[DeTomaso] Ford 392 rev limit?

Fri Sep 6 08:22:13 EDT 2013

Rich

Wow, 6200 RPM for 90 minutes.  I am impressed.  Have you refreshed the 
motor since?  Did you notice any performance change after the event?  
That is quite a testament and I am glad to learn it.

JT

On 9/5/2013 8:10 PM, 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>
> 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#>
> 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#>/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#>
> torque= 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> 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
>> 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.
>>>
>>>
>>>
>>>
>>>
>>> Thanks for any input,
>>>
>>>
>>>
>>>
>>>
>>> Jeff Cobb
>>> _______________________________________________
>>>
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>>>
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>>>
>>
>>
>> --
>> Sean Korb spkorb at spkorb.org http://www.spkorb.org
>> '65,'68 Mustangs,'68 Cougar,'78 R100/7,'60 Metro,'59 A35,'71 Pantera #1382
>> "The more you drive, the less intelligent you get" --Miller
>> "Computers are useless.  They can only give you answers." -P. Picasso
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>>
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