No Go said:
Great info!
I've read about the rake as well. I mainly wanted the car lower, but I also followed the factory rake angle as much as possible (ie lowered front and back equal amounts) I would have to say with a full tank of fuel, the car is level front and rear...as gas burns off, the rear rises. Course I did the run with a full tank, so probably little rake. It was stable.
I see on the airdam. 180 mph wind is something that is hard to imagine. I suppose if I were to add something to the front, a semi flexible piece that reached almost to the ground would be ideal. Not a real player for me as car is driven daily to work.
I doubt fuel burn is going to be much of a balance issue unless your talking a third a tank or more consumed in one run. The rake has to work in conjunction with less rear roll stiffness to truly make the car more stable. This is why the record run cars had no rear antiroll bars. Also, on the lowering, you want to be careful to not lower the car so much that you run out of suspension travel. I say that because I do not know how smooth the surface is which you are using. If you have to trade off ride height for travel, always take the travel and keep the suspension off the jounce bumpers. That will make the car more stable over rough surfaces. At 60 a road might seem smooth as glass. At 180, it might be a hell of a ride.
I have datalog of over 7000 rpm as well. On the dyno, it was solid pull to 6800 rpm (last rpm tested) and that was my goal in building the engine which is a SCAT 4340 3.75 crank and 5.7 H beam rods with SRP pistons. I have confidence on rpm capabilities of the engine especially after a One Lap of America and 12,000 miles under its belt already.
If you have a steel crank and those rods, forget what I said about running to 6900. My guess is your engine limiting speed will be more valvetrain related than bottom-end related.
I remember reading about Lingenfeltor having difficulties with the Cats getting damaged after topend run. I've done several, so hope they aren't already. My AFR is still rich (12.7) at high rpm, but not as rich as stock (11.8). A set of headers would really help the combination I believe, but my first instinct is way too much work to be doing for the number and then switch for emissions.
It's a very difficult problem because with long periods at WOT cat temp gets high enough to destroy the cats but then, once you add a little extra fuel to cool them, you kill power and, if you add too much extra fuel, you melt the cats due to rich mixture. Really, for what you are doing, you need to take the cats off and use headers, make sure your injectors have no more than 2% flow spread, baseline your AFR at 12.5, then chassis dyno test to get best AFR. Once you have the AFR set, you need to make sure the engine is not getting any knock retard. If you see that with your scans, you need to increase the octane of the fuel.
This winter I'm looking at freshening the engine (rings/bearings) a bit more hand porting to the heads, and switching to a smaller GM 846 camshaft maybe with some 1.7 rockers to get the valve lift up to my near .600 goal.
I wouldn't be looking for a specific valve lift. I'd be looking for the max. area under the curve you can get then I'd look at port work such that flow is maximized, ie: you need a professional head porter and a flow bench. You might even find that no amt. of port work is going to make the LT4 casting work well at .600 valve lift. If that's the case, AFR's got some great aftermarket head choices.
But...to be really honest, if it were my motor, I'd forget more head work and more lift. I'd concentrate on getting the cats off (because at your current power level, there'll probably be a 30hp gain by taking them off) and optimizing the AFR, becuase I think you've got more than enough horsepower. I think I'd look at chassis set-up and finding a 4-5 mile distance as ways to get to 190-195.
Nitrous is little further down the line, but your suggestions have been part of my concerns. Besides losing an engine at that speed could more than hurt my wallet! I wonder how much gas would be consumed in a 3 mile run if set at 100 hp.
If you're going to use nitrous oxide in a long-duration WOT situation, there are many issues you're going to need to address....fuel flow, nitrous and extra fuel distribution, nitrous pressure, weight and packaging of the hardware and the bottles. You may even need to address cooling issues. To be honest, for this type of race-only application a properly installed and calibrated centrifugal supercharger might be a better choice.
BTW: When I first posted to this thread, your car and testing seemed familar. After talking to Jim Mason by email, I know who you are, now. Just curious, where are you doing your "test" runs. Maybe you'd better email me the location off-line.
