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Sonny Leonard designed this GM hemispherical cyli...
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Engine Builder Jon Kaase shared this great tech t...
Jon Kaase has racked up an impressive number of M...
Stroker Tips from the Pros
Some great advice from the folks who build big strokers for a living
By Len Emanuelson
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.
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.
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.
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.
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.
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.
“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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.