[DeTomaso] Ford 392 rev limit?

Daniel C Jones daniel.c.jones2 at gmail.com
Fri Sep 6 13:36:01 EDT 2013


> Set the vehicle up to keep the engine at peak torque rpm on the street.

Why would you want to do that?

Dan Jones

On Fri, Sep 6, 2013 at 9:44 AM, Doug Scott <doug at pickbbs.com> wrote:
> There is a big difference between a drag racer taking his purpose built
> 428FE to 8 or 9k at the track and taking a crate engine that was built for
> 6k to a higher rpm on the street.  Piston speed is not the issue anyways, it
> is the piston having to stop to change direction twice each revolution that
> creates the broken parts.
> Set the vehicle up to keep the engine at peak torque rpm on the street.  If
> the car currently can't hook up in the first three gears, you don't need
> more rpm, you need more traction.
>
> doug
>
> -----Original Message-----
> From: detomaso-bounces at poca.com [mailto:detomaso-bounces at poca.com] On Behalf
> Of Will Kooiman
> Sent: Friday, September 6, 2013 7:49 AM
> To: Boyd Casey; Rich
> Cc: detomaso at poca.com; Jeff Cobb
> Subject: Re: [DeTomaso] Ford 392 rev limit?
>
> 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> 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 ; 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.wik
>>>ipe
>>>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/adv
>>>anc 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/adv
>>>anc 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/adv
>>>anc ed-engineering-tech/1037077-basic-math-formulas.html#>
>>> >
>>> torque= flywheel
>>><http://www.ebay.com/sch/i.**html?_nkw=flywheel<http://www.ebay.com/sc
>>>h/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/sc
>>>h/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> 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.b
>>>aco matic.org/~dw/engine/dehaven/dehaven.htm>
>>> On Wed, Sep 4, 2013 at 10:40 PM, <MikeLDrew at aol.com> 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>
>>> > Subject: Re: 351 rev limit
>>> > Date: August 29, 2013 1:54:13 PM CDT
>>> > To: "detomaso at poca.com List" <detomaso at poca.com>
>>> > Cc: Jeff Cobb <zumzum at cox.net>
>>> >
>>> > 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 this strong engine will instantly bang off the
>>> > limiter
>>> too
>>> > quickly in the first three gears. Engine is very smooth at high
>>> > rpm's
>>>>
>>> and
>>> > feels as if it would rocket well past past 6,100 with no problem.
>>> >
>>> >
>>> >
>>> >
>>> >
>>> > I do not know cam, rear end ratio, compression or any other specs
>>> > and
>>>I
>>> > will try to find out today. Torque peak feels the  strongest about
>>>3,000
>>> to
>>> > 4,100 with 6,1000 coming in too soon.
>>> >
>>> >
>>> >
>>> >
>>> >
>>> > What rpm limiter chip do you all recommend for what I have described?
>>> >
>>> >
>>> > I have little history with higher rpm ceilings of large FORD engines.
>>> >
>>> >
>>> > Owner is a 64 year old just math professor hot rodder who drives
>>>little
>>> and
>>> > want to have fun before his last personal rev limit.
>>> >
>>> >
>>> > We don't desire to scatter his engine and he might buy and use some
>>>of >
>>> my
>>> > VP 110 leaded race gas which will also help.
>>> >
>>> >
>>>
>>>
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