Pistons and compression...

[mrjody]

1999 mecury mountaineer..bone stock bottome end(crank rods and pistons)..142k miles
"about" what would the compression be running twisted wedge track heat heads..58cc I believe

 

[CDW6212R]

Do you mean like the 58cc heads I have to build a 332 next year? I'm sorry, had to throw it in there, I was glad to find them last year.

Here's a link to an online compression calculator;

Calculate Engine's Compression Ratio (CR)

speedmaster79.com

I'm not sure what the stock GT40 or P heads have for chamber size. If you swap heads, try to keep the compression ratio near stock unless you include new programming for the computer. Moving far from the stock airflow, cam, or compression can alter the A/F ratio with the stock tune. With a new PCM tune you could make use of more compression, and a custom cam could help a bunch with higher compression.

Stock is about 9.0:1 so it's rather low to start. The SOHC V6 has well over 9.5:1, and they still use regular fuel. With the right cam it's possible to run similar compression in the 302, or premium could get it to 10.2:1 or more. But without the proper cam and tuning, don't push it far away from stock.

Which heads are those, the original 170 twisted wedge track heat heads? Mine began as those, and the first buyer never used them, and somewhere along the line had TEA port them, with a 205CNC program. So they are great heads in any form, with excellent potential.

 

[CDW6212R]

With a short search it looks like several places say the GT40 heads are about 63-66cc heads, and the GT40P is around 58-61cc's. That means that your 58cc heads should not alter the compression enough to upset the PCM. The airflow is much much higher than the GT40's though, going from 195cfm to 240cfm minimum(some later on are above 260cfm, ported they can go over 300cfm), that might be noticeable to the computer. That's a lot more airflow, which is a great thing.

 

[mrjody]

Thanks. Its getting dyno tuned so really doesn't matter.
I was hoping to be around 9.6.

 

[CDW6212R]

Gotcha, then the sky's the limit. By the online specs mentioned, and don't rely on those as gospel, I'd guess the stock compression is near 9.1:1 for P heads, and the 58cc might put in around 9.2:1, using the good calculators can be more accurate. Most people don't have every dimension to fill in those calculator details, so guesses are common.

Which version of those heads do you have, and what are you aiming for later on? The shortblock is your bottleneck after the exhaust, and you know how expensive both of those are.

 

[mrjody]

58cc I believe. Ill check when I get thwm back from the porter..
If I like how the truck acts and feels with the stock short block, with the anderson n412 cam, ill pull it back apart and put a blown 331 in it..daily driver with ac

 

[CDW6212R]

That sounds like a good plan. Do check the PTV clearances, even with just .512" lift you need to be sure there's plenty of clearance. If you do build an engine you'll want the TW type pistons so you can use cams that take full advantage of the heads.

 

[mrjody]

Absolutely. Once this motor decides to get weak or whatever it decides to do..then ill pull it back apart for the good.motor. using a blower so it can still be streetable at that power level

 

[CDW6212R]

Very good. I'll let you lead the way.

My plan is to do the trans and exhaust first, then the manifolds, blower, body/paint/interior, and end with the nice 332.

 

[mrjody]

Yeah Sounds about like mine.. once I get it running and can enjoy it..see if I even like how it drives.. then alot of things are going to take place. If somw.reason I dont likw the truck..Ill go back to a foxbody

 

[CDW6212R]

It will be down a good 75hp in the way it feels versus a light Fox, my guess. These GT40 302's in a Mustang were called the Cobra, and they were fairly quick. It'll take probably a good 50-75hp added to get close to that level of pull. These act like a Mustang towing a heavy trailer.

My Mark VII feels like it's down about 40hp or more than a Mustang, and it has the identical HO engine. The extra weight and a tighter converter make it slow off the line, slower than the Explorer too.

 

[mrjody]

Yeah man. Im just a musrang guy. I love them little cars. Had several over the years. Last one was a vortech blown calypso green coupe, 91. Ice cold ac, 520 to the rear on a very safe tune..daily driver street car. Now, 520 hp is nothng in today's hp world. BUT..it was a fun car and if you dont enjoy them, why even bother building them..
It was bone stock bottom end, blower cam, trick flow top end, 42 lb injectors, methanol Injection, t5.
Loved that car.. so thats where I'm getting the ranger idea from..that mustang except it will bave a 331