Thread: 4.6 can't breath

Not sure what you're basing your conclusions on. What intake arrangement was the engine running with before you did all the work?

Unless you've done substantial work to the heads I would not expect you'd need to change much.

If you are running an Impco system, there's your problem right away. Impco model 200 converters are what you often find on D1's and they are a menace to the power output. Chuck it out ASAP and see how the petrol power is, work back from there.

If you are running a venturi type mixer the one you have could be too small, very few LPG fitters actually understand gas performance. Optimum ID of a good mixer for a 3.9/4.6 is 46mm. Most mixers fitted to D1's range from 38 to 42mm, way too small.

a bit more info, it is running a impco 200 and yes that is to small as the power is better without it on(have tried that) even with the gas system in place when playing around with it there is significant improvement if i remove the filter and take it for a drive, so to me both gas and filter are restricting it so i am just looking for the cheapest option, was quoted heaps for a mixer upgrade and would have to remove the washer bottle as the 450? mixer wont fit.

the heads and ECU are standard, the cam is a high torque custom grind from crow cams to suit gas and it is balanced, its a 97 so origionaly had the flat filter in the plastic air box( butchered by another crappy gas fitter installed by previous owner) so when i installed the snorkel i swopped over to the canaster air box. With the origional filter i had it down to 13.5 l./100 now the best i can get is 15.5 l./100

Bin the Impco system, buy a used OMVL converter or similar, and a suitable less restrictive mixer ring. As for the loss of economy you really need to adjust the main mixture after changing to a restrictive filter. On Impco 200's its on the side where the gas hose is connected, marked L and R for lean and rich.

I'm also not sure why the canister cleaner is such a problem, I've fitted quite a few to 3.9's 4.6's and so on. Make sure the element is the paper one and the retaining cover inside isn't sealing off the airflow. You have to use the paper element as the air runs into the pleats endwise. Leave the cover off for a bit more airflow.

Also what is your ignition timing set at and is the compression raised?

How about some pics of the air filter, does the air feed in from the front or has it been adapted to the side of the canister, Regards Frank.

Easy solution for you, I have a 3.9 that I will swap for the 4.6 .

sorry mate can't swop 3.9 for 4.6 as much as i would love to help u out

the snorkel is plumbed into the side of the canister which forces all the air through a very small portion of the filter which would be 1 of my issues, but to help it along i have also set up a cold air intake from the front of the canaster to behind the headlight which i can easily remove and replace with a blanking plate for river crossings.

after yet more sole searching and research i think that i will have to live with it, for the time being. To get it all sorted 100% i will have to upgrade my air intake and gas mixer, then ignition system and a bigger exhaust.

After checking out the standard exhaust on a mates 4.6 P38 i think i will try that first, then ignition system.

it is a long road and will have to wait untill funds are better

thanx for your input and feel free to keep the advice comming, this rig is turning into 1 hell of a project but is slowly getting there

cheers Matt

Have you stuck a vacuum gauge in your inlet tract? Just thinking that that will help you sort out whether the engine is being strangled from lack of air or whether it is being clogged up from too much back-pressure.

Are you running a stock exhaust system? The stock headers are so-so at best but you to gain a great deal by simply doing a muffler back system. The stock muffler and resonator are quite restrictive. Years ago I changed mine out and had a 2 1/4" or 2 1/2" system fitted. The economy was much improved, as was the sound and responsiveness.