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Loss of torque with valves open??

Newbe

Montreal
Joined
21 Apr 2019
Messages
587
Over the last few years I've carried out a number of mods to my 997.1 C4s, I just love tinkering!!

New Topgear stainless steel manifold
New Topgear big bore X pipe
New Topgear valvetronic back boxes
BMC airfilter
IPD Plenum
82mm throttle body
Remap on a rolling road (380bhp/315 torque)

Not huge gains over the standard 355 BHP but I've had great fun doing it and loved the rolling road experience, I wasn't expecting much to be honest. The car runs really well and sounds great with the odd crackle and pop.

So hears the thing,

I've noticed if I leave the valves closed the car appears quicker to Rev between 1500/3500. Once over 3500 it revs quicker with the valves open. I'm guessing due to my set up the valves being closed increases the back pressure which helps the torque low down between 1500/3500. I'm considering putting the old manifolds back on the car to try and even the torque across the Rev range. I will lose some BHP right up the top end but I don't spend that much time in the top end.

Has anyone experienced this before? What did you do?
Has anyone found the original exhaust manifold work better than aftermarket? Or should I leave the manifolds and look at my Xpipe (although I like the sound)

Any thoughts???
 
Well, I have TG manifold, TG 200 cell cross overs, and no silencers. Mine revs much quicker, and also drops revs quicker, along with it being extremely loud, but I also have noticed a drop in torque lower down, but when it's up above 3000rpm, it really responds well.
I'm not sure what I'll do. I may add silencers, will lose a few bhp, but will tone it down a bit, but I may also go with an X pipe instead of the crossovers.
Are these TG manifolds a bigger bore than standard?
None of that's any help I'm afraid, I'm just in the same boat as you more or less 😀
 
arthouse said:
I was just reading an article on Sharkwerks' website about a similar scenario with Gen 2 GT3 exhaust mods making the car lose torque low down in the rev range, their answer is a software tweak, article hear if you want a gander -

https://www.sharkwerks.com/tuning-guides/997.2-gt3-2010-2011


Interestingly they said 'most of the lowdown torque' , which would suggest although they can improve it they can't get it all back. It's that trade off against torque and BHP I guess.
 
matt997s said:
Well, I have TG manifold, TG 200 cell cross overs, and no silencers. Mine revs much quicker, and also drops revs quicker, along with it being extremely loud, but I also have noticed a drop in torque lower down, but when it's up above 3000rpm, it really responds well.
I'm not sure what I'll do. I may add silencers, will lose a few bhp, but will tone it down a bit, but I may also go with an X pipe instead of the crossovers.
Are these TG manifolds a bigger bore than standard?
None of that's any help I'm afraid, I'm just in the same boat as you more or less 😀

The Topgear manifolds are slightly bigger bore, plus being stainless will be louder. I like the sound of the Xpipe because it doesn't really kick in until it goes over 3000 rpm which makes for good town driving. However the Xpipe is also known for impacting low down torque :dont know:
 
I'm sure just about everyone here knows that when it comes to mods i have no idea on faults on the cars ..

If i may i'll explain how i .. in my own mind understand this system .. right or wrong as i am willing to learn ..

Exhaust back pressure .. the cylinder is on a power stroke .. exhaust back pressure keeps the burning and still expanding gasses in the cylinder to give a full power cycle .

A lack of back pressure and the still burning gasses exits into the exhaust faster thereby not giving full power on that power stroke .

The exhaust is dumped far to soon in other words .

I can't really see a remap helping this .. a remap is literally adding more fuel at certain rev ranges . it cant help with a mechanical issue such as larger exhaust and a lack of back pressure .

Above 3K rpm then you are into valve high lift .. a totally different setup and where the real power comes into effect .. back pressure has minimal effect at higher rpm .

Whether its the manifolds or the lack of silencers i cant say .. although i would lean towards the silencers due to valves closed .. sports valves ? .. technically that would increase back pressure i would guess at .

Either way .. to me and my lack of understanding of modding then i would lean towards the exhaust needs altering back to a more normal setup if you want lower down torque .
 
deMort said:
I'm sure just about everyone here knows that when it comes to mods i have no idea on faults on the cars ..

If i may i'll explain how i .. in my own mind understand this system .. right or wrong as i am willing to learn ..

Exhaust back pressure .. the cylinder is on a power stroke .. exhaust back pressure keeps the burning and still expanding gasses in the cylinder to give a full power cycle .

A lack of back pressure and the still burning gasses exits into the exhaust faster thereby not giving full power on that power stroke .

The exhaust is dumped far to soon in other words .

I can't really see a remap helping this .. a remap is literally adding more fuel at certain rev ranges . it cant help with a mechanical issue such as larger exhaust and a lack of back pressure .

Above 3K rpm then you are into valve high lift .. a totally different setup and where the real power comes into effect .. back pressure has minimal effect at higher rpm .

Whether its the manifolds or the lack of silencers i cant say .. although i would lean towards the silencers due to valves closed .. sports valves ? .. technically that would increase back pressure i would guess at .

Either way .. to me and my lack of understanding of modding then i would lean towards the exhaust needs altering back to a more normal setup if you want lower down torque .

Thanks for the reply Demort, I still have all the original parts so I'm going to have a mess around with different parts and see how it feels. The car is actually running great but I think I can get a better balance. I plan to have a go next weekend, fingers crossed.
 
Its a pretty common situation tbh. A lot of aftermarket exhaust systems, for any car, will deliver an increase in top BHP at the loss of some bottom end torque.
Fine for a race track, good for bragging rights but less benefit on the road.

Having a great sounding exhaust makes up for it anyway. :thumb:
 
Perhaps you would like to look at these comparative graphs of BHP and Torque from our tests of std and capacity conversions from which you will see that extra capacity gives both more torque and BHP (although probably only worth thinking about if your engine goes wrong or a pre-emptive rebuild is being considered).

At any revs Torque is proportional to BMEP and that is proportional to the volumetric efficiency (VE) at those revs (i.e. roughly proportional to the amount of air trapped when the inlet valves have closed) * the C/R (up to the point of ignition).

As De-Mort absolutely rightly points out - spilling air back out of the exhaust lowers the amount trapped and therefore lowers the compression pressure reached at the point of ignition and the resulting torque is proportional to the compression pressure at the point of ignition.

The problem for engine designers is the amount of time to move air changes with revs - so at 1,500 rpm there is 4 times as long for air to move in and out of the engine than there is at 6,000 rpm (5 times as long as @ 7,500 if you have extended the rev limit) - that makes the charge speed and inertia vary enormously and this influences what revs the engine has been designed to operate best at and over the widest range.

So you can move where the best VE is but its difficult to spread it when the whole engine was designed for a different chosen power band. This usually means you can get more top end and less bottom end (or visa versa) but not both - horses for course - you can choose to change it but not stretch it.

However a capacity increase does two things. It increases the pressure drop at the bottom end and mid range (increasing the inflow and VE) giving more mid range torque and it also increases the top end (usually not by quite so much) because although the amount of air it can ingest is starting to be handicapped by the flow limits - the piston area is bigger so the resulting combustion force acts on a larger area.

Another benefit is that if the extra torque is a larger proportion than the increase in capacity - it means slightly additional BMEP = better economy as well.

Just something to consider when looking at other tuning aids especially for a high mileage keeper or failing engine.

Baz
 

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Hi Baz

Thanks for your reply and explanation.

I hope it doesn't go wrong as you guys fixed it 2 years ago :thumb:

And you done a great job! Full rebuild, I think it was at the time you were just releasing the 3.9.

Thanks for posting the graphs, it was really interesting to compare them to my own, I could only find a couple of the pulls but I thought I would attach it out of interest.
 

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Newbe said:
Hi Baz

Thanks for your reply and explanation.

I hope it doesn't go wrong as you guys fixed it 2 years ago :thumb:

And you done a great job! Full rebuild, I think it was at the time you were just releasing the 3.9.

Thanks for posting the graphs, it was really interesting to compare them to my own, I could only find a couple of the pulls but I thought I would attach it out of interest.

Is that just a direct 3.8L replacement you went with? Quite impressive numbers.
 
sja360 said:
Newbe said:
Hi Baz

Thanks for your reply and explanation.

I hope it doesn't go wrong as you guys fixed it 2 years ago :thumb:

And you done a great job! Full rebuild, I think it was at the time you were just releasing the 3.9.

Thanks for posting the graphs, it was really interesting to compare them to my own, I could only find a couple of the pulls but I thought I would attach it out of interest.

Is that just a direct 3.8L replacement you went with? Quite impressive numbers.


I originally had the 3.8l rebuild by Hartech but my Dyno graphs are based upon all the parts I added on my original post above (2 years after the rebuild) , although it appears some of the parts I've added has impacted the low down torque. Although the graph doesn't really show that I can definitely feel it in the drive, it's slower to Rev. Still goes well though !
 

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