brianfromhawaii

iTrader: (3)

I have a s480 on my 408 with the truck/corvette manifold setup... 6 speed.

i am not getting any boost in first or second, and in third it doesnt come on until about 3700rpm

it goes from a 4 inch downpipe into a 3 inch catback... i was told by a few guys this would not be a problem, and a few others have told me it is a problem

what is normal backpressure on the downpipe side of an exhaust? post turbo.

95bowtie

You should still see boost even with 3 inch exhaust. It would limit you at a certain hp/rpm level though.

Especially with a lot of cubes.

Maybe there is something else going on like a boost leak on the cold side or wg isn't working right maybe.

HexenLord

A small exhaust will cause issues with peak power at high boost. It won't stop you from building boost. If you aren't building boost, then there isn't much back pressure being generated.

AKAFRED

iTrader: (1)

do you have a bum waste-gate?

Sarg

iTrader: (6)

Or large exhaust leak?

Sales@Tick

iTrader: (3)

This. You have a boost leak.

Ferocity02

iTrader: (60)

Check to make sure the wastegate is working, and the fire ring is installed. Also check for boost leaks.

brianfromhawaii

iTrader: (3)

I do not have a boost leak, I checked for this. my wastegate opens at 5psi... its an OBX 60mm, i do not know what a fire ring is, but i can confirm at an idle the wastegate does not leak.

I can make boost, its just it comes on slow in third... there is also a small exhaust leak in one of my manifolds... sounds like header tick... does that make a huge difference?

CameronVic

iTrader: (5)

Make sure your BOV is hooked up right. I could only make 7psi for the first two days my car was together. Had it hooked to the side port. Lol.

Oh, and are you saying it goes 4" DP into dual 3" or 4" to single 3"?

brianfromhawaii

iTrader: (3)

my bov is hooked up to the manifold, post throttle body... i believe that is correct...

brianfromhawaii

iTrader: (3)

oh you ment on the bov itself, no its on the top port. for sure.

BMF_Racing

iTrader: (6)

Any leaks will cause turbo lag so it is important to have a sealed hotside (exhaust port to turbo). You have less load in first gear so it will spool less in the lower gears, but it should def have boost in first. What does the car do in 4th as far as boost?

brianfromhawaii

iTrader: (3)

The car in 4th is an animal, the boost must come on around 3200 rpm and it does 5 psi as well, as i said, i can hear the wastegate opening...

thanks for all the help guys

SPRAYED 01

iTrader: (22)

That turbo going to a 3 inch is not good. What muffler? Back pressure after turbo is not what you want to measure. You need to tap into a pipe going to the turbo and hook it up to a boost gauge. I bought one on ebay that goes to 60psi, it was like $30.

brianfromhawaii

iTrader: (3)

just a glass pack. i am going to tap into the downpipe in a few days when the o2 bung comes in.

SPRAYED 01

iTrader: (22)

Why are you tapping into the dp again? You need to tap in before the turbo.

brianfromhawaii

iTrader: (3)

because if there was restriction it would be being caused post turbo, so i should measure that...

HexenLord

I'm still wondering why you want to measure back pressure because of no boost in first or second gear. If you aren't building boost, you arent generating much back pressure.

I've seen more people than I can count make over 600whp and run 10s on a single 3 inch exhaust. I've also seen several over 700whp and in the 9s. And I've seen a turbo v6 Buick dip into the 8s with an 88mm and 4 inch downpipe that went into a 3" with a muffler.

Stop wasting time thinking back pressure is stopping you from building boost. Boost causes the backpressure. Without boost, the back pressure won't be enough to even measure.

brianfromhawaii

iTrader: (3)

was going to measure back pressure in third and fourth

SPRAYED 01

iTrader: (22)

Is it possible you are just spinning the tires so bad that there is no load? If i rev in neutral or park I get no boost, kind of the same idea.

I had a bad turbo once with 4:1 bp. It still made 11psi, just took an a boost controller of like 18 to get it.