twin turbo T/A ideas...

[Phallic_Apparatus]

hey everyone,

I've got this 86 trans am that i love to death, the only problem that I'm having with it is the stock LB9 TPI engine leaves a lot to be desired in terms of power. Who the hell wants to drive a slow car with a v8 anyway?

So I got this crazy idea. I've heard of guys putting the 305 heads on their 350s to raise the CR, so I thought about putting some aluminum 72 cc 350 heads on my 305 and lowering the CR. to about 7.0, flipping the exhaust manifolds upside down and running dual t3 turbos with some custom intercooler piping.

im still debating on whether to stay EFI or go carbed. if i go EFI im definitely getting at least an LT1 intake because at high rates of flow the TPI runners suck. if i go carbed, i already have a 4 bbl intake sitting around all id need is heads and carb, and i could recoup some of the money by selling the tpi setup.

does anyone have any suggestions as to which stock gm cars came with 72 cc aluminum heads that flowed well? carbed or efi, if efi maybe the vortec truck heads and intake? also, is the 305 a destroked 350 or a debored(lol) 350? cam suggestions? im wanting to rev this thing pretty high and i will have a strong valvetrain no matter which route i go.

any suggestions/criticisms are welcome and much appreciated i just had to throw this out there and hear what some of the GM experts on the board think about it.

edit: i botched the CR calculation earlier, its really ~ 8.7. what's the thickest head gasket that they make for the small block? were there heads made that will fit the 305 with a bigger chamber than 72 cc? thanks

[Syclone_jak]

Check out TPiSpecialties for a nice fulie intake. You are barking up the wrong tree with the GenII intakes, they don't have coolant crossover passages, so they will not work with your GenI heads.

If you are doing this as a junkyard project it should be doable and neat, besure you use a non H.O. passenger side manifold as it has a shorter outlet than the HO versions, also check your hood clearance Im sure it will be a problem.

If you are building a motor I would suggest forgeting the 5.0 (some will disagree). 1295.00 goodwrench 350 3year 36K 8ish:1 compression why bother with the 305?

Search the board there is alot of good info here, it will save you from being ignored and asking question that have already been answered.

[Phallic_Apparatus]

well the reasoning behind using the 5.0 is that i have it already, and i can lower the compression ratio by using some heads that were made for a 350 with a larger chamber and a really thick head gasket, all without having to crack open the bottom end at all to rebuild it, or buy an expensive new rotating assembly. the t3's i can get for 150-200 bucks apiece and the heads will probably be cheap as hell too. when i flip the manifolds over im gonna chop them all the way down to where they turn into one piece, so im hoping hood clearance wont be an issue. this is definitely gonna be a junkyard project. i would search the board but i know noone has done this before...i just have a feeling. im really thinking about going carbed, maybe a holley 800 cfm or something. i already have a gen 1 intake thats a 4 bbl. so all i need are heads and carb to finish the top end...well after a valve job of course. and once i blow this bottom end up, i might get that bottom end you were talking about and turn up the boost...hehe. also i either need two small intercoolers or one large one...not sure yet but i know im going to have cold air in abundance, ill probably relocate the battery to the rear and cut some holes in the sheet metal on either side to get the intercoolers and air elements in front of the fog light holes ive left open. ;)

[AgFormula02]

Do not plan on your bottom end lasting long under a boost application. A turbo comvenrsion is a lot more intense than what you plan.
Aside from the plumbing for the turbos in an already crowded engine bay, keep in mind that you will need a lot more fuel.
There is a reason why these kits sell for several grand.

[orangeapeel]

Why not a single turbo?

[Phallic_Apparatus]

Why not a single turbo?

well i would need a very large and very expensive turbo to feed a v8, when i can get two t3 turbos for ~ 150 bucks each and then it would be like having two turboed 4 cylinders attatched at a 90 degree angle....heh.

[yyz28]

Wow... That's a pretty wild plan.

First, you CAN mate a GENII intake on your GENI car, it's been done time after time after time, with EXCELLENT results. HOTROD and SUPERCHEVY mags have done writeups on using LT1 intakes on GENI engines and are fans of the idea. See www.thirdgen.org for more details. I considered it on my 3rd gen for a while... now I'm carb'd.

Second, Larry (02Formula) is RIGHT ON... Be prepared to be replacing a thrashed bottom end unless you build it right from day one.

I would consider dumping the TBI, go with a TPI or Carb'ed system, and you'll be SHOCKED with the gains that alone gives you. TBI is a total POS, and you can net 50 ponies OVERNIGHT with a swap to TPI or Carb.

[Doc]

He's already got TPI Scott...;)

[wil442]

YOU CAN DO IT!! YOU CAN DO IT ALL NIGHT LONG!!

How much boost are you planning on running?

OR

How much HP are you shooting for?

Turbo motors don't need to rev high, the reason is at a higher rpm you increase stress on the rods/bolts due to the switching direction at higher speed.. yada yada. The better way to go to increase HP instead of high rev's is to set higher boost levels, then you can develop the HP at a lower rpm keeping the bottom end happier, I know there is more stress due to the increased boost, but it will be less than the stress of the high revs. That's the only advise I have for your plan. Otherwise it's a sound plan. The smaller twin turbo's will have lower surge limits so you'll develop full boost sooner, but will lose a ton of efficency (turbo efficency) at higher engine rpm's, another reason i'm in favor of lower rpm / higher boost.

What kind of transmission/rear end are you running?

[Phallic_Apparatus]

i was thinking of running maybe 6 lbs of boost to start with, and then turning it up as high as it would go. as far as my hp goals, im shooting for as much as possible ;). im running a 700r4 tranny and i have a 3.42 posi/disc rear end out of a 95 z28, so im guessing thats a 7.5" ring gear.

something interesting that the guys from thirdgen.org let me in on is that there are people doing this already with the L31 vortec truck heads. im assuming that i would need the vortec intake and possibly the EFI stuff from it, but i remember hearing something about a vortec intake that was made for tpi, maybe even using the sensors and stuff from the tpi setup with the vortec intake...im not sure though. im still very much in the planning stages on this one. i did see someone's twin turbo setup on a camaro, and it will fit in there, ill have to get rid of the emissions and A/C stuff, but the A/C being gone wont bother me, thats why i got a T-top car in the first place.

what compression level do you think i should shoot for? it would be crazy if i could run 87 octane....just for the sheer novelty of it, but im almost sure that it would blow the engine apart from detonation. i was also thinking about a huge alky/water injection system to help keep detonation down.

[AgFormula02]

You need the base of the intake that is desinged to adapt the stock runners to vortec heads. I believe S&D has that part.

[Phallic_Apparatus]

You need the base of the intake that is desinged to adapt the stock runners to vortec heads. I believe S&D has that part.
so this means i still have to use the piece o shit TPI plenum and runners?...that sucks. might be fun though, long tubes = more velocity = more torque and that sounds kinda fun...

[wil442]

I now it's possble to use an old school carburated intae manfold on Vortec heads you need to either redrill them or modify the existing holes, or something. You might be able to modify any non-vortec intake the same way.

As far compression ratio...it's kinda tricky, it's easier if you know how much boost you will run. The reason I say this is because if you plan on only running say 6 lbs max, then you could go with a higher ratio, like 9-9.5 CR. That way when you aren't under boost you have good performance. If you were planning on running 20 psi of boost you would want about 7-7.5 CR so when you hit the 20 lbs detonation is controllable. So it's kind of a tricky answer. I have the formula somewhere to calculate final CR under a given amount of boost with a starting CR, just can't find it right now....

Here's a few other things to concider for a buildup...

I'm not sure how long your tranny will last if it's stock.

Boosted charges in the cylinder burn faster, so you need to retard the timing slightly when under boost. There are a couple ways to do this, use a controller, like MSD, or retard your distrubitor all the time.

Spark plug coldness range may need to be changed.

If you go carburated make sure the carburator is made for boosted application.

You'll need to add more fuel while under boost, either larger injectors or raising the fuel pressure in relation to boost (rising rate fuel regulator)

You want the exhaust manifolds to retain as much heat before the turbos. As air cools it expands, as it expands you now have more mass in the manifold and it gets bunched up and looses velocity, velocity is what makes your turbo go weeeeeee!!!!!

Try to stay away from turbos that are oil cooled only, the life expectancy of water/oil cooled is much much longer.

I plugged a 305 into a calculator I have that calculates HP in relation to boost pressure. It gives a high and low. This is based on a typical/not too crazy performance oriented engine. The main difference between the two is caused by Volumetric Efficency. I'd say the low is an engine with about 70% VE,the high is about 95-98% VE. A typical V8 sits about 80-85% VE, so you should be somewhere in he middle.

Amb.--- Boost--- min HP max HP
15.7---- 1------- 249.002 368.7145
16.7---- 2------- 264.862 392.1995
17.7---- 3------- 280.722 415.6845
18.7---- 4------- 296.582 439.1695
19.7---- 5------- 312.442 462.6545
20.7---- 6------- 328.302 486.1395
21.7---- 7------- 344.162 509.6245
22.7---- 8------- 360.022 533.1095
23.7---- 9------- 375.882 556.5945
24.7---- 10------ 391.742 580.0795
25.7---- 11------ 407.602 603.5645
26.7---- 12------ 423.462 627.0495
27.7---- 13------ 439.322 650.5345
28.7---- 14------ 455.182 674.0195
28.7---- 15------ 455.182 674.0195

[Racer Dave]

I'd personally suggest staying with an EFI system. A blow through carbbed system can and will work, but typically wil only work well while under vacuum or under full boost. And, for me anyway, an EFI system is actually pretty easy to tweek in order to adjust your air/fuel mix to keep your motor safe.


Runnign an alcohol/water injection system would be very beneficial, especially if your compression ratio is going to be higher than expected. You could even go non-intercooled if you're running low boost(like 6 psi) and a water/alky injection system. I can currently run 15 psi of boost on 89 octane gas with my water injection system with no detonation on my SRT-4. I don't do it, but I can.

As far as a fuel system, I'd first recomend upgrading the fuel pump. A friend of mine in his V-6 Cavalier runs an aftermarket 255 lph F-Body pump with running boost and has not had any problems with running out of fuel, even with running 15 psi with 52 pph injectors. After that it all depends on how much money you want to spend on the fuel system.

If you want to go as cheap as possible, then buy yourself an adjustable rising rate Fuel Management Unit(FMU). This will allow you to adjust the base fuel pressure, then increase the fuel pressure while under boost on a 1:1, 2:1, 3:1, 4:1, ect., ect. bassis. So if you were running a base fuel pressure of say 30 psi, running 6 psi of boost, and had a 4:1 FMU then you'd end up with a fuel pressure of 54 psi when running 6 psi of boost. The big downfalls to this system is that you have no control over the fuel if it is running too rich at certain rpms but lean in others. This system is also hard on the fuel pump and injectors. You've got to make sure the fuel pump can handle high pressures while still being able to maintain proper fuel flow, and you've got to make sure your injectors won't stick open when they see high pressure like this.

Another option is to go with bigger injectors then what you need and some kind of piggy back fuel computer like an Apexi Super AFC. This computer alters the MAF singal in order to give more or less fuel and is adjustable at differant rpm points. You just have to make sure you are not exceeding the MAF sensor's flow limits.

The other option which would be best is to use a true stand alone computer system. You can actually build these for pretty cheap if you know electronics. Mega Squirt is such a system.


Hopefully this didn't confuse you too much. I'm currently in the process of turbocharging my Geo Metro(just because I can) and turbocharging my little brother's 4.3L Chevy truck.