Will a 3.5 motor replace a 4.6 in a P38?

Some might say I'm crazy, and maybe I am, but that's what I've done!

Obviously, it's not as simple as just bolting it in and the job's not finished yet. The 3.5 motor is in and runs ok but has a misfire and won't rev past about 1600 rpm. As I open the throttle the revs start to lift then seem to hit a wall.

So, I started with my 3.5 and 4.6 motors side by side on stands and compared the two. What I've ended up with is a 3.5 block, crank, rods and pistons bolted together with the 4.6 heads on.

It's got the 4.6 camshaft in it and the block has been drilled and tapped to accept the camshaft keep and the timing gears, timing cover/oil pump/water pump from the 4.6.

I've turned some billet down to fit into the two central core plug holes on each bank. These are drilled and tapped to accept the knock sensors. They're a tight fit into the holes and I've gone round them with chemical metal for good measure.

I cut a hole through the web on the back of the engine and fastened the crankshaft position sensor mounting plate in there. I've used the adaptor plate from the 4.6 so that it's got the lugs on the engine side for the sensor to 'see'.

Well that's about it!!

So why won't it rev? I've had her running in the dark and can't see any arcing on the leads. It's got a new set of BPR6ES plugs.

My thoughts are...

Maybe the knock sensors are not reading right, so the ecu is retarding the timing right back?

Fuel problems? She does smell a bit rich.

Timing problems - maybe the crank or cam position sensors are not right?

Any body got any comments or suggestions?

No error codes stored in the ECU and all the readings look ok except the air flow value which is 0.07 g/s at 600 rpm and 0.12g/s at 1600 rpm. That seems a bit low, although I wouldn't know what it should be.

ecu?

ECU was working fine with the old engine Nathan.

im no expert, the ports in the 4.6 heads are bigger with bigger valves? wud it be an issue with fueling/air?

I'm not sure that they are bigger? They looked about the same, but I didn't measure them. I think the ECU should figure it out - it knows how much air it's getting so it should put the right amount of fuel with it. The lambda sensors will tell it if it's lean or rich, so it'll adjust the mixture one way or the other as required.

At least that's the theory!

start it up and let it get hot, then unplug mas senser se if that makes it rev any better

the 'bigger' was summat i read on the net so maybe not correct?

Done that J. No different. It's like it's hitting the rev limiter, but at 1600 rpm. I tried spraying a bit of brake cleaner around the engine to see if there are any intake leaks but the revs didn't change.

after all the work you have done this must be doing your head in. i put my thinking cap on

have igot this right you used the push rods from 3.5?

J, doing my head in is right!

The push rods, lifters, rockers and timing gears are from the 4.6.

Only the block, crank, con rods and pistons are from the 3.5.

I'm going to do a compresion test on her when I get home. I'll let you know the results.

I know nowt about your V8's but unless a 4.6 is just a stroked 3.5 the bore diameters will be different as will the combustion chamber sizes in the head. (unless the heads are totally flat) Even if it's just the stroke thats different it will affect the compression ratio. So just maybe you have a compression ratio like a lawn mower motor of about 5 to 1??? Or maybe you know all that and have done the calcs?????

theres info on rpi's website about going from 3.5 to 4.6 take a look it may be of some use

have you got it running dave?

had a problem with my 4.0 recently.would rev to about 2000rpm and then started to missfire and would not pass 2500,as though it was hitting a limiter.tried lamba sensors,disconnecting maf sensor,it turned out to be a broken pin on flywheel and crankshaft sensor replaced.that was 9 days ago and 2000+ miles later it is running better than solihull ever managed

Checked the wiring between TPS and ECU and guess what?

Fixed that and started her up - shot up to 2000 then climbed to about 3000 and stayed there! So I knocked her off, it's a brand new engine after all! Stuck the machine on her again and the throttle position was reading 12%. I read up on that a bit and found that your supposed to 'reset the TPS voltage'? I've no idea how to do this so I elongated the holes in the TPS casing so I could get it to read 0% and bingo! She's as sweet as a nut!
Took her out for a spin and I can notice the difference - she's not the animal she was, but for a fat lass she can still move! I didn't give it the full beans either so once she's run in she might still light the tyres!

So that, gentlemen, is how we do that. Easy peasy, lemon squeezy! I have to say, I'm quite pleased!

Thanks for your responses guys, you've been a great sounding board.

Great stuff glad uv sorted it

well done and best of luck for the future

My 110 has a 3.5 with 4.6 heads and cam and the comp ratio is 10 to 1
Heads have been skimmed but dont know by how much, Goes quite well but as I didn't do it I'm not sure how much the valves are shrouded etc.