MildMax951

Using an lbe I boost to 15 psi at 3250 - 3500rpm
thereafter as is well known the boost drops off
and by 6000 I have 8 psi.

I have a std k26/6 turbo and the std wastegate(not shimmed) and altitude is 5000 ft. Is there any point in shimming the wastegate or getting a dual port wastegate/mbc combo.

I searched the archives and found a post from Rage2 where his k26/6 dropped to 8psi at redline
even with the hose to the wastegate disconnected.
I understand that depending on the condition of the wastegate spring exhaust pressure alone is capable of opening the wastegate at high enough exhaust flow.
Rage2 is in Calgary, also high altitude.

I am specifically looking for any feedback that includes the high altitude factor.
As much as I love my 951 I'm not about to
relocate to the coast for the sake of more HP.

coolhand

I live in Colorado, also about 500 feet, and my stock 951 holds 12psi most of the way through the rpm's and only drops off a few psi at redline. Maybe shimming the wastegate might help

coolhand

ooops, meant 5000 feet

MildMax951

Thanks for the info coolhand.
This is the problem, there are conflicting opinions. I'm sure I read a thread from Rage2, although I can't find it now, where he had tested his car as well as a friends car at altitude and came to the conclusion that the k26/6 due to it's flow map / size couldn't hold boosts in the 12 -15 psi range at high rpms no matter what you did
in terms of shims, dual port wastegates etc.
Any comments Rage2.

I want to try and pin this down before I spend time and money chasing something that perhaps isn't possible.

adrial

At sea level or thereabouts my 26/8 drops to 10-12psi at redline.

I think with an upgraded wastegate or maybe shimming the stock gate, I'd be able to hold to redline...

Moral of the story, people have gotten 26/8's and 26/6's to hold 15psi or more to redline at sea level. So in theory, you should be able to hold 15psi or more - the difference in atmospheric pressure from sea level to 5k feet.

Rob

I'm at 7200 ft and if I recall correctly, my boost drops off to about 9~10 psig at 6k. This is with an open 2.5" exhaust, dual port waste gate, and EBC.

I also know that I don't have any boost or exhaust leaks. The open exhaust, waste gate, and ebc have all helped with spool up time, but have not affected the boost drop off significantly.

Rob

guardsred951S

High altitude = thin air, so it's more difficult for the compressor to push this thin air into the engine and since the K26 gets out of it's sweet spot after 5000 rpm, it's not surprising you see less boost at redline. I think a shimmed or dual port wastegate would help but ultimately, altitude will kill performance. My non-shimmed K26/8 will hold 12-13 PSI to redline (at sea level) if not more - I'm only reading this from the stock gauge so that may not be too accurate.

If you're looking for better straight line performance, I noticed a lister that put a 944 S2 tranny in his 951 to help since the turbo takes longer to get the air flowing. He was in Denver I believe and the S2 tranny has a lower final drive ratio.

<img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />

rage2

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by MildMax951:
<strong>I'm sure I read a thread from Rage2, although I can't find it now, where he had tested his car as well as a friends car at altitude and came to the conclusion that the k26/6 due to it's flow map / size couldn't hold boosts in the 12 -15 psi range at high rpms no matter what you did
in terms of shims, dual port wastegates etc.
Any comments Rage2.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">The final K26 test I did before I upgraded the turbo, I could only get 9psi at 7000rpm. I had an AVC-R set to 95% duty cycle (the max) at this time. I also crimped the signal to the wastegate so the car was set to give "unlimited boost". Rolled on the throttle, 9psi at 7000rpm.

This was with a deltagate, k26/6, AVC-R. Now I can hold 28psi all the way to redline with the HR Stage 3 .

GrantG

Morale of the story is to get a bigger turbo. That's the way to make big power at altitude, but the lag gets really bad - can't have it both ways (unless you get a 3.0L with a big turbo...)

MildMax951

Thanks for all the info guys.
Based on Rob's and Rage2's replies, both using dual port wastegates with ebc's (surely the ultimate boost control combo) I'm just going to leave the car as it is.
It's a street only car, in fact it's my only car
Im getting 260 lb/ft rear wheel torque at 3250 rpm
and that wallop in the back at low rpm feels great. The car is very reliable so it's a case of if it's not broken don't fix it.

jchaley

I live in Denver (5000') and have played with the waste gate and boost settings. The stock wastegate is pushed open by exhaust back pressure. I have a stock waste gate that is shimmed, my spring is weak so the boost line is disconnected and plugged. I get 16lbs in 5th (on a aftermarket VDO gauge) and that falls off at high RPM. Keep in mind that 16lbs (on mechanical gauge) at 5000' is only about 13lbs of sea level boost (it takes 3lbs just to get to the same air density that you would have at sea level. That said I played around with the stock waste gate to see it the turbo or waste gate was limiting boost. I pulled a vacume on the wastegate to keep it from opening and took the car for a spin. The boost pressure very easily shot over 20PSI. I did not keep the car floored to redline but this simple test leads me to think that a dual port wastegate that holds the wastegate valve closed until the desired boost is reached would make a big difference. That said I think the bigger question is if the turbo is efficient at 15lbs at redline or if all you are doing is heating the air.

John
86 944 turbo (daily driver, S2 tranny)
86 944 turbo (48k miles for sale)
78 911SC (track car)
72 911T (wifes porsche)
88 323 GTX (everyone needs a 250HP snow car)
02 WRX wagon (wifes car)

MildMax951

Thanks jchaley, that's some interesting data.
But we still don't know what happens at high RPM
as in "I did not keep the car floored to redline"
and I certainly don't expect you to sacrifice your car to science to find out.
to Illustrate the point here are some boost figures from a dyno run of my car.
The boost was read by a carefully calibrated MAP sensor teed into my vacc line, so the boost is in absolute psi.
3500 15psi
4250 13psi
5000 11psi
5500 9.5psi
6000 8psi

I can easily dial up more boost at 3500 by adjusting the LBE (that ties in with easily hitting 20 psi with vacc to hold wastegate closed)
but I feel I will suffer exactly the same drop off
as the volume of air that the turbo has to move increases with rpm, ie I suspect the turbo is the limiting factor. However short of holding the wastegate shut with vacc and running to redline
where If Im right the boost will drop off to the 9psi range and the car will survive, and if I'm wrong the car will go up in a puff of smoke, it is difficult to determine the limting factor.
If all a dual port wastegate/mbc is going to get me at 5500 is a 1psi boost gain then it's not worth it.

rage2

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by jchaley:
<strong>That said I played around with the stock waste gate to see it the turbo or waste gate was limiting boost. I pulled a vacume on the wastegate to keep it from opening and took the car for a spin. The boost pressure very easily shot over 20PSI. I did not keep the car floored to redline but this simple test leads me to think that a dual port wastegate that holds the wastegate valve closed until the desired boost is reached would make a big difference.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">If you kept it floored (and didn't blow anything up ) it would've dropped boost while heading towards redline/fuel cut. I did the same test except I rolled on the throttle so I didn't get the 20+psi spike. With the AVC-R controlling boost, I slowly closed off the wastegate as it went up the revband (again, so I dont get that huge spike). By 5500rpm, the wastegate was fully shut and boost started to drop all the way to 7000rpm.

jchaley

I am not saying the stock turbo can hold high boost to redline. I think the k26/6 is too small for this (if it cant hold at sealevel it will really have problems at altitude) but I think the stock wastegate design causes the waste gate to open even with no boost reference. I am planing on upgrading to a garrett hybrid turbo and a dual port. I want something more efficient in the 15-17lbs of boost (mechanical gauge) I plan to run. A more modern design like the garret should have less lag and heat the air less as well, all resulting in more power. I hope that this combined with the short gears in the S2 tranny will make a really responsive street car.

John

MildMax951

I just had a thought, on this thread and a lot of others members write in and compare boost figures in psi and sometimes bar.
I don't have an aftermarket mechanical boost guage, the dyno operator I used has a very accurate MAP sensor. My stock boost guage tracks his map sensor pretty accurately but under reads by just over 0.1 bar so I can use it for pressures up to 1 bar obove ambient at sea level.

To get to the point, are mechanical guages referenced to ambient air pressure or are the somehow calibrated to ambient air pressure at sea level at manufacture.

If they are refd to ambient air pressure and everyone on the list lived at sea level we would all be comparing the same thing. However if the guage is used at altitude (ambient air pressure decreases by roughly 0.5 psi every 500 ft increase in altitude) and you take the guage out the box and it reads 0 psi then at 5000 ft you would actually be reading 2.5 psi higher than at sea level.

So getting back to this thread if someone living at 5000 ft using a mechanical guage wrote in said his k26/6 held 11.5 psi @ 6000 rpm, I might be encouraged, but in reality his 11.5 and my 9 psi ABSOLUTE (ie reffed to seal level) are actually the same thing. Am I talking a load of rubbish, or is this correct?