TurboTommy

Thanks for all your responses, guys.
Yes, the ratios that are being mentioned here are full boost readings (whatever that amount is, with the wastegate open), and that's normally how the back pressure vs boost pressure is tested.
I just happen to be the dufos that decided to test at moderate acceleration levels to see what's happening, especially because I have a little more overlap in the cam.
I have the LR 3" downpipe flowing into a 4" exhaust, so post turbo should be taken care of.

blown 944

I have measured it and have seen 2:1 even it higher boost with smaller hotsides.

It seems to me that if you want under a 1.5:1 at high boost you have to have a housing that will spool the turbo above 4k rpm. The only hotside I used that was under 40psi at 25 psi MAP was a T4 .69 a/r.

Right now I have a T4 .48 twin entry that has a little trickery done to it to spool fast and I see well over 2:1 on spoolup. Then when it opens to full it is ~ 40Psi in the crossover.

Highest I have ever registered was ~60psi. Too much of course.

I would love to see others measurements to see what elevation and camshafts effects can be.

333pg333

So in essence for 99.9% of our applications we are going to have to have not the ideal amount of backpressure?

gt37vgt

ignition timing and fueling will have an effect as well .. as it creates the after burn hey ?

thingo

Thomas if you can measure the exhaust back pressure (after the turbo) it will give you an idea if gains are possible.I have seen (on other cars) less than 1:1 and no exhaust back pressure after the turbo(with a 3" exhaust!), and it was no drag car either.

fast951

Thomas, Unless there is some pressure in the exhaust, the turbine will not spin. Once the turbine starts spinning things get different.
As I recall you run high boost with the aid of water Methanol, and your engine is fairly strong. You have been running this setup since 2006, don't you think if there was a back-pressure problem, you would have had engine problems by now. Based on the insane top speed you ran, it sounds that you don't baby the car, and it likes it.

Oddjob

For reference, I have measured around 1.6-1.7 backpressure ratio on two different K26/8 cars with 2.5" open exhaust, stock manifolds, crossover, & downpipe, running 14-15 psi boost.

ehall

OT

Oddjob were you the one that created the 968 mirror install write-up?

Fishey

First, I want to mention this 2:1 etc.. ratio is called EBR (Exhaust Backpressure Ratio)

Second, I want to mention that having anything with a 1:1 or lower (.5:1) EBR is physically impossible. The reason being is the exhaust pressure between the manifold and turbo is not greater then the intake pressure the turbo will simply fail to spin or will spin in reverse. This is the reason we all run this cool little thing called a Blow off valve.

Third, EBR can change all over the map and to be honest it really doesn't make much of a difference except that it will make it so your overlap with a higher EBR causes more reversion. However, on a turbo car overlap is always left to a minimum to stop reversion in the first place.

gt37vgt

its rare perhaps imposible to maintain but sure as hell not impossible to achive .
its the pressure differential across the turbine hot side that makes it spin not the pressure differential of the hot + cold side .

TurboTommy

Yeah, basically what you're saying is right about my car.
However, as you know me by know, I'm **** retentive, and like to have as much information as possible to find out how things work, and then possibly improve on them even more.

So, doing a bit more research it now seems obvious that, of course, you need above atmospheric pressure in the cross-over to spin the turbo, and that will happen even with low intake mani absolute pressures and small throttle openings.
I just didn't think it would be that much.

A.Wayne

Sounds Fishey to me ......

toddk911

Which is the purpose of VTG turbos correct? A LOT of backpressure initially for super spool up and then rapid reduction in back pressure for the top end.

Chris White

That’s part of it – the VGT set up also ‘aims’ the flow at the outer edges of the exducer to help with initial spool up.

Chris White

After years of trying I have achieved the perfect 1:1 ratio on my track car set up. After thousands of hours of testing and I can’t count how much money I have found the way to achieve the 1:1 balance….

It simple…I have no pressure on either side…0:0 is 1:1…of course spool up time in literally infinite. Or at least until I get it running again.