The limits of the OHV are more so caused by the poor flow limits of the stock heads and the camshafts. To give you a bit of perspective I only know of one OHV 4.0 like ours that has made over 300 rwhp and it has a turbo on it. Most with well built engines and ported heads make around 250-275 rwhp. My truck makes 286rwhp and 391 rwtq on a 125 shot. N/A it makes 196 rwhp and 254 rwtq. The OHV engine is a Torque monster that is based off the old FE engine line, like the 390, 427 and 428. It has rail type rockers that I used to have Billet aluminum rocker arms made for but stopped after quality control and truly never selling more then 8 kits total.
One major deal with these engines is the Airflow and RPM's. They are NOT made to turn 6000rpm +. Most make there max power around 4600-5000 rpm's My engine will all of its airflow mods makes peak power at 4600 rpm's and that's with the 422 cam. Remember this mainly in things like custom turbo applications and turbo selection, you want to size it for full boost around 2500-3000 rpm's to get a full power range from it, same thing with Torque converter selection.
ON a stock bottom end the pistons are going to be a limiting factor. They are cast from the factory and weak. Granted I have cast pistons in my engine but I have alot of cylinder head work and a Comp Cams #422 cam. Most s/c 4.0's running on stock engines make around 200-225 rwhp which is good since they make around 130-145 rwhp stock or with bolt ons.
When you install a cam, the only two available for the 4.0 OHV are the 410 and 422 from Comp Cams, you really do a good thing for the overall performance from this engine. The 410 is what I would recommend for a 4x4 application but if you are looking for the most from your engine then the 422 is the way to go. Don't listen to Comp cams crap about hard idle as I've installed the 422 in stock trucks and drove away without any tuning. Sure, tuning will help in the overall picture but it is not required for either camshaft.
The cylinder heads, the 1990-94(90TM and 94TM casting numbers) have a large combustion chamber and intake and exhaust port. The 95-97(95TM) have a 10cc smaller chamber and the same intake and exhaust port as the earlier 90-94 heads. The 98-00(98TM) heads have a small chamber and a round exhaust port that in stock form is much smaller then the earlier cylinder heads. Most aftermarket cylinder heads are based on the 98TM castings but I think you can also get the 90-94 in the aftermarket castings as well.
The 98TM heads in stock form don't flow as well as the 90-97 due to the smaller exhaust port but the round exhaust port allows for better port work to be done to the exhaust port and then can be configured to outflow the earlier style heads. So in stock form I would opt for the 95-97 but if you want some to do port work to then go with the later 98TM's and ALWAYS use aftermarket castings. They are thicker and stronger then the Ford castings and they are less prone to cracking, and come with new guides.
All of the cylinder heads will bolt to any OHV 4.0 engine, 90-00. The piston dishes are where the difference in the overall combustion chamber sizes are changed. They ALL had the same compression ratio of 9:1. The later model SOHC has a 9.7:1 Compression ratio.
I would have to say that the OHV in completely stock form should be good for upto around 225-240 rwhp before you are putting so much stress on the bottom end that it gives way. Now put ported heads and camshaft in it and then you can step it up a bit more. Eventually if the pistons don't give the rod bolts will.
A few tips to building a 4.0 OHV, use a SOHC oil pan because of its built in stud girdle. You may have to use a washer under the engine mounts to clear the frame but its a good idea. Always get new pushrods and rockers for rebuilds, get them from
www.Deltacams.com. Pushrods for custom lengths can be made by SmithBros Pushrods at
www.pushrods.net.
Tuning 4.0 OHV's, I hardly recommend ANYTHING more then a 24lbs injector, even in supercharged applications. 24's will hold 250rwhp with NO problem and won't give you any idle issues like larger injectors. 30's are good to but make sure you need them. Anything more, like 42's are just stupid big for these engines and shouldn't be used. You'll never make more power from a OHV 4.0 then a 30lbs injector will support. I've made 370rwhp from 30's on 4.6 V8's all day. I use 24's on my truck but I also have a wet shot that adds the extra fuel under load so the 24's make my idle nice, well as nice as you can get anyway.
The largest throttle body made is 66mm from BBK, Edelbrock used to have BBK make them and put Edelbrock on them but they are exactly the same.
MAF selection. Now I'll have to do this in year models. For the 90-94 or EEC4 trucks with the 4.0 I recommend the C&L 73mm MAF with the appropriate sampling tube calibration for the injectors you are running. One great thing about the C&L is you can use it for stock 19's and then when you do need larger injectors you can just buy a $40 sampling tube update for the new injectors instead of having it retuned or recalibrated. DON'T EVER USE A PRO-M OR ANY OTHER CALIBRATED ELETRONIC MAF!!! They suck and throw the engines electronics for a loop.
For the later model 95-up OHV's using the larger SOHC maf which is 65mm in Diameter works great. If you just have to have something larger then get a 99-04 Mustang GT MAF which is 80mm in diameter and is all you will ever need, the 90mm that everyone loves to beat their chest about is too large for a 4.0 OHV and is like using 42's. THere again don't get calibrated MAF's for these year models but rather since these are EEC5 you can have the PCM recalibrated for the larger injectors. BUT remember, if you swap injectors then you need a dynotune or atleast a tune using a wideband to get it right, there's no way around this.
Same goes for tuning, for the 90-94 using the right MAF you really won't need much tuning, except for s/c applications then you will need a custom dynotune. You wont find anyone who will do a mail order chip for s/c applications for EEC4, and if you do then you don't want it.(Just ask Bird
The 95-00 is a great PCM system with datalogging and reprogramming abilities and can be mail order tuned n/a with stock injectors and MAF but if you change either you will need dynotuning. Same goes for S/C tuning.
A few things I can thing of right off my head, you can swap cams with the heads on but you cannot remove the lifters, just wire them up. Don't spend alot of time ported the lower intake as its not a huge restriction. The SOHC under drive pulley works on the OHV but even on the SOHC's I've tried it on it doesn't make much if any HP. The C4 transmission is best behind it as it will hold more hp then it will make, if built of course. The 700 R4 is an option as well from Advanced Adapters and will do the same but with overdrive.
The main thing to remember with the OHV, it's not made to turn 6000rpm's so with each mod you plan review the operating RPM and the gains in which the mods makes it set to make it in.
** I forgot to mention the blocks and their construction and year models. The 4.0 has a 3.952 bore standard, and as tempting as it may sound to do a 4.00 bore and use just about any standard off the shelf Forged piston ONLY the 90-94 blocks will safely hold a .048 overbore. The later 95-97 and 98-00 blocks were made from a different material and weren't as strong and could only hold a .030 overbore safely.
Also, you can't use a early model block, 90-94, in a 95-00 truck because they didn't have the casting for the motor mounts from the later model trucks in the side of the block. In other words there's no enough holes in the block for the bolts for the later model trucks.
Now, you can use a late model 4.0 in a early model truck, say 90-up. Basically the run down is 98-00 works in all year models that have the 4.0, then the 95-97 fit everything upto that point and so on. Ford added the holes as the years progressed and they didn't take any away.
Just some points I thought I'd jot down for you guys with the older OHV. Some of the things I've said here do apply to the SOHC but the SOHC will make alot more power because of the improved flowing heads and it will turn more then 6000 rpm's. And will bolt in where a OHV was...
Later Doug.
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