miked6187

1)What is different from the pi 99-04 block that allows that engine to have the pi heads but when they are put an a 96-98 engine it raises the compression?

2)How much gain should I expect from just doing cams and intake with ported and polished npi heads vs swapping in pi heads too?

3)If I do change the heads I have been told that the compression will rise which means I will have to run premium gas which I want to avoid because where I am premium is 50 cents more expensive per gal. Is there any way to change the compression ratio back to what it is from the factory?

MaxxRPM

Welcome to the forums Miked.

I'll give it a shot;
1). It's not the block that's different but rather the pistons. The PIs have a slightly deeper dish piston compared the NPIs.
2). The PI heads have slightly bigger valves and bigger ports so they flow a little better. The "best" reason I switched to PI heads is to run the PI intake manifold without it leaking coolant.
3). Not really. It's not that big of a jump in compression. It has more to do with your tune than anything else. You should be able to run your stock tune (but would benefit more with a good tune).

poxenham

The compression bump is really a great thing. More compression = more power. Not so fun if you're planning on going boosted.

You can run your stock tune completely fine, but your car will get an even greater hp gain when you couple the PI swap with a tune. The MAF on the car meters the air so the engine will adjust for the added airflow. Premium fuel is a must.

Soldier GT

96-98 stangs a PI swap is a must.

MaxxRPM

I never got to run my car after the PI swap with the stock tune (stage 1 cams), but running with the Bama/SCT tuner, their 93 octo tunes (Bama or SCT) would give me detonation (pinging). I'm currently running SCT's 87 octo tune and it runs great. For my motor atleast, the 93 octo tunes pulled too much timing.

lizzyfan

If you are going to do a PI swap, a dyno tune is a must!

HeilmanMotorsports

I did a pi swap on my 98. I bought a complete 99 pi motor and used my coil packs. The dyno shop told me that any modular computer will run any modular engine. So I did not get tune.
The npi block has dished pistons and higher cc combustion chambers in the npi heads.
The pi block has flat pistons and also smaller combustion chambers that have a little different design to help improve flow. The cams are also different improved for more flow.
Also the pi intake is deeper it goes down into the block further than the npi intake.
the engine blocks are physically the same. Different pistons heads cams and intake.
Npi engines are 215hp and pi engines are 265hp.
I think if doing just the head intake and cams swap I would expect a little more of a gain since raising the compression ratio...I think it would be close to 10:1. If I remember correctly.

swarthyfellow

I believe the compression goes from 9.5:1 to 10.5ish:1, I run 89 in my car with tune I did myself and it runs great. Put I was able to get a much better tune running 93. I believe the math calls for 91 octane for that C/R. If you want to get a little creative, you can us a rotary sander to carefully gain back the difference in volume in the combustion chamber, or swap pistons. Personally, I'd not take the time to do that and run 89 its usually only 10-15 cents more. I would also highly recommend doing cams/spring/seals at the same time as the swap. I regret it every day I drive.

HeilmanMotorsports

Yeah +1 on springs and seals. For sure.

dawson1112

I never messed around with the 4.6 engines much, the only one I ever owned was a 2000 and it was already PI. But I would think that you could use a thicker set of head gaskets to gain back some of the combustion chamber and keep compression a little lower.