Under drive pulleys

[darkirv]

whats an underdrive pully do?

[sooty]

http://en.m.wikipedia.org/wiki/Underdrive_pulleys?wasRedirected=true

[tomburke7]

whats an underdrive pully do?

I used one on a Mazda RX2 I used to own. I had modified the engine to make good power up to 8000RPM, however at those revs the water pump was cavitating badly. This situation could lead to engine failure if you held those high revs. For a few $ I put large pulley on the pump and slowed it down a bit. Once I was there I also put a larger pulley on the alternator to slow that down & lower the amount of energy it took from the engine.

In summary: you need an underdrive pulley if you're going to rev your engine hard (above its original build specification).

[Nurb608]

I've got one on mine as on track days there are quite a few times i'm up in the high rev range, i.e. up around 6500 - 7500. Obviously not constantly though.

[tomburke7]

I've got one on mine as on track days there are quite a few times i'm up in the high rev range, i.e. up around 6500 - 7500. Obviously not constantly though.

The problem of cavitation causes air pockets to pass through then engine, air does not absorb heat like water so it causes local hot spots, and the high temp can damage the engine - quite a problem on rotaries with their fragile apex seals.
It is only a problem if you hold the high revs (e.g. down I long straight)

Does you Nurb happily rev out to 7500? (wow!) I thought they had a rev limiter just after 6500.

[Nurb608]

Does you Nurb happily rev out to 7500? (wow!) I thought they had a rev limiter just after 6500.


Yeah, it's lifted up to 7500 with the stage 3 map, and with the cam etc holds the power all the way up there :)

[kabel]

I was tossing up whether to buy one of these from Regal`s when I ordered my cam and EDS..........decided against it for the short term.
I don`t usually drive the car that hard for too long !

[tomburke7]

I was tossing up whether to buy one of these from Regal`s when I ordered my cam and EDS..........decided against it for the short term.
I don`t usually drive the car that hard for too long !

And of course the other problem is reduced output from the alternator at idle. The lights on my RX2 went quite yellow and dim at idle - only just tolerable, and this is on a 1971 car with very few electric accessories. No air-con, only an AM radio driving one 3 inch speaker[that was never turned on] no onboard computers. -There was insufficient current to drive just two 55watt sealed beam headlights (the amp meter went quite negative at idle), so I recon it would be a real prick to drive a modern car with an under drive pulley.

[gman]

I have one on my car and it hasn't caused any issues with available voltage at idle, even with my very small gel cell racing battery..The Astra alternator is pretty good and the stock battery is HUGE so in a stock application it should be OK...

The main issue would be IMHO with harmonics, vibrations and possible engine gearbox issues....

[lithium]

yes i was going to say, do the aftermarket underdrive pulleys act as a proper harmonic balancer? i'd be worried that in the long term, the crankshaft would rattle itself to bits!

[gman]

No, usually they don't...They are balanced by themselves but they do not act to reduce the harmonics inheirent in all engines, especially inline 4 cylinder ones.

I did a whole spiel on harmonics etc around removing the balance shafts. Underdrive pulleys are the same.

The harmonics would affect the gearbox more than the engine IMHO as they would be transmitted directly into the box along the crank.

[tomburke7]

I did a whole spiel on harmonics etc around removing the balance shafts. Underdrive pulleys are the same.



I'd like to read that spiel, what is it under?

[sooty]

I'd like to read that spiel, what is it under?

Nope your right...The balance shafts are mainly for NVH reasons..

The balance shafts in the Z20LET and dual mass flywheel in the later Z20LEH are there to try and compensate for the secondary imbalances (or secondary vibrations –twice engine rpm) that are found in all in-line 4 cylinder engines due to the inherit asymmetry of the layout.

Despite what anyone tells you, these cannot be eliminated with internal component balancing, no matter how well it is done or by whom. End of story it is simple physics. This is because it is not generated by a weight imbalance, but by an imbalance in the speeds and actual movement (descending and ascending) of the rod/piston assembly which is not always completely equally through their 360 degree arc.

The vibration is generated by the fact that the movement of the rods is not symmetrical throughout each crankshaft rotation. Descending rods are moving at differing speeds through their rotation in relation to ascending rods, causing a net vertical imbalance twice each crankshaft revolution. Increase the stroke and there is a greater difference in speed and acceleration because of increase distance traveled. Increase bore and there “CAN BE” an increase in mass (of the piston and rod assembly) which increases inertial forces and hence increase the imbalance.

The big issue is that these forces increase 4 fold/4 times/quadratically as RPM increase, no matter how well balanced the bottom end is because its not a weight imbalance issue, it's a distance vs. speed issue.

By removing the balance shafts you can increase the secondary vibrations in the engine, simple. The Z20LEH works around this with a heavier dual mass flywheel to isolate the vibrations from the gearbox and chassis.

So why does GM use balance shafts and DMF's while Nissan and Toyota don't in the SR20DE/DET and 4AGE series engine?

It is mainly done for 2 reasons:

to reduce Noise/Vibration/Harmonics in production cars making them smoother and quieter, and
to reduce component stress particularly on the gearbox/transaxle.

So removing the Z20LET balance shaft will slightly increase HP, might increase the noise in the cabin of the car, could increase wear in the engine (slightly) and increase the risk of gearbox component failure due to increased harmonics and vibrations.

So remove or keep??

My guess would be, if your going to do it keep the standard flywheel and remove the balance shafts.

If you really want the extra “urge” then go the whole way and lighten the flywheel as well but do the rods and rod bolts too.

It's that one i assume

[tomburke7]

It's that one i assume

thanks!

[gman]

That would be it...

[tomburke7]

It's that one i assume

Yes, I'm not too happy with the throttle response of my 2.2. It feels like the flywheel is too heavy, but it could be something built into the 'drive by wire' throttle. Any thoughts?

[sooty]

fly by wire throttles always have lag unfortunately, the sport button should improve it a little, but i can't ever recall reading any viable form of getting it to be as responsive as a cable throttle

[tomburke7]

fly by wire throttles always have lag unfortunately, the sport button should improve it a little, but i can't ever recall reading any viable form of getting it to be as responsive as a cable throttle

there is a thing called a sprint booster, but I am not convinced.
http://www.sprintboostersales.com/
here is the blurb:
The ETC (Electronic Throttle Control), which is also known as Drive-By-Wire, has the unfortunate downside of delayed response and subdued acceleration, which can create problems in certain situations such as up-hill starts, quick gearchanges and overtaking.

SprintBooster overcomes this throttle response delay for your vehicle by providing crisp, on-tap acceleration for whenever the driver demands it.

http://www.sprintboostersales.com/images/graph.jpg

.

[sooty]

there is a thing called a sprint booster, but I am not convinced.
http://www.sprintboostersales.com/
here is the blurb:
The ETC (Electronic Throttle Control), which is also known as Drive-By-Wire, has the unfortunate downside of delayed response and subdued acceleration, which can create problems in certain situations such as up-hill starts, quick gearchanges and overtaking.

SprintBooster overcomes this throttle response delay for your vehicle by providing crisp, on-tap acceleration for whenever the driver demands it.

http://www.sprintboostersales.com/images/graph.jpg

.
that essentially looks very much like what your sports button does. But if you look at it, you're not really getting the response any sooner, and with the speed you can move your foot between say, 1/4 and 3/4 throttle i can't see it doing any difference.
There's actually quite some contention in the UK forums about which method the sports button uses.

Whether without the sport button on, full throttle is only equivalent to 80% throttle, and everything is scaled accordingly
Or.. whether with the sport button on, 100% throttle is achieved at only 80% throttle application, as per that sprint booster

[Nurb608]

that essentially looks very much like what your sports button does. But if you look at it, you're not really getting the response any sooner, and with the speed you can move your foot between say, 1/4 and 3/4 throttle i can't see it doing any difference.
There's actually quite some contention in the UK forums about which method the sports button uses.

Whether without the sport button on, full throttle is only equivalent to 80% throttle, and everything is scaled accordingly
Or.. whether with the sport button on, 100% throttle is achieved at only 80% throttle application, as per that sprint booster

On the VXR's with sport button off if you have the pedal flat to the floor it reads as 70%, sport button on reads as 100%. Just different scales that the throttle position sensor uses and tells the ecu.

[sooty]

On the VXR's with sport button off if you have the pedal flat to the floor it reads as 70%, sport button on reads as 100%. Just different scales that the throttle position sensor uses and tells the ecu.


Ahh cool, very interesting, so the "seat of your pants" quicker feeling isn't just a placebo effect, the car really is quicker with the sports button on.

[Nurb608]

Ahh cool, very interesting, so the "seat of your pants" quicker feeling isn't just a placebo effect, the car really is quicker with the sports button on.

Yup ;) It's similar to the ecoboost, however the sports button gives you the true 100% throttle where as I think the ecoboost still keeps the maximum value already used and just changes the rest of the scalling (By the looks of that graph anyway.)

[gman]

I have a sprint booster (for the Astra G) and it makes a big difference to the throttle response. I took it off after I had the ECU re-flashed as it was too touchy...


If the car feels a little slow or "doughy" to respond, than as Sooty and Nurb have said, there isn't always a direct correlation between what your right foot does and what the ECU thinks it should give you...

Theres always an interpretation between the two...I hate the ECU intervention, but hey...What ya going to do...