[DeTomaso] Ford 392 rev limit?
boyd411 at gmail.com
Thu Sep 5 23:34:02 EDT 2013
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.
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
> 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.
> -----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
> 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 *
> 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
> *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
> = (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) 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-**
> 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
> 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.
> 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
> 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.
> 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
> > 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 >
> > while I was tuning up a 57 Ford Fairlane Skyliner with a Police
> > 392 engine.
> > Just bought by a faithful customer and he wants me to do what I want to
> > to make it better and faster.
> > My question to you guys is about the safe rev limit. It has a 351 taken
> > 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
> > quickly in the first three gears. Engine is very smooth at high rpm's >
> > 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
> > 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
> > 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 >
> > VP 110 leaded race gas which will also help.
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