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Discussion Starter · #1 ·
In reading about this and a buddy of mine doing the mod, this lead to us talking. In your lower to mid range of the revs…for optimal power on motorcycles, some back pressure is good. It’s very popular and fairly cheap to put exhausts on bikes and generally you will gain power in the upper ranges but lose bottom end and mid range. Not that big of a deal on my bike since I don’t race it off the line, but it’s being raced in the upper rpm range. But for the car, most of the time you’re taking off from the light. So while you get more power from doing the ‘golf tee’ mod/trick to keep the valve on the second exhaust pipe open the entire time, aren’t you losing some power as well in exchange? Aside from sound, my initial guess would be that this mod will cause a decrease in power/performance. Any ideas?
 

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nope... backpressure in no way produces torque... in some cars the loss of backpressure followed occurred at the same time as the loss of torque (very common in your 4 cylinder honda) but this does not equate to cause and effect. Even in bikes that is a design of a poor pipe not of backpressure creating torque....

sorry this just always bugs me..

throw a turbo in and this make it even more untrue.
 

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Discussion Starter · #3 ·
Ok assuming all else is untouched, isn't more air flowing through and the car running more lean then?

On the speed talk forum in one of the tuner threads, the guys will still change their FI maps even when the modification on the exhaust happens after the Y pipe.
 

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Better exhaust flow is a GOOD thing.

The one case where it isn't:
If exhaust scavenging is TOO good at low rpms, then yes, on older performance cars with lumpy cams (think high-RPM racing cams), you lose power due to intake charge being pulled out the exhaust when both intake and exhaust valves are open simultaneously. In enginespeak: racing camshafts often exhibit lots of intake/exhaust overlap. This is one of the main reasons why you have lumpy idles, and low torque down low with racing cams.

HOWEVER, we have double VANOS, which lets the computer control intake and exhaust timing completely. Non-issue.
 

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Discussion Starter · #5 ·
Better exhaust flow is a GOOD thing.

The one case where it isn't:
If exhaust scavenging is TOO good at low rpms, then yes, on older performance cars with lumpy cams (think high-RPM racing cams), you lose power due to intake charge being pulled out the exhaust when both intake and exhaust valves are open simultaneously. In enginespeak: racing camshafts often exhibit lots of intake/exhaust overlap. This is one of the main reasons why you have lumpy idles, and low torque down low with racing cams.

HOWEVER, we have double VANOS, which lets the computer control intake and exhaust timing completely. Non-issue.
I agree that better exhaust flow is a good thing so long as the ECU sends more gas when the airflow increases. On bikes it's more limited and if you do a full exhaust system the bike will run very lean since generally much of your performance exhaust work is in the headers. So if the ECU can toss more fuel with the increased airflow then the power thing would be a non issue, just a teeny dip in gas mileage.
 

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No, it lacks Valvetronic. It has double VANOS.

In other words (if I'm recalling correctly): the N54 has fully-adjustable intake and exhaust valve timing, but lacks fully-adjustable valve lift (which is what Valvetronic's brings to the table in addition to full valve timing).

However, this is a good thing, sort of: I've read that the mechanicals of the Valvetronic system actually causes some level of valve float at high RPMs, which effectively limits the power you can make at high RPM. At least this is what I hear--I don't know Valvetronic very well.

This is why you'll never see Valvetronic on an M-car, I'm told.
 

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I agree that better exhaust flow is a good thing so long as the ECU sends more gas when the airflow increases. On bikes it's more limited and if you do a full exhaust system the bike will run very lean since generally much of your performance exhaust work is in the headers. So if the ECU can toss more fuel with the increased airflow then the power thing would be a non issue, just a teeny dip in gas mileage.
Ah, but with the magic of fully-adjustable exhaust valve timing and duration, you can have it all! For low RPM, you can have late exhaust timing with short duration, effectively canceling out the freeflow exhaust headers/collectors/mufflers/etc. You can then take advantage of the freeflow headers etc. at higher RPM and/or higher throttle positions by increasing duration and advancing exhaust valve opening timing.

Having dealt with old-school performance engines, with their oddities and compromises, for way too long, you just gotta love the new variable-timing systems out there.
 

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No, it lacks Valvetronic. It has double VANOS.

In other words (if I'm recalling correctly): the N54 has fully-adjustable intake and exhaust valve timing, but lacks fully-adjustable valve lift (which is what Valvetronic's brings to the table in addition to full valve timing).

However, this is a good thing, sort of: I've read that the mechanicals of the Valvetronic system actually causes some level of valve float at high RPMs, which effectively limits the power you can make at high RPM. At least this is what I hear--I don't know Valvetronic very well.

This is why you'll never see Valvetronic on an M-car, I'm told.
Yup - Valvetronic increases intake efficiency which is redundant on a forced-induction engine. It's also heavy and limits the redline - that's also why it won't be seen on an M car. Still, it's a brilliant piece of technology, no throttle needed!
 

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I think two turbines is enough back pressure enough :)
 

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It's true.

And turbo cars always respond well to exhaust mods that lower restriction.


It take force/energy to spin up the impellors of the turbos. This slows down and slightly cools the exhaust coming out of the engine. This causes backpressure.
 

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...
 

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I am wrong in that the 335i does not have double vanos?
The 335i does have double-VANOS. It does not have Valvetronic like the 328i has... see above.

Edit - You deleted your comment, therefore I retract mine as well. ;)
 
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