I've read over and over how the stock lsx and truck intakes create a situation where the 7 & 8 cylinders run leaner than the rest and can cause detonation problems and piston failure due to running lean. Supposedly this problem is solved by going to a single plane style intake manifold like the gmpp one. I have a few questions about this topic.

1. Is the problem with the stock intakes due to the different theoretical runner lengths for the cylinders?

2. Why wouldn't this still be a problem with a single plane? I know the runners are closer to the same length but the inboard ones are still smaller than the outboard ones...

3. I've heard that you wouldn't want to do a efi conversion on a dual plane intake manifold like the edelbrock one. Is this because it would have the same problems due to different runner lengths like the stock intake manifolds?

4. Does forced induction make the problem worse?

I'm asking this because I was considering doing a efi conversion on a dual plane manifold and was told not to do so, but couldn't be given a reason why.

 

I have heard a couple different answers on this topic. One is that oil ends up in the manifold from blow-by and is consumed mostly by the rear cylinders because it gravitates back under acceleration. The ls1/ls6 manifold is believed to be worse because of the flat floor on them(letting oil drain into #7), opposed to the truck manifold runners comming off the top of the plenum where there is less liquid oil to consume. The oil consumption sets up a knock-prone condition in #7 or 8. The single plane Edelbrock would have a more equal air/oil distribution because the runners are on the same level and very similar in length. I don't know what the exact cause or solution to the #7 problem would be, but a single plane short runner manifold seems to be the best choice for FI in general.

 

Interesting, I hadn't heard that one before. If it's more of an oil issue rather than runner length then, I wouldn't think using a dual plane manifold converted to multiport efi would be detrimental. Obviously a single plane would be ideal, but if I used a dual plane, it's not going to blow up my engine right?

 

The manifold itself won't cause your engine to fail, I think its more of an issue with extreme HP engines than with a mild build. What kind of HP do you expect to make and at what RPM? Sorry I don't have any answers, I am learning these things for myself right now.

 

I'd only be going for 400-500 fw hp/tq and wouldn't spin it more than 6K tops, and I wouldn't want to make more than 9-10psi doing it. I know that's still alot but then again, it's really not when alot of guys are layin down 1000+. This is actually a SBC buildup in an OBS truck to replace the crappy stock CPI vortec manifold. The same should apply to the NBS intakes since the stock intake places the TB at one end of the manifold instead of smack in the middle like the GMPP or edelbrock one does.

 

Forced induction increases the chance of #7 going lean or higher hp applications with heads/cam. The air coming into the manifold is forced into the manifold. When it hits the back of the manifold, the air goes into cylinder #7 creating a lean condition. (this is from what I heard)

I would not worry about this. There are tons of people using the factory lsx manifolds and not blowing there engine. Parish had his stock truck manifold up to 1000rwhp. Many are doing it. It is critical to get the air/fuel dialed in and i think you will be safe.

 

dont you mean oil from the stock pcv system?
If that much oil is going past the rings youve got bigger problems than an intake.
Either way people who have deleted the pcv system still have problems (me for one) I think Kano hit the nail on the head in his post about why it leans out actually tracing the events to the way air exits the plenum and the firing order.

 

Okay, so correct me if I'm wrong but it sounds like it is a manifold design issue in that the throttle body is at one end of the manifold and the other end is basically a wall that the air runs into in a FI setup, forcing the air into the rear two cylinders. So then it would have less to do with unequal length runners then? That would make sense to me regarding a FI motor, which is why for example a NA TPI motor like in the older camaros sign off pretty early, but when running a blower or turbo, the rpm limit is extended quite a bit (I'm speaking from what I've heard others say, not necessarily from firsthand experience).

 

Exactly, I was talking about oil coming into the manifold from the pvc, not actually blowing by the rings. If you guys have deleted your stock pcv and still have problems with #7, then the oil theory doesn't make as much sence. I read a post saying that the pressure(boost) differences of the stock intake were evened out using the single plane which would help balance AFR across the cylinders. Kano's firing order chart makes good sence too.

 

Yes its the intake design Kano outlined why the firing order causes specific cylinders to lean out, its a theory but it makes sense.
I made a post trying to get people to poll on who has fueling variances hoping to definatively nail it down but no one voted
It does seem that alot of guys have this problem in particular with the truck intake whether FI or NA. I really think its the way air exits the plenum and in particular the shape of the exit, theoretically that would make for different runner sizes since the air travels farther.