I was all ready to order a set of GT45's to run TWINS on my 468 BBC when a good friend asked me why? Some good points were made during our conversation as to "Why" I would need two 76mm turbo's to make a low boost combo. In the CONS of not needed debate he says LS guys are running 20-30# of boost on 5.3's with a SINGLE 76-88mm turbo. To make half as much boost surely doesn't require two BIG turbo's??? Does it?

This will be a pump gas Street/Strip engine purpose built for boost but not huge boost. How does the CID and engine RPM factor in? I'm lost now, my original plan of twin 76mm is way over kill IMHO, the recipe I found on the Internet was for a 1500-1800 HP WAY too much for what I'm wanting to do. HELP?

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

First off, if a turbo is said to make 900 hp, it will do that at a high pressure ratio, or boost level, so just because it will make 900 on a small cid engine doesn't mean it will make that hp at a lower boost level on a larger engine. Another way to look at it.... larger cid at low boost will require more compressor than a smaller cid at a higher boost level making the same hp. He is right though you probably don't need two 76mm's for a mild bbc, depending on your n/a hp. How much n/a hp, or whats the combo? Rpm, and cid is used more for turbine choice.

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

You will need some decent size turbines with that displacement. The gt45 is a good choice IMO, price and compatibility wise. You can run them around 8 psi and they should be decently efficient that way.
And your buddy sayin people running singles on a 5.3 doing so well... gotta take into acount you have over 100 more cubes to fill up. The gt45 has almost a 70mm comp, but a hefty 88mm turbine which wont be a choke point on a 468ci. Otherwise, you should look into something in the upper 80mm or 90mm compressor and those get pretty expensive

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

I was looking at some CVX? GT45 ball bearing Turbo's for $565.00 or make offer. I just wish I knew more on how to interpret the specifics on the turbo's. What is "Trim" I've noticed the trim can vary a lot between GT45's some might be .50 while others are .69!!! Which is better?

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

The Engine: "PUMP GAS" May end up with water/methanol injection
.060 4-bolt main 454 block. 468 CID (Will be studded top and bottom)
Scat 6.385" H-beam rods
STD/STD Factory 4.00" stroke crank
Pistons: Still shopping to get the compression finalized.

Cam:Sending to (Bullet for regrind) Currently too BIG Bullet Racing billet solid roller used in a high compression 509 BBC NA. .710/.690 @.050 .276/.284 adv 310/314 LSA 110. I'm looking at lobes in the .630-.660 lift range but will call them to see what they recommend for a boosted combo.

Heads:
#781 Heads 2.19/1.88's ported intake and exhaust runners w/5 angle valve job on the intake seat. Manley Severe Duty valves, Crane Gold roller rockers, and Moroso stud girdles.

Intake/Carb
Old School single plane Holley "Street Dominator" Oval port with port work in the runners and the plenum. Gasket matched to the Large Mr. Gasket openings and the plenum enlarged and blended.
Holley #4781-8 850 DP. I will need to modify for the blow-through setup.

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

Trim is large vs small diameters of a comp/ turbine wheel... the larger the trim, the more it flows for a given diameter. turbobygarrett.com has lots of info if you're looking. Sounds like a good combo, any rpm range you're shooting for? I'm just going to guess 500-525 hp n/a? So using 550 hp at 10 psi gives you 925 hp, so it looks like two of the 68mm gt45's work be a decent fit.

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

The gt45 with 76mm or 80mm is going to get you into the hp range you didnt want. You can get 2 of the regular gt45s for less than the ones you mentioned. Those will still have room to grow, 1200hp or so before theyre spent. The 76s about 1400, the 80s about 1600... they are brutes. One gt45 will do 650hp proven

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

way cool, youd be the only other big block turbo guy ive seen run the street dominator, i have that exact intake on my setup with a single t6, but my compressor is about 100mm
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Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

I bought the Street Dominator after deciding to go large Oval port on the heads. The intake came from a high school friend and will need porting especially in the plenum area to handle decent power. Your SD might be different but mine has the spread-bore carb pad and a tiny/small plenum opening and restricted inlets to the runners. I'll gasket match the runners to a large oval gasket, open the perimeter of the plenum and blend the larger opening into the runners. Hopefully the mods will really wake this intake up to handle flow!

One step at a time! JD

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

I could get the regular brass/bronze bushing GT45's for between $220-$270.00 each. I would like to see 1000HP at the crank and +- 770HP to the wheels. I'm not worried about turning really high rpm's but would be willing to see 6000-6500 to get there. I'm not sure the heads I have under 8-10# of boost could handle over 6000 rpm's & 468 CID not sure. JD

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

I need to settle on camshaft selection to compliment the head flow and RPM of the CID. There's so much to consider when everything needs to work together. I'm hoping to get to the 1000HP mark if possible. The goal is to use an "Old School" engine and hang with the Turbo LS guys around here. I have NO chance of being the fastest but I'd like to be respectable. JD

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

ya my intake is the same way, tiny plenum, im just planning on turning the boost up. if it runs horrible ill buy a brodiz single plane or maybe a pro comp china one. i just like th eroom the street dominator allows under the hood. its the shortest single plane ive ever seen

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

I've been comparing my expectations to see if it was impossible. A mildly ported 781 head flows around 300-310 cfm as far as I can tell. I would agree an 8.5-1 468 with a decent set of 781's even with a roller cam might make +- 525-575HP. I've seen estimates of boosting in the 10-12# range increasing power by between 65-75% hopefully this is close. So, it looks like I might be in the range of 866-1006HP potential. Does this sound close?

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

Sounds about right. 10 psi should theoretically add 68%. I wasn't sure if you'd be able to make 575hp on 8.5:1, but shouldn't be too far from it. If you're going to keep boost low, and the setup allows it, i'd think about 9:1 or so, although you have cubic inch working in your favor.

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

Thanks for all the feedback! I can see why "Blackbeard" cut and changed the turbo flanges on these headers! Overall the headers seemed to be well worth the cost but the turbo flanges are screwy. They angle in/out and down.... Definitely will need changing once we can mock it up in the car.
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Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

Yep, I don't. Understand how they can't even make them level, but they weld good, I installed my v bands on them and they welded actually pretty good. But the v bands just look so much cleaner and easier to deal with. My compressor is 106mm but I got the turbo for cheap and I know I'm on the big side of the scale

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

Stalled on the project family stuff, kids just had their tonsils out. I'm looking at going "Twins" because it's just plain cheaper than one big turbo. JD

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

id like twins. but idk if you pan on running power steering bt the factory pump wont clear these headers. well it does but barely. i think it would cook the fluid in the pump.

 

Re: Low BOOST 7-10# BBC 468 cid what size turbo do I need: Single or Twins? Head spin

I think on the 55 car ill need to run a rack. Ill go manual rack-n-pinion keeps everything simple. Good info though !