I’ve started collecting parts for my Miata conversion. So far I’ve got a 5.0 engine from a ‘91 mustang with all of the wiring harnesses. I’m adding a 65m throttle body with a gt40 intake from a explorer. I also have heads from a ‘93 cobra with 1.7 rocker arms. I would like to have it tuned once the car is together. I would like to get to 300hp or as close as possible while still reliable.
What else do I need for the engine? The intake and heads are not ported. Would it be worth doing? I’m thinking of keeping the stock cam with the 1.7 rockers. Thoughts? What size MAF sensor do I need? Injector size?
i realize I’ve asked several questions but any suggestions would be appreciated.

 

Not sure if you will get there with those heads, they will need a lot of work and still may not get there. Cam will help, but you may be tight with 1.7 rockers (depends on cam). Porting not worth it on those heads in my opinion. Better to spend the money on diff heads (can find used aluminum trick flows for 500ish, save weight and make more power). If you google 93 Cobra heads, you will find a ton of info/opinions as well.

Regarding MAF, if your TB is 65 mm, no need to go much larger as the 65 mm is your restriction (think explorer intake is 70mm opening). 24 lb injectors should be good for the 300 hp target, just get matching MAF.

 

So is it worth upgrading to a 70mm TB?

 

Not without a better set of aluminum heads and upper/lower intake manifold(s)

 

The heads on these engines is where they bottleneck. You’ll need some new heads. Some GT40 heads can be had for pretty cheap and will get you about where you want to be with the proper cam. The steel ones are petty cheap if you dont mind the weight.

 

The Cobra heads are basically iron GT40 heads. Still a bottleneck and need work to get anywhere near the other A/M heads, and you will still have weight disadvantage for all of the time/money put in to them..

Regarding TB size, if upgrading to Explorer/Cobra/GT40 intake, then go with 70 as that is the port size.

 

Talk to the guys that you are going to have do the "tune" on your car. Tell them your goals, and they should be able to point you in the right direction.

 

Hi,

For 300 HP you wil need the following

Good compression in your bottom end 170-190 psi cranking pressure.
Heads and intake system that flow about 230 cfm on the intake. The GT40P heads will get you close at around 215cfm and the explorer intake with a 65mm TB will be able to flow 215 cfm as well.

A cam with about 215-225 deg duration at 50 thou, with about 0.5-0.55 in of lift.

Injectors of 24-30lbs at 43 psi and a 75-80mm flow meter.

That will get you 300hp for sure.

But I can suggest an alternative path.

Ditch the GT40 heads and explorer intake. They flow OK but not much better than that.

Find a decent set of Trickflow twisted wedge heads that flow 250cfm or more with a 60cc chamber, track heat or trickflow R intake and a 220-230 deg @ 50 duration cam with 0.55-0.6 lift.

Top it off with a 75mm TB, 90mm Lightning MAF and 36-42 lb injectors

This setup will get you a solid 350hp for not much more money.

Having done both myself, the latter is a far more satisfying outcome,

Cheers

 

Also the TB is the least restrictive element on the engine. 65mm is fine for 300 hp. You only need a larger TB if its the most restrictive thing in the intake and the car is pulling a vacuum at full throttle.

The GT40 Heads and explorer intake are the choke points, both in equal measure.

A 70-75mm wont hurt though, if you happen to find a cheap Accufab unit grab it...

 

I had bookmarked this some time ago. I came across it randomly...

In short, 400 crank hp, junkyard motor, $1000 budget.

Building 400hp Into A 5.0L Small-Block Engine - Muscle Mustangs & Fast Fords Magazine