LemansbluebyU

So I had my car on the rollers today and all went well except that the boost would only go to 17psi at 5800 rpm and held steady there up to 7000 rpms. This a billet impellor YSI on a 427 cu in motor with mast heads, custom cam, holley high rise intake and American Racing ZO6 racing headers...... it has a 10% over driven crank pulley and a 2.75 s/c pulley. I'm thinking I have a boost leak somewhere but I've not yet looked over everything. I do have a dual nozzle alky kit on there which isn't being used and was wondering if maybe at that boost level boost is leaking past the nozzles, they are just in one of the silicone couplers and pretty wobbly. I also have an unrestricted 4" intake tube with an air filter on the end so I don't know if that is maybe restrictive. Just looking for opinions and maybe it anyone else has had a similar situation. Thanks for any help.

ysb02

That’s a very bad idea to put the meth nozzles there regardless.

do you have a. 8/10 rib drive setup? Flip drive?

LemansbluebyU

8 rib with secondary drive

Detoxx03

Gotta move those nozzles to a more secure location.

winters97gt

Not sure why you’d put them on the coupler as well.

See if you have belt slippage. That’s what it sounds like to me.

Boosted04

Sounds pretty normal to me, imo. I had a stock headed 402ci with a billet YSI, 15% OD balancer, and 2.70 pulley. The most boost I ever saw was about 20psi.

ysb02

It could be belt-slip but I'd dyno and tune the car with a bellmouth on the centri unit to see what it makes. Most air filters choke these setups.

LemansbluebyU

I was going to look into getting a bellmouth for it. I removed the meth nozzles and put a new coupler on, I can't see any evidence of belt slip....no belt dust anywhere but that doesn't mean it isn't slipping. I will check the belt and try to tighten it as well. I just found it strange that for 1200 rpm the boost didn't increase at all so there has to be either a restriction or a leak or possibly belt slip.

silver408z

What kind of power are you making? You have a big engine and big heads. You might not see more than 17 psi. Isnt KPA how the computer measures air? What does the kpa do?

xBoostx

X 2 on big cubes and big heads no restriction means less psi curious how much power seems you are doing real well.

If you put some BHT track bite on the belt will not slip.

Unreal

Big overlap cam?

I see 23-24psi on my Ysi setup. I find the people who throw donkey dick big lopey cams so they sound cool have this exact issue. Put a blower limited setup on, then bleed off a good amount of boost right out the exhaust with 20+ degrees of overlap. On a 427 with big heads, you are just doing a disservice pushing your limited amount of incoming air right into the exhaust.

c5mtl

I made 21 22psi with a 408 decent size cam and heads that flowed 345cfm 8inch crank and 2.75 pulley....and a bellmouth vs a filter and a cast impeller ...I would think 17psi sounds about right at 5700 but would expect it to climb moving towards 7000 or whatever your shift point is..... I’d be looking for signs of belt slip

Unreal

Agreed it is probably slip, as it shouldn't level out unless something is just off on the setup. At this level, even having a few month old belt, or a 1/4" adjustment is the difference between 17psi and 20+.

LemansbluebyU

I will check on the cam specs tomorrow, I had Pat G spec it out for this set up so I would think it would be ok. The mast heads are LS7 285's with titanium valves. The whole set up is brand spankin new, this was the first time one the dyno. I know the belt is going to stretch a bit so I will check that tomorrow as well, no evidence of slipping that I can find but that doesn't mean it isn't. 1100 rwhp@6200/940trq@5200

gameover4

my 427 build with regular ysi makes 15.5-16 psi (my cam isnt ideal either)

silvrhand

I'm only pulling 9.6 PSI on my build and put down 900rwhp, the heads had a lot of work done by Greg Good here locally so I'm pretty happy with the efficiency. That's with the stock throttle body and the filter on it.

xBoostx

Hah I new you were doing well no need to check cam specs as a culprit on any fault if spec'd by Patrick G.

How many lbs of boost?

At that power level lots is been asked of the belt so you will have to tighten it up more than you would think, again add some VHT it helps quite a bit.

You will not see trace of rubber dust if belt slips unless there's a jammed idle pulley.

gotjuice?

I only see 15 psi on my ysi setup, spinning a little above the max, 427 ci.

realcanuk

Do a dyno pull showing boost vs rpm and you should be able to see any noticeable belt slip.

My YSI boost looks like this.

gameover4

exactly as stated above. you can see belt slip in boost graph. your making 1100hp on e85 i assume. Cant ask for much more power than that out a ysi.