65mph@3100rpm!!! Help PLZ

[Guest bastaad525]

I couldn't say for sure, 0% or not, but I know my speedo now is VERY accurate with my current setup. And it's the stock '72 speedo. At 75mph it is dead on (speed and engine rpm) with what the transmission calculator says it should read.

[Tony D]

"The car is screaming on the hwy 65mph@3100rpm (not cool). Is there a way to resolve this problem? "

 

 

60 or 65mph at 3100rpm IS cool, and NOT a problem!

 

You are just below peak torque at cruising speed on the highway, making for PERFECT top gear roll-on throttle response. You will be in the HEART of the tourque band on a stock engine, and will accellerate briskly.

 

This is not a problem, and is not affecting your terminal top speed, either. If you speedo was DEAD ON, you top speed would theoretically be 120 to 130mph at 6200rpm.

 

My old Fairlady Z with a factory direct-drive four speed and a 3.70 gearing, with STOCK HEIGHT TIRES would pull 6350rpm at 200KPH. Some math involved, but that's dead on, and I NEVER had a problem with that car.

 

Currently my 75 2+2 runs at around 3500rpm at 70mph, though that may have changed since installing the 45 series tires, so I may be running slightly higher.

 

For a Z-Car whatever your average cruising speed on the highway is determined to be, you should gear it so the vehicle is turning around 3000 to 4000 rpm (depending on perfomrance bias) at that speed.

 

From what you tell me, your car is running perfectly fine, and there is absolutely no problem.

 

It's not a V8 that will float the vlaves at 4800rpm...

[fl327]

My z gets its best mpg right around 3200-3500 rpm with about 14hg manifold vaccum.

[Chemicalblue]

Thanks everyone for al the great input. Have a Great Thanksgiving CB 8)

[Tony D]

327 brings up a good point: peak torque, or where the "flat spot of the torque curve starts" is actually where you want your engine to be for top fuel economy.

For more perfromance, yo ucheat the curve towards the "peak point", but ALWAYS want at least 500 rpom below the peak so when you stomp on it, you get IMMEDIATE response (and might I also add....BOOST!) while still having a meaningful range of rpms to run through before upshifting.

If in top gear, you pull storngly to the top speed.

 

My car gets 22mpg towing an 800# trailer with these specifications above, despite "screaming" at close to 3500 (anywhere from 3100 to 3500, with occasional extended speeds of 4000rpms...)

So higher engine revs don't necessarily kill gas mileage, as well as lower revs don't automatically give better mileage!

 

327 has a good point and should be well heeded.

[Guest bastaad525]

But how do you determine the torque peak point for a turbocharged engine? Obviously, the torque curve on the dyno, at WOT/full boost, is not going to be the same off the boost driving on the street. Or is it?

 

anyone know about where the L28ET makes peak torque off the boost?

[Tony D]

yes, it is.

The torque peak only moves north and south on the dyno sheet, depending on the intake manifold pressure. The curve should be identical for torque, only near the top of the graph as you add more boost.

 

The peak moves east and west depending on variables like camshaft, camshaft timing, possibly ignition timing, and maybe carburettor size...

 

But generally speaking, "peak" torque on a turbo Z will be nearly identical to a stocker, just further north on the graph when you crank in boost.

 

Stock N/A L28 makes peak HP at what? 5.5K or thereabouts? Where do most guys dyno out at with peak HP? 5.5 to 5.8K? Not that big a difference to worry about.

 

But the "flat spot" of the torque curve remains fairly constant.

 

You also have to consider "how much torque do you need to accelerate the way you want" which enters into "performance -vs- economy" argument. Some may want the engine cruising at 4000 rpms so the boost response is instantaneuos, and gives a much harder pull when used.

 

others can wait a bit, and gear so they are around 3000 so response is still instantaneuos, but you may have to pull a bit to really get going...

 

This is BTW the origin of the commonly misused term of "turbo lag"... People confuse what happened in 60's vehicles with improper driving technique.

Drive a turbo corvair and you will know what REAL turbo lag is about. The engine made seriously variable boost in each gear. Maxing out around 10psi in fourth around 5500rpm (where it was rated as a magnificiently peaky 180HP!)

 

Caomparing that response to what a turbo ZX does is like night and day!

[Guest bastaad525]

Tony - I don't know why, but my motor is proving to be the exception to the rule. It's power and torque curves are almost identical to that of the '80-90's Mustang 5.0 engine, making big torque early on and falling off early on, with peak power coming at around 4200rpm. My numbers to the wheels are almost exactly what the 5.0L engine was rated for at the flywheel, and peaks are almost at the exact same places, much lower in RPM than a typical L6 motor.

 

 

Also, in regards to the original topic of the post, it just occured to me no one has mentioned the possibility that his TACH is off, instead of or in addition to his speedo. My stock tach was reading off by a few hundred RPM and got more and more inaccurate as revs increased. Heck even my autometer tach is off a consistent 100-200rpm.

[Chemicalblue]

My curve flattens out at 3500-4800. Both hp and tq are in paralell curvature. And stays realatively flat for the duration of that rpm range. It did produce (last dyno run) 321hp&300lb tq. Allot of changes have been made since then. My goal is to pull a C6 to 150mph and right now it rocks all the way to 125mph, but has nothing left. I guess I would like more top end which would transalate into a lower cruising speed (rpm) at 65mph in 5th. This next tune we are expecting 400 hp which will be even more fun but even more uncontrollable due to the gearing. Larger tires will help 10mph mabey; but my real ? is to find a # or something on the trans and diff that tell me what they are. Gearing is something I know nothing about and now that its a issue its time to learn. Thanks again for everones help

[Tony D]

oh dude, you are way off base there! Tires will not add "10mph" nor will regearing the car.

A vehicle with a 3.36 and late ZX tranny using stock height wheels will have speed capabilities at 6500rpm of 160+, but not with the HP you are talking about.

 

If anything gearing would only lower the roms you are at during your drag-limited terminal speed.

 

This has been discussed elsewhere on the forum. But as a Crewmember on a Land Speed Record Setting S30 I will tell you there are three words that are MANDATORY if you want to break beyond 150 in an S30 body:

 

G

Nose,

Aerodynamics.

 

Horsepower only gets you so far, after that you can not jump the cube unless you CUBE the HP available from the engine.

 

But aerodynamic aids can make a DRASTIC difference in top speed.

 

Little tail spoiler everybody loves to run, Remove it and add 3mph at 140!

I shudder to think what the 5" "Racing Edition" cuts off!

 

New Corvette Advertising: What is the difference between a 170 mph Corvette and a 186 mph Corvette? The Headlights for one thing. New Vette has flush headlights covered with plastic (Aero Headlight Package) BIG differences in drag on things like Mirrors, Windshield Wipers, Wide Tires at speeds above 130mph.

 

Going much faster than 150 in an S30 is not as easy as most would like to think...

[Guest bastaad525]

Okay so, let's assume you've got the G nose, headlight covers, no tail... maybe a few other little things like body pan covering the bottom...

 

 

THEN how much HP would it take to break 150mph?

 

 

How much hp is needed to break 150mph in the more aerodynamic 280zx?

 

The powercurve of my motor as it stands now... sucks I'm hoping the better exhaust will reclaim the 4000-5000rpm range and move the power peak up.

[Tony D]

we exceeded 150 with under 300hp to the rear wheels under those conditions. Around 286 I believe.

 

Then again, with 236 we only went 143 in 1.3 miles, and with 286 it took closer to three miles to go 150+ But with the radiator opening blocked off and the full bellypan there wa a run over 170...

 

But without the G-Nose, you are literally pissing in the wind trying to simply add HP and go faster. It don't work that way with a small engine vehicle, nor with a V8. There reaches a point where a car won't go any faster unlesss you strap external combustion rockets on it's back due to the HP required to go faster.

 

But a bodypan covering the bottom of the car is no "little thing"! The difference between the car with and without the pan is alomst another 10mph at speed. Figure out how much Horsepower yhou are getting "back" to use for forward motion for each little thing you do: remove wipers, tape seams, louver inner fenderwells to let airpressure out, remove rearview mirror, shave sidemarkers...

 

1# of thrust = 2HP...

 

A JATO Bottle gives 1000# for 12 seconds...

 

Ever watch that "mythbusters" episode? Muahahaha...

[Guest bastaad525]

236rwhp in an S30 with g-nose, bellypan, and headlight covers only 143mph??? Do I have that right?

 

 

Shattering a few dreams there, I think

 

 

I figured I was good for at least 140mph with 230rwhp, stock body (well with front chin spoiler).

 

 

They really do SUCK aerodynamically don't they.....

[johnc]

Then again, with 236 we only went 143 in 1.3 miles, and with 286 it took closer to three miles to go 150+

 

236rwhp in an S30 with g-nose, bellypan, and headlight covers only 143mph??? Do I have that right?

 

Yes, that's probably correct. Remember, the LSR guys are accelerating from a dead stop on a low traction surface (dirt, salt) so they have to go easy on the throttle. LSR record attempts are made on a measured course and at the lower speeds (like we're talking about here) they run on something called a "short course." They don't have 5 miles to accelerate up to speed. Its more like 1 mile, then the speed trap.

[Guest bastaad525]

So then theoretically the car could have gone faster than 143mph with that amount of power, given more space?

 

 

If a stock 240z with some 125 net flywheel hp can hit 125mph, and my built up N/A motor in a 240z with 170rwhp hit 130mph with some still left to go, verified with the tranny calculator, shouldn't the extra 60rwhp of the turbo motor be good for at least another 10mph? I'll be the first to admit I really have very little (more like NO) understanding of the forces involved here, but I find that hard to believe.....

[Chemicalblue]

So if a car was geared to go 100mph and it had 10hp,and you up the hp to 100 would it not still go 100mph? just get there faster right?

[Guest bastaad525]

That's the other reason I find it hard to believe that the car wouldn't be able to even break 140... this thing is geared WIDE. With the 3.54 diff and '82-83 5 speed, it would hit 140mph at just over 5000rpm, with another 1000rpm of flat power band before it starts to fall off. For totally useless comparison, my Sentra barely does about 95 at 5000rpm in 5th (5 speed) I hate the gearing on that car.

[johnc]

it would hit 140mph at just over 5000rpm, with another 1000rpm of flat power band before it starts to fall off.

 

But the big question would be, is there enough torque below 5,000 rpm? My car could pull up close to 150mph with 284 whp but I was running a 4.38 rear gear and a custom, close ratio transmission that would only see a 600 rpm drop from 4th to 5th (7,500 rpm to 6,900). A stock transmission that would drop the engine below its torque peak would probably see equal top speeds in either 4th or 5th gear and would most likely see a higher top speed in 4th gear.

 

I'm not saying the turbo car won't see 140, just trying to show how hard it is to really go fast.

[GabeRoc]

chemicalblue, i think you have asked a good question. it is doubtful, regarless of gearing, that a 10 hp car would go 100 mph. it problem is that the power your engine produces needs to overcome the resistance of the air arround you, and the amount of power needed for that gets quite large at high speeds, even for very aerodynamic vehicles.

 

wind resistance is a function of geometry (how aerodynamic something is) and velocity squared, so if you double speed you quadruple resistance. the generic equation is x*V^2 where x is a constant based on geometry (the smaller x, the more aerodynamic) and V is velocity. this resistance is a force, so to move the car a distance requires energy (force* distance). the faster you go, the more energy required per second. energy per unit time is, of course, power. so the power required to maintain a certian speed is x*V^2 (the force agianst the car) times V (the speed at which the car is going) or x*V^3.

 

so as speed increases it starts taking a lot of engine power to maintain speed, not leaving much power for accleration.

[Chemicalblue]

So take the resistance out, its a z car not a semi. Put the car on a dyno and if you are geared to only go 100mph at max rpm. you can only go 100mph, even if you have 1000hp. My previous set up I hit 176 on the dyno. Now that I changed the rear end to the 88 model and 17" wheels I would be lucky to hit 130mph. I would like to know If any one else has a 83 trany and a 88 vlsd w/ 17s. If so What do U cruise at around 3000rpm?