The combo is in my sig. My HP kit came with a 3" crossover. Spool up has always been slow. What kind of gains could I expect by dropping the size down to 2 1/2" or even 2 1/4"? The car is mainly street driven, so giving up a little on top is not a big deal. I was also thinking of swapping out my 3.27 gears for a set of 2.73s. Thanks.

Jess

 

Is the 3" wrapped?

 

Yes everything is wrapped, and the turbo has a blanket.

More info on combo;

Down pipe is 3 1/2", then split to two 2 1/2" with Dynomax Ultraflows and tailpipes.

Turbo has a .96 A/R exhaust housing, which I thought was correctly sized for my motor. 3200 stall converter.

Cam is 221.5* at .050" int., 230* at .050" exh. lift .554" int, .526" exh. lobe separation is 116*. I was thinking of swapping in a F-303 in the near future.

Jess

 

To answer your question about spool, simply changing the size down isnt going to do much for ya. Might pick up 100-200rpm of spool while loosing flow potential and increasing backpressure.

 

I'd drop the exhaust housing to a .81 A/R to pick a little spool up before changing the crossover.

 

Changing the exhuast housing ar doesn't effect spool that much either. I tested a .96, .81 and .68 on my setup on the dyno and spool changed maybe 50-100 rpm for each size and everyone of them made the the same power and torque. At $175-200 a pop for a new turbine housing its just not really worth it imho. If you want to radicaly increase spool get a billet wheel turbo and/or go ball bearing and you will pick up 1000-1500 rpm in spool. The rest of these ideas are really fruitless efforts. Coating all the piping helps a bit better, usualy see around 300-400rpm difference there.

 

Was these test on an engine are chassis dyno?

 

When i helped dyno tune my friends 417 mopar it would hardly spool with 3.5" crossovers so i reduced them to 2.5" right on the dyno and it was night and day. It would come right in to boost after that change only and this was on a 1137hp engine making 18psi non intercooled.

I just finished my new turbo kit on my mild 5.0 and eded up making my crossovers 2.25" to aid spool and it works awesome and i'm glad i did it.

 

Looks like I will try 2 1/4". Have it coated, should not cost too much for one pipe. If I am not happy with that I will try a smaller exhaust housing. Thanks.

Jess

 

If you can, get a measurement of the back pressure in the crossover before and after making the swap. You SHOULD end up with less back pressure and a quicker spool by switching to the smaller piping. I'm very interested in what the findings would be. I think the 2 1/4" is a good choice for your setup.

 

Snow/salt on the ground here now, so the car is parked. Also, I am installing t-tops, so back pressure readings won't be possible. I will however add a fitting so I can check with the smaller crossover. The smaller pipe should free up some space too. I am going to try and make the waste gate dump into the down pipe. The HP kit just has the waste gate open to the atmosphere. It is kinda tight around the alternator, but I will see what I can do.

Jess

 

you shouldn't need anything bigger than a 2-1/2 crossover unless you're running over 400" and really spinning it

my buddy Manny ran NMRA SSO with a 360" SBF, Victor heads down into the 7's with 2-1/2" crossover piping

 

I always hear that its just best to run 2 1/2" cuz its best of both worlds, flow and spool, and it works for Upto 1000 hp. I'm currently running 2 1/2" now on my 351w with a t76 & .81 housing and q trim wheel. Spool is extremely fast, 10 psi by 2900! I would have to disagree with changing housing and not much happening, I know if you dont buy the correct housing for the exhaust wheel it wont change much, like buying a .96 q trim housing when you have a s trim wheel, more flow is possible but the wheel won't be as close to the housing as it should be. Also, a . 96 twin scrol housing flows less than a .96 single, something to think about. I read a lot of people with a 351w,t76 and a .96 housing q tirm wheel and spool 10 psi at about 3500 rpm, so housing has got to make a big difference. Right now I keep getting an exhaust leak off the header from back pressure. I'm about to move to a larger housing.

 

How much does the cam play into how quickly the turbo spools up? My current cam I bought used from a member here. It is a Jay Allen cam specked for a 393 with AFR 205 heads and a 88mm turbo.


duration 221.5* at .050 int, 230* at .050 exh.
lift .554" int, .526" exh
Lobe separation 116*

I was thinking of swapping it out for a f-303 with 1.7 rockers.

Duration is 226* at .050 int and exh, lift .512"(.544" with 1.7rr) both int and exh, Lobe sep =114*

Smaller cam, more low end torque, faster spool? This would be done in addition to the 2 1/4" crossover pipe. Thanks.

Jess

 

Well Brent's Fox is making over 1000rwhp with F-303 on his setup right now. He's running I believe a 347 with a T88 on it right now and moving it. Check out project cars at the top of the forums. So that F cam is definitely capable of moving the air needed for big power and I think your on the right track with it and the swap with 1.7rr

 

I really think there is something else wrong on the spool up. Maybe timing or fuel? I had a 76GTSH on my 363 and it was instant. I would describe it like nitrous. It came on very fast. I was using a 2.5" crossover with a 3.5" downpipe with a B&G kit.

 

What intake are you running?

 

I am ashamed of the 3" crossover video so I've never posted it, but the after is below.

After 2.5"

I had to turn the boost down to get a cleaner run...

 

Intake is a single plane with injector bungs, 90* facing towards passenger side with 75mm throttle body.

The tune could be some of the problem. I uses Moates Quarterhorse. I got the car to run well and stopped messing with it. Defiantly could use some more dialing in. I can't do much about it now, other than to build the smaller crossover and wait for spring.

Jess

 

Since it is a street car you need to ditch the single plane manifold and get a long runner manifold. That will make significant differences in turbo response.