I will have to work on getting that image larger.

 

Each turbo is seeing only 270 inches, so I'd estimate it would be OK to go with a smaller exhaust side for that size motor. You have twice the flow area with two turbos. The rest of the exhaust system after the turbos really needs to be up to the task, for sure.

Maybe you'd help spool if you went wider on the lobe centerline to isolate the intake and exhaust sides of the engine, but that is a pretty big change to make. It would be easier to bolt new turbos in. Andy, you're correct that the OP wanted to bolt on a 4" inlet, but from the looks of the pictures above, the only thing attached to them is an air filter -- an adapter might be pretty easy or a new filter wouldn't be a huge deal. The exhaust side looks like a four bolt setup rather than a v-band which would be a pain to find in a smaller size.

If spool is an issue and you want to stick with 72's, a nitrous oxide system works pretty well. N2O creates a lot of exhaust gas, and this can spool up things in a hurry. The system can be set up so that it's on only at WOT, only above 2500 and a Hobbes switch can turn it off when you hit 2 or 3 psi.

Before you make changes, though, you'll want to double check that you're not missing anything -- boost leaks, timing, air/fuel, throttle opening, all cylinders firing, etc. It would be bad to change a bunch of stuff and find out that a spark plug wire was bad.

 

to 35spline: Is that a wide band o2 sensor in the passenger side exhaust?

 

Can't you get a 60-1 or 62-1 in a 60 series comp, 4 inlet 2-1/2 outlet?
There older wheel designs but either one should make enough power for
a street car.

 

Yes, I am running the Accel Thruster EFI

 

I disagree on the first part.....I have yet to see someone who builds a 540 and puts crap heads on it. Profiler 320cc heads + a 244 @ .050 cam flow a LOT more air than a run of the mill 270ci motor. If I were to change anything, it would be to get a cam from a very reputable turbo cam builder. I don't think N20 is needed....I mean the reason people go to turbos is to quit filling bottles right?

I completely agree with the 2nd part.... I suspect either hotside leaks or coldside leaks (or both). It is very surprising to see how big of a difference even a few pinholes can do. FYI injector o-rings have a really bad tendency to leak pretty bad. And yes double check the tune-up (OP Can you post a bigger timing table? the pic is too small to see).

 

The DP flanges are 4 bolt (it is an on center .96 AR housing) the DP is 3.0" but the outlet out of the turbo is 2.5"

It is not that big of a deal since I only have air filters attached, but I would like to the on center housing so I do not have to re-fab the 4 bolt DP flange

 

I have a buddy with a nova 383 , same weight as your car, that makes 500 or so hp(he is NA) and doesnt start to wake up till about 3500. he also uses a manual transmission. It is plenty of power on the street(I think too much) as first and second are pretty much useless.

I say that to say, Im finding it difficult to imagine how 650hp NA in a torquey 540 bbc would be "weak" for the street.

having said all that, have you leak checked the exhaust and intake piping?

also, the spool up you desire would come from changing to .68 exhausts.

 

The 630HP NA 540 bbc was before I added the turbos. The 3400 lb car ran 10.9 in the 1/4 at 124 with a crappy 60' time and that was with Gm 781 iron heads and a carb. It was a beast not it doggy under lower RPM before boost. I would like to get this combo a little more streetable ie. more fun to every day drive.

 

I will not be able to answer any more post (vacation) until next Friday Sept 17. My friend Greg who is helping me with the Accel tuning might be able to answer some posts while I am away, but I am not sure he is a member here. Thanks for all of the reply's and I look forward to figuring this thing out!

 

methinks more tuning is required of that efi conversion.

also, if you are trying to drop the spool rpm, just going from .96 to .68 should get you there.

Please keep us updated

 

No question that the Profiler heads flow really well, close to 400 cfm at peak lift, but I'd maintain that each turbo is only seeing half of those 540 inches with a twin setup. A 270 inch four cylinder motor that makes 320 hp is still pretty good, there just happen to be one on the drivers side and one on the passenger side in this case. If you put a single t72 on a 302 inch engine, I wouldn't expect boost until north of 3500 rpm either. I'd agree that going with a more turbo friendly cam would help.

With a nitrous system, a ten pound bottle will last a really long time because it is only on for maybe a second to spool up the turbos, then it shuts off. Filling the bottles gets old in a hurry, though you don't have to do it that often. I don't know about the Accel setup, but with a F.A.S.T. or a Motec you can control it easily, just use one of the outputs to ground the nitrous solenoid. This lets you tailor it to exactly what you need and works really well. The Buick guys use this on street cars with big turbos, and the bottles last "forever" which may be a day or six months, depending on who you talk to (YMMV!) and if they forget to shut the bottle off when the park the car.

But this car, with 540 inches, a manual transmission, 9.2:1 compression, etc. should have really outstanding off boost performance and then just tear your head off under boost. To the OP: It might be timing, but I can't see the tables either. What is the A/F doing under non-boosted conditions?

 

I think something is off. Boost leak(s),exhaust leak(s),timing, or fuel. Those turbos should come alive sooner than they are. I'll give you an example of one of my setups.
505ci bbc
320cc ProComp heads
Crane cam bigger than the one you have^^^^ just stabbed in, not degreed. 114l/s
timing locked at 32* msd 6AL
2" primary headers, 3" cross overs
C&S 750
3.5" cold side
3.27 rear gear and before that 3.73
turbo 400 with a TIGHT! conv.
2900# fox body

All this with twin TV7101 Garrett large frame turbos 1.32 turbine A/R

I would be at 12# by 3500 rpm. Off boost it was a dog.IMO But when I got into boost it would come on really quick and hang the hell on! So yes I think something in your tune is off. You should be spooling quicker.

 

Heres another one of my builds I just got it running and still working the bugs out but so far this is what I got.

498 ci bbc
781 cast iron heads
comp cam even way bigger than yours^^^ just stabbed in, no degreeing. 114 L/S
timing locked at 32* MSD digital 6
2 1/4" primarys,3.5" collectors
1050 sqaure bore carb built by myself
3" cold side
3.27 gear
powerglide trans with an off the shelf TCI 3500 super street fighter
1985 Chevy S10 3200#
Twin Garret turbos. Strait off a Volvo truck. GT40 cold side/GT 42 hot side with 1.1* A/r turbine


Same thing all 6psi is there by 3500 rpm. Its sluggish off boost but not as bad as my other combo above. I know for a fact I could have these turbos lighting off sooner but think its fine the way it is. It spends most time on street tires so when the boost comes on the power is pretty overbearing and your just aiming at that point.

 

I will also add that the above boost #'s are where I have the gates set at. Also the 505 ci I started out on 91 octane pump gas it was ok. Then coverted over to E85........ WOW what a difference!!! The 498 is currently on E85 and it WONT ever see 91 octane.

 

I think the .68 A/r would choke the hell out of a 540 on the top end. But hey, you never know till you try ..Right.

 

More thoughts:
1. There is no way a guy with six mufflers and exhaust all the way out the back of the car is going to have a hotside leak of any significance and not know it.
2. An engine with big heads, cam and single plane intake is not going to have good low end torque. Don't be fooled into thinking that the big cubic inches makes up for it because it doesn't. The shape of the torque curve dictates the "percieved" low end. A 500+ cid engine will have gobs of torque at 3000rpm but if it has significantly more at 4000rpm than 3000rpm it will "feel" lazy.
3. A lazy engine will not spool turbos at a low engine speed until you make the ehxaust housings small enough that they will hurt the top end.

 

I have to agree with 1 and 2^
on the 505 I had 5" open down pipes on and then went to 3" down pipes with glass pack mufflers. Both times I ended up with header leaks and didnt know it.
The glasspacks really didnt even seem to effect power that much either. :noidea:

 

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.I got T4 60mm stuff on my 401 sbc... its just about instant spool when you jump on it with the 4000 stall. .68 a/r and 195cc heads but single plane EFI. Plenty of torque on the low end. Car moves extremely well even in thirdgear with the TH400 and 2.73's out back!

It has 4" compressor inlet and 2.5" outlet. 3" downpipe outlet.

I think they would be small for a 540 big block as I think the are small for my setup.

I dont consider a 72mm too large for a street 540. Q trim .96 may be delaying spool alittle bit. I'd be curious to try .81 housings. P trims may hurt top end too much as a big block will flow a ton of air, more so than my 6000 rpm 401 sbc.

 

a lot of the 500 cid engines ive seen have more than 500ft lbs of torque by 3000 rpms. some have 600ft lbs.

there should be no traction in first gear and most of second. though the op mentioned that he has only tested in 3rd and 4th so that could be a big part of the perceived laziness.