Hi all, I've got a 1992 BMW 850i with an L59 5.3 iron block from a 2003 Yukon in it. Goal is ~550whp, or wherever it lands with conservative timing.

Relevant specs are as follows:

• Log manifolds to single 88mm GT45 ebay special
• Open dump 44mm wastegate off the hot side of the turbo
• Completely stock longblock
• Stock L59 Camshaft
• Stock 5,200rpm redline
• 4" to 3" downpipe splitting into dual 3" pipes out the back, each one with a cat, resonator, and muffler.
• Bigass tube and fin intercooler
• Pro Built Automatics 750fwhp 4l60e with 2,800rpm stall Circle D 300mm converter

Problem I'm having is that the spool is absolutely atrocious. Gets up to ~11 pounds of boost at 5,200rpm redline in second. If I set the boost target higher (tried to run 14 pounds the other day), it doesn't hit max boost until it's in 3rd gear when starting from the bottom of second. I knew it wasn't going to be great, but this is effectively unusable. Without wishing to go into detail, I will simply say that due to my location, a loud/decatted exhaust and/or a lopey idle are not options.

The way I see it, I have a couple options:

• Smaller turbo like a 7875
• Small cam like an LS6 Cam or similar
• Larger cam (been eyeing the Sloppy Stage 1.5 cam) and idle it at ~1,000rpm to hide the fact that it's cammed
• Any combination of the options above

Would a small cam like an LS6 cam make any significant difference to spool? Would a larger cam idled up sound like a normal car? All the videos I see if idle on cams has people setting the idle as low as possible, I have yet to find an example of someone idling one up to sound "normal".

Bearing in mind the restrictions on the build due to my location, what would you do were you in my situation?

 

Something like the VS 2.5 78/75 would have been ideal, I dont know if adding a cam would make it worse or not, you would be taking some off the bottom end. Some of the guys can fiddle with the tune and make them better, but that's a big turbo for what you want. Just an example, my 5.3, tiny cam, 78/75, 93 octane, makes 500/500 on 10 PSI, 5900 RPM, full exhaust, no louder than a pickup truck. Instant spool.

 

88 seems HUGE. Definitely consider 78/75...

 

Let’s confirm the measurements first. Some of those eBay turbos have poor nomenclature at best. Could be a 69mm compressor with an 88 turbine inducer that they’re calling an 88mm.

 

^^^ that's a good point.

 

1. Ebay GT45s are 69mm, not 88mm, and are based on 20 year old turbo technology.
2. What spring pressure in the gate, and do you have a boost controller?
3. Stock 5.3 cams are bad for power, but the worse part is that this probably means stock valvesprings as well.
4. You're not going to be making much power only revving to 5200 RPM anyway.
5. A 7875 is a larger turbo, not a smaller turbo. It just has smaller housings.
6. If you're worried about idle, choose a real cam that can get you good power with a good idle. The sloppy cams were specifically chosen because they were the CHEAPEST cams out there and aren't as one-size-fits-all as everyone claims.
7. Your combo seems thrown together without much planning and some potentially unreal expectations.

Get a mild cam and some real valve springs. Raise the rev limiter no less than 1000 extra RPM. Consider a better turbo.

 

I'd do a good set of valve springs and a mild cam.

 

Anything that makes more power NA will improve spool. Its more flow through the unit. I wouldn’t’ bother with another factory cam. You will get a night/day difference with an aftermarket unit. Something like a 212/212 @ 112 with PAC1218 springs will idle like stock and give you a 50-60hp gain across the board in most cases. Id go even tighter on the LSA, but you will start to get a choppy idle if you get it too tight.

That said, measure what you have. As mentioned, you likely have a 69mm turbo, not an 88. Believe they have a 77mm turbine whee. They are not super lazy turbo for a 5.3. (Not that there aren’t quicker spooling units out there.) Which makes me think you have other issues. Did you use the fire ring in the WG? Confirm no exh flow through the WG while revving etc? No boost leaks?



If all that checks out, a lot of your “spool” characteristics come from the tune. Zero reason a gt45 won’t make decent boost quickly on a 5.3.

A few companies have put larger compressors on the gt45. If you really have an 88mm compressor you will never get what your wanting from that unit. If you really want 550hp you need to look in the 66-72mm range IMO. I just did a setup with the ebay 78/75 that has the .68 T4 instead of the usual .96 T4. Run 7-8lbs and it hits full boost by 3k in a factory 5.3 with a small cam. You don’t need or even want a 78mm compressor. But price wise it was hard to beat. A Borg S366 would be my choice if money wasn’t an option. They are about $575-600.

 

Ive got a GT67 turbo as well as a billet 7665 turbo for sale that will be worlds better than that thing.