ptuomov

ptuomov

ptuomov

Here are the more exact numbers from a recent run at the same power level. Barometer 14.80 psi and intake manifold absolute pressure 30.69 psi. That's 211.6 kPa absolute, compared to your 186 kPa. The absolute pressure ratio between your engine and mine is 0.88. On the other hand, the displacement ratio is 6.54/4.95 = 1.32. Peak power rpms are similar. Multiplying the absolute pressure ratio with the displacement ratio gives 0.88*1.32 = 1.16. The combined effect of higher manifold pressure and lower displacement is that your engine has a breathing potential advantage by ratio of 1.16x. Yet, by my accounting, even with pump gas and the resulting very retarded ignition timing, my engine is within 5-6% of your power level on race gas. My engine is basically all stock or modified stock components, none of the fancy or expensive stuff inside. What's going on? Is this just the belt driven supercharger being that much less efficient than a correctly matched turbo? Or is there more to it?

Carl Fausett

OF COURSE there is more too it. Much more.

What have you changed the exhaust valves to in order the may tolerate the additional heat that the turbo will produce? Valve guide materials? Exhaust seat materials?
What have you done to the cooling system to ensure that you will be removing enough heat from the exhaust valve seat areas in the heads?
What cooling system pressure are you going to to raise the boiling point of the coolant?

Time will tell the story - getting your TT up and running once to match HP with one of the SC engines is just the start of the job. We'll see how it holds up over time.

Rob Edwards

I suspect that the valve seats and coolant system are both stock. Nothing about the cooling system will ever be a limiting factor in Tuomo's ability to use this motor on the street, he'll never be close to full load for more than 10-15 seconds at a time. At which point he'll be going 150 mph. How many times can one do that in public before attracting too much attention?

ptuomov

Last iteration already got a lot of miles on it. The first shakedown at 600+ rwhp level was a 1000 mile continuous run from Kentucky to Boston. That was five years ago and with a completely stock engine and the turbo system. No issues.

The original 1980's S4 engine was much more overbuilt than what people give credit for.

The blue engine in the car has Ferrea stainless steel exhaust valves. My opinion is, however, that like almost all aftermarket "performance" products, the original 928 exhaust valves from the 1980's would hold up to the turbo heat better. The same goes for the valve guides: I have supposedly durable aftermarket guides, but in reality the 1980's stock guides are probably far better in quality. Most aftermarket components are overhyped $hit. Fortunately, the valve seat inserts are stock so I am confident that at least they'll last with the turbo heat.

Heat management is always an issue with the 928. The stock radiator is great, but the stock airflow thru it is not. You really don't want to place an intercooler in front of the radiator, if you run anything like the stock S4 bumper cover. Instead, you need more and/or bigger holes! That's why the intercoolers are in the fenderwells, with their own bumper cover openings, and the air flow paths are sealed to the maximum extent possible. The thermostat, coolant reservoir cap, etc. are all stock. Sharktuner logs haven't shown any temperature problems so far at any iteration.

Underhood temperatures get up, if one doesn't run the car with belly pans. With belly pans installed, the underhood temperatures are normalish. Kind of like stock S4, right? What helps is that the exhaust manifold pressures are kept in check with free-flowing turbine and turbo-back exhaust. In terms of EGTs, what helps there is compression and cams allowing sensible ignition advance. EGTs are well within Garrett's limits. Choose the turbo incorrectly and you get in trouble, but these turbos match the engine.

Internally, the cylinder towers will be cooled very well as they are the original casting aluminum towers. I don't have any heat transfer inefficiencies that cast iron or steel sleeves would cause. The the top of the cylinder towers will be well cooled, certainly better cooled than with an equal-power sleeved motor. The cylinder heads have additional cooling passages around the exhaust valves, mostly to let out any gas bubbles that could inhibit cooling. Seat widths are appropriate for turbo use, but so are the 100% stock 928 S4 valve seat widths. The heavy pistons have massive thermal inertia and could be out of a diesel engine, so they can live with the turbo heat as well. Top ring land is so thick (wide?) that the ring is far down in the bore, absent detonation we're going to be well below the critical temperature for the top ring from 944 turbo. Again, the original engine from the 1980's can take almost anything except high rpms and dumb "performance" modifications.

ptuomov

The 928 S4 cooling system is great, except it needs more air flow.

The problem is that if one modifies the engine in a stupid way, one can easily create various cooling problems without adding meaningfully to the power.

hacker-pschorr

Well, I guess if we are starting the machismo dyno comparisons, why not add one more to the mix:

Who's going to be the first 928 to beat Todd's 9.5 seconds @ 152.7mph 1/4 mile run?
Bonus challenge: Tires no larger than DOT rated 255's out back.
At the end of the day, isn't the goal going fast?

His car is currently 408lbs lighter than it was during that run (with the same weight distribution). Not sure when he'll be back at the drag strip again. He is slowly dialing in more power in 2nd gear and still not achieving wheel spin.
He thinks there is another 250hp on the table with his current setup, but it's running so well, and so ungodly fast, he's not making major changes to the power-band above 2nd.


Silliness aside, can we keep this thread on track please? Over 1,900 posts, lets not go off the rails now. Totally different builds, totally different purposes, different fuels, 3 different ECU's........it's fun to make comparisons but lets not get stupid.

FredR

I take it you dumped the cooling air slats long ago?

Carl Fausett

I completely agree. 1/4-mile racing isn't my thing. I like to build for endurance.

The other non-comparable (whether you like it or not) is that Todd's build is a one-off. One-off builds are simpler - you can do whatever you need to get it to work on ONE car.
My builds tend to be reproducible and able to be purchased as a kit, installed by others, and fit many models. That's a BIG difference. Not same.

ptuomov

Todd Tremel's comparison is fair and relevant, I think, as it's another 5.0L turbo car.

The main differences are E85 fuel and nitrous shot. My car is a street car driven in New England, so it'll have to run on 93 pump gas. With turbos, 93 isn't going to ever keep up with E85 + NO2, so in that sense while it's fair and relevant comparison, his car is going to come ahead because of the fuel.

One thing that I am going to find very interesting is comparison of Todd's custom intake manifold vs the stock S4 intake manifold in my car. I am thinking that in the 2000-6000 rpm range, the turbocharged stock S4 intake manifold may get surprisingly close to Todd's custom sheetmetal intake. Above 6000 rpm and turbocharged, I think the short-runner sheetmetal intake is going to walk away from the S4 intake. For this purpose, I am wondering if you or anyone else has a pump gas dyno run from Todd's car. I know he did one, I just don't remember where it is. I have a vague recollection that it was in the high seven hundreds in rwhp at close to 7000 rpm, that's just a vague memory. This isn't intended to be a pissing match comparison, I am legitimately curious how much of a difference the intake makes under boost. If it only makes a difference above 6000 rpm with pump gas, then I am likely to hang on to the S4 stock intake for the foreseeable future, as the rest of John's turbo kit is designed to run the best in the 3500-6000 rpm range.

ptuomov

Those in the radiator shroud? Yes those are gone, mostly to make room for the compressor inlet ducting.

ptuomov

I'm not selling anything.

How many hours or miles do you have on your 6.54L supercharged 900hp engine? I am fairly certain that my completely stock '87 engine had more turbocharged hours on it. Both bumper to bumper traffic and some faster little runs. The blue engine has fewer hours.

I am wondering if there would be a way to test this reliability in some setting, car against car? Any ideas? Some sort of endurance test?

Strosek Ultra

Tuomo, the optimum reliability test used to be starting in Hamburg and go WOT all the way down to Munic. The problem nowadays is the heavy traffic and you car being too fast in order to perform the test in full. I have heard of stock 928s especially GTSs failing from bearing failure.
Åke

Carl Fausett

I agree. Its a lot better comparison that to compare it to my build or application.

6 years of racing from 2009 to 2015, including many road races (Road America, Gingerman, The Autobahn CC, etc) plus Pikes Peak and Bonneville. Videos of all of this is posted.
I shouldn't have to tell you that racing is far more strenuous on a motor than street.

I wish you continued success with your project.