DeTomaso Mailing List: December 98, Message #71

Hi guys,

Here's some good food for thought.  Guy on the Nor-Cal Shelby forum came up
with a laundry-list of kick-ass parts, intending to build a high-compression
393 Windsor stroker track motor for his '92 Mustang, and asked for inputs from
the crowd.  Noted tech guru Don Wollesen came forth with the following science
of strokers post (and then privately sent me an addendum with Cleveland stuff
for you mugs.):

He writes:

>An opinion and a couple of technical comments.

First, the opinions. 

In my limited experience doing NORCAL Shelby Club open tracks since 1990 and
SCCA
racing since 1994, two things stand out to me. At open tracks, I am a
relatively fast driver. At SCCA events I am amazed at how many guys are
5 seconds a lap faster than I am with exactly the same car (Larry Oka's
Datsun 510s and Mazda RX7s) and the fact that I struggle to avoid being
the slowest driver in the class. This even at SPIR where I took
Bondurant's 4 day competition driving school with 2.5 days in 5.0
Mustangs (in 1989, 1990 being the last year Bondurant was at Sears) and
know the track fairly well. Of course I have to re-learn Sears everytime
I run there.

The other opinion (based on the above which implies that horsepower
isn't everything) is that it's really fun to go to a track and drive
with a minimum of attendant fiddling with the car inbetween. It's also a
joy when the thing lasts all weekend without any troubles. Come home,
check the pads and change the oil.

Another opinion is that there will always be SOMEONE out there with more
acceleration than me  and you ...  and you have more horsepower than I
do. Or stated differently, I naively expect that the guy who I follow
through turn 11 for 3 laps in a row despite the fact he blows me in the
weeds on the following straight ... will be gentlemanly enough to let me
by to see where I make up all that distance in the following corners.
Well, at least at open tracks maybe.

So, in summary IMHO I recommend doing a reliable motor for open tracks.
For one thing the expense ain't really worth it for a "killer motor" and
if the motor craps out too often, it really ain't worth it. Maybe in
real races when you are in the points chase for the season, but not open
tracks. For this reason, I have personally always frowned on using
blowers; too fussy on getting everything right; too easy to cook pistons
and stuff like that. They DO add horsepower, though.

You said you want horsepower and torque isn't quite as important. Of
course "Ol Shel" said, "Horsepower wins on the dyno, torque wins at the
track." Nice saying that explains the 427 Cobra nicely, but if you
notice that Porsche actually won anything then what "Ol Shel" said is
not really quite so. Horsepower IS what you want, but you want it fairly
flat over a reasonable RPM range (you can calculate your shift points
using your transmission ratios to see the RPM range of interest for YOUR
gearbox).

Airflow enables horsepower. Period. End of statement. What I mean by
that, is you get your set of heads, get 'em tweaked and flowed, and (in
theory anyway) that determines what horsepower y'alls gonna git
regardless if ya got a 289 or a stroked 351. It's just that the redline
will have to higher on the smaller engine in direct ratio of the cubic
inches difference.

So ... what determines redline anyway? (answering this will give insight
on whether to stroke it or not). Once you are not airflow limited
anymore (cuz of those big bucks ya paid fer the heads) you will be
limited by reciprocating forces on the rod bolts (good thing you
mentioned ARP here) and/or that pushrod valve train. 

For the bottom end, 7,000 RPM on a 393 stroker will DEMAND forged
pistons if you want it to stay together; it will also DEMAND racing rods
(from Carillo or whoever else), because there will be LOTS of force on
the wrist pins and connecting rod bolts when that piston goes through
TDC and you want these two components to be both light in weight AND
STRONG. e.g. the force on the rodbolts is probably around 350,000 pounds
(175K# each bolt) for a 393 at 7,000RPM (I didn't figure this out right
now because I don't know what your piston and rods will weigh, it's just
a rough estimate based on a number I got on a calculation on this with
stock parts in a 289 a couple of years ago); this is right at the limit
of premium ARP rodbolts. And ... uhhh ... oh ... ummm. by the way, this
force is a function of the SQUARE of the RPM ... so if you ever join the
"41 club" (that's where you shift from 4th to 3rd, but really get 1st),
you have just exploded that engine. So this fits with your statement
that you don't want to stroke a 351 block past 400 CI, as then the
redline will need to be decreased for the thing to hang together. If it
were me, I would stick with the standard 351 bore and stroke; it's
cheaper, more reliable (gives you more 41 club margin) and I assume that
this IS an open track car. You can get over 400 HP out of a 351 and
maybe put up with it on the street (you didn't mention about
streetability). I also recommend a viscous pulley damper if you want
this kind of RPM out of a 351 stroker, as crankshaft flexure starts
raising its ugly head with this kind of reciprocating mass and energy;
viscous pulley dampers help reduce the twist of the crankshaft on V8s at
high RPMs (I've heard claims of up to 15 degrees from front to rear, but
don't believe it because the crank would probably break before that).
But suffice it to say that V8 crankshaft twist does occur and these
help.

If you trailer it, you can get a lot more horsepower; NASCAR guys run
~8,500 RPM on 351s and get over 700HP out of 'em or so I am told. You
can get a partially used NASCAR test engine (never raced!) like Dave
Peck did if you want something strong and really loud. Steven Weaver
awhile back that said his 289/302 based 347cube stroker came in at 420HP
on the dyno. I think Dave Dwelley's Falcon 351 came in above 400HP too,
you might contact Dave and chat with him about it.

Of course, the FOX chassis notoriously understeers, and the extra weight
of a 351 just aggravates this. I've never owned a FOX, but that's what
Bondurant had when I went through his school ... man did those 5.ohs
"push" in a corner! So, weight savings on the engine (even when it's 5.0
based) and front end of the car is really helpful. Lot's of fun to drive
though, either way!

The other RPM limit is valve train. If you really want >400HP, doing
this with a 289/302 gets to be a chore because of the need to exceed
8,000 RPM redline to accomplish this. Several vital things here: Good
pushrods, good roller-tip rockers, and good valve springs. Roller
lifters weigh more (not good for really high RPM),but TechCraft or
another competent shop can tell you the right stuff for the target
redline. Also the cam requirements  for a really strong 302 would
probably keep the car off the street. But ... you can pull over 350HP at
7,000 RPM with a 302 and your car will handle better than with the 351
block. 

Valve train is the limit for the smaller displacement engines, and rod
bolts are the limit for 351 strokers (assuming the same cylinder heads).

Cam lift is a good question. I have never heard anyone give this advice,
and I dreamed it up myself. Actually, I figured it out using simple
geometry equations. Any valve lift past 0.25 of the valve head diameter
is basically a waste of time (or lift, more accurately). So, a 2" valve
needs no more than 0.50" of lift. A 1.6" valve head (like an exhaust
valve) needs no more than 0.40" of lift. Flowbench results clearly
demonstrate this when you pay attention to it. Yeah, maybe flow goes up
a bit past this 0.25/1 lift/valve head dia. ratio, but not much compared
with what it does at lesser lifts. I have seen flowbench data where the
flow actually drops off much past the 0.25 ratio point, although that's
a bit unusual. Yeah, most cams you buy have the same exhaust lift as
intake lift despite the fact on the exhaust side that it doesn't help;
it's just convenient for the machinist to do it this way. Cam duration
(all other things equal) will be affected by the rod length/stroke
ratio. More stroke (for a given deck height) results in a smaller
rod/stoke ratio and that needs a bit more cam duration ... to get the
same timing for piston position (which is what you are trying to get the
valves to open and close for). I've seen articles where a long rod
engine and a short rod engine (otherwise as identical as possible) were
both run on the dyno and the long rod engine gets more horespower with
the same cam. The assumption here is that cam timing is cam timing is
cam timing. But this ignores where the piston is and where the piston is
with a short rod isn't where it is with a long rod at the same crank
angle (like halfway between TDC and BDC for example). So the piston
position with the short rod is NOT the same as it is for a long rod and
THEREFORE with an identical cam the VALVE opening does not occur at the
same piston displacement with different rod ratios. From a piston
displacement viewpoint, a long rod engine gets more cam duration (with
the identical cam setup) than the short rod engine and therefore gets
more horespower ... in addition to lower piston drag which also helps
the longer rod ratio engines. If you get the same valve opening for
piston displacement, the short rod engine needs a bit more cam duration
to accomplish the same actual piston displacement timing. OK, I need a
blackboard to really explain this. The easier thing to do once you get
your bore, stroke determined, and head flowed, is give these numbers to
Dema Elgin to do your cam, he knows all this stuff and is not a big fan
of too much valve lift.

These are just my summary inputs on the 302/351 and stroker argument.

Way back when, I used to drag race. That acceleration IS fun. At the
drags nothing beats cubic inches other than more cubic inches. But the
track isn't the drags. Decreasing time spent on a straightaway helps,
but it doesn't guarantee the best lap time. 

BTW, the Cleveland motor has smaller main bearings, smaller rod bearings
than the Windsor. What this means is that the engine can go to somewhat
higher RPM (compared with the Windsor) because of proportionally lower
surface velocity on the main and (more importantly) the rod bearings.
Because of this bearing issue, all other things equal, the Cleveland
block will result in a bit more HP than a Windsor block with Cleveland
heads adapted to it. The flip side is the Windsor block and crank are
stronger (e.g. better for supercharging) and stiffer so are better as an
endurance engine. Used to be, of course, that Cleveland heads flowed
better than Windsor heads. But with today's aftermarket Windsor heads, I
doubt if that advantage still exists. Intake port velocity is too low
(ports are too big) on Boss and 4 barrel Cleveland heads, and these
heads demand high RPM if you want to take advantage of their flow
capabilities. It's a real chore to get the Cleveland valve train to go
to 8,500RPM  which is where it needs to be to take advantage of the
heads for a Boss 302. That would work out to 7,300RPM for a 351
with the same heads and would give similar horsepower with an easier
valve-train challenge. Cleveland heads also suffer from combustion
chamber distortion that subsequently results in cracked heads (on real
serious race motors). Knowledgable (like the NASCAR guys did when this
engine was the hot item) engine builders drill holes in the right places
on the head and put a steel dowel in the hole to address this problem.
I've got a book at home that describes this issue.

Life is complicated. That's what makes it fun!

Ennyway, that's my input for ya. Hope it helps in some small way.

Regards

Don Wollesen