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Discussion Starter #1
The rpm limit on the 4.6L SOHC is, I think, somewhere around 5500 rpm.

The rpm limit on the 4.6L DOHC (Mark VIII) is, I think, somewhere around 6000 - 6500 rpm.

SOHC = Iron Block (2 bolt mains)
DOHC = Aluminum Block (4 bolt main)

SOHC = Press fit powdered metal rods
DOHC = Full Floating powdered metal rods

SOHC = Coated hypereutectic, press fit pistons
DOHC = Coated hypereutectic, full floating, pistons

So why is there a difference in the maximum rpm between the two?

The short blocks seem close enough for it not to matter that much (IMHO). (The rods are the weak link and they both suck. :D)

Is it the heads, and the possibility of floating a valve in the SOHC at the higher rpm? The DOHC valves have a lot smaller distance to travel than the 2V.

Reason I’m asking is a 4V head swap onto a 2V block: what would your rpm limit be?

Opinions? :2huh:
 

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I'm not sure but it may have more to do with the rest of the drive train. I know that 2 piece drive shafts were used in T-Birds. Not sure about Mustangs. They would have a lot of trouble at higher revs. The MK VIII, Cobra etc used much stronger drive shafts and could handle the higher revs.

Just my guess.
 

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The SOHC mustang (same as ours from 96-97) has a rev limit of 6500 also. Our MN-12's are limited at 5500 because of the POS 12" converter and the restrictive exhaust (no point to revving it to 6500 when it doesn't make any power above 5K).
 

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Discussion Starter #5
Cool!

I didn't know the Mustangs had a 65K upper rpm limit.

I hadn't even though about the torque converter. duh! :leftright

Thanks for the replies. :thumbsup:
 

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fordchip has my rev limiter set at around 6750. Hard to tell exactly where it is on the stock tach but it is somwhere midway or slightly passed between the 6500 and 7000 marks.
 

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Discussion Starter #8
Ooops.... LOL :uppoint:

Didn't you hear.... the new Mustangs have a gas turbine engine in them now. Yeah, that's the ticket...
 

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Discussion Starter #9
Not Ooops to you 4 Cam.... You posted while I was typing. I was refering to my "65K rpm"....

When I do my DOHC swap, I'll probably have them set the rev limit at 6500. From what I've heard the engine is pretty much done pulling at that point. And this will still be my daily driver so I have to lean on the "safe" side.
 

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Yeah, I completely found the rev limiter by accident, no reason to rev it that high at all. It is done dealin way before that.
 

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gzj14v said:
I'm not sure but it may have more to do with the rest of the drive train. I know that 2 piece drive shafts were used in T-Birds. Not sure about Mustangs. They would have a lot of trouble at higher revs. The MK VIII, Cobra etc used much stronger drive shafts and could handle the higher revs.

Just my guess.
The driveshaft wouldn't necessarily be affected by higher revs. For example, in my Cougar, the driveshaft is turning faster at 80 mph at just over 2000 engine rpms than it is at redline in first gear just shy of 50 mph.


With regards to the rev limit, is it true that the valve springs etc. can't handle more than 5500 or so? I know the TC is a piece of crap and there is no power above 5k, but can the stock valvetrain handle that?
 

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The stock springs are supposed to be rated up to 6,500 ( this little bit of info was on the corral). Run a SOHC mustang to it's redline and you'll see. With a chip, my stock 110,00 mile 4.6 in my 96 cat shifts at 6,100 in first without a hint of float. That's one of the good things about the OHC engine. Since the valve lobe itself is what opens the valve, and is removed from the valve by RPM instead of the actual valve spring tension, the OHC motor can achieve higher revs than the OHV with the same amount of spring tension.
 

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".......on a stock 2V there is like 60# of seat pressure........."

"...... the moral of the story is, you need better springs.
The stock springs are basically junk and have a huge tolerance."


I'm not even sure if I should've pulled these quotes......I don't think anybody
would argue with the quality of the source, if ya know what I meam. ;)

Let's face it, the non PI heads and cams runout of breath at 5K rpms (more
or less), combined with all the other "non rpm" friendly parts, our oem redline
comes as no surprise.

Admittedly OHC design requires far less spring tension than OHV, but still.....
I'm amazed our 4.6L's rev as well as they do....with those tongue depressor
sized rods I'm amazed guys are pulling 6.5K+ rpms out of the engine on a
regular basis......
 

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The full floating rods are referred to as 6000 rpm rods by ford .... TheT/C in the birds and other modular engines with the 6 bolt crank(nodular) has a rev limit of about 5500 rpm before it starts balooning due centrifical force, which causes a bearing to separate, and go banging around inside, a definite "ungood Thing" in the venacular..... the cobras , MK VIII's, and 99+ Gts have a 8 bolt crank(forged), and a smaller, higher stall speed T/c which "will go over 6000 rpm before balooning starts''... therefore a higher redline is assigned to those cars... Std transmissions change the rules a bit tho'... as for" 94 daily Driven's" post, I take exception to the main bearing bolt count, as the alum. block has 6 bolt mains and the cast iron block has 4 bolt mains.... The limiting factor in all these engines rpm is principly the T/C, unless a stick tranny is involved , then its the rods and pistons, however, being a square engine(Bore & Stroke are the same), they do not lend themselves to higher rpms as well as the 302, with its 3 1/2" stroke vs 4" bore, which will rev a lot more freely....What is truley amazing is that these modular engines in their various forms have the durability to stand up under our abuse day in and day out, and even with po-po pistons and rods they hold up alot better thanthey ought to................
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'95LX 5.4L , more chassis than motor 'till the blower gets installed
 

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Now I'm about to purchase an '02 Mountaineer engine and a Mark VIII TC (figured while I have the engine out I might as well do the TC), but I'm not too familiar with the power characteristics of the newer PI engines. I will be using the Mountaineer's EEC, correct? So it will have an adjusted redline, not my standard 5250? If I am using the Cougar computer, I will have to (or should) get a chip, correct?

Where is the power peak on these newer engines?
 

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You want to stick with your electronics, and have a chip burned by Dennis Reinhart or one of the other "tuners" in that group... give him all the specifics, :engine, mods, T/c ,rear end ratio, etc.. . and code off the ECU box....they will be incorporated into your chip for a workable combination, with perhaps a higher power peak (Both Hp and rpm), but the "red line" really doesnt change all that much,as the rods and pistons are the same or nearly so, but up around 6k is a rational max, bearing in mind that these engines usually have a falling hp/tq curve at 5k without a power adder... your shift points should end up with the next gear kicking in before tork and power curves cross... the tuners know and can tailor for your type of driving... I want a lower rev limiter on my chip cause the 5.4 I have does most of its work in the 1st 1/8 mi, then peters out and tho its got 6k rods and a MKVII T/c , and has sounded sweet at 6k, it just doesnt make any sense to have a rev limiter above 56oorpm, or shift points much over 5000 rpm on my car, as its not a dragster, but a daily driver.. Trust your tuner and tell him what you want... he may recommend somethings to optimize the whole set up, and you would do well to listen to him .... Good Luck ...
PS, the Mountaineer EEC will have a compatibility prob. with your older transmission and other stuff , so save your money, use the cars box and a chip to compensate for better breathing,etc
 

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Thanks a lot! I wasn't sure where the power peak was on these engines but you answered just about everything I needed...I appreciate it.

BTW, whereabouts in FL is Homosassa?
 

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Homosassa, The town that time forgot....We are in Citrus county, 73 mi north of clearwater on U.S. 19 (The Iowa Trooper didnt understand that the reason for the 8pt. roll cage, and 4pt. harness was 'cause I drove on hwy 19!!) also about 10 mi. So. of Crystal River, another booming metropolis....LOL
 

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Discussion Starter #20
Leland Jacobson said:
as for" 94 daily Driven's" post, I take exception to the main bearing bolt count, as the alum. block has 6 bolt mains and the cast iron block has 4 bolt mains....
Agreed. I guess I should have said the "difference" was the two extra bolts on the Aluminum blocks. I knew they were 4 and 6 bolt, but since they are both cross bolted, I just didn't think to mention the cross bolts. Oops :D

But anyway thanks for everyone's input! I'll have a T-45 attached to this engine so I'll have my tuned rpm limit set to 6300, with me shifting NLT 6000 rpm. The biggest thing is that I'll have to get it dyno'd and see exactly where peak torque and hp is, and then adjust my shift points/rpm based on that (no use having extra rpm, if the power isn't there!).

Does anyone have a dyno run for a stock Mark VIII engine? I'd be real interested to see where the peak torque and hp is along with how much area is “under the curve”.

Thanks again for the input. :thumbsup:
 
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