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-   -   4.0 ohv compression ratio (https://www.ranger-forums.com/4-0l-ohv-sohc-v6-tech-33/4-0-ohv-compression-ratio-103231/)

dahunter_2007 03-08-2010 10:48 AM

4.0 ohv compression ratio
I plan on swapping the heads from a 95-97 4.0 onto my 94 4.0 to make it a little higher compression. I was wondering if anyone could tell me approx. what compression ratio I would have? Any input is greatly appreciated. Thanks!

My91Ranger 03-08-2010 01:12 PM

I've never heard of this before but its a neat idea. If nothing else mill the heads and check your PTV clearance.

dahunter_2007 03-09-2010 12:48 AM

yeah, the 90-94 4.0's had a smaller dish in the top of the pistons and a larger, oval shaped, combustion chamber. In 95-97 4.0's it was the opposite, larger dish in the top of the piston and a smaller, heart shaped, "fast-burn" combustion chamber. So i figured why not use my stock pistons which have the small dish and combine them with the small combustion chamber in the newer heads and raise the compression? lol I'm just curious what the compression really would be. So far I have not found anywhere on the internet that tells the volume of the piston dishes, the combustion chambers, and what the piston deck height would be, otherwise I could calculate it myself. :)

My91Ranger 03-09-2010 06:05 AM

I do like the idea, would be simply enough but you would have to measure the combustion chambers to find out exactly or you might end up with to much compression. The only other problem I could see is the piston to valve clearance.

Alex98 03-09-2010 08:29 AM

I cant tell you what you will have but usually the 4.0 has a 9:1 compression ratio and taking parts from older 4.0's and newer 4.0's and mixing them can be trouble they changed the motor from year to year very slightly, it could prolly work but i wouldnt recommend it.

dahunter_2007 03-09-2010 11:12 PM

Well I have a couple spare motors so if I ruin this one I have more can experiment with. lol Thanks yall for the advise, I really apreciate it. How would I go about measuring the valume of the piston dist and combustion chamber?

morris 03-09-2010 11:28 PM

this is something i've never given much thought towards. since our motor isn't really a HiPo motor. please tell us what you find out.

how would one measure compression?

dahunter_2007 03-09-2010 11:38 PM

Engine Compression Ratio (CR) Calculator Just fill in the blanks here. lol but I don't know the combustion chamber volume, piston dish volume,or the piston deck clearance. So Ican't do that yet lol

Downey 03-10-2010 12:19 AM


Originally Posted by Alex98 (Post 1615808)
I cant tell you what you will have but usually the 4.0 has a 9:1 compression ratio and taking parts from older 4.0's and newer 4.0's and mixing them can be trouble they changed the motor from year to year very slightly, it could prolly work but i wouldnt recommend it.

plz read below b4 passing out this info


There have been three distinctly different heads with five different casting numbers installed on the 4.0L since 1990. The combustion chamber was changed in ’95 and the exhaust ports were revised in ’98.

1990-’94 All and ‘95-’96 Aerostar
The original 90TM casting had an oval-shaped chamber that was slightly bigger in diameter on the intake side. It was replaced in ’93 by the 93TM-AA casting that was exactly the same. These heads can be identified by the letter "T" located on the top of the right rear/left front exhaust port.

1995-’97 Ranger, Explorer and ’97 Aerostar
The 95TM-AD casting that came out in ’95 had the heart-shaped, fast-burn chambers that shrouded the intake valves. It was replaced by the 97TM casting in ’97, but it was the same, so they can be used interchangeably. These castings were used on the Ranger and Explorer from ’95 through ’97 and on the Aerostar in ’97. They can be identified by the letter "U" that’s located on the top of the right rear/left front exhaust port.

Ford introduced another new head in ’98. The exhaust ports on the 98TM-AD were much narrower than they were on the earlier castings; they measured 1.40˝ across the port compared to 1.70˝ on the 95TM/97TM castings. According to the engineers I have talked to, the smaller ports increased the velocity of the exhaust gasses so they carried more heat down to the catalytic converter. This helped the converter “light off" sooner, so it did a better job of reducing emissions during the critical start-up and driveaway phase of the EPA emissions test. With that in mind, it’s probably not a good idea to swap these heads back and forth with any of the earlier castings.

All of these heads have the letter "A" cast into the head right above the right rear/left front exhaust port.

That’s the story on all the castings and the major components. Most of the differences are specific to given years or applications or both, so they can’t be consolidated or interchanged. The only exceptions are the rods that can be used in matched sets, no matter when they were made, the 90TM cranks with the notches that can be interchanged with the 90TM/96TM cranks with the long keyway as long as the matching crank gear is used, and the various cam consolidations that are open for discussion.

With all this in mind, there are some other things that rebuilders should be aware of when working on these engines:

Rebuilders should not install an engine with the 95TM heads and deep-dish pistons in a ’95 or ’96 Aerostar. All of these engines had the same compression ratio whether they came with the original heads with the open chambers or the newer ones with the heart-shaped chambers, so they would seem to be interchangeable, but the computer calibration that was used for the old-style heads with the open chambers will not work with the newer heads with the fast-burn chambers. In fact, "It will burn the engine down in a few thousand miles," according to a Ford engineer who worked on this engine program. Ford continued to use the early heads on the ’95 and ‘96 Aerostar because they still came with the early calibration, so rebuilders must do the same.

Many of the heads that were produced for the 4.0L in ’89 and ’90 tended to crack around the spring pads on the intake side and had porosity problems down in the corners where the oil drained back into the valley. The cracks on the top can be found by Magnafluxing the heads, but the porosity problems won’t show up unless the head is pressure tested at about 70 psi. Some of the later heads have cracks around the oil drainback holes and quite a few are cracked in the chambers, so they should all be carefully inspected before being rebuilt. The threaded plugs on the top tend to leak, too, so they should be removed and reinstalled with sealer on the threads. Ford says they should be torqued to 80 ft.lbs after resealing them.

Rebuilders should also be aware that there are new aftermarket heads available from several different sources. Some shops have found that it’s cheaper to buy new ones than try to repair the used ones that have a lot of cracks.

All of these engines had four studded mains that were used to hold the windage tray in place under the crank. We recommend installing them in every engine in specific locations according to the diagram so the installer doesn’t end up moving them around and disturbing the torque on the main bolts in the process.

The two steel balls in the lifter valley that are used to plug the oil galleries must be removed in order to clean the block properly. They are almost impossible to remove from the top, but they can be driven out from the bottom by coming up through the oil holes in the mains.

The early crank gear has caused problems for many rebuilders. The crank gear was timed to the crankshaft by the small tab on the back of the gear that fit into the shallow notch that was machined in the step at the back of the snout. There is nothing to hold the gear in place against this step until the damper is installed, so it wants to slide forward and jump out of time during assembly, test and installation. Most rebuilders knurl the shaft or stake the gear in place to try to prevent the gear from moving, but that doesn’t always work, especially if the installer isn’t aware of the possible problem. Be sure to warn your assemblers and your customers so you don’t have to pay somebody to take it apart and do it over again.

Ford solved this timing problem in model year ’96 when they machined the crank and the gear for a full-length Woodruff key that indexed the gear on the crank and held it securely in place. Ford offers this revised gear (p/n F5TZ-6306-A), but it’s readily available in the aftermarket for a lot less money. The key can be a little hard to find because it’s a metric size; it’s available from Ford as well (p/n W702979S300).

The ‘90-’96 crankshafts can be interchanged as long as the matching crank gear is used, but always remember to stake the early gear in place in one way or another whenever using a 90TM crank with the short keyway and the notch in the step.

The roller lifters are held perpendicular to the cam by a steel pin in the side of the lifter that slides up and down in the groove that’s machined in the side of the lifter bore. Having this pin in the side of the lifter required a special design that had an inner and an outer body, so there’s some space in between them that can trap a lot of debris. That makes it very difficult to get them clean, so they’re hard to rebuild, and that’s a real problem, because new lifters are very expensive.

The rear seal on the 4.0L has always been prone to leakage. It appears that the original factory finish on the seal surface was too slick, so the seal just couldn’t control the oil. Ford offers a repair kit that comes with a sleeve and a seal (p/n F5TZ-6701-A), but it costs about $50 at the dealership. Rebuilders should definitely install a sleeve over the factory seal surface, but there are better alternatives in the aftermarket; Micro Sleeve (800-475-3383) and Classic (800-393-0544) both offer sleeves for the 4.0L cranks.

The pushrods and rockers on the 4.0L tend to show a lot of wear, even on engines with low miles. We suspect that there are two reasons for this problem: (1) The material in the rockers appears to be too soft, so they wear on the tip, and; (2) there’s not enough oil getting to the pushrod socket, so it gets worn out, too.

There’s pressurized oil at the rocker shaft to lubricate the rockers, but there’s no direct way for that oil to get to the pushrod sockets. There’s a passage in each rocker that allows oil from around the rocker shaft to migrate out to the small hole in the top of the pushrod socket, but it’s wide open on the outer end so there is no pressure there to feed the oil down into the socket.

Apparently the small hole in the pushrod socket that intersects this passage is supposed to meter oil down into the socket somehow, in spite of all the motion that’s trying to sling the oil out from the open end of the passage in the rocker. It’s hard to believe that any oil can actually get into the sockets, so it’s no wonder they wear out prematurely. Rebuilders should expect to rebuild or replace most of the rockers. The tips can be ground if they’re not worn too badly and the sockets can be repaired by installing a special insert that’s available along with the tooling needed from Silver Seal (800-521-2936) or Goodson (800-533-8010). Or, if you prefer to have someone do them for you, rebuilt rockers are available from Delta Camshaft at 253-383-4152.

You may want to consider flame-hardening the tips, too. Ed Davis at Waterhouse Motors in Tacoma, WA, has been doing this for awhile to eliminate wear on the tips. He found that the wear on the tips was loading the valves so hard to one side that they were wearing out the guides and causing other problems in the process.

Plan on buying a lot of new pushrods, too. Most of them are scuffed and worn on the tip due to the lack of oil in the socket.

That’s the story on the 4.0L. It’s pretty straightforward if you keep the castings straight and pay attention to the details. This has been a common engine in some of Ford’s most popular vehicles including the Ranger, Explorer and Aerostar, so there are over 3 million of them on the road and plenty of business waiting for the shops who know how to rebuild them and make them live.

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