I finished my first turbo build on my 68 Camaro and it looks like I screwed up on turbo selection for a 100% street car. My turbos are just starting to build boost after 3500 rpm which is OK for 1/4 mi car but not much fun on the street with a stick shift. I am looking for ideas on what to do.

Here is my combo: 68 Camaro, 3400lbs, TK0 600, BBC 540ci, Profiler 320cc heads, 9:2 to 1 comp, hyd roller 244@ .05 114 lobe sep. 6000 max RPM Accel thruster EFI .
turbonetics t72 Q-trim, .96 AR on center housings. Tial 44s with 7.5 lb springs (I have not heard these open yet). Until the turbos get going my turbo housings make it act like my motor is trying to push the exhaust of through 2 inch holes on each side. Looking for 900 HP. 630HP n/a + 7.5 lbs boost = 900HP

I am looking for faster spool up at a much lower rpm (2500) so what should I do? I see several options, might be more.

1) sell my current turbos and replace with what? I would like to stick with on center housings and keep this a bolt on mod. I have 4" inlet and 2.5" outlet.

2) Change to smaller .81or .69 AR on center housings.

3) Change to P trim turbine whee and p trim housing , keep 72 comp wheel.

4) Replace my current turbo combo with ???

 

Somethings not right..... I've seen SBC 406s spool twin 76mms with .96 A/Rs just as quick.....Whats the ICL on the cam?

 

What rear gears?

What exhaust system size? mufflers? Have you measured pre and post turbine pressures?

What gear does it spool that slow in? all gears or only 1st maybe 2nd gear? what does it do in 4th?

 

Same questions as above, and what is that master cylinder setup? I could guess clutch and brake, but there's a third resivour that looks like its plugged? And that tire wheel combo is sick! Also what is your timing looking like?

 

I gave the cam card to a friend who is helping me with the EFI tuning I will see if he can post the cam info. I have to admit that I did not degree the cam just stabbed it in straight up. I am seriously considering swapping the cam to a true "turbo cam" with a lower duration @ .05 for better street manners and quicker spool up.

 

The exhaust is dual 3" all the way with one 18" Magnaflow and 2 bullits per side. So yes there are 6 mufflers on the car but they are all straight flow through design. I have not tried to measure the back pressure. I am still breaking in the clutch so I have only tried 3 and 4th gears. Here is a pick before I ran it over the rear axle and out the back.

 

My thoughts:
1. Those turbos are too big. To keep it a bolt on affair get the smallest turbos you can get with the 4" compressor inlet. They will still be on the big side for your setup.
2. 2500rpm full spool and 6000rpm shift point is expecting too much. Smaller housings will definately get you the spool but will also definately start to choke the top end. Full spool by 3000rpm with reasonable backpressure at 6000rpm is achievable.
3. Better exhaust would help. Disconnect at the downpipes and test to verify this.
4. Don't get too carried away and get a "turbo cam" for 7.5psi. Get a cam that favors the low end a little more as this will have more impact on spool than any of the above.
5. That setup should not feel like the turbos are hurting off boost performance. Check the tune. It sounds like you need a lot more off boost timing.

 

The timing is the Accel basic 2 bar timing map. I used a proven tune from a similar BBC TT setup (only difference, it was a smaller 454 engine) as the starting base tune. The most confusing thing was how far off my VE target table was from actual. I had to lower my VE target values by a lot. This could be a clue that something is not right, maybe the cam? These 320cc profiler heads are really good flowing heads so I thought this was unusual to need lower VE's. I do have 120 lb injectors but that is accounted for in the engine spec set up. I thought VE should be VE regardless of injector size.

 

I did a compression check and all 8 cylinders were 160-165 psi

 

This is a dual MC set up like the circle track guys use. The plugged MC was for the clutch but I have removed and use the stock mechanical Z bar linkage. Much better peddle feel. Here is a link to my install write up. http://www.pro-touring.com/forum/showthread.php?t=48160

 

don't those procharger bov's stay open at low rpm?

 

Yes, but it closes immediately when the throttle is opened up.

 

Wow -- that is a really well done car...very nice.

I think you're right about the turbos, though. The turbonetics t72s flow enough air for 850 hp each (1700 hp which you would have if you ran 21 psi), but for a street car that is too much. The t60's flow enough for 550 hp each, and that would be pretty good on an engine that is 270 inches (i.e. half of 540), they would also spool by 2500 and give you enough airflow to get to 6000 rpm which would be a nice broad power band. Since each turbo sees half the motor, you'd want to size it like that.

 

Problem is finding a smaller turbo with a big enough exhaust side.....you need a HUGE exhaust side for a 540, even if shifting at only 6000.

 

Also I don't know of a 60mm turbo with a 4" inlet.

 

Why is the 4" inlet so important andy? As long as the discharge lines up what would be the problem?

It does seem strange that a 540 inch motor with good heads can't spool twin q trims. What are your A/F ratios and timing at right now? What rpm are you make any boost by?

 

It may not be. I'm just going by our givens from the original poster. Hed stated that his turbo had a 4" and wanted a bolt in affair.

 

I would just assume that as long as he uses an on center t4 turbine and the DP flange is the same that the compressor side would be easy to mate up. Silicone adapters are alot easier to deal with than welding.

To the OP are those DP flanges the old 4 bolt or v band? 3.0" or 3.5"

 

Here is my timing table