I've exhausted(pun) my local resources and knowledge base on turbo builds and need outside help.
I have a Ebay turbo that's roughly equivalent specs to a gt2876r
The motor is a 1.8 liter 4cyl.

I am able to pull from a stop through the gears just fine. From low rpm to red line, achieving 15 psi boost without issue.

Where I run into issue is at mid rpm/ highway cruise speeds (no overdrive - 3500rpm) when I gently accelerate, it spools to about 5-6 psi and is hit by surge. turkey gobbling, car bogging / shaking not pulling any farther.

possibly happening on longer boost loads rather than short span quick spool ups?

regardless, could this be caused by a massively over size turbo for my poorly breathing motor?
could it be a crummy BOV not behaving? does the BOV have any funciton at speed on the hwy without fully letting off the gas?


Has anyone else experienced long low boost surge or part throttle surge ? what fixed it?

I don't mind buying another turbo but I certainly need to know that it'll fix the issue before just throwing money at the problem.

Thanks for your help!



1.8 liter MGB with siamese 5 port head. 2 intake 3 exhaust ports.
Full sequential spark and fuel with a 45DCOE throttle body

turbo specs are:
T25/28 chineseum ebay special
0.60 A/R Cold Side
Ind: 54.00 mm
Exd: 70 mm
Trim: 60
Turbine Wheel
.64 A/R
Ind: 52.90 mm
Exd: 45.47 mm

 

Yes, a large compressor can cause surge. Sometimes a simple turbine housing change to a larger ar can fix the problem. Plotting it on a compressor map could be helpful. The bov should be held shut by boost and only open in a high vacuum situation.

 

Got any logs of when it happens?

 

I wasn't able to upload them through the forum, so here's a google drive link with multiple. modified 2 is a snap of highway toe in conditions.
I am running an MS3x gold box

 

could very well be that the fuel and timing table in this area is not tuned correctly if the motor is laying over when it does it.

 

I did some looking and a rough plot on a 2876 map. I could see where you'd easily be in surge at mid rpm. (Given ~100 hp na). Like others have said if you're lucky, you can make a little more hp in that area and move it back on the map otherwise you may be looking at a .86 housing or turbo swap. Does your turbo have recirculation or anti surge slots in the compressor housing?

 

How about this?
T25
Compressor Specs:

• Inlet: 1.70 inch
  Inducer: 42.87mm
  Major: 60mm
  Compressor: .80AR
  Compressor Housing: M24/ T2
• Turbine Specs:
  Exducer: 1.850"
  Turbine: .86AR
  Major: 53.10mm
  Minor: 46.25mm

 

I do not have anti surge slots or holes. just a plane jane housing. Is this something that could be added by a competent machinist or is it designed into the housing?

I'm on GT's site and looking at bw's matchbot playing with values.

what do you think about a gt2560 or gt2554? both small frame turbos appear to have .80 a/r compressor housings
their smaller two 2052 doesn't appear worth considering... am I correct?

 

after more looking at the logs I identified that it was consistently crapping out under longer duration of boost conditions. It was only in 4th gear and not in first or second. Previous post from tbird mentioned that the motor could be laying over in this range... misfire? paired that thought up with temperature gain and went on a hunch that the spark plugs might be overheating.
Sure enough. Went to a colder range iridium, fine wire, plug. Now it pulls cleanly all the way through the rev range in 4th gear for any duration I throw at it.
Also, bought a proper spring matched tial bov just for good measure. It's on the way to install next week so it'll remove that from the mix.

The turbo is definitely too big and spools too late, but I'm happy to report that the problem is identified with a simple solution. Now off to the dyno to dial in the timing.

Thanks for your advice. I appreciate y'all not laughing me out of the room!

 

Thanks for the follow up! Glad it's fixed!

 

Feeling dumb here, please be patient with me. Plugs solved one problem but not the primary issue.

After some time away from it and a refresh on the project, it is still surging hard, mid throttle, on highway. It will pull without surge all the way to redline if I hold WOT.
Once I started driving it calmly vs nailing it every time I got in it, it would surge at part throttle and hwy revs. I went back to the chart and plotted my points to found myself way way off to the left.
I purchased a gt2252 turbo which I plotted and found it more appropriate. Attached you'll see the before and after maps. I plan to put the new turbo on this evening or next... so we'll see what happens there, but I would appreciate input or thoughts on my selection.
I'm still a little to the left side of the island, but much closer. The turbo itself is substantially smaller and I can only imagine how much closer I am to getting it dialed in.

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In to see the results.

 

If you have vacuum after the throttle blades during this condition try a bypass valve (cent. Supercharger) instead of a BOV.....and make sure it is open with the vacuum you are seeing during the surge

 

no, it's definitely in boost during this condition... it's mega surge. I plotted it out on the map that's most similar to the compressor I bought on ebay and it's flat out ugly...

new turbo arrived this week and it has a different exhaust outlet flange. I'll make a flange and downpipe elbow next week and should have something to test next week. Attached are plots of the new turbo and my specs as well as a side by side... hilarious. "upgrading" to the little one!

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That big turbo isn't a copy of a GT2876. It's not even a copy of a GT series compressor: it's an ancient (1970s) T04-B wheel. If the numbers are to be believed, then it's something akin to a V-trim. It's an awful wheel profile for your engine and it's made colossally worse when combined with a roughly stock-sized T3 turbine. (or T25? Who knows...)

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This is another example of why China turbos are garbage: even if they don't physically fail, the ad/listing is full of lies and the buyer has no idea what they are really getting. I'm not ragging on you, just trying to stress that buyer beware.

I'd be curious to know how much better the small replacement works.

 

That's some great feedback! thank you!
I never had any grand expectations of quality with it, just that I could purchase a "small" turbo with all the ancillary items for under 500 bucks to play with... I bought some cheap "universal turbo kit" on ebay to prove the case before I sunk any real money into it...

I got the materials to start making this flange... should have something driveable by the end of the weekend. I'm sure it'll be lightyears better, but can't wait. I'll post results.

Sorry for posting this in the advanced tech section... I think it's pretty clear that I'm far from advanced when it comes to turbos... If a mod can move it to the appropriate section, please do.

 

I am the Moderator for this forum and do not see any reason to move your posts.

Your thread just helps others to understand how important it is to use compressor maps
and other tools to get the right turbo(s) for each application. Some of the old turbo stuff was
Bullet-proof and other turbos in the same time frame were poorly designed. The old RAJAY
Turbos falling in the Bullet-proof category. High MacInnis, who wrote several turbo books was the Chief Engineer of Rajay Turbos.

Tom Vaught

 

Night and day difference.
No surge, instant power, smooth delivery. So much better.
Running 15 psi now on the smaller 2252 turbo.
It's not as 'dramatic' as the larger ebay turbo but it's definitely faster and more usable with a wide powerband.

will post a link to a log file once I get it tuned to the new turbo

Thanks for all your help and advice

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So here goes the next stage of tuning and development.

With the larger turbo running between 13-15 psi I was pushing a substantially higher CFM flow through the motor (when it would accept it without surge).
In the case of the smaller, lower flowing, turbo, can I expect to run higher PSI to achieve similar cylinder pressures? How does that math work?
15 psi with X turbo = 20psi on Y turbo.
Air density and CFM have to have a calculation to find the equilibrium. Is this something that is easily calculated?

I'm high as a kite after this success and am anxious to find the next step forward. Any suggestions are appreciated.

*not concerned about blowing it up... have another one waiting in the wings to be built with quality guts once I find the limit of a stock mgb motor.
**current configuration is on bone stock 9:1 mgb motor with plenty of miles. cast pistons and worn out road cam. nothing special.

Cannot upload this file so here's a shared drive link


<>

 

Read up on MASS FLOW thru a Compressor and Engine vs speaking about PSI and CFM.

Mass Flow is in Lbs per Minute of flow or in Kilograms per second. Mass flow is on the horizontal line of a compressor map. Example: 70 lbs per minute of mass air flow

Pressure Ratio (a pressure ratio of 2.0 is about 14.7 psi of boost pressure), is on the vertical scale to the left of a compressor map. example 2.1 Pressure Ratio is about 15 psi of boost pressure.

Tom Vaught