Originally Posted by bonose
First post so be gentle... Thinking of of Roadster build using Mk VIII donor. Have a few questions for the experts and not so experts.
Big Picture: Looking at econo build using Mk VIII donor. I've been reading a lot and am a little slow so...
1. Goals: 300-350 rwhp w/dohc 4.6L. Looks like the 96 Mk VIII offers better valve springs and more flexible (yet complicated) wiring harness. Wouold you all have issues w/ donor w/100k-125k miles-all other things being equal.
2. Anyone running stock Mk VIII dohc engine in FFR w/manual trans that has done dyno runs to verify the stock hp potential?
3. The Mk IRS works but need shorter axles and konis to replace the Mk shocks, correct? Any vendors/parts to make the axle swap?
4. What would be the most effective value for cost/hp gains? Seems that Mach 1 intake w/C heads are very popular upgrades but the intake swap is challenging. What about EFI parts/mods?
5. The FR500 heads look good but $$$. Anyone modding stock heads or even C heads and how much hp gain for the effort.
Gotta go fly now, so I'll check back and add more later. Have really enjoyed the FFR community and expertise and I really look for ward to some great inputs.
First - Great to have you here. I think you'll find this to be one of the best forums out there so don't worry about questions. We all started where you are. And I'm a huge fan of the 4.6 DOHC; although, I think if I were to build another I'd probably lean towards the new Coyote if I could find one at a good price.
If you are set on a 4.6 DOHC, the Mark VIII is a good short block (Teksid) but I don't think you'll make your 300-350 rwhp goal without some work. The intake and heads will hurt you there. The stock rods will not handle boost too well so as long as you are staying NA, you'll be OK with that short block.
NA power for these is basically all in the heads and intake. As you noted, the C heads will be your best bet for power along with a 99-04 lower intake that was common on the Mach, Marauder, or Cobra (99-01). The C heads actually flow as well or better than the FR500's and are easily found for a decent price. A pair of 96-98 Cobra intake cams will provide a little improvement in power as well since they have a little more duration for a stock cam. Later stock cams have reduced duration and that is really what counts with a modular DOHC - valve lift is not as important. But there are other aftermarket cams out there that will improve power more. Long tube headers from FFR will help with power as well. C heads, 99-04 intake, stock cams, and long tubes should get you well over the 300 hump. Add aftermarket cams and a ported intake to the mix and you can be up in the 350+ range at the rear tires.
The MKVIII IRS is a good starting point but the differential is not a posi so you may want to install one during the swap. The shorter axles are included with the IRS kit from FFR but you'll have to swap them out with your donor assemblies. There are vendors that sell the entire axle assembly ready to install but that will cost you and will not be the most economical path since you stated you were doing a donor build. And the FFR IRS option includes the Koni rear shocks.
The fuel injection can be a little challenging at first but there are options available to get you up and running easily. Wirediet (Art Cuesta) is very popular so do a search on that. I used a Telorvek panel from The Detail Zone and it was straightforward and works well.
Do some research and searches here on the forum and you'll begin to get a plan together. And again, all questions are welcome. Best of luck on your build.