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Underbody braces, turbos and more!

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PostPosted: Sat Nov 16, 2013 10:32 am 
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Location: Spruce Grove Alberta
I was tinkering with my turbo car testing tuning. awhile back. Yes it looks like I am keeping HER.
I was also reading some things in regard to turbos and cams with overlap. so a installed a second boost gauge on the exhaust to see what my garrett T25BB would produce

I was surprised to see how high the back pressure is !

Its hard to totally compare both while driving completely, but, it does seem the back pressure is higher than the boost alittle once its 7-9 psi. about 1-2 PSI

Not good for performance with these cams!

my question is which new turbos have less back pressure in relation to boost produced?

in my reading some say many new turbo applications do not have this issue because the turbos are more efficient.

i love the sound and feel of the cams I have installed but .... i think it is effecting power and tuning up higher. Its not the same as my original cams.

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PostPosted: Sat Nov 16, 2013 1:09 pm 
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Location: Winnipeg
You have to remember that the exhaust pressure energy used to spin the turbine wheel is the pressure differential from the inlet, to outlet of the exhaust side of the turbo. Your gauge is only measuring the pressure before the turbo. If you have a restrictive exhaust you could have considerable pressure after the turbo that you are not considering. It's common to see a turbo have the intake pressure higher than the exhaust pressure, until higher RPM, when it will cross over and have more exhaust pressure. This is one of the factors considered when picking turbo component sizes/trims.


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PostPosted: Sat Nov 16, 2013 3:37 pm 
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Quote:
i think it is effecting power and tuning up higher. Its not the same as my original cams.



There's no way that's true.

You're flowing so much more air with that cam set, that you're making significantly more power, with less boost pressure than the stock cams.

Pipe is right. What you're seeing is normal. You should see the pressure we were seeing with some of the turbo 3's.

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PostPosted: Sat Nov 16, 2013 11:17 pm 
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Location: Spruce Grove Alberta
suprf1y wrote:
Quote:
i think it is effecting power and tuning up higher. Its not the same as my original cams.



There's no way that's true.

You're flowing so much more air with that cam set, that you're making significantly more power, with less boost pressure than the stock cams.

Pipe is right. What you're seeing is normal. You should see the pressure we were seeing with some of the turbo 3's.


I may not have this straight in my head but - this much I will add. My exhaust is not stock 2.5" straight through - magna flow muffler - which is a open pipe basically. Medlows it out!

I do not think it is restrictive but... at new power levels maybe. It could just be my conservative tune.

My question is - is it possible to have a turbo with correct exhaust sizing, to have lower up stream pressure on turbine, than the boost pressure in the intake manifold? I understand the pressure diff required to power etc. but ..

The article I was reading indicated the problem with these cams in turbo applications ( cams with over lap like the one I have ) is exhaust backing up against the intake pressure and diluting the mixture in the cylinder to some degree. when both intake and exhaust are open.

In the end the effective amount of fuel an cylinder can/ could be less.

Is this not the same reason - a naturally aspirated engine is better - (same cams ) are better with straight pipes , (it reduces back pressure) and cross over from other cylinders.

Some fuel air flows right through into the exhaust purging exhaust better.
It make sense to me ???

As far as power comparisons with original cams to me (seat of the pants) the 210/340 cams are much better up to low boost then my memory fades out, or I am just getting used to the power ..LOL
I thought it was better at top end before.

I have not dyno'ed - the car for some time -

The principal is what I am interested in right now not my application specifically.

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90 Swift GT - GT-BBT25 turbo Red - For Sale
99 MX-5 10th AE Saphire Blue
2011 Infiniti G37x
2010 Equator White
2000 Solara convert


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PostPosted: Sun Nov 17, 2013 7:19 pm 
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Theories are just that, theories. They don't always work out in practice. The turbo is a significant restriction, so you're always going to have some pressure, no matter what cams you use. In a performance application, the stock cams really have no place. It makes no sense to install a turbo with the intention of making power, and leave the huge restriction in place that are the stock cams. No better evidence of this is the Mk1 turbo3. With the stock cam you can make 20+ psi boost on the stock turbo. Add a mild performance cam, and it's all that little turbo can do to make 15 psi. Less restriction = more flow and less pressure build up. More flow at less boost pressure and less heat = more power.
While some stock cams are definitely better than others, they must be considered during the build up. To put it another way, 10 psi with your stock cams is flowing a lot less air than 10 psi with a performance cam set, and we know that air flow equals power.

You still have to choose your cam profile based on your intended RPM. If you have the 210 cams, and only rev to 6500 RPM, you've made a poor cam choice. If you want to make power to 8000 or more, and you are using the stock cams, you've made a similarly poor choice, because you'll stop making power long before that. Each choice will involve compromise, you just have to decide what end of the power band you want it to occur.

I put the overlap theory in the same league as the rough intake port theory. Everybody knows it, it seems to make sense on some level, and it's almost never a consideration.

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