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Stroker Tips from the Pros

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Engine Builder’s Stroker Motor Resource Guide has become an annual
feature. Last year this publication ran an introductory story about
increasing your business with the growing demand for stroker engines.
It was focused on popular O.E. and aftermarket-based engine
combinations, and covered the basic dos and don’ts of building a
stroker motor.

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We are fully aware that the “bread and butter” stroker business in your
area may be the 383 cid small-block Chevys and the 347 cid small-block
Fords, but the growing trend is for really big stroker engines like the
550-630 cid Chevys and the 420-450 cid small-blocks. Getting there with
O.E.-based components is just not practical in most cases, and simply
impossible in others.

In this installment on strokers we will take the bigger perspective
consulting several pro engine builders who build 600-900 cid engines
for a living. Much of what they have to say will apply to building any
stroker engine, regardless of make or size.

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We have a stellar lineup of professional race engine builders
including Sonny Leonard of Sonny’s Racing Engines, well known for
building 700-900 cid IHRA Mountain Motor Pro Stock and Pro Mod engines;
Jon Kaase of Jon Kaase Racing Engines, also known for his prowess in
the IHRA Pro Stock wars and winner of the 2003 and 2004 JEG’s Engine
Masters Challenge; and Scott Shafiroff of Shafiroff Race Engines who
probably builds and sells more stroker engines and stroker kits than
anyone else in the country. Scott also fields Vinny Budano’s NMCA
three-time championship winning race team out of his shop. And finally,
Tony Bischoff of Bischoff Engine Service (BES) who won the 2006 and
2007 JEG’s Engine Master Challenge and was the ’07 NMRA Diablosport Pro
5.0 champion in his own racecar. Besides all of his racing
accomplishments it is interesting to note that BES started out as a
small engine rebuilder like many of our readers and has made the
transition to focus on high performance and racing almost exclusively.

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Aftermarket Blocks

A key bit of advice provided by the experts in last year’s Stroker
Guide was to start with a good aftermarket block because they solve a
lot of problems encountered with building the larger displacement
motors. First, there is more room for the rotating assembly. Many
aftermarket blocks have wider pan rails, eliminating a bunch of
grinding for rod and counterweight clearance for medium stroke
applications.

You can purchase several blocks with raised cam locations so that you
can run a standard base circle or even a larger cam core. Another great
feature available in some aftermarket blocks is a taller deck height
that enables you to run a decent length connecting rod without having
the piston pin ending up in the oil ring.

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Thicker cylinder walls, allow for even more displacement and can handle
the higher thrust loads created by greater rod angularity. Granted,
you’re not going to be able to start with a $150 core, but your engine
build won’t be compromised at every step of the way, and the end
product will make more power and be a better value for your customer.

Getting Started

This year’s esteemed panel of pros had some good advice about managing
your customers and their expectations at the beginning of the project.
According to Scott Shafiroff, it is extremely important to consider the
total engine combination because frictional losses from the longer
stroke and higher piston speeds drastically increase with a stroker
engine.

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“Some of your customers may want to build their engines in stages,”
says Shafiroff, “doing a really big shortblock now with plans on
purchasing the correct cylinder heads later. Many times that never
happens.” Scott’s advice is that, if you are definitely limited to your
present cylinder heads, build a motor appropriate for those heads,
because it will make more power and be much more efficient. Another
recommendation is to call your suppliers and get advice on what
combination will work best.

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Brian Adams of BES claims that one of any shop’s biggest problems is
having a customer wanting to build an engine around the parts he
already has. This always causes problems, he warns, and ends up costing
the customer more money in the long run. BES recommends that when
available, start out with a pre-engineered stroker kit that you know
fits together.

“We’ve had good results with companies like Eagle,” says Adams.
“Their kits are almost a bolt-together deal.”  The real high-end race
engines usually require piecing together.

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Block Notching

Due to both Sonny Leonard’s and Jon Kaase’s high-end racer clientele,
the O.E. parts issues posed by Scott and Brian simply don’t come up,
however, the same assembly problems confront them too. When asked what
the number one problem is in prepping a stroker engine the conversation
always turned to notching the block for connecting rod clearance.

Scott said that the biggest problem he sees in block prep is that
people “over-notch” the bottom of the bore for rod clearance. This
allows the piston to pull out of the bore and rock, losing ring seal
and power.

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Jon Kaase concurred, stating that they maintain at least .500? of
piston skirt below the oil ring in the block at bottom dead center. He
also said that when prepping an aluminum block for a really large
(5.750?) stroke crank, it is very tempting to throw the block on a mill
and make the needed cuts. Jon cautions against doing this because two
things can happen: first, the notches will be larger than they need to
be reducing piston stability, and second, the end mill will probably
grab the sleeve and crumple it up like a beer can. At Kaase’s they
tediously grind these notches by hand.

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A neat tip that Jon shared with us is that he cuts the top off of a
piston so he can look down through the bore to see any clearance
problems. “You can see things from the top that you just can’t see from
the bottom side.”

Sonny mentioned the cylinder notching issue as a big problem too, and
he also brought up the point that when you run these large 5.5?-5.75?
stroke motors, crankshaft flex is a big problem, even with a high
quality billet crank. He allows .080? of clearance all around the
counterweights and for rod-to-cam and rod-to-cylinder clearance. BES
still does quite a few strokers based on production blocks and Brian
mentioned that it’s not uncommon to hit water when grinding the bores
for rod clearance. He also added that the widened pan rails on
aftermarket blocks save a bunch of time and money.

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Rod Ratios

There’s been a lot written over the years about rod length-to-stroke
ratios, and some experts consider a 2.0-rod ratio optimum for best
power. With a big stroker motor you can forget all that. You simply put
the longest rod you can physically fit in the motor.

Shafiroff warns that “rod ratio is a dangerous way to judge an engine –
intake manifold design has more importance on how an engine will run
than rod ratio.” Kaase said, “don’t ever do the math on rod ratio or
piston speed, it’ll scare you.” Sonny Leonard stated that some of his
larger drag race engines have a 1.34-rod ratio, but he likes to stay in
the 1.43-1.52 range for his marine engines that require a lot of
endurance. Lower rod ratios load the cylinder walls extremely hard
requiring more frequent rebuilds.

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Tony Bischoff the owner of Bischoff Engine Service slightly disagreed
on this point telling us that he builds a large number of short deck
598 cid engines that are very popular for bracket racing, and his
customers get up to 400 runs on an engine. Tony recommends that they
get freshened up every 250 runs, but it’s up to the customer and some
extend the rebuild intervals without catastrophic results.

Camshaft Timing

Scott Shafiroff stated “stroke and cam lobe centers are connected –
more stoke generally requires wider centers.” He also recommends as
much lift as possible with shorter duration. Sonny was more specific
saying that in general his 4.5? stoke motors use 116-117-degree lobe
center cams, and his 5.5? stroke motors get 121-degree lobe center
cams. (That of course depends upon application.) Sonny also stated that
he runs more duration or rocker ratio on the exhaust to broaden the
horsepower curve with the wider lobe centers. Kaase also subscribes to
the wider lobe center school of thought stating that the piston
acceleration is so high in a stroker that the power stroke is probably
over when the piston is halfway down the bore.

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Cylinder Heads

Like rod length, stroker cylinder heads defy
traditional logic. As a general rule they can’t be too big. In fact,
Sonny builds his own heads for his 800cid-plus engines. He has both
Hemi and wedge versions to meet the demands of 800 cubes at 8,000rpm.
According to Scott, “you can never have enough cylinder head,
especially when you are dealing with the 430cid-plus small-blocks.” On
the cylinder head topic Brian concurs with Scott, “aftermarket is the
only way to go, it’s silly to use an production head on any serious
stroker motor. We’ve had good results on our big-inch small-blocks
using CNC ported heads that have 230cc and larger runners.”  The good
news is that the aftermarket cylinder head manufacturers have realized
what the stroker market needs and is doing a good job of filling it for
the more popular engines.

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Parting Shots

We asked our panel if they had any other advice about stroker engines.
The following are some of their parting thoughts on the subject. Jon
Kaase volunteered some information on crankcase vacuum. He said that in
his wet sump motors he likes to see at least 10? of crankcase vacuum,
and for his dry sump engines he looks for 20?. Jon left off with, “When
the pistons are going up and down, there’s a lot of air trying to swap
places at 8,000rpm!”

Sonny Leonard reminds us that stroke isn’t everything – use as much
bore as possible too. It will allow you run bigger valves and un-shroud
the valves in the combustion chamber. He also mentioned clearances.
Sonny adds .001-inches of piston-to-wall clearance to the
manufacturer’s recommendations because the piston speeds are so high
that it produces more heat and expansion. Piston speeds in 800 cid
Mountain Motors is approaching those of an F1 engine at 19,000 rpm. He
also advises a little more deck clearance due to the heavy pistons in
these big motors. About .070?-.075? is the minimum that you can safely
get away with for one of these motors at 8,000 rpm.

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Scott Shafiroff made the point about how important it is to
calculate the compression ratio before doing any machine work – “it’s
easy to get stupid compression in one of these motors!”

Brian also added that BES tries to steer its customers to EFI because
the driveability is so much better and EFI is easier to tune. Like
cylinder heads it’s difficult to get big enough carburetors that will
perform on the top end and still be responsive on the bottom end. With
EFI it’s possible to tune for the full power band. Tony Bischoff made
the excellent point that when selecting individual stroker components,
check to see what the shortest rod length is that will clear the
crankshaft counterweight in the piston pin boss area. This is often a
problem area that can be easily avoided with a little planning.

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There you have it from a team of engine builders that have won more
championships and races with stroker engines than just about anybody on
the planet. Their advice is straightforward, specific and easy to
follow.

When asked about what else they do differently when screwing together a
stroker the answer was nothing. “Just assemble it like any other race
engine.”

So the next time a customer comes by wanting a 632cid Chevy street
motor, have no fear, order up the parts and follow the few simple tips
provided here.

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Click on the following links to download charts: 


Stroker Resource Guide (pdf)


Stroker Kit and Component Suppliers (pdf) 

written by engine builder contributor len emanuelson. len has spent the past 37 years reporting on the performance and racing market, notably as technical editor and publisher of such titles as popular hot rodding, hot rod, car craft and circle track. see the bottom of this article for stroker chart pdf downloads.sonny leonard designed this gm hemispherical cylinder head for the 5.300?-bore-spaced cnc blocks he uses on his big-inch engines. one of the real problems on 700 cid-plus engines is getting a cylinder head that can feed the 
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