sleeper101

So I have seem to run into a problem this yr. basically I started out 5.3 stock 862 heads comp cam .614lift .230 duration dual plane intake 76mm turbo 14psi pump gas. then I wanted to make more power and I went 862 heads ported polished. bigger valves. single plane intake race gas 18psi and basically picked up no et at the track at all. And I have also blown 2 sets of cometic head gaskets with arp studs. and it doesn't seem to pull much over 5600rpms... I'm leaning towards cam being too small not allowing the valves to be held open long enough and causing it to build cylinder pressure. am I on the right track? any suggestions?

Silver_TransAm

iTrader: (2)

What is your timing and air fuel ratio? What 76 turbo? What lsa on cam?

Sounds like valve float though. What valve springs do you have?

GrimReaper

iTrader: (33)

I do not see that camshaft being to small. Hopefully others will chime in with more head gasket info than me.

sleeper101

A/f is 11.7-11.8 turbo is on3 performance. Springs are 165# seat, 510# at .650 lift. Lsa is 114 its a turbo cam

sleeper101

Timing is 18 degrees overall @4000rpms on 112 octane car made 692whp on a dynojet on this setup with 40% leak down in 1 cyl which was a headgasket blown between cyl.

Preston99WS6

iTrader: (7)

I Dont think it has anything to do with the cam at all if its a 67/76mm turbo then the drive pressure is nearly 3 to 1 and the test I am referring to was done on a 4.8 This test was done by FOOSE04. It will be worse on your 5.3. The problem is the turbine side is too much of a restriction and youve hit your limit increasing the boost wont do any good if it cant flow any more past the exhaust all turning up the boost will do is lead to increased exhaust back pressure. So based off the 4.8 data your exhaust side pressure would be nearly 54 psig since your motor is 10% bigger your probably more likely to be at 60 psig plus which is retaining more heat and pressure from the onset, leading to increased cylinder pressure, most likely causing you to blow gaskets.

Option 2: The heads could possibly be warped have them checked, also have the short block deck checked. If they are straight then you have too much back pressure.

Bottom line is you need a turbo with a bigger exhaust wheel to go any faster.

Here is a link to the thread/test I am referring to. FOOSE04 posted it up on our local board but I don't think it made it here to tech. It pretty much proves these turbos are not sized any where near correctly for any LS motor including the 4.8 not saying it wont work. FOOSE04 went pretty fast on it I had one and ran a 6.38 with it on my 6.0 but there was nothing else left in it at that point turning the boost up past 14 psi gave me nothing. I got rid of it and swapped to a S480 and havent looked back and it was a world of difference and isn't laggy at all.

http://www.carolinahorsepower.com/fo...ad.php?t=13109

Hopefully this helps.

snyousef

iTrader: (11)

What temps is the car running when operating?

sleeper101

Well mine is a 5.3 base bored .120 over so basically a 5.7 the exhaust side on turbo is .96 ar and cold side is .80 I'm not real familiar with exhaust pressure side of the turbo. but I do know going up 4psi never made a difference in et. never checked it in hp. Temps norm runs 190 degrees it will run up to 205-210ish after a pass. but never over 210. this last time I blew the head gaskets it blew it in the water jacket side and torched my head. the heads were brand new 2 months ago my machinist always does a mirror finish on the heads.

jridenour31

Preston is dead on. The cam is definitely not too small. The exhaust wheel on that turbo is too small. You're not going to make any more power with that turbo.

tta656

iTrader: (5)

the turbo Foose04 had originaly was What size??? and did he swap to a BW300,if thats a BW300 as the box states ,that thing is a monster..

Preston99WS6

iTrader: (7)

Just look at the data in the thread I posted it also has a video explaining it while it was running down the road. Sorry about the heads, your best bet if you want to maybe go faster is take the boost back down to 14psi and add some more timing to take advantage of the octane but it will probably only net you .1-.2 tenths best case scenario.

Sorry for the bad news though, good news is a S475 is cheap and can be had new for around 600.00 cheaper if you can find them used. I would check out the yellow bullet classifieds, all said and done you should be able to swap over for about 300.00 out of pocket and I guarantee you will go a second faster when you get it full tilt.

sleeper101

Problem is I'm limited to a 76mm max in my class I run and I really need to find another .3-.4 to be competitive.

Preston99WS6

iTrader: (7)

It was a Magnum T76mm with a .8 A/R compressor housing and .96 A/R exhaust with the 67mm turbine. These turbos are all pretty much the same all the wheels ect come from the same company they are just built at different locations, As far as I know there is really no difference between the magnum and the On3 unit just the name of the factory where it was assembled.

This turbo is very common on here because it is cheap, and for a hot street car it performs well and is very responsive on automatics and 6 speeds. It will do 600rwhp all day long but when you lean on it you run into the above posted back pressure issue. You really don't even have to get the converter right because the exhaust is undersized and it spools so quick.

To answer the second question yes he did swap but hasn't posted the data he still has the rig so I will ask him if he ever got around to testing it, he stays pretty busy as he is active duty.

Preston99WS6

iTrader: (7)

Run the S476 it should meet the class rules and it will be the ticket DR Turbo might be able to chime in on this but I am pretty sure it was designed just for the 76mm class its also sized perfect for your 5.7 and if you do some research you will find most of the really fast guys in that class are running it. Another good option is one of the precision 76/75s either of these will get where you need to be, me personally I like the BW units there a little large but there reliability is unquestionable.

You got one hell of a car for that class as it will be pretty competitive because it is lighter than most of the fox bodies running that class, and its LS powered. Which means its going to hurt a lot of peoples feelings when you get the right power adder on it. You know what to do from here.

LS1NOVA

iTrader: (12)

I have a PT7675 for sale and Jax04 has a S475 for sale for cheap. Either of those used turbos would make a night and day difference on your combo. You spent money on cam, heads, etc. Now spend it where it counts.

jridenour31

That's not a problem at all. You don't need more compressor, you need more turbine. A 7675 or s475/476 would go considerably faster.

NicD

iTrader: (7)

Yup, more turbine.

Louis

iTrader: (7)

3:1 drive pressure ratio is absolutely asinine. 2:1 at most is where you would like to be. I shoot for 1:1 drive pressure to manifold pressure.

youll find that the car actually spools faster when drive pressure isnt so high.

sleeper101

Thanks for the info guys. I'm seriously looking into a turbo change.
Has anyone tried porting the housing and seeing if that makes it better?

hellbents10

iTrader: (10)

I agree, get a 476 in there with a 96mm turbine wheel and a 1.32 AR and hang on!!!