Pi Head Swap: Worth The Effort?

Discussion in 'SN95 4.6L Mustang Tech' started by bhuff30, Oct 11, 2013.


  1. bhuff30

    bhuff30 Founding Member

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    I am considering the PI head swap for my daily driver: 97 GT, 217k miles. She already has a PI intake, PI cams and some other basic bolt ons (OR H-pipe, underdrive pulleys, upper plenum / 75mm TB, drop in K&N, tuner). Only current issue on the top end is some slightly old valve seals that occasionally leave a puff of blue smoke at a long stop light. Timing chain guides were replaced at 175k, so I don't worry about the timing set too much.

    In the garage, I have a low mileage Romeo PI engine with a smoking problem. From testing, I think it's a blown head gasket, or maybe a ring pack issue. I will be tearing down the engine to part it out anyway, so I will have the heads literally in hand.

    So is it worth the effort? The gains would only be 5-10hp, even with my other mods? The time required is around 12-16 hours to do it yourself? I guess the only cost would be the FMS head change kit at around $120. Seems like too much work, so I am thinking it makes more sense to sell the heads, and use the time and money for the roadster instead, so I'll be curious your replies.
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  2. sneaky98gt

    sneaky98gt Mustang Master

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    I'll leave a longer reply later tonight when I get time.

    But short answer? Yes.
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  3. bhuff30

    bhuff30 Founding Member

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    Doing a quick search, it looks like the power gain may be closer to 15 hp. Of course, torque should be up throughout the range due to the increased static compression. I will probably hold onto the heads and look for a good chance to install them.
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  4. Three50won

    Three50won Member

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    They will probably add more than 15 hp because I think you will get more compression. PI heads on a non-PI bottom end usually make more power than a full PI engine. And since you already have them you might as well get them ported by a good company. They will make a night and day difference with a good stage 1 cam. If you upgrade the springs and go with a stage 2 cam then you will gain even more!
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  5. Three50won

    Three50won Member

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    To add, if you take your time (if it isn't your DD and you can spare downtime), then why not just build a short block. You already have the block. The GT crank is good for probably like 700 or 800 hp. Tear that block down, get it machined, add some H-Beam rods and forged pistons with a good 9.0:1 - 9.5:1 compression ration (if planning to go FI) or like 10.5:1 - 11.0:1 (if staying NA). Then get the heads ported, add some aftermarket cams, and do the engine install. You may be looking at spending about $2500-$3000 but that engine will be rock solid and a beast.
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  6. bhuff30

    bhuff30 Founding Member

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    She is a daily driver, so I don't want to put big money or effort into the project. Yeah, an aftermarket cam might give me 30hp, but at $600, I will pass. Then again, 15hp for the cost of a $120 head change kit sounds pretty good.
    My money and effort (in terms of cars), is getting wrapped up in the FFR.
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  7. sneaky98gt

    sneaky98gt Mustang Master

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    Alright, 2 days later, I finally get the time, haha.

    I think you're going to see a good bit more than 10 horsepower. I think that the better heads, bump in compression, and (I'm assuming) tune afterwards, you'll see more like 20-25 on the tires. Basic bolt-on (CAI, exhaust, tune/timing adjuster), PI head-swap cars will make 260ish on the tires (amirite? @Winters98GT ). At the same time, I remember someone on here a WHILE ago had a 100% full bolt-on, PI cam/intake setup with NPI heads that made 225 on the tires. That's 35 rwhp. So I think you're selling yourself short on how much you'll pick up.

    Granted my case is under boost, I picked up a TON of power simply by going to PI heads. My semi-old setup (blower, blower cams, NPI heads, intake, exhaust, tune) trapped a best of 112 mph in pretty decent weather. When that motor popped, I went to a full PI motor on the exact same components, and it went 118 mph in similar conditions. So even without the bump in compression, I picked up 6 mph in the 1/4 just from PI heads. That's a solid 60 rwhp.

    And on top of all this, just how much do you think you could get if you sold the heads? I know in my area, you'd be pretty lucky to get $150 for them. I see complete top ends going for $250. For the cost of a head-changing kit, I'd definitely say go for it. Also, if you wanted to save a few more bucks, I've read many times now that you can re-use the head bolts at least one extra time. So you may could pass on the head-changing kit and just get the gaskets.
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  8. Mr. Rustypwnz

    Mr. Rustypwnz Advanced Member

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    It depends how much you want to put in the engine, but if you just want to rebuild it throw some 4v rods and pistons in it. Makes 11.8-1 comp runs great off pump gas/e85(with tune).
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  9. bhuff30

    bhuff30 Founding Member

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    Sneaky, thanks for the info. A gain of 20-25hp would be great, and certainly worth the time and effort.

    Rusty, your 11.8:1 compression setup runs fine on premium? what cam are you running? I can see it working with an aftermarket cam where the cylinder pressures are a little lower, but probably not on a PI cam. Anyway, I will stick with 10.5:1 CR of the PI heads for now. If I ever need to replace the shortblock, I may pull a teksid and run it as is with flat tops.

    I started tearing down the PI engine and managed to twist off a valve cover bolt in the head. That should be fun to remove. Hopefully I can swap the heads in sometime before the end of the year. Again, daily driver, so probably around t-day or x-mas.
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  10. Mr. Rustypwnz

    Mr. Rustypwnz Advanced Member

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    My old car had stock cams. Perfectly fine on pump gas. You have to run really high timing to get one to knock. I have a race tune that will barely knock, and its like 26-27deg. at WOT. Stock timing on 96-98 cars are like 10-12 at WOT.

    Ive built 5 of the 4v rod/piston with pi head engines. Its a very fun/cheap engine. Even more fun on a 125-50 shot!
    #10

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