Backpressure problem

[jskarateka]

I've been focusing on tuning for years but I seem to have a lot of exhaust backpressure that I believe is hurting my 1/4 mile times (11.9-12.2 @ around 110-112mph 1.7-1.8 60ft) . Had the problem with my stock unopened block and heads, but it got worse with the new engine build.

I have to plumb my regulator type boost controller as shown in the attached picture, to hold the wastegate closed so that I can get 25 or more pounds. With it plumbed the correct way, back pressure will open the wastegate.

With the regulator plumbed the correct way on the old stock long block (and all my current bolt-ons) the most I could get was about 20-21lbs at the beginning of the run, with some creep to about 23-25lbs at the end.

With the regulator plumbed the correct way on the new block/heads/cam it was worse when I tried it. 17-18lbs max at the beginning of the run with creep to maybe 21lbs.

If I plumb it like the picture, I can adjust the regulator for whatever boost I want over 20lbs, and it will spool fast to that amount and hold steady throughout the run.

Nothing appears to be wrong with the tune or exhaust or otherwise. Opening the dump does not help.

Does that sound like I'm getting way too much back pressure? Any thoughts on the best way to correct?

Too small turbo maybe? I got the TA49 turbo before I had the new long-block build (blew a rod bearing on the old block). As I stated above though, even then I had the issue to a lesser degree.

 

[TurboBuRick]

Jim?

When you say plumed the right way, Do you mean this?

Also do you know what spring is in the gate?

Rick

 

[dank GN]

yes your boost controller is hooked up wrong for sure.

 

[jskarateka]

Jim?

When you say plumed the right way, Do you mean this?

Also do you know what spring is in the gate?

Rick

Yes. It has a 7 pound spring.

The wastegate seems to be fine. Here are the results of some testing I did when I was originally trying to get my car to boost to 25lbs with the old motor:

If I run a vac line from the turbo compressor direct to the inside vac connector of the wastegate (no regulator), and leave the outside vac line of the wastegate off and vented to atmosphere, It will boost to about 8 lbs and hold there at WOT.

If I run a vac line from the turbo compressor direct to the outside vac connector of the wastegate (no regulator), and leave the inside vac line of the wastegate off and vented to atmosphere, It will boost up until it needs new head gaskets at WOT.

Here's the kicker; If I leave both vac lines of the wastegate off and vented to atmosphere, It will boost to about 15 lbs and hold steady there at WOT. So basically backpressure alone will open my wastegate at 15lbs. That's why I have to plumb it like my photo, to bleed some pressure to the spring side to keep it closed.

Isn't that way too much back pressure?

The only 2 issues I can find that may be 'wrong';

1) The down pipe I have, had do be dented in 3 spots to not rub anywhere with the china headers I have, but they are not very big or deep dents. Nothing that seems like it would cause that much disruption, but maybe I'm wrong?

2) The exhaust is a 2.5 inch hooker cat back. I have a new Pypes system that I'm about to put on because the hooker system is starting to rot out. There is a hole in one of the mufflers now. But again I wouldn't think that old hooker mufflers would cause that much back pressure.

 

[murphster]

Yes. It has a 7 pound spring.

You need a stiffer spring for sure. Your backpressure goes up more than 1:1 vs boost. For a small turbo, let's say its 1.5:1 for high boost. So at 28psi boost, you'd have 28x1.5=42psi pushing against the top. You'd need at least a 14 pound spring to add to the 28psi to get 42psi pushing down.

You are saying hooked up correctly you had 17psi max before creep. With the 7lb spring that means you had 24psi total pushing down on the wastegate before it would open. So that means 24psi backpressure. 24/17 = 1.41 ratio even at low boost....

So put in a stiffer spring and hook it up the correct way and you should be good to go. I'd get something like a 20lb spring. You may have to stack them to get that much pressure. This is why people go to CO2 with an electronic controller. 50psi plus to the top of the wastegate with the touch of a button and no more creep... Only other way is to play with springs and fine tune with the knob. Lot of guesswork and trial and error.

 

[murphster]

The other thing with your setup is that there is usually some boost creep at higher rpms. If the boost going past your dial a boost knob also goes up linearly, it will magnify the boost creep. Having a reference boost to the bottom of the gate will help nullify this as it will increase the pressure to the bottom of the gate linearly instead.

Look at the Turbosmart manual boost controller setup with the single port to the bottom....
Go to Instructions.. User Manual:
http://www.turbosmartdirect.com/Product-Categories/Manual-Boost-Controllers/GBCV-Boost-Tee-Blue.html

Now if you run a really stiff spring for a max boost of say 30psi, you'd get that with the boost controller mostly closed (turned down) and then as you open it up and let more air to the bottom of the wastegate you run less boost. Seems like a better way as it minimizes creep and you won't run more than your spring pressure on accident.

 

[jskarateka]

You need a stiffer spring for sure. Your backpressure goes up more than 1:1 vs boost. For a small turbo, let's say its 1.5:1 for high boost. So at 28psi boost, you'd have 28x1.5=42psi pushing against the top. You'd need at least a 14 pound spring to add to the 28psi to get 42psi pushing down.

You are saying hooked up correctly you had 17psi max before creep. With the 7lb spring that means you had 24psi total pushing down on the wastegate before it would open. So that means 24psi backpressure. 24/17 = 1.41 ratio even at low boost....

So put in a stiffer spring and hook it up the correct way and you should be good to go. I'd get something like a 20lb spring. You may have to stack them to get that much pressure. This is why people go to CO2 with an electronic controller. 50psi plus to the top of the wastegate with the touch of a button and no more creep... Only other way is to play with springs and fine tune with the knob. Lot of guesswork and trial and error.

Murphster thanks for the info. I'm OK with leaving the regulator plumbed like it is in my photo because it actually works very well. No creep or surge, holds steady at the boost I set it. My main concern, which you answered, was that there was too much back pressure.

I guess it's back to the tuning drawing board then.

 

[jskarateka]

The other thing with your setup is that there is usually some boost creep at higher rpms. If the boost going past your dial a boost knob also goes up linearly, it will magnify the boost creep. Having a reference boost to the bottom of the gate will help nullify this as it will increase the pressure to the bottom of the gate linearly instead.

Look at the Turbosmart manual boost controller setup with the single port to the bottom....
Go to Instructions.. User Manual:
http://www.turbosmartdirect.com/Product-Categories/Manual-Boost-Controllers/GBCV-Boost-Tee-Blue.html

Now if you run a really stiff spring for a max boost of say 30psi, you'd get that with the boost controller mostly closed (turned down) and then as you open it up and let more air to the bottom of the wastegate you run less boost. Seems like a better way as it minimizes creep and you won't run more than your spring pressure on accident.

You mentioned 30psi. I never really tried past 25-26, should I be going for 30 with my combo to get to mid-low 11s? Monitoring closely for any knock of course.

 

[Chuck Leeper]

Maybe plumb in a bp sensor, and find out what it really is?

 

[Mr.Spool]

You mentioned 30psi. I never really tried past 25-26, should I be going for 30 with my combo to get to mid-low 11s? Monitoring closely for any knock of course.

With your motor combo you may not be able to make 30psi.on the stock motor no problem 30psi with all things in order.on champion heads and 212 you should be going mid 11s on a lower boost number with the car 60fting 1.5/1.6. Mph 116/119

 

[pacecarta]

champ irons 212 roller stock dutt shouldn't need more than 22 psi to reach mid 11s on a ta49 ..

with the better flow heads your moving more air ..more pressure will be on the exhaust side and youll need more spring on the gate , general rule is spring should be no less than half the desired boost when run dual port with regulator as pictured . , small turbo asked to flow high on the map and BPressure goes up, the slic is also seeing more air flow which greatly increases the restriction so the turbo now works harder for the same boost level .. fix the spring and intercooler restriction and should see low 11s

 

[Mr.Spool]

As guys have stated put a stronger spring in it to keep things sealed up