What is a good Rpm for the turbo to come on?

bb · 12-11-2005, 06:02 PM

I am wondering, before I start working on my exhaust this winter, what is a good RPM for the turbo to come on? The car is a 76' 2.7 turbo, and the boost is coming on at 3500 with the new exhaust that I made. I was going to modify the exhaust to see if I can get it to come on earlier, but maybe 3500 isn't that bad?

Thanks
Oscar

RarlyL8 · RarlyL8's Garage

A good rpm for the turbo to come on would be 1500rpm. 'Corse that's not gonna happen with a turbo.
Better question - what rpm would YOU like the turbo to come on? Pick a range and work toward it using the vast knowledge of this bbs.

WydRyd · 12-11-2005, 06:29 PM

When you say "coming", do you mean it starts spooling/building boost at 3500, OR reaching max boost?

What size turbo are you currently running?

What's your HP goal?

Craig 930 RS · 12-11-2005, 07:07 PM

FWIW I get full boost at 3,200.....3,500 sounds good for the engine you've made, as it is smaller and IMO boost would come on up higher - just a guess.

herman maire · 07:41 PM

Just wondering how may LBS of boost are you running on youre 2.7?
Are you still running the CIS pistons?
What HP are you puting out?

I am also it the early stages for my home made twin turbo 3.0. From the research I have done 3-3500rpm is pretty much a decent set up for a turbo with minimal back pressure for a air cooled motor.
If you start wanting boost too low in the rpm range the turbo will create too much backpressure wich thermally kills the exhaust valves . Water cooled motors can handle hight back pressure much better.

Food for thought I guess

My goal is for my boost to come on around 3-3500 and pull to around 7000rpm.
That would be sweet. But lag does suck.

Emission · Emission's Garage

Quote:

Originally posted by RarlyL8
"A good rpm for the turbo to come on would be 1500rpm."

Ya stole my smart-ass comment!

Mine, with a cat-less fairly open exhaust, gets boost around 3000 RPM, a bit better than it did with the stock exhaust set-up.

juicersr · juicersr's Garage

What turbo are you running with ? Exhaust pix?

bkreigsr · 05:44 AM

i've only had mine since april, but it seems that i can feel the lump get on the cam about 2,200, then its a quick jump to boost at 3,200, then make sure its pointed straight, then hold on.
anyone else feel the 'on the cam' hit?
Bill K

slow&rusty · 12-12-2005, 04:52 AM

I would estimate on my K27-7200 boost starts "coming-on" (to use your words) just before 2000rpm, its pretty seamless transition between on and off boost.

The best way to know for sure is to hit the dyno.

Yasin

bb · 12-12-2005, 05:37 AM

When I say comes on I mean when I am at 100kpa on my Map pressure. With the Megasquirt I have been able to see that "every" time at 3500 rpm I reach 100kpa. I am reaching 140 as my maximum boost. Right now I am working on getting my system dialed in. I still am trying to get the wastegate to work right. For some reason I am maxing out at 140 and the wastegate never opens, or is openning and keeping the boost at 140. My next thing is that I want to make a run with the waste gate removed and someone keeping an eye on the boost to see if I am losing boost at the wastegate.
For some other questions. I am running the standard cams and CIS pistons. I want to be at 7lbs of boost and am going to set up an intercooler over winter.

bb · 12-12-2005, 05:42 AM

Juicersr:

I am running a Rajay turbo,( not sure what size yet), with a custom exhaust. I bought a set of used BB headers, and shortenned them. I had to shorten the headers since the flanges were too far to the rear of the car to plumb the turbo. I had to take them down about 4-6 inches. Then I mouted the headers and started welding pipe until I ended up where the turbo was mounted. Last year I made the pipe that went from the headers to the turbo from 2-1/4" down to 2" to see if the turbo would come in earlier. I am not sure if it worked since I did not have the megasquirt hooked up then. I am debating making a 2-1/4" pipe again and making a couple of runs now to see where it will come in.

Oscar

Emission · Emission's Garage

Quote:

Originally posted by juicersr
What turbo are you running with ? Exhaust pix?

K-27 Turbo.

Fabspeed dual exhaust (click on the link for a pic).

http://www.schnellauto.com/images/930fabspeed.jpg

bkreigsr · 12-12-2005, 09:33 AM

Emission:
dats a clean machine my friend!

BlkBird · 12-12-2005, 09:57 AM

Start feeling boost at 2400 rpm or so.
Solid rocket fuel booster at 3200 rpm.
Wastegate opens at 3800 rpm. Full boost achieved (1 Bar).

sammyg2 · 12-12-2005, 10:23 AM

My boost pressure goes positive at around 2900 rpm depending on what gear I am in. In 1st gear I don't see boost on the guage until it is time to shift because everything happens too fast. In second, it shows up at 3100, in 3rd I see it at 3000, in 4th I see it at 2900. The difference is due to lag, not threshold.
I would prefer it to hit earlier. When i re-do my muffler I think I can lower it to 2500 rpm.

herman maire · 12-12-2005, 10:41 AM

Quote:

Originally posted by BlkBird
Start feeling boost at 2400 rpm or so.
Solid rocket fuel booster at 3200 rpm.
Wastegate opens at 3800 rpm. Full boost achieved (1 Bar).

That sounds like a real nice set up.
What is the power band like? with this quick spool set up does it still pull strong to around 7000rpm or is it fading off a little also is there any chance you could post some pics of youre turbo set up.
Oh and what turbo are you using?
thanks

BlkBird · 11:10 AM

HM,

The turbo started off as a standard K27 7200 but it just came back from a trip to a modifier. I couldn't tell you exactly how it was tweaked but it puts out an incredible amount of HP and torque.

See the dyno results at this thread. You can see the old "original" K27 HP and torque results and the new post-tweak numbers. If you look you can see a slight dip at 3800 (waste gate opening) and another where the Andial Fuel Enrichment system cuts in.

Other modifications include GHL header and dual outlet muffler, 1 bar spring and a very attentive and patient mechanic.