be used for ideas on how to do it, unless you just plain don't want an intercooler.

[patkeefe]

Well, after a couple of months of trying to get my CIS working corectly...the thing got real rich at the track in May...and going through all sorts of histronics and hand wringing, I finally decided I MUST have a leak somewhere. Sure enough, the bottom seal of the SS airbox blew out on a backfire, probably right around when I last took it to the track.

So, I'm going to pull the engine, fix the airbox, tweak a few other things while the engine is out, and fit the airbox with a popoff valve, which I had sealed closed to keep the boost from escaping (he original problem which stated this thread). I have some springs which will likely work, and I will test the pressure relief function when I get the airbox sealed up again.

It still amazes me, that even with a blown up airbox, that little 3 liter makes a lot of power. If I ever get it sorted, it should be fun.

[patkeefe]

I guess these butt welds on the stainlees steel airbox can't take a pressure wave too well.

[patkeefe]

[patkeefe]

I took the wirefeed Lincoln to it, and J-B Welded the crap out of it for good measure.

[patkeefe]

No photos, but I have the test setup. The airbox holds 80 PSI now. I am still working on the relief plate to get it to stay closed below 20PSI.

I am convinced that every SC airbox on this planet leaks, and probably all 930"s with mixture problems have a leak on the airside somewhere.