[DeTomaso] Ford 392 rev limit?

Thu Sep 5 14:35:13 EDT 2013

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
> > _______________________________________________
> >
> > Detomaso Forum Managed by POCA
> >
> > DeTomaso mailing list
> > DeTomaso at poca.com
> > http://poca.com/mailman/listinfo/detomaso
> >
>
>
>
> --
> 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
> _______________________________________________
>
> Detomaso Forum Managed by POCA
>
> DeTomaso mailing list
> DeTomaso at poca.com
> http://poca.com/mailman/listinfo/detomaso
>