Intake Manifold Choice for EFI - Pontiac

[andrewb70]

About Camshafts: Mine has a 110 degree lobe seperation (Comp Cams XR288HR-10). How much of a problem is this going to be?

(perhaps this should be on a different thread...)

How much vacuum does it pull at idle?



Andrew

[Dave]

My vacuum at idle depends a little on temp, but it pulls 10~14 (in what ever units vacuum is measured in...)

[andrewb70]

My vacuum at idle depends a little on temp, but it pulls 10~14 (in what ever units vacuum is measured in...)

14 is OK, 10 might be a little low. The units are inches of Hg (Mercury). The reason you want more vacuum is because most aftermarket EFI systems use a speed density calculation to meter fuel. Speed density relies on the MAP (Manifold Air Pressure) sensor. In a naturally aspirated engine there is only vacuum. In a forced induction application there is both vacuum and pressure. The less vacuum the less usable scale on the load axis of the fuel map. This will not really effect WOT tuning but part throttle operation will suffer. Drivability will also suffer.

Does anyone know a conversion from in Hg to kPa?

http://www.ilpi.com/msds/ref/pressureunits.html

Andrew

[EFI69Cam]

In this application I don't think it would matter a bit. I mostly like the cast boss for the fuel rail attaching point. The short injectors allow the height of the manifold to be shorter and the throttle body mounting pad to be lower. Makes for easier packaging, especially in Trans Ams.

Andrew

I agree the cast boss is nice. My new edelbrock victor jr EFI has cheesy bent stainless strap stands for the rails.

[68Formula]

14 is OK, 10 might be a little low. The units are inches of Hg (Mercury). The reason you want more vacuum is because most aftermarket EFI systems use a speed density calculation to meter fuel. Speed density relies on the MAP (Manifold Air Pressure) sensor.

Andrew

To add to that, the measurement you're taking with a analog gage is not as sensitive as the map sensor. The map is providing the ECU with digital data at a very high sampling rate (comparitively). With tight LSA the map sees pressure waves you won't read with a gage. The ECU attempts to average these waves but has a difficult time determining proper fuel map and ignition to match. So vacuum readings can give you somewhat of an idea if there would be a problem, but it's not necessarily the root cause. If it's less than 14" it's probably going to be a problem. If by the range you're indicating it's fluctuating between 10 and 14" then you definitely will have a problem.

Your cam would probably work great with the same lift/duration specs but ground with a 114° lobe seperation. Don't forget to try that ignition timing like I mentioned earlier.

[rockvillespeed]

well, half of PY is in on this thread it seems!
hi guys
i'd use the smaller intake for a purely street car; your shop is a bit off in their understanding of manifold applications, it seems.
a victor is gonna behave like a voctor, efi or carbed - it's short large runners are set up to fill at high rpm only; you will lose good torque in the rest of the band.

[Skip Fix]

The Torker II/Victor one is BO until Feb 16. Got one on order just to check it out.

[Doug F]

Low vacuum does make it more of a challenge to tune. I've tuned speed desity systems at about 6-7" of vacuum. You really have to have a system that allows you to scale the MAP axis of the fuel map. If you don't you could have trouble with a manual trans or a tight torque converter when tuning.

I generally try to open up the LSA a bit with big cams. I run a XR292 solid roller in my 491 on a 112 LSA and it has plenty of vacuum (idle about 60 KPa). I also run power brakes no problem. You need a lot of timing at idle 22-28, that helps a lot.

For a novice tuner, the low vacuum could make it harder if they don't konw the little tricks, but is doable.

0 KPa = 29.92".
100 KPa = 0".
50 KPa = 15"

[Hammered]

So Doug, being a novice with a low vac engine (about 9" at idle) I'm interested in what the little tricks are. I have a manual trans and am pretty happy with the tune so far, but would always prefer to make it better. Should the granularity of the MAP be made finer at idle?

Thanks in advance.

[andrewb70]

So Doug, being a novice with a low vac engine (about 9" at idle) I'm interested in what the little tricks are. I have a manual trans and am pretty happy with the tune so far, but would always prefer to make it better. Should the granularity of the MAP be made finer at idle?

Thanks in advance.

You want finer granularity in the range that is most used by the engine. So if you are at 9" at idle right now that is about 70 kpa. Your engine on decel in gear may pull down to maybe 50 kpa. So everything below 50 kpa is not being used. So what you want to do is start your load scale at 50kpa and scale it up to 100kpa. So now instead of having 16 row going from 0-100kpa, you have 16 rows going from 50-100kpa. Now you have much finer control in the range where your engine actually operates.

Andrew

[Hammered]

Duh!!!! How come I figured that out for RPM but not vacuum? Thanks Andrew!

Okay, anymore low vac tricks?

[rockvillespeed]

here's my crossram efi project in-progress; at this point it's very nearly ready to roll.
still have plenty more to do unrelated work before it's running, though, like efi, fuel system, pull and port or change heads.
probably mucho grande camshaft, too.

[andrewb70]

here's my crossram efi project in-progress; at this point it's very nearly ready to roll.
still have plenty more to do unrelated work before it's running, though, like efi, fuel system, pull and port or change heads.
probably mucho grande camshaft, too.

Very cool! Are those the pico style injectors?

Andrew

[rockvillespeed]

they are picos; 35 lb, sourced from edelbrock.
i was really amazed by how tiny they are; huge advantage in packaging flexibility.