My car is slower after mods

[cleong]

I first took my car on a top speed run when it was about a year old. At that point of time, the only mods that it had were lowering springs, and aftermarket rims with 195/55/15 tires. It posted a maximum of 194kph or 120mph (GPS recorded).

Since then, I have modded the car with a DC header, and Fujitsubo axle-back exhaust. I have also switched to Iridium spark plugs, but now I'm running 195/60/15 tires. This setup was dynoed to produce 102 whp which I believe is significantly more than the stock 90-odd whp. However I was only able to post a maximum of just 181kph or 112mph, measured with the same GPS on the exact same run, and I have repeat this run a few times with the same result.

I'm wondering where the speed went. Did I get a tailwind on the first run, or did my mods actually hurt the car's overall speed? I was under the impression going to a taller tire profile would actually help the car's top speed. Keen to know what you guys think of this.

[thebarber]

you have tires that are 5% taller....this will make your speedo 5% slower than your first run.....that would explain most of the top speed you think you've lost....

[scape]

your tires are taller and may be heavier, this could be a problem for top speed output

[supmet]

Personally I don't take GPS speed seriously, unless its is a "GPS speedometer" not just a "GPS with a speedo"

And ya, a headwind or tailwind on different runs could easily change the top speed on a car this light and lacking in power. And as scape said, heavier tires, even though they are taller, could slow you down.

[supmet]

Originally Posted by PK198105

why not , if you have a good reliable receiver it can be as accurate if not better than the actual speedo - the lag time.

Well for one, if I keep a steady speed and go up a steep hill, the GPS drops a few mph even though I'm going the same speed.

[yaris-me]

If you have bigger tires and more HP, then the only plausible explanation is higher air density or head winds.

[silver_echo]

stupid question... from my understanding, we have a speed limiter in the US... is there a limiter on yours? if so, what speed is it set at? is this the difference between the tire sizes?

[cleong]

Thanks for the suggested theories guys.

The GPS is a regular Garmin with a speedometer function. Its accuracy has in the past been separately verified to be within 1km/h (0.6mph) accurate to another much more accurate externally mounted GPS unit (10 refresh cycles per second). So I'm quite confident that the speed measurement is consistent.

Singapore is typically hot and humid all year round, typical days run above 86 degrees.

I do not think there is a limiter on mine, as I don't feel a rev cut the car simply runs out of puff to push the speed higher. At that speed it is already spinning around 5500rpm in fifth gear and it just holds it there.

It could be that the extractor isn't a good match for the engine, but that'll be the first time that someone says something negative about the DC header. The dynosheet shouldn't lie, though, with 102whp. Unless the car is losing torque at the very top end of the RPM range. Do engines begin losing horsepower with more miles clocked?

The only way to verify it would be to return the car to its original speed run's configuration and test it again.

[ChinoCharles]

Step one would be to go over everything one more time. A loose MAF seal, for example, could cause something like this. You're talking about a big difference. Its easy to start pointing fingers at the parts, but its rarely true.

Check spark plugs, make sure your MAF isn't dirty, check couplers, exhaust leak... something like that. DC header causing an 8 MPH difference in top speed? Mmmmm, no.

[cali yaris]

The difference is 6.66% to be accurate.

How does the "cat-back" exhaust mount? You had to cut and weld, I presume? Because we don't have a bracket to bolt on after the cats.

but the biggest question is: Have you done a compression test? Maybe cylinder 4 is weakened.

[ChinoCharles]

^^^ Its the mark of the beast. You're doomed dude.

[YarisPR]

Originally Posted by ChinoCharles

Step one would be to go over everything one more time. A loose MAF seal, for example, could cause something like this. You're talking about a big difference. Its easy to start pointing fingers at the parts, but its rarely true.

Check spark plugs, make sure your MAF isn't dirty, check couplers, exhaust leak... something like that. DC header causing an 8 MPH difference in top speed? Mmmmm, no.

As Chino said any issue with the MAF really f*cks things up believe me!!!! I had a custom made CAI and after 2 years it began to let air in from the seal where the MAF was and the car chaged dramatically!!.
How about...
-20-25MPG
-Tried top speed runs and never hitted the Limiter
-The car had a "missfire" (sorry I don't know the right term in english) between 20000-3000rpm
-And worst of all..... Yaris I usually beat easily running by my side...

[cali yaris]

And worst of all..... Yaris I usually beat easily running by my side...

AMEN.

[cleong]

Oh f*** me, sorry for the brainfart. I do not have a catback exhaust, I have a regular axle-back that didn't involve hacking and welding.

[ChinoCharles]

Donut gasket? How good is your camera? What does it sound like?

NEXT ON ER: YARISWORLD...

[eTiMaGo]

Hmmmm when you are at 180km/h you are in 5th gear right, at what kind of RPMs? I don't remember the dyno specifics of your car but maybe it is producing a lil less power at that point, meaning that it has less power to overcome wind resistance and thus cannot reach a higher speed?

I myself have experienced a similar problem, when I first got the car and the first round of mods (wheels, intake, HKS exhaust), I was able to reach 200km/h according tot he speedo (didn't know GPS would be more accurate at that point)

Couple years later I had the opportunity to really floor it again but could also only reach 180ish, acording to the speedo again. Could have gotten a bit more but ran out of road, so I did not hold this as a conclusive result. At this point I had changed the exhaust for the full Nitto piping, and I might have also installed the NST lightweight pulley (which should have no effect on power output if the engine speed is relatively stable)

So, as I never dynoed the car before and after, I suspect I may have lost some high-end power (the car pulls well at low-mid RPMs), or it might just be an effect of the engine aging, who knows, really.

What I did notice though, and maybe this is related, as I have a basic AFR gauge, at WOT the AFR goes very very rich, if the gauge is to be trusted I would see as rich as 10:1 AFR (on the second run, didn't look the first time around), which would have a negative effect on power output. So another possibility is that the stock open loop fuel map is not adapted for the intake and exhaust mods. Anybody wanna sponsor an AFC Neo or similar?

[cleong]

At 180km/h I think I'm somewhere between 5000-6000rpm. You must excuse me for not being precise, I was busy trying to keep the car going straight!

Here's how my dynochart looks like:



You can see how it really falls off after 6000rpm. I daren't go back to 4th gear to eke out the last few km/h though. Worried that my pistons would come shooting through the bonnet!

[kmf123kmf]

Originally Posted by supmet

Well for one, if I keep a steady speed and go up a steep hill, the GPS drops a few mph even though I'm going the same speed.

I would expect this result if the GPS was not aware that its altitude was changing. Does anyone know if (or to what extent) GPS maps incorporate altitude data?