LT4 New LT4 Topspeed!

[No Go]

184 MPH in calm conditions, flat concrete, headlights up, 95% humidity, but 60 degrees.

My data logger shows ~6300 rpm in fifth gear with my 275/40-17 tires.

3 miles from a stop covered in less than a minute-all WOT!

Combination is stroked LT4 (381 CU IN) with 11.9 CR, GM 847 camshaft, CNC ported heads/intake, 58 MM TB, Corsa exhaust, stock exhaust manifolds, and 36 pound injectors.

Disappointed with the results, but nonetheless it was smoking!

 

[FX GS]

Disappointed with the results, but nonetheless it was smoking!

What were you expecting?

 

[B17Crew]

184! :eek

B17Crew

 

[Vettefan87]

Nerves of steel man. Way to much for me to think about doing.


Justin

 

[No Go]

I was hoping for a number around 190 mph considering the HP increase over stock.

Mine dynoed at 260 to the wheels on a Mustang 1750 dyno, now 363 WHP. This equates to 410 WHP on a Dynojet.

Car was rock solid...just a matter of keeping the throttle pegged for such a long time...

Car is geared for 200 mph at 6800 rpm in fifth gear (OEM 3.45 axle).

 

[8388]

Nice run

 

[Hib Halverson]

I was hoping for a number around 190 mph considering the HP increase over stock.

Mine dynoed at 260 to the wheels on a Mustang 1750 dyno, now 363 WHP. This equates to 410 WHP on a Dynojet.

Car was rock solid...just a matter of keeping the throttle pegged for such a long time...

Car is geared for 200 mph at 6800 rpm in fifth gear (OEM 3.45 axle).

Lower the front of the car slightly.
Brace the front air dam.
Do it in daylight so your lights are retracted
If you really want to get into this, temporarily remove your side-view mirrors.

Try it on a day where there is no crosswind on your test track
Run it out for 4 miles....that you made 184 in 3-,mi. tells me your car will do 190 and maybe then some. The problem with top speed testing is the last 10-15 mph comes very slowly and requires a test track with a long distance available.

The ZR-1 "record run" car of 1990, averaged 190-mph at WOT with only 440hp and that was on a 7-mi. low-banked oval. I've been in a 475hp, C4 which made 0-185-0 in about 4.5-mi. at a GM test facility.

It seems that you've got somewhere between 440 and 500 hp, depending on which dyno you're on, so I'd think, given the right race track, you could make 190 mph.

 

[No Go]

Hello Hib

All good points...

- I have lowered the car about 1 inch...any more and getting in my own driveway will tough.

- Does the front airdam flex during this slipstream?

- Headlights retracted would make a big difference, but since I have no experience one way or the other, I could only guess at the improvement.

- Side view mirrors: I would like to keep it as I drive it on the street.

The last bit of MPH does come slow. I viewed the speedo only 3 times during the run...once at 153, then 175 (last topend), and finally 184 when I approached the shut down area. I do not know if the car was still accelerating, but very slowly if so.

- The temperatures and wind were great, but the humidity was unreal.

- The engine combination peaks at 6300 rpm, but does not drop significantly to fuel cutoff (6900 rpm). I would expect the camshaft to make power a bit higher as well considering the duration (296/304) and .575/.595 lift. This winter will come with some more changes...not all those for more power unfortunately as emissions are part of the plan...

- I guesstimated 455 engine rated hp (compared to my original 330 hp rating) so it checks with your comments.

Thanks

I do have a nitrous port in the TB...

 

[Hib Halverson]

Hello Hib

All good points...

- I have lowered the car about 1 inch...any more and getting in my own driveway will tough.

- Does the front airdam flex during this slipstream?

- Headlights retracted would make a big difference, but since I have no experience one way or the other, I could only guess at the improvement.

- Side view mirrors: I would like to keep it as I drive it on the street.

The last bit of MPH does come slow. I viewed the speedo only 3 times during the run...once at 153, then 175 (last topend), and finally 184 when I approached the shut down area. I do not know if the car was still accelerating, but very slowly if so.

- The temperatures and wind were great, but the humidity was unreal.

- The engine combination peaks at 6300 rpm, but does not drop significantly to fuel cutoff (6900 rpm). I would expect the camshaft to make power a bit higher as well considering the duration (296/304) and .575/.595 lift. This winter will come with some more changes...not all those for more power unfortunately as emissions are part of the plan...

- I guesstimated 455 engine rated hp (compared to my original 330 hp rating) so it checks with your comments.

Thanks

I do have a nitrous port in the TB...

It's not that the car needs to be low (that does help, I admit) but that it needs to be raked slightly. That actually may hurt aero a little, but it makes the car more stable over 150 mph. If it were me, I'd make the trade-off for safety. The C4 exterior has insufficient downforce at the back above 150. That tends to make the car really, really sensitive to steering input and cross winds and, even, slight troughs in the road. The two record run cars back in 90 not only were raked but they had a lot of caster dialed in the front and no rear stabilizer bars. All that was in effort to make the cars more stable above 150 mph.

The air dam will flex backward at high speed. That degrades aero. If you're really serious about this high-speed stuff you'll need to make some aluminum or hard plastic braces. Probllem with that is, the dam is no longer flexible for normal street use.

Headlights out is a very large hit in aero. I'll bet that's 2-5 mph, right there.

Good air, obviously helps the engine, but interestinly does not help aero. Humidity helps the engine a lot...cools the intake charge and makes the engine less likely to detonate. If you were taking engine controls data during your tests, was the engine getting any knock retard in the long WOT ruin in high gear?

You've got your limiter set at 6900?!!
Brave soul, you are.


Seriously, I think you've got a car that capible of well over 180 and perhaps a little over 190...and you're not going to need nitrous oxide injection to do it. All this aero stuff aside, the car might do 190 as is, if you ran it out for another mile. The only thing I'd watch, if this engine uses a stock LT4 crank and rods, is how long you've run the engine at WOT above 6500.

if you do decide to use nitrous under those conditions, make absolutely sure of the system's ability to supply gas at a consistent pressure for more than the length of time of your run. Also, you want to make sure you've got no problems with detonation and no problems with lean AFR during sustained WOT operation with nitrous. Any of those problems in a long period of time at high rpm and WOT will cause catistrophic engine failure. Setting up a nitrous oxide system for a long period of WOT in a top speed run is a lot more critical than setting it up for drag strip use or brief "squirts" on the street.

Lastly, you mentioned this car has stock exhaust manifolds. If it also has cats, if you live in a state that has an emissions test, considering the cost of replacement cats, I'd take them off for this top speed stuff. With sustained periods of WOT, exhaust temperatures probably are high enough to damage the cats. If you need them to stay street legal, there's no point in destroying them.

 

[CKA_Racing]

You could have saved yourself the trouble and just ask Bluewasp what his top speed is. He has the same HP as you give or take a few. He has done some top speed runs on the autobahn in germany. Matter fo fact I think his top speed was 184mph also.

 

[No Go]

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 was expecting the lights to hurt, just not sure. I will have to try to tuck them in next time...

Humidity was thick. I would expect that to remove more available horsepower then help via less detonation, cooling, etc. Otherwise things were quite optimal. Maybe next time...

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.

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.

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.

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.

Thanks!

CKA Racing...who said this is work? This was my goal for owning this vehicle. Besides Bluewasp is making about ~65 less WHP than I...don't get caught up in different dyno numbers game...

 

[Hib Halverson]

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.

 

[bob eshleman]

200+

HI I have a 96 /lt4 RED VETTE.All external mods so far approx 400 hp.I am building a 396 stroker that I believe will take then her over the 2oo mark.Afr 210 heads 11.5 comp.,bore 4.345 J/E pistons ,3.875 callies.5.85 rods, cam is still open.not much sense to twist over 7200.Already have brethless cold air,h/p MAF, T/B by-pass, 53.5 t/b,TPIS headers,3" exhaust with Dr Gas X pipe and P/Effects.C5 front brakes/ceramic pads.285 up front 315 rear. Brake bias spring ,5 spoke and custom rotors /metal hoses.525 hp is targeted for this.Please advise, will see limited use @ W/Glen. thanks bob.

 

[No Go]

Hib: Well, I guess I expected more from the 'lumpy' combination. I agree headers is something that will probably be worth considerable amount-I would expect the increase be in the topend region as well. Headers and Corsa exhaust becomes very loud-no cats to dampen the effect...so just not sure if that is worth it too me.

I have nearly reached my streetability point in other words.

The .600 lift is a self imposed rule again referring to streetability. It appears any more lift and statistics show quicker failure down the road due to valve spring fatigue, etc. I run the new Beehives by CC and am very impressed. I had a set of double with damper springs and Ti retainers...valve float after 6300 rpm. Add the beehives and no changes had solid valve actuation to 6800 rpm. I do wish I tested higher...I was just glad at the time it didn't blow since I assembled it myself. It seems understood that 7000 is the limit with the factory PCM as well so only a couple hundred rpm left anyhow.

My cylinder heads are somewhat professionally done...CNC Cylinder Heads in Florida...I have flow sheets before and after so they are good, not awesome, but nice increase in flow...catch is they are 219 cc capacity in the intakes! Huge...figured good as long as I can deal with the sogginess of the slow flowing ports at low rpm. Inreality, it does have less low rpm torque than the stock LT4, but I really noticed this only after adding the GM 847 camshaft so really blame the huge overlap for killing the extreme lowend...I'm talking 1000-1500 rpm. Above that it acts like a stock LT4-not bad at all. I'm looking at some hand touch ups to get the heads over 300 and 200...if that is possible. But goes back to the exhaust plug...or system diminishing the effects.

I have/had enough budget to do this once with minor changes...so a blower is out...besides they seem way to finicky...ie...2001 OLOA-every car with a turbo or supercharger blew up before the 4000 miles was complete...unacceptable!

bob eshleman: Your combination is similar to mine other than the extra cubic inches and headers/3 inch exhaust. It definitely takes a lot of power at the right rpm range to make it happen. I chose a high rpm combination as 5th gear at 6800 rpm with my 275's and stock 3.45 axle is 200 mph.

I know it can be done. Good Luck!

 

[Hib Halverson]

200+
(snip)
525 hp .

To go 200+ in a C4, you're gonna need more than 525 hp. You'll also need a deck spoiler and huge freakin' cojenes.

 

[bob eshleman]

200+

How much more than 525-550 HP will I NEED ON A 396 SMALLBLOCK? I crewed on a 3000lbs 500hp Buick Sommerset with 302 cubes that went 180 at Daytona while having to use the inner track and 167 up the hill at W/ Glen with track gears and that IMSA car was as not clean as a c4. I also have 2 more gears and more motor. THANKS BOB.

 

[No Go]

I don't honestly know.

Many folks have said that 180 is where things get really tough...aerodynamic drag increases significantly of course.

Your 525-550 hp is what kind of hp? Wheel on a dynojet? Wheel on a mustang? Crank horsepower?

I have:

I have ~500 crank horsepower right now. I don't doubt headers, cubic inches, and larger exhaust will gain you an extra 50 horsepower, but if I'm maxed at 184, then it will take more than 50 hp to gain 16 mph.

I wish Bonneville or Maxton was closer, I would find out...