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Porting the 5.8 GT40 lowerr

Unread postPosted: Sun Oct 04, 2015 4:20 pm
by PRHutch67
Have you ever done any porting/testing on the GT40 5.8 liter lower? I'm planning a street build 408 and would like to use the GT40 setup from the Lightning motor. I opened up the upper intake and radiused the entries into the runner tubes. The lower already seems pretty generous but porting it seems like it may help due to the extra cubes. I just wanted your input on the matter. Thanks!

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Sun Oct 04, 2015 8:55 pm
by Tmoss
The Lightning lower flows about 275 cfm stock and once ported to a 1262 size port, it will flow over 300 cfm. So, it is the Lightning lower that is the big choke point at about 260 cfm. On a 408, I'd suggest running it with a GT40 style box upper if you want more power, like the Cartech/Downs or Comp Polymer to let the cubes breath better.

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 3:40 pm
by PRHutch67
Thanks for getting back to me so quickly! I've been reading your stuff for years and I really appreciate you taking the time! Am I right in gathering that the UPPER is the choke point at 260 cfm, not the lower? I'll definitely look into those box uppers. Since I'm still in the acquisition phase I'll just ask. Is there a single manifold that you'd recommend for a street driven 408?

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 4:00 pm
by StangD
No, the lower is the chokepoint, which is the case with most Ford intakes. Have Tom port it if you really want to use the intake. He did my last combo and it worked very well.

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 4:15 pm
by PRHutch67
Then I'm still a little confused. If the stock lower can be ported to 300 cfm, then where is the 260 cfm restriction coming from?

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 4:26 pm
by StangD
It must be ported to flow more than 260. The 300cfm flow is for the upper is how Tom's post reads to me. I'm sure he knows about what the lower can be ported to as well.

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 4:35 pm
by PRHutch67
Porting a gt40 tubular upper sounds like an interesting trick.

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 4:44 pm
by red5.0fogger
Lol ya the tubular is run wat cha brung

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 4:57 pm
by PRHutch67
Yup. My take on his post is this. The lower flows 275 cfm stock and can be opened up to 300 cfm because there's enough material for porting. The upper on the other hand is limited by the diameter of the tubular runners and can't really be ported. I did cut open the upper and radius the entry into the runners but on a 408 that's only going to be a band-aid. I think that's why tmoss recommended the box upper.

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 5:33 pm
by Tmoss
Yes, the length and limited cross section of all 3 of the GT40 style uppers is the choke point on bigger engines, the 9.2 deck Lightning 351W lower is a GREAT piece, far better than the 8.2 deck model. If hood clearance is not an issue, I like the TFS-R Box on larger cubes. The EFI Super Vic is also a good choice but has an impact on inlet hardware selection. Next, maybe a Victor EFI.

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 6:03 pm
by PRHutch67
Thanks again! How much extra clearance does the TFS box r require? Could i get away with a 2 inch cowl?

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 6:35 pm
by StangD
PRHutch67 wrote:Porting a gt40 tubular upper sounds like an interesting trick.
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I suppose it has trouble standing up on it's own fairly rapidly. :) And sorry for my misunderstanding. Didn't realize you were talking about a tubular upper.

Re: Porting the 5.8 GT40 lowerr

Unread postPosted: Mon Oct 05, 2015 6:48 pm
by PRHutch67
No worries stangD. I appreciate anybody who's willing to help!