Thread: LIM Modification

One thing that has troubled me since I first saw the inside of a LIM is how its designed. first, the outlet is not centered so flow to the cylinders Im sure is not equal nor optimum (Ive wondered if this may contribute to KR) and then the ridge that runs down the middle of the LIM. It seems the idea of it was to split the air flow, but the ramp is so steep that I highly doubt that it does even that effectively. Looking down at it it looks like the air would be coming out at such a velocity it would just bounce off the bottom of the LIM and then just brute pressure and velocity would force some of it to the cylinders rather than it being guided there with minimal resistance.

So I would like to know if anyone has experimented with any internal LIM modifications that would improve air flow? No offense to anyone, but please dont post and just say it wont work or you dont see how it can be done. The whole purpose of this is to see if it has been done, can it be done and whether or not there would be any real benefits.

So if you have modified the LIM with this, then can you either post up what you have done, pictures would be a HUGE bonus, or PM me if you dont want to post it in the open forum.

If you dont think this will work then I would much rather you post up WHY it wont work with any supporting information.

I have some working ideas and am working on some mock ups, but I would like to get some input from anyone that has attempted this.

do you have a pic of the LIM. im just kinda curious as to what exactly your talking about.

I truly feel that if you could put some type of inlay in there that would organize the air flow better then velocity would go up significantly. I have long personally question why there is only a 40hp difference between the NA cars and the Supercharged particularly in comparison to other supercharged vehicles on the market that see much more gains. I am nearly convinced that the largest reason is the way the LIM is designed. It does not promote good air flow at all.

Theres not much you can do with centering the outlet due to the way the supercharger outlet is, but I feel channeling the air flow is very possible. I would be really curious to how badly the #5 & #6 cylinders are starved compared to the rest of them since the placement is more biased to the first 4 cylinders.

blowfish, I dont have any pics of the LIM here with me. Ive got some service work to do after work but if I have time I'll post up a couple.

Okay, I borrowed an image from the classifieds:
I forgot already who the owner was, but if you dont want me using the image just let me know and I'll take it down.



Blowfish, if you look in the square area you will see a ridge down the middle. That is the splitter I was referring to. But if you see one up close, you can tell it does not angle the air flow much if at all and does little more than just split the air flow. And the square area is slightly forward of the middle cylinders so the #5 and #6 cylinders, I believe, probably dont get the air flow that the others do. There is little more, as I mentioned, than vacuum and brute force of the air flow that gets the air into the cylinders.

The easy solution is just to put some type of metal strip in there that would provide a more gradual ramp with perhaps some type of grooves or something to bias the flow to the cylinders. You're not going to be able to fit anything in there like full fledged channels and such.

I would like to have the resources, machine equipment and such to extend the ports to the heads up to the square area. Some tubes as it were extended up to the opening of the LIM so that when the supercharger was bolted on it would be sitting directly onto the the ports to the cylinders. But you would still need some type of splitter in there and I dont know how practical all of that would be.

The outlet of the supercharger is so small in comparison that I think at this point its more practical to just split the air better, angle the split better and put in some bias to each cylinder.

ahhh okay. WOW that does not look very effective at all!! and i see how the 5 and 6 cylender is getting starved. now that i look at it that looks like a very poor design. might just be me not knowing anything (shut up tdc) but it deff looks like it could be better. does anybody make an after market LIM???

deff very intrested to see how your research goes

Im kinda at a stand still with putting the Grand Prix back together until I can get some more money together so Ive been looking at stuff like this. This is what happens when you miss your car and start looking for things to do that dont cost a lot to concieve or try out.

thats why if you were ever to tap you lim for some alky, youd need to add two more on the 5 and 6 cylinder side. without those youd get uneven distribution. im not sure this would have anything to do with your car knocking, but it definately doesnt help with flow issues.

no one that i know of has done anything to remedy this. but then again, no one has found anything wrong with how it works either.

Originally Posted by SyntheticShield

Im kinda at a stand still with putting the Grand Prix back together until I can get some more money together so Ive been looking at stuff like this. This is what happens when you miss your car and start looking for things to do that dont cost a lot to concieve or try out.

haha i deff agree with that. question. isent boost messured by the amout of air built up?? so going down into the lim after the supercharger the air is just being stored in there before it gets used??

well if air is built up then i see it hard for cylenders to be starved. air is going to be on tap when it needs it. after i thought about it i was like wait....

well i was just throwing out the starved theroy. there is always room for better air flow.

Intense racing offers a ported L.I.M for $199. I don't know how effective the porting they do is but they do hold quite a few 3800 powered records.

It is in the induction section on their website but they don't reccomend them for stock heads. theres a bunch of pics showing the difference between stock.

These BRAND NEW lower intakes have been professionally ported to match our INTENSE™ Stage 2 and Stage 3 heads. They willl help increase airflow and horsepower gains even further with those heads. We do not recommend these for those with stock heads as air turbulence will occur causing reduced laminer flow and a reduction in performance.

i bet they port matched the outlets to mate up with the heads. that has nothing to do with the inlet side being offset from cylinders 5 and 6.

I agree, that due to the forced induction and that not all of them are open at the same time that they are probably not starved as it were. However, there could be differences in how each cylinder fills with air, that is to say that even under forced induction I doubt with the design that they all fill equally. Furthermore, if you are having to add extra nozzles for the alky kits on the #5 & #6 cylinders, then there is definitely flow distribution inequalities, otherwise there would be no need to add the extra nozzles.

I think you could even out the distribution at least and probably improve the flow characteristics. Again, if you looked at factory supercharged applications, the Grand Prix has to be among the lowest in gains compared to its NA option. 40hp is a fairly low increase for an extra 90 cu. in of air being put in the motor and I am unaware of any other application that comes in that low for roots style blowers anyway.

Now, all that said, myself not being a trained automotive engineer it could be that the LIM is designed to hold air in reserve. In other words there is the volume of capacity in there that there is for the purpose of making sure there was air available for each cylinder during high rpm events in which the time to fill each cylinder would become shorter and shorter as rpm increases. If that is the engineering purpose behind the design, then simply adding some type of ramp to the splitter would better aid distribution. In addition it would add some tumble/turbulence to the flow which would aid velocity.

These are the times I wish I was buddy buddy with a GM design engineer to ask them such questions. Though, from a design perspective, more emphasis is given to emissions control than anything else.

The intense racing LIM is ported on the runners which I already planned on doing. The plan was to gasket match the ports and try and match them up as much as possible with the ports in the heads. If you think of it from the air flow perspective, it is leaving the supercharger and being slammed to the floor of the LIM and then it flows from there to the lowest pressure point which would be the cylinder that is on the intake stroke. In which case, it has to cross over the ridge in the LIM and that for sure is not good for flow. The design lends itself more to the air going where its needed when the vacuum is greatest. Granted that low pressure is going to pull in some air to be sure. But is it the best set up? Can it be improved upon? Would there be benefits to channeling the air any?

For sure questions that could at least be looked at. Ahhh the experiments I could run if money were no object. The problem in experimenting here is that, to be done correctly, you have to swap out the LIM on a dyno, or at the track or something like that where you can see the results in as close to real time as possible. This is potentially problematic primarily due to the potential for coolant leaks. However, if it were simply a matter of changing the ramp/splitter angle, then you could probably just take off the blower and make a modification and put it back on and see if there were any positive changes. If there were, then it could warrant further study and/or engineering changes.

TDC, thank you for your constructive view. This was the input I was looking for, not just "It wont work", "Dont waste your time". Its easy to say those things but it takes more of an investment to explain why in a constructive, informative and factual way.