"the GT40 lower may be a little better than the truck lower but the truck upper is less restrictive and has more throttlebody attached so it's probably a wash."

That's why I'm porting my gt40 upper TB opening from 65 to 75mm. Spent a good portion of the weekend filing but it looks to be a very smooth transition into the "S". The 75mm blade is still .25 in. sq. less surface area than the stock truck dual 54mm TB blades, but I'm hoping, for all the hype, the Accufab 75mm TB flows well (I've talked to folks useing them on turbo'd 5.0 stangs and SC'd lightnings so I'm pretty sure it's plenty for NA 5.8).

It's possible that with the smaller plenum box (smaller than a normal 5.8 truck upper), port velocity will be higher, yielding more torque. I read a mustang article a few mins. ago where they did an intake shootout with a 347 and different intakes. The GT40 (using a 70mm throttlebody) beat out all competition on the torqueband and didn't lag too far behind the high dollar intakes (TFS-R, Holley systemax, edelbrock victor, some using 75mm TBs) on peak/super high rpm HP (not too important for me). Plus I can have a Lightning plate on the intake in my Bronco.

Its also the only lower I can use with my Kenne Bell lightning kit. Before I got the twin screw I was set on running a Performer Truck EFI intake, but was told they kill high HP/big inch motors airflow (not totally convinced there, though).

 

Yes on a naturally aspirated motor it does, several guys here have installed it on a stroker and regretted it, lots of TQ but absolutely no top end. The problem with the truck Performer is the length of the upper intake runners, they help TQ on a smaller or stock motor but become restrictive on a big inch or high flow motor, the regular performer upper doesn't have that problem and was probably one of the intakes in the comparison test you saw.

 

I know its an old thread but....

Having spent a good while porting a gt40 upper and lower, I think I understand why the Eddy truck performer EFI manifolds aren't good for big inch strokers, and I'm not so sure its the runner length. I compared the 351w gt40 lower intake ports at the exit of the lower as compared with the eddy truck lower and the gt40 351w lower has a considerable amount more port volume then the eddy truck manifold or the oem 5.8 truck manifold. You'd have to shave a ****load of material from all sides of the runner all the way from the upper plenum all the way to the lower manifold exit on the Eddy manifold to have the same runner volume as the 351w gt40 lower.

I'm hoping that the large 75mm tb used with a thoroughly ported upper/lower/gt40 lightning head combo at around 9:1 compression will have good lower rpm port velocity for a good torque curve. It'll be all rollerized valvetrain (got my comp 1.7:1 adjustable race roller rockers) and probably the stock roller cam.

 

It seems the runners in all Ford intakes narrow towards the head.. probably because the ports in the stock heads are so small. I can't see that helping performance though. Overall none of the stock or next level aftermarket intakes flow near enough to supply a deep breathing stroker, that's when you have to step up to the TFS-R or Edelbrock Victor intakes that have much larger volume runners.

 

I just found this thread and jerg_064 said:

Quote "We'll it's going to be hard to pick up the GT40 lowers for a 5.8, as far as I know they only came on the 93-95 lightnings. There could be more, but nonetheless they're very rare and hard to come by."

Now,
I have a GT40 lower off a 5.8 taking up space in my garage. It was on a marine engine with F3ZE heads.
Could this be worth something to sell? If so, where should I advertise it? Shipping could be expensive since its a cast iron piece. I think the ones that I saw in FRPP were all aluminum. I just checked and it weighs in at a hefty 44 lbs. Probably wouldn't be worth shipping a long distance from here in S.E. Michigan.
Numbers cast into the front are RFF2JE 9K461BA if that means anything.

 

Can you post a picture of it?

 

Any thoughts on a price?

 

Gonna try posting pics here,
Hope these aren't too big.

 

there was one on ebay a few months back same as you are describing,cast iron,marine,and it starting price was $100. I did not check back to see what it went for after the auction ended,but it had less than a day left and there weren't any bids yet.

 

"It seems the runners in all Ford intakes narrow towards the head.. probably because the ports in the stock heads are so small. I can't see that helping performance though."

That would truly suck if that narrowing was somehow important to performance or fuel atomizaton. I'm hoping that by keeping the volume as consistent as possible through the lower and having it go into head intake ports that are slightly larger than the lower port exits, I'll get air into the CC more efficiently and it wont become lazy on the way.

 

It's not on multipoint EFI motors that's taken care of by the injectors which squirt right at the back of the intake valves.

 

"I wouldn't bother with porting the GT40 stuff. Won't see any gains, you'll lose velocity, and they'll be worthless to anyone after you're done with them."

Does this sound right? I could see if one was just blindly removing tons of material, losing velocity, but it at least looks to me like the porting I'm doing won't kill velocity. I explained to this gentleman:

"All I've done is try to smooth out the path and enlarge the sharp/small radiuses near the port exits to be more parallel with the opposing wall, focusing on the runners with the most deviation from straight."

I haven't yet messed with the exit port size, i'm saving that for matching with professionally ported gt40 irons. My assumption is that well-worked gt40irons will like the increase in flow that I assumed a smoothed/straightened lower would provide.

 

There a guy that frequents the Mustang Corral forum that has a lot of experience porting the HO and GT40 intakes and heads.. goes by the handle TMoss. The flow and dyno numbers posted by him and those that have built motors using parts he has worked indicates there is significant power to be gained here.

 

Tmoss's experience has shown that there is gains to be made mostly in the lower intake. Just as you described- short turn radius and ports 1 & 5. In most cases the intake cant support the same flow numbers as the heads.

 

fox lake performance who offers 3 stages of porting on the 5.0 motorsport/gt40 claims the out of the box lower makes 230cfm which they say they can get up to 70-80 cfm over, (300-310cfm) which is enough to suppy most popular aftermarket aluminums (afr 205cc flow 310cfm/intake @ *.700"* lift with a huge 4.125 bore). I wonder if the 351w gt40 lower can be stretched any further than that by virtue of the wider lower intake/straighter runner paths...