Large Twin Turbos for Road Racing???
 

Humor me here :drive:

I have an old aussie Commodore - had it for years - same platform as a 2002-06 GTO but with 4 doors. It's had more evolutions than a swamp pond tadpole and now has wide body (custom) so I can fit 315's out back and 275's up front. Yeah that helped handling. But I'm needing to build a new motor at some point as I'm leaning on it a bit hard so it can't last forever.

I run a stock LS3 with a mild cam (218/222 cut on 114/116LC) and 040 cometics to get tight squish - so it must be right on 11:1 with a TDC deck clearance that'd make baby Jesus cry if he got his pinky stuck in there.

At first I just kept the old rear mount turbo setup that was the original LS1 powered. The rear mount was a Bazman/RSL/Torque Performance rehash of the original STS kit i.e. we got the kit, looked at it, shed a tear and said "poos and wees"; threw everything out and did it again with better shit. This worked wonders on the LS1 with an auto. Boost from standing start with no tricks and 31mpg at cruise.... but with the LS3 the turbo was too small - it was like trying to pass a 3" turd through 2" plumbing, and if you've ever tried that you know it can cause cramps and the blow off valve starts to scream.

It made for a hell of a lot of bottom end (over 300rwhp by 2200rpm lolz), but it tapered off faster than Hillary's bunghole when the votes were in.

SO.... what do you do when your family has grown up, the car is basically track, show, road race, cruiser and general funster? I didn't want to run a big rear single, nor did I want to put 2 red hot glowing snails under my hood because I do week long events of closed roads etc and that is not where you want red hot glowing thingies. So you rip the back seats out and swap in twins, build a window'ed firewall behind the front seats and run the turbo exits right out through the doors. At least that is what I did. Twin Bullseye Power billet turbos - little suckers and they get the job done real nice.

Car made best of 663rwhp on a 10lb spring on pump gas. It peaked at 13.7psi but came back to around 10 up top. Not bad on pump at 11:1. No detonation, has done a few 1000kms now, much of that in anger so let's assume the tune is ok (thanks Robin).

Car has lost 100rwhp and compression is down on 1 by 15psi (not a lot) so I'm thinking boost leak - loose pipe somewhere. While I'm thinking that I'm thinking the LS3 needs forged pistons if I'm going to keep my foot in it on long closed road events and track days. Track days esp build a lot of heat.... as in a LOT of heat... as in last time I took my gf out she dropped the "F" bomb 3 times. 1st was the acceleration out of the pits onto the track, 2nd was braking into the 1st corner, then finally as she acclimatised to the power/braking, the 3rd F was when we caught fire.

Fire does dampen ones enthusiasm for another lap. Was power steering fluid on exhaust, nothing serious - but smelt like a burnt fart. Drove home, no damage, added steering cooler, happy man again.

Geeze is this going to be a book, I haven't got to my question yet... ok car has clutch rated at 1300hp, carbon driveshaft rated at more, Tick built T56 rated around 1300hp, so only weak link is diff (Harrop true trac rated at 737rwhp....but if I avoid drag racing on slicks and change the oil often it might hang in or I'll need to go stronger)... wait my diff already screams louder than grandma when her tits got caught in the ringer, so I'll give it one rebuild and try changing oil often before splurging 10k on a friggen diff

TURBOS: Thoughts are when it is time:

A Budget Plan:
LS3 block
Dart MID sleeves for 427 c.i.
Callies crank, CP rods, coated pistons
Best heads I can afford
FAST intake - because the sexier ones won't clear my hood bulge and a bigger bulge under the hood will look like Kim's ass - too big

Win Lotto Plan:
Dart Next block with MID sleeves to 440ci. (4.185" bore x 4" stroke) then as above and a dry sump. Dang, I need to move on or I'm going to have to sit in a corner with my box of tissues just thinking about what a bad boy I intend to be

Ok - turbos. I need to go bigger to feed 427 or 440ci at 7000rpm. Bill Devine set me up with exactly what I needed to date. He was going to supply twin 75's then thought it over and went with billet batmowheel 52/73 compressors and 69/61 turbines with SVO small A/R whatever that is. Power delivery of these little puffers is linear and smooth. I can give it full throttle top of 3rd gear in the wet. In the dry from 1st gear. Yeah I use soft DOT tires, so normal rubber would make it a handful.

The only battle I keep having on longer runs is heat soak after front intercooler fitted - added cowling to direct air down radiator and made big difference. I still have back pressure, and I have lag in 1st gear (autos and turbos are made in heaven, but no way can an auto survive road racing at this power level).... so here is my thinking and I'm keen to hear your thoughts:

For road racing/closed roads my needs are somewhat opposite to drag racing. I do not want maximum power and high boost. Big cubes plus high boost = uncontrollable track car and too much heat over a long course. I want excellent off boost torque and useable top end that has good response but predictable delivery not an extra 600hp rush in a 500rpm nanosecond... and less heat.

My theory is - bigger motor built to make great response and power off boost. eg. 440 built to around 650-700hp NA will make enough bottom end and mid range to warm up the tires in most conditions ie. match the bottom end (to 3000rpm) of what I have now but without needing boost to do it. Then - instead of going next size up as would be logical - go BIG. Big as less backpressure, better throttle response, more air, less heat.... more lag sadly but my hope is that big efficient state of art turbos will start making boost by 3000rpm and be all in by 5k for a wicked top end. If I limit boost to 7.5 to 10lb the rush should be manageable, but for longer events I'd likely use a 5lb spring to keep power at or below 800rwhp which is the top end of what is usable.... 700rwhp is about perfect for being able to use all of it lap after lap IF you can control heat.

Thoughts? Smaller turbos to make the power at higher boost - or bigger puppies like twin 76 (would like to stick with Bullseye as I like em) billet BB batmowheel, whoodackies that laze along at low boost?

Strange application I know - but 1000+rwhp is a no go on my events - you could not keep heat out or fuel in long enough to compete. Ok for the odd dial up, dyno wank pull and drag day, but I need a solid cool, reliable, driver friendly yet responsive 700rwhp- 800 max on the normal setting. On a 440 DART setup 800rwhp would be around 5lb. Tell me I'm a dickhead?

yeah heat control. oil cooler, piston squirters...the whole nine yards.

All of my turbo experience is from Subarus, so I don't have answers, just some food for thought... But a GM V8 with twins is not so different from a pair of single turbo 4 bangers is it? Or is it.

Big turbos at low boost doesn't seem like a winning strategy to me, because you're getting the late boost onset of a big turbo but you're not getting the high power that a big turbo was made to deliver. Seems like the worst of both worlds. Unless you find one with a short/wide compressor map that's really optimized for 5-10psi, in which case, forget I said anything. But when I was looking at bigger turbos (years ago, I must admit) it seemed like they were optimized for 20-30psi, so running them at 10 psi would just mean waiting longer for boost to show up.

What do you want your powerband to be like? Would you be happy with something that hits full boost at 4000 RPM, and hits the rev limiter at 8000? Or do you need something that hits at 3000? Or do you want to build a motor that revs to 8000? Make sure you pick a turbo that comes on at a low enough RPM, and that can compress efficiently at a high enough RPM.

Since your power goal is 700whp, and you want twins, you're looking for turbos that can flow 350whp each, so maybe 400 fwhp, so you're looking for turbos that flow around 40 pounds per minute. Those are rough estimates, feel free to disagree, do your own research, etc, but I think they're roughly in the ballpark. Maybe fudge that up to 45 lb/min to make sure you'll hit that whp goal and have a little bit left over for a "dyno wank pull." (I'm totally stealing that, by the way.) Find a couple examples, look at the compressor maps, use an online compressor map plotting tool to see if your intended powerband will fit within those compressor maps. Whatever boost pressures those turbos want, give it to them. I'm guessing high teens?

I suggest spending all the money you are talking about putting into turbos on better brakes.

I cant even figure out what you're saying you have now, what is or isnt working now, vs what you actually want or need ?

You said Bill specced you a pair of turbos....which seem extremely small, but you seem to suggest you're already running these ?

And are these twins you seem to already have ( or not ? ) going up front, middle, rear, outer space ?

There's absolutely nothing wrong with a low boost big motor build and it will aid reliability, much as per OEM would do....but even on a road race...just exactly how much time for say any given mile are you at WOT ?

You are right about that sir. For now, at up to 663rwhp I think I have the cooling issue under control for closed road events, for track hot lapping I'm not so sure but have 2 oil coolers to add yet. Piston squirters are a must for any rebuild.

I suspect you are right and will be looking at some maps once I figure out how to read them. The big difference between a built 7L motor and a Subaru/Evo type turbo build is that the 4 banger needs all the help it can get at lower rpms to get going while a built crisp 7L motor (with tighter lobe centers not 115 lazy ones) can be built to make enough power below peak torque rpms that boost is not needed, and adding small responsive turbos to a big motor simply gives more torque than you can use on street rubber. Hence my thoughts to us NA power up to say 4000rpm (by which time big turbos may only be making 2-3psi) and use the big huffers to crank up the top end from 4k to redline (7000rpm is enough for a reliable OHV motor if I can get a longer rod in).

For drag racing my idea sucks.... if drags were my main goal I would simply only go bigger turbos to make incredible power at higher boost. I also note that most street turbo systems copy in part the drag racing formula toned down a notch, because let's face it most guys want that big dyno number and they are not going to lean on it at WOT for several miles at a time hot lapping around a track.

Not many try to make twin turbos work for road racing and circuit race on big engines. The upside of going tis way would be if I do want to drag race I simply need to add more boost, and I'd likely swap an auto back in just for the weekend out of sheer sympathy for the gearbox/drivetrain at that power level.

Why? I have 6 pot up front and 4 pot rear now with track day friendly enduro pads. I haven't failed to get around a corner from lack of brakes yet.... touch woody.

Are you at 150MPH at the 500 Foot mark at the end of a 1/4 mile straight?
Brake upgrades are also:
Air ducting
Pad type
Rotor material
Size of drivers balls... ;)

I would be way surprised if a turbo engine in a amateur level car, outperforms a well built N/A engine over a season...
IE, how many DNF's, how much cost to run, etc.. I watch guys overbuild themselves out of a fun class..
GT1 is for space frame cars.. TUB cars just won't hang.. And thats where turbos will take you in a hurry.
Heck a well built N/A GT2 will eat a lot of TUB GT1 cars for lunch... cause at that level its the whole package..

Cheers Stevie, sorry my post rambled. Setup is per image below and I've done a few 1000kms in several events like this. Apologies it looks a bit rough, the interior has not been finished off yet and the ugly mufflers are simply there as a temporary measure until my custom stainless double skinned ones get made. I have run it with straight polished pipes out the doors but it was a cop magnet, ...now it is quiet. So the front is on left and the air filters are in the trunk to right. It gets plenty of air in there via vents behind rear wheels flares that look like brake cooling vents but work in reverse. Car is 6'7" wide front and rear, so a bit wider tham most GTO's lol.

https://cimg9.ibsrv.net/gimg/ls1tech...35a7d0ad92.jpg

https://cimg4.ibsrv.net/gimg/ls1tech...7f6d54f20f.jpg

Those small billet Bullseye turbos do a wonderful job on a stock LS3. I have climbed all over the back of a McLaren out of turns from as low as 80kph (50mph) in 4th gear and just leave it in 4th while the MacLaren used all their gears and rpms between turns. It's got incredible torque. Boost comes in low, under 2k.

My thinking is when I can afford it - to build a bigger motor, rely less on turbos to provide bottom end kick and more on cubic inches, then use bigger turbos to give a top end kick. More air, less backpressure, cooler than smaller turbos pushed harder. But unsure how the power delivery would come in - brutal or linear. I'd cam for tight lobes and crisp response.

Time at WOT varies, but track days would have sustained throttle out of every turn lap after lap. Road events are closed roads up to 20-30 miles x 5 stages of those a day for a week, but lots fo turns and speeds limited on straights so not long at WOT on any given straight, just out of turns to get up to speed again, so on and off throttle all the time.

Really...if your current engine works, has been reliable etc etc....why not just throw a pair of slightly larger turbos onto it ?

a Pair of 62mm SXE's or something ? ( need to add turbine housing )

https://agpturbo.com/borg-warner-s36...8-13009097056/

Or cheaper again,

https://agpturbo.com/borg-warner-s36...-60-68-177272/

And as to how power delivery will come in, will largely depend on tuning and what you do with boost control. There will be a multitude of options.

Bullseyes Batmowheels are ridiculously overpriced, unless you're buying them for a similar price to the SXE's

And TBH, unless I'd seen solid evidence they could perform better than a proper BW unit...I'd be buying the BW's...( which I did lol )

You are very right... some guys over here are spending over 1m a year on their racing! It's nuts.

For me, this is all about fun, having a car that can do a bit of everything, and that spends less time in the garage. It was at its most dependable at 450rwhp on a single turbo LS1. But that got slow and so the chase began.

I'm in so far now it seems silly to back off and go back to say 500-600rw when everything is set up to take the odd dial up to 1000+rw - which I would like to do at times - just not around a track. 7-800rw is where I think will be the sweet spot for hard duty, maybe even less. I can say that 663rw is real usable out of corners -all of it, even in the wet at top of 3rd gear! Car could become a handful beyond that, aiming to find out then dial it back to where it can be used fully.

7-800rwhp reliably is just a decent forged engine away (with piston squirters), add coolers for the trans oil and engine oil, upgrade diff oil (and change it after every event). The question is how I make that 7-800rw - a killer NA stroker with big turbos at low boost, or just forge the Ls3, add squirters and wind those little turbos up more - they'll handle 1300hp rw at 30psi, but I'd have to drop compression and for 99% of driving high compression is better.

Brake upgrades depend on what you want. Stay 330mm and sacrifice rotors with aggressive pads, or go bigger and have same stopping power with longer service intervals but much higher up front cost

A ls7 slams down about 800 on race gas and N/A with the right parts,, of course thats a 30K engine build after you have a motor.... LOL

Of course your dependency on others to help keep it running is a big factor too! a do-it-yourselfer who's not worried about breaking stuff can get away with a lot.. :)

I've seen evidence of fatter power curves and quicker spooling, top end is a function of airflow so I wouldn't expect they make much more power that another turbo rated at the same cfm.

Those BW turbos are cheap - that's a good deal... can you get them with ball bearings? BB are likely faster to spool and should last longer

Yeah a NASCAR build would be nice.... but as you say, I'd need a mechanic as a co-driver. I kinda like pulling up to the bar at the end of each day not the workshop. lol

BTW thank you to everyone that has contributed. Appreciate it

Nice car and setup, I'm definitely in to see how this turns out for you. I'm happy if my heap makes it down the track for a 1-2-3 pull, doing what you are doing is on a much higher level.
Have any pics you could share of how you keep assemblies (mainly engine & trans) cool in its current configuration? I'd like to steal some ideas for my car. Also any more pics of current turbo pipe routing? What scavenge pump are using and what's your scavenge "sump" like? Just thinking that it's obviously working very well to handle the constant thrashing and hot oil temps.
Sorry for only asking questions and not giving any advice haha.
Thanks,
Jarrod

Given the application....justification for BB's would be difficult vs a well selected JB turbo. If you have the money though and are happy to spend, a pair of Precision Gen2 6062's whilst still small...would offer very fast spool and still good power up top. I'd go for the biggest turbine housing they offer.

Or I see they now have a Gen2 6266 which should be a good step up but still staying small, if that makes sense.

But a pair of those vs a pair of regular JB BW's....there's a substantial price difference.

Or of course you could opt for the BW EFR series, which I guess you could consider as the BB version of the regular 300's SXE series, although they'd be better than that as they use a much better turbine setup although some compressor wheels are the same.

As for block or bottom end...I'd stick with alloy, and if you want squirters consider a factory LS9 bottom end or at least the block.

LS9 block has squirters already, steel main caps ( dowelled ), stronger alloy for the block and M12 head stud fasteners vs usual M11. So it is a very good base to start from, and doesnt carry the weight penalty of iron

But I'd be throwing a pair of better suited turbos onto what you have first and a decent management system that can give you proper engine control...and proper boost control to cover your usage.