Trying to figure out how to get a little more juice out of my car, specifically the SC.

This is what I have:

V1 SC with a 3.25" Pulley - Pushing 8 PSI regularly, 9 on a good day.
MEVI
Cartech Adjustable FMU
Blitz BOV
WSP Y-Pipe
WSP B-Pipe "Fix"
Stillen Exhaust

Fuel is tuned to a curve between 11.5:1 (low end, 0-3000RPM) to 12.5:1 (6500RPM)

My vacuum reads 15 at idle in P and 13 in idle in D. I know it's low, but i have literally done every test from re-installing my manifold to a compression test to re-zip tying every hose and replacing all check valves - and trying 4 different gauges (and 3 mechanics) - no luck.

My EGT's are around 1000-1200 degrees on the freeway, and 8000-10000 driving slowly. When fully boosting, I touch 14000-14300

So what's next? If I go with a smaller pulley (2.87"), will I need to upgrade my fuel? Will a S-AFC do this, or do I need more?

Will a high-flow Cat help? What about a bigger B-pipe?

FYI - Adding just the exhaust gave me +7hp and 19tq at the Dyno.

Any ideas? I want to up my hp, of course, but not sacrifice my engine or any major components.

No flames or arguments, just advice and discussion - Thanks.

 

Try a RT High-Flow cat, wouldn't hurt. I have the WSP aluminum y-pipe, RT cat, and Greddy SP cat-back. I installed my boost and fuel pressure gauges and according to most people, my car is running pretty good, vacuum is at 20 at idle, I believe slightly below 20 or still at 20 in D , fuel pressure is at 30 at idle, I believe slightly higher or still at 30 in D. I have the 3.33" pulley with factory NGK's and everything else, and I was able to see 9psi easy and consistently, one time I hit 10 but I believe I was redlining it. So, I guess the RT high-flow cat could help your Max breathe a little better by having your exhaust more uniform.

 

V1 or V2? The V1 boosts about 1 PSI less than the V2.

 

Oh I didn't know that. I have the V2, and to answer your question on the other thread, I have the Greddy SP, its usually quiet, maybe it was somewhat loud because I was WOT in that clip.

 

"So what's next? If I go with a smaller pulley (2.87"), will I need to upgrade my fuel? Will a S-AFC do this, or do I need more? "


From what I understand about these things, the question is whether the stock fuel injectors can supply enough fuel without having to boost the fuel pressure up to dangerous levels. Dangerous being over the 100-110 psi range, at which point you are close to locking up the injectors. The duty cycle is going to be pretty high (if not 100%) as well, which can result in shorter injector life. Dashingmax is pretty knowledgeable on this subject.

This is regardless of what kind of fuel management system you are running. The S-AFC will help you tune the fuel distribution, but it won't overcome the basic fuel capacity limitation of the stock fuel injectors, which is why Mardigrasmax went to larger injectors, but he may have done that because he is also running nitrous along with a small pulley.

The stock fuel injectors may very well be adequate with a 2.87" pulley and a V1 blower. Others are running the 2.87" and not gone to larger injectors, I'm pretty sure.

Your low vacuum level disturbs me. Have you done a compression or leak-down test? Got an aftermarket cam with lots of overlap you're not telling us about?

 

At 100 psi you most likely ARE locking up your injectors, shouldn't go over 95...

 

I've seen claims of running 110 psi from org members. But you're right, better to stay below 100.

 

Thanks for taking the time to write a thorough post on the subject, Stephen.

As far as my vacuum and compression test, I stated in the original post that yes, I have done a compression test.

Compression:
Front bank: 190 190 190
Rear Bank 210 210 210

Spec I believe is 145-185, or something like that (Don't have my JDM Manual near me right now)

I'm well above spec.

IanS

 

Ooops! Sorry, I guess I didn't read very thoroughly. Yeah, you're compression looks good - about what mine is, and my vacuum is at 20-21 inches at idle. How mystifying!

Getting back to your original question about how to bump up your power, have you considered a 40 or 50 shot of nitrous? That shouldn't overtax your fuel system with the 8 or 9 inches of boost you've got, and it will add gobs of low end power while you're spooling up to full boost.

 

Yep - I've done everything from spraying carb cleaner around to find the leak, to soapy water, to just listening, to covering up the SC with a pillow and listening.

Also, 4 mechanics have spent an hour plus looking at it, and they all say they can't find any kind of a leak - I've reinstalled my MEVI twice and replaced every gasket, seailing with form-a-gasket also just to be on the safe side.

My engine is just strange, that's all I can figure out.


As for the N02, I dunno about that - It only works when you turn it on - It'd be good for the track, of course, but the track in Seattle is only open 3 months out of the year on Wed and Fri nights, and I have to work Wed nights.....so it doesn't seem worth it.

I'm looking for a more drivable solution - smaller pulley, tuning, exhaust, ECU, maybe even just give it all up for a Turbo.

Although if anyone has any ideas I haven't thought of for the vacuum thing, (although i can't think of what i haven't thought of ) then maybe fixing that will help my boost in itself.

 

The fact that you have one entire bank of cylinders that are 20 psi lower than the other bank makes me wonder if it is a cam timing issue. Maybe it would be worth it to take your timing cover off and take a look at the timing marks on the front bank. Oh yeah, you've got a SC to contend with. Whenever you decide to put a smaller pulley on, take a look at your cam gears while you have the blower off. Maybe you can verify cam timing by just pulling the front valve cover and verifying that the exhaust cam isn't closing the exhaust valves too late. Not sure how you would know, though, except by comparison with what the cams on the other head are doing.

 

Heh....I have no idea what you just said - well, I sort of get it, but i have no idea what to look for.

 

I have to leave to work early today (daughter's car needs working on ) but I'll try to explain in more detail later this weekend. But late closing exhaust valves due to an exhaust cam that is not timed correctly will result in both reduced vacuum at idle and lower compression for an entire bank of cylinders.

 

Might that be why I get what almost (but not quite) sounds like a backfire when letting off the gas sometimes?

 

Timing overlap can cause backfiring, but I think the rumbling you're hearing is normal for Maximas with modified exhausts. I get it too.

What I was trying to tell you earlier is this. As I'm sure you know, the intake and exhaust cams control opening and closing of the intake and exhaust valves. They not only control how far the valves open, but also how fast they open, how long they stay open and when they open. Valve lift and duration are controlled by the cam profile, and when the valve opens and closes relative to position of the piston and relative to what is happening to the other valve(s) is controlled by what position the cam is in when the engine is assembled. If you look in a FSM or Haynes manual, it will talk about making sure that the timing marks on the cam gears are lined up when you put the timing chain on. In addition to the timing chain, there is also a smaller chain that connects the intake cam gear with the exhaust cam gear on each cylinder head. So you have a total of three chains. If you look in a manual, you will be clear.

Now, IF your timing chain was assembled with the intake and exhaust cams oriented so that the timing marks don't line up with the engine at TDC for cylinder no. 1, then the timing will be off. This means that the relationship between the exhaust valve opening or closing relative to the intake valve opening or closing will be off, too. This situation can result in low compression for an entire bank of cylinders, which is the situation you have (although 190 psi isn't all that low, it's just low compared to the rear bank of cylinders, all of which were 210 psi). This can also result in low vacuum at idle.

 

Wow. That was extremely informative.

Something I'm going to have to check out when I take my SC off again in the coming months.

Thanks!

IanS

 

There is one small thing.

When installing the SC, I busted the spot that the screw goes into the Crankshaft Position Sensor. I was still able to screw it in, and it absolutely looks perfectly in place, but, hypothetically, if it were turned slightly, would that cause a problem?

Anyway, I've got a new CPS coming off of mtrai760's wrecked Maxima later this week - may as well replace it.

IanS

 

no

 

Like I said, I didn't think so.

IanS

 

I was just htinking that i should be able to test this with a timing light.

Correct?

IanS

 

Just a question....hypothetically, if the compression tests each cylinder at max compression (upward most stroke from the crank), then the timing valves wouldn't have anything to do with it....since the cam would have gone through a complete revolution. While having poor synchronization between intake and exhaust is something to be concerned about, a compression tester reports the maximum compression experienced during a complete stroke of the piston in that cylinder.

??