[DeTomaso] Ford 392 rev limit?
Jeff Cobb
jeffcobb1 at me.com
Fri Sep 6 09:39:01 EDT 2013
I would like to thank everyone for their input about the rev limiter cutoff question.
The customers other two cars are a Viper and a two year Porsche, so he is used
to higher rpms and would enjoy if he could spin higher on this Superformance Cobra.
Even though this engine spins free to 6,100, I think and hear from all since it is a
stroker that 6,100 to 6,300 would be a fine limit.
Other info came in about the engine last night which may confirm this belief;
M-6010-A451 Sportsman block
Gt40 aluminum heads @ 9.7:1
a cast stroker crank
4.030" x 3.850" b+s
430hp 450 trq
cam lifts of Int .566" Exh .576" and duration is 232 int and 240 exh.@ .500 lift.
I think I will put in a 6,300 and tell him to have fun and watch it.
So again, I thank all for the help.
Jeff Cobb
On Sep 6, 2013, at 7:25 AM, Boyd Casey wrote:
> Will, your 4.6 cobra is an entire different ball game it has DOHC, not push
> rods ( as you know ) so I don't think the comparison is valid. SInce this
> Windsor stroker is supposedly only putting out 390 hp on the dyno I am
> suspect about the quality of the build.
> Since rich stated that the car reached the current 6200 rev limit to
> quickly in 1st, 2nd and 3rd maybe his rear is geared to low ( he did say he
> didn't know what kind of rear was in the car. He also said the customer
> wanted the car to be faster, does he mean quicker ( like 0-60 ,or 1/4 mile
> or does he mean faster like higher top speed) with the high torque of most
> strokers and the average rpm that peak hp and torque are generated at ( on
> the dynos I have seen for stroked windsor's on sale) I don't think pushing
> the motor to RPM's over 6000 will accomplish his stated goal.
> But who knows? I'm glad that I have a Cleveland!
> Boyd
>
> On Friday, September 6, 2013, Will Kooiman wrote:
>
>> I am not a professional racer or engine builder, but I have blown up a few
>> engines.
>>
>> Here are some data points:
>>
>> Assuming the valve train is a new design, with modern 1 piece stainless
>> race valves, the valve train will probably not grenade the engine. Valve
>> float will occur depending on the RPM, valve train weight, springs, and
>> camshaft ramp speeds. But valve train float doesn't cause engines to blow
>> up. Failure of components do - like the keeper in my Viper that split, or
>> the one-piece street valves in my Pantera that lost a head (at about 5,000
>> RPM), or the multiple occurrences of two piece valves failing.
>>
>> I have spun *many* rod bearings. It has always been well over 6,000 RPM
>> in a 302W with a high volume oil pump and a stock pan. I was too stupid
>> at the time to realize the oil pump was sucking the pan dry. It never
>> blew up, but I still had to pull it apart to replace parts.
>>
>> My brother spun a bearing in a 340 Dodge Dart at well over 6,000 RPM with
>> a high volume pump and a stock pan (trying to catch a 302W that had a much
>> higher top gear).
>>
>> There are 428FE drag racers that hit 8,000 or 9,000 RPMs with a 3.98
>> stoke. So, I don't buy the piston speed limit of 6,000 RPM with a 4"
>> stroke, especially with good components - h-beam rods, forged pistons,
>> high quality valve train, etc.
>>
>> My 4.6 DOHC Cobra would easily rev to 7,000 RPM. I bet it would hit 9,000
>> RPM, if the rev limiter were removed. The stroke was something like 3.4".
>>
>> We all know a stock block 351C will not handle high revs.
>>
>> We have all heard about cranks cracking.
>>
>> So, if we are talking about an engine with a race block, proper valve
>> train, h-beam rods, forged pistons, and everything "right", 7,000 or maybe
>> 7,500 would be safe. Just because some hit 9,000, that doesn't mean it is
>> safe. That just means that some people do it without blowing up.
>>
>> But in this case, you don't know anything about what's inside. If you say
>> it is safe to hit 7,000 RPM, and it blows, you're going to look pretty
>> stupid.
>>
>> THAT's why the limit should be stated as 6,000.
>>
>>
>> --
>> Will
>>
>>
>>
>>
>>
>> On 9/5/13 11:34 PM, "Boyd Casey" <boyd411 at gmail.com <javascript:;>> wrote:
>>
>>> Rich,
>>> That's the cool thing about owning and driving your own car , if you throw
>>> a rod and have a hole the size of a grapefruit in your block you get to
>>> take all the credit for it. There's nothing wrong with running your engine
>>> in the 6k and 7 k rpm range if it was built for those rpm ranges.
>>> According
>>> to Ford a stock Windsor 351 has a 5500 rpm redline. If you stroke a stock
>>> Windsor and the only thing you change is the crank and connecting rods
>>> your
>>> not going to have an engine that can run between 6500 and 7000 rpm for any
>>> length of time . Mad dawg runs his 377 ci stroked cleveland at 7000 rpm
>>> for
>>> the entire Silver state and has only caught on fire once! But he has a
>>> professionally built race engine. Driving an engine at the limits that
>>> it's
>>> designers have built it for is not babying it, in fact trying to run a
>>> motor at RPM's 1000 to 1500 above what the engineers at ford recommend (
>>> as
>>> in a stock Windsor) is childish . It's like using a hammer to drive a
>>> screw
>>> it might work but it's not going to work consistently and sooner then
>>> later
>>> something is going to break. Your engine was obviously built to a higher
>>> standard then a stock Windsor . A boss 302 could rev to 8000 rpm a Honda
>>> 2000s has a 9000 rpm redline, a formula 1 engine revs to 18000 rpm. The
>>> point is all these engines were built and engineered to operate and
>>> withstand the stresses associated with operating at their respective
>>> piston
>>> speeds since the only thing we know about the engine in question is that
>>> it
>>> has been stroked to 393 ci and without knowing what kind of crank,
>>> connecting rods and if it ha s solid lifters one has to assume that
>>> operating it at RPM's above 6000 would not be prudent . Since the person
>>> who wrote. The first post was asking for advice telling him " not to baby
>>> it" would be irresponsible , unless your going to pay for a rebuild when
>>> it blows up because he followed your advice.
>>> Boyd
>>>
>>> On Thursday, September 5, 2013, Rich wrote:
>>>
>>>> I am not one to baby an engine -- it was built to run and have fun
>>>> My 351c was basically a boss -- I would rev 6500 and it saw 7000- never
>>>> failed.
>>>> My 383 stoked cleveland I ran silverstate at 6200 rpms for most of 90
>>>> miles-- I had a 6800 rpm chip in it and would bump it at times.
>>>> My 358 windsor with a lot of good stuff in it-- I bump the 7800 rpm chip
>>>> in it.
>>>>
>>>> Why build it and not run it. It should run 6500 no problem unless it
>>>> was
>>>> not built right.
>>>>
>>>> Rich
>>>>
>>>>
>>>> -----Original Message----- From: Boyd Casey
>>>> Sent: Thursday, September 05, 2013 11:35 AM
>>>> To: Sean Korb
>>>> Cc: detomaso at poca.com <javascript:;> ; Jeff Cobb
>>>> Subject: Re: [DeTomaso] Ford 392 rev limit?
>>>>
>>>> I am not an expert but the one thing that I do know concerning rpm
>>>> limit is
>>>> piston speed.
>>>> As an example a Formula 1 engine with an 18000 rpm and a bore of
>>>> 98mm
>>>> and a stroke of 39.7mm has a
>>>> Piston speed at 18000rpm of 4689 fps. This is an engine where every
>>>> single
>>>> component is built at the ultimate technological level available. So
>>>> calculate the piston speed of your stroker motor and remember that your
>>>> build is not to the same technological level as a formula one motor.
>>>> See this ink for the formula for piston speed and the relative piston
>>>> speeds for different kinds of engines.
>>>> Wikipedia piston speed article and
>>>>
>>>> formula<http://en.wikipedia.**org/wiki/Mean_piston_speed<
>> http://en.wikipe
>>>> dia.org/wiki/Mean_piston_speed>
>>>>>
>>>> Remember that stoker motors are generally built to increase HP and
>>>> *Torque*
>>>> at the expense of RPM. Most of your high reeving engines have very short
>>>> stroke. When you stroke an engine you increase the distance the pistons
>>>> travel and as a result for each RPM you have increased the piston speed.
>>>> You can't expect to run a stroker motor at the same RPM as the engine
>>>> used
>>>> to run before it was stroked unless you have made substantial
>>>> improvements
>>>> to the strength of the rotating assembly , valve train, and every other
>>>> component that is going to be effected by the higher stresses of the
>>>> increased piston speed.
>>>> Here are some of the more common internal combustion engine formulas
>>>> courtesy of LS1 Tech:
>>>> *[i][b]formulas for bore, displacement, stroke *
>>>> pi=3.1415927
>>>> pi/4=.7853982
>>>> cylinder volume= pi/4 x bore squared x stroke
>>>> stroke= displacement / (pi/4 x bore squared x no. of cylinders)
>>>> bore= square root of above formula
>>>> displacement(in cc's)= (pi/4 x bore squared x stroke x no.of
>>>> cylinders)/1000
>>>>
>>>> *formulas for compression ratio*
>>>> cylinder volume= pi/4 x bore squared x stroke
>>>> chamber volume= cylinder volume/(compression ratio - 1.0)
>>>> compression ratio= (cylinder+chamber volume)/chamber volume
>>>> displacement ratio = cylinder volume/chamber volume
>>>> amount to mill= (new disp. ratio - old disp. ratio)/(new disp. ratio x
>>>> old
>>>> disp. ratio) x stroke
>>>>
>>>> *formulas for piston speed*
>>>> piston speed in fpm= stroke in inches x rpm/6
>>>> rpm= piston speed in fpm x 6/stroke in inches
>>>>
>>>> *formulas for brake horsepower and torque*
>>>> horsepower= (rpm x torque)/5252
>>>> torque= (5252 x horsepower)/rpm
>>>> brake specific fuel consumption(bsfc)=fuel pounds per hour/brake
>>>> horsepower
>>>> bhp loss= elevation in feet/100 x .03 x bhp at sea level
>>>>
>>>> *formulas for indicated horsepower and torque:*
>>>> horsepower= (mep x displacement x rpm)/792000
>>>> torque= (mep x displacement)/150.8
>>>> mep= (hp x 792000)/displacement x rpm
>>>> mechanical efficiency= (brake output/indicated output) x 100
>>>> friction output= indicated output -brake output
>>>> taxable<http://ls1tech.com/**forums/advanced-engineering-**
>>>>
>>>> tech/1037077-basic-math-**formulas.html#<
>> http://ls1tech.com/forums/advanc
>>>> ed-engineering-tech/1037077-basic-math-formulas.html#>
>>>>>
>>>> hp
>>>> = (bore squared x cylinders)/2.5
>>>>
>>>> *air capacity and volumetric efficiency:*
>>>> theoretical cfm=(rpm x displacement) /3456
>>>> volumetric efficiency= (actual cfm/theoretical cfm) x 100
>>>> street carb cfm= (rpm x displacement)/3456 x .85
>>>> racing carb cfm= (rpm x displacement)/3456 x 1.1
>>>>
>>>> *formulas for weight distribution*
>>>> percent of weight on wheels = (weight on wheels/overall weight) x 100
>>>> increased weight on wheels=<(distance of cg from wheels/wheelbase) x
>>>> weight)> + weight
>>>>
>>>> *formulas for center of gravity*
>>>> cg location behind front wheels = (rear wheel
>>>> weight<http://ls1tech.com/**forums/advanced-engineering-**
>>>>
>>>> tech/1037077-basic-math-**formulas.html#<
>> http://ls1tech.com/forums/advanc
>>>> ed-engineering-tech/1037077-basic-math-formulas.html#>
>>>>> /overall
>>>> weight) x wheelbase
>>>> cg location off-center to heavy side= (track/2) - (weight on light
>>>> side/overall weight) x track
>>>> cg height= (level wheelbase x raised wheelbase x added weight on
>>>> scales)/(distance raised x overall weight)
>>>>
>>>> *formulas for G force and weight transfer*
>>>> drive wheel<http://ls1tech.com/**forums/advanced-engineering-**
>>>>
>>>> tech/1037077-basic-math-**formulas.html#<
>> http://ls1tech.com/forums/advanc
>>>> ed-engineering-tech/1037077-basic-math-formulas.html#>
>>>>>
>>>> torque= flywheel
>>>> <http://www.ebay.com/sch/i.**html?_nkw=flywheel<
>> http://www.ebay.com/sch/i
>>>> .html?_nkw=flywheel>>
>>>> x first
>>>> gear x final drive x 0.85
>>>> wheel thrust= drive wheel torque/rolling radius
>>>> "g"=wheel thrust/weight
>>>> weight transfer= <(weight x cg height)/wheelbase> x g
>>>> lateral acceleration= 1.227 radius/time squared
>>>> lateral weight transfer= <(weight x cg height)/wheel track> x g
>>>> centrifugal force= weight x g
>>>>
>>>> *formulas for shift points*
>>>> rpm after shift= (ratio shift into/ratio sift from) x rpm before shift
>>>> driveshaft torque= flywheel
>>>>
>>>> <http://www.ebay.com/sch/i.**html?_nkw=flywheel<
>> http://www.ebay.com/sch/i
>>>> .html?_nkw=flywheel>>
>>>> torque
>>>> x transmission ratio
>>>>
>>>> So as a more direct answer to your question ( without knowing your exact
>>>> bore and stroke I would estimate that at 6100 rpm you are already
>>>> pushing
>>>> the limits of what I would consider "prudent" ( I would think that 5500
>>>> rpm
>>>> would be a safer limit)
>>>> There is one other way to find out ,but it involves the risk of pushing
>>>> it
>>>> until it throws a rod or something and then you now you have exceeded
>>>> the
>>>> safe RPM limit for your build.
>>>>
>>>> Boyd
>>>>
>>>>
>>>>
>>>> On Thu, Sep 5, 2013 at 1:35 PM, Sean Korb <spkorb at gmail.com<javascript:;>>
>> wrote:
>>>>
>>>> I would stick with the 6100 rev limit. Actually I might go down a bit
>>>> too. I think after checking the car on the dyno you should be able to
>>>> find
>>>> your maximum power and where it is. That's where you should consider
>>>> shifting to the next gear anyway.
>>>>
>>>> Without knowing how the valve train is configured, there's no reason to
>>>> think it can withstand going over 5500RPM for an extended period. The
>>>> camshaft, weight of the valves, spring pressure, valve train stability
>>>> gadgets (roller rockers, guide plates etc) all contribute.
>>>>
>>>> I'm a little sensitive to that since I had an early experience with a
>>>> 351C
>>>> in my Mustang. Everyone told me I should remanufacture the heads with 1
>>>> piece valves, but I thought all I had to do was put rollers into the
>>>> rocker
>>>> fulcrums and I could do 6100RPM (just like your rev limiter). Big
>>>> mistake. My car swallowed one of those ancient 2 piece intake valves
>>>> and
>>>> the cylinder wall had an argument with the piston.
>>>>
>>>> It's best to find someone who has already destroyed a few motors, take
>>>> their setup and go a tick under what they were doing. You *will* break
>>>> parts if your motor doesn't have some thought put into spring pressures
>>>> and
>>>> valvetrain girdles. We only have 2 valves per cylinder and they're
>>>> *heavy*.
>>>>
>>>> That said I have a 375W in my Cougar with roller rockers and a flat
>>>> tappet
>>>> cam that has seem 7000RPM (an accident) with more to go but I feel a
>>>> lot safer down at 6100RPM. On the dyno I start to plateau at 4800RPM
>>>> and
>>>> it falls off at 5500RPM so I shift well before than anyway. I've tuned
>>>> it
>>>> more since than so I need to go back to find my new shift points.
>>>>
>>>> http://www.bacomatic.org/~dw/**engine/dehaven/dehaven.htm<
>> http://www.baco
>>>> matic.org/~dw/engine/dehaven/dehaven.htm>
>>>> On Wed, Sep 4, 2013 at 10:40 PM, <MikeLDrew at aol.com <javascript:;>>
>> wrote:
>>>>
>>>>> Hi guys,
>>>>>
>>>>> During the dark days when the forum was down, Jeff Cobb posted a query
>>>> and
>>>>> it was rejected, so he asked me to post it for him (below). There
>>>> are
>>>>> actually two posts--start at the bottom one and work your way up.
>>>>>
>>>>> Mike
>>>>>
>>>>> ====
>>>>>
>>>>> From: JEFFREY COBB <zumzum at cox.net <javascript:;>>
>>>>> Subject: Re: 351 rev limit
>>>>> Date: August 29, 2013 1:54:13 PM CDT
>>>>> To: "detomaso at poca.com <javascript:;> List" <detomaso at poca.com<javascript:;>
>>>
>>>>> Cc: Jeff Cobb <zumzum at cox.net <javascript:;>>
>>>>>
>>>>> Hello again about the rev limit question,
>>>>> The engine specs are:
>>>>> Ford Racing 392 Windsor stroked,
>>>>> 360 hp at rear wheels from the dyno sheet
>>>>> GT40 Heads--9.7:1,
>>>>> Mighty Demon 750
>>>>> Ford Racing steel flywheel
>>>>> Tremac TKO 3550 5 speed and 3.73,
>>>>>
>>>>> Any help would be appreciated,
>>>>> Thanks,
>>>>> Jeff
>>>>> ______________________________**______________________________**
>>>> _____________
>>>>> On Aug 29, 2013, at 6:13 AM, Jeff Cobb wrote:
>>>>>
>>>>>
>>>>> Hello group,
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Hope you all can provide info.
>>>>>
>>>>>
>>>>> A 2000 Superformance Cobra came back in yesterday for some finish up >
>>>> work
>>>>> while I was tuning up a 57 Ford Fairlane Skyliner with a Police
>>>> Interceptor
>>>>> 392 engine.
>>>>>
>>>>>
>>>>> Just bought by a faithful customer and he wants me to do what I want
>>>> to
>>>> do
>>>>> to make it better and faster.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> My question to you guys is about the safe rev limit. It has a 351
>>>> taken
>>>> out
>>>>> to a stated 392, the largest Demon carb I've ever seen and a MSD 6AL
>>>> with a
>>>>> 6100 rpm limit chip.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Problem is that>_______________________________________________
>>>
>>
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