Horsepower Calculation

[Camfire]

My '81 280zx N/A should be putting out 145 net HP stock. I've got:

 

Header

Straight pipe, no muffler

Cool air intake

-100 pounds weight redux.

20 Degrees BTDC timing

93 Fuel

 

BUT: according to my calculations, I'm only pulling about 100 HP

 

This is the formula I'm using:

 

HP = Weight/(ET/5.825)³

 

Does this sound right to you guys? My ET is about 18 seconds, but I've only clocked it once. It was a pretty rough estimate, because I was using my trip gauge and stopwatch. so I may bring in better/ worse timing the next time I clock it.

 

Even with my rough results, I'm pretty sure I should be showing higher HP. What have I overlooked? I'm too poor to take it to get dyno'd. Is something just probably wrong with my engine? I'm going to buy a compression gauge tomorrow.

[Mikelly]

I wouldn't doubt that it is correct... Sounds about right actually.

 

Adding the header, free flowing exhaust, and the CAI won't help overcome the limitations of the stock AFm and the stock OEM tune of the ECU. Also, running that restriction free exhaust may actually be hurting you. That motor, with the parameters mentioned above, needs a little back pressure to assist you.

 

Mike

[240zphilly]

Using an e/t to estimate horsepower can be wildly inaccurate simply because you may be driving the car like crap. If I jump in there and we do the same test and get a 20 second e/t did you just lose 50 horsepower? No, you just lost a driver... Now, when you account for thirty years of use and estimate a 15% parasitic loss of power through your drivetrain, your 100whp calculation seems realistic. If you want a more accurate figure, pay 9.50 and get the car weighed with you in it on a certified scale, then download a dyno app on your gps equipped smartphone. If you want more power... I defer to greater knowledge contained here:

 

http://forums.hybridz.org/index.php/topic/95316-braaps-l6-efi-induction-advice-and-tips/

 

Take a deep breath and read that a few times, in its entirety. Refer back to it when you get stumped, and dont be discouraged, horsepower is just a number.

[BLOZ UP]

That motor, with the parameters mentioned above, needs a little back pressure to assist you.

 

Backpressure is a myth: http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

 

You can tune exhausts, but that's for optimizing velocity, not backpressure.

 

 

The motor makes 145HP at the motor. I'm not sure if 1970s HP rating standards are the same as they are now (like allowing race pipes, lack of accessories, etc.). Assuming 15% loss that's 123 HP. The rest is probably a tired engine.

 

Going to a drag strip and get a few trap speeds. You can estimate your HP much better that way--especially if the track has a weigh station.

[Leon]

Backpressure is a myth: http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

 

You can tune exhausts, but that's for optimizing velocity, not backpressure.

I'm with you on the backpressure deal, but exhausts are "tuned" to a certain harmonic resonance not velocity.

 

I think Mike meant that the engine is not "self-tuning" nor the fuel maps readily adjustable and thus the engine cannot be altered much before fueling problems show up, he just phrased it poorly.

 

With that said, I don't think a free-flowing exhaust would hurt an otherwise stock ZX. The header, maybe, but not the exhaust post-collector.

[Tony D]

Exactly, the context in which Mike was referring is correct.

The Parroting of a generality found on the internet is incorrect.

I expect better here, alas...

[BLOZ UP]

Exactly, the context in which Mike was referring is correct.

The Parroting of a generality found on the internet is incorrect.

I expect better here, alas...

 

Here he is, in his typical passive aggressive fashion.

 

First things first: So, the parroting that a car "needs" backpressure is incorrect? Or the parroting that backpressure is a myth is incorrect?

[Pyro]

Too much ignition timing can reduce top end power and that is where the max HP is being made. Jacking up the base timing also increases the total timing, unless you shorten the length of the advance slot.

 

Have you performed a valve adjustment lately or had the 30 year old injectors cleaned and checked? Lots of things can reduce hp if not maintained.

 

Trap speed at end of 1/4 mile works much better than et.

 

Using your odometer and stop watch is very inaccurate.

 

Get a g-tech meter or make an official run down the 1/4 mile and use trap speed for your HP calculations.

[78zstyle]

Here he is, in his typical passive aggressive fashion.

 

First things first: So, the parroting that a car "needs" backpressure is incorrect? Or the parroting that backpressure is a myth is incorrect?

 

Hey bud when are you moving to jersey?

[Gollum]

I'd agree as well that you're "in the ballpark". Remember that ET and Trap calculators will be giving you ACTUALLY WORKING HP, meaning AT THE WHEELS. The drivetrain soaks up power that simply can't be exactly predicted no matter WHAT anyone says. The only way to know exactly what's at the flywheel is to put the engine on an engine dyno which for most people is pointless.

 

Your location doesn't show what state you're in, so I'm not sure what could be deemed "legal" as far as modifications go, but if you really want to be serious about getting any extra power you're going to need a better ECU, prefferably one that's programmable to some extent. The really sad part though, is that I could pull off a turbo conversion for what it would cost me to do a fancy megasquirt with COP and such, meaning that no matter what significant gains will be spendy. If you want cheap HP look to a different powerplant, or turbocharging the one you have (and doing it correctly with programmable fuel and spark at the minimum).

 

Sounds though like your best dollor to HP mods will be getting that thing running in tip top shape. You should be able to pull low 17s with basic weight reduction like you've done, paired with a strong running stock motor.

 

If you don't go to the track all the time and that's not really what matters to you, and you care more about having a fun street car that you enjoy, I'd say one of the best investments you can make is on a GOOD lightweight flywheel and clutch. This will help the engine feel more lively and responsive and give the impression of having a much more agressive engine than you do. I know it sounds silly, but it's true. An engine that rev's fast and readily can be substantially more fun on the street. Also, if you do decide to go turbo or swap a turbo motor down the road, you have a flywheel and clutch to go with it.

[Tony D]

"I think Mike meant that the engine is not "self-tuning" nor the fuel maps readily adjustable and thus the engine cannot be altered much before fueling problems show up, he just phrased it poorly. "

 

 

I think that was quite clearly phrased, and said EXACTLY correctly. "In the context mentioned, he was correct"...

You fixated on 'backpressure' and jumped on the internet search engine to parrot something you didn't actually research, answering a suggestion which never was really offered as a solution.

You missed Mike's point completely.

Is that a bit more directly aggressive for your tastes?

Like I said, I would hope someone here would take a bit more and actually READ what someone posted, and let the meaning, and point sink in before mindlessly posting a link.

And as Leon pointed out, the exhaust is tuned more to a harmonic resonance point speaking in those terms.

His contention that the header will likely give him nothing, but after the collector a free flowing exhaust may likely help I agree with.

The front end is very touchy about tuning, after that, lower is pretty much better in terms of restriction. Most oversimplify that as 'eliminate all backpressure' which is a terrible way to phrase it.

If you want to post a link, post a relevant one at least!

[Camfire]

Everyone, applause all around. you have been exceptionally helpful and courteous.

 

I tested my compression: 170, 170, 160, 170, 160, 170... so that's practically a "like new" engine, right?

 

so arguing aside, the verdict is that I'm not actually hurting anything inside my engine with my exhaust system?

[Tony D]

Absolutely not. Zoomies out the fender...probably. Extractor and exhaust pipe with some sort of muffler...pffft! Not an issue.

 

May not be doing yourself any favors, but not harming anything.

 

My experience on a stock engine was that changing the exhaust from the cast manifold back reaped great rewards.

 

Changing several years later to the 'matched extractor' (header) for the exhaust I originally installed netted me considerably more noise, but nothing practical to speak of in terms of real-world horsepower.

 

The biggest change was effected from reducing or eliminating backpressure AFTER the collector (wherever that may be), and not from the point of the head-manifold gasket surface to the collector (whether that be near the head as in the case of the stock 75 Cast Iron Manifold, or down by the Transmission / Engine Joint in the case of the Extractor.)

 

Frankly, the biggest thing I got from the extractor was an exponential increase in noise outside and inside the vehicle. If I had a suitable stock manifold to put in there, I'd swap back in a second and not cry about it at all. The 'performance' achieved was not worth the penalty incurred IMO.

 

Exhaust, yes.

Extractor, no.

 

And this, I believe, was Mike's Point to begin with.

[Tony D]

BTW, with the cast iron manifold, and 2.5" exhaust and two mufflers on board I Dyno Jetted to 147, run the 1/4 mile in 15.30-15.50, and was quiet as can be. My consistent time in the 1/4 was 15.45~15.55.

After adding the header to the existing package, I ran consistent 15.40~15.50. So I don't see the expense and noise as an acceptable tradeoff. I was within a 0.1 range before, and the header didn't lower me even a full 0.1 second.

 

I'm not a drag racer, but I was consistent. Car was 75 2+2 weight of 2695 with my 255# in it.

My 260Z Coupe all stock similarly equipped (no weight) runs about 1-1.5 seconds slower, and my Dyno Jet on that one showed close to 30HP less---around 120.