[Steve C.]

The larger 4.5 stroke engine will be sensitive to under-flowing cylinder heads. Something to consider with the use of iron heads, If so keep the RPM down. To illustrate, grab a bicycle tire pump some time, and first try to operate it as quickly as you can. Then, slow down the pumping action. You will notice that it is far easier to move the pump handle slowly.

Of interest, Dave Bisschop on the subject of a 4.5 stroke...

Having actually built a number of identical combo's with the only change being the stroke, I can tell you with 100% certainty that in each case the engines made more hp and torque with the 4.5" stroke vs. the 4.25" stroke running the same bore, same head cfm (iron as well as aluminum), same compression ratio's and the same cams, headers etc.

This doesn't mean I recommend everyone go to the 4.5" stroke as the gains when looking at it from hp per ci shows the efficiency is no doubt going down. The biggest difference is the low rpm torque increase with the longer arm which can make things more difficult to get the power to the ground and will be harder on drive-train parts as well, but in a heavy moderately geared street car that is set-up well it sure can work well too. From a longevity standpoint it's uncertain in the long run how well these engines will last with the additional side loading on the lower part of the cylinder walls, but so far we haven't really seen any issues, but I think the key there is too keep the rpm's down, which kind of happens automatically with all things being "equal".

To me if the car is set-up 100% to compliment the engine combination then the performance difference will most likely be little to none at the drag strip. The shorter stroke engine would need more rear gear (multiply the power) and will operate at higher rpm. I look at Lisa's 65 GTO racecar, the car weighed 3580lbs with driver and would run 10.20's @ 129-130mph all day long on a foot brake. The 413ci 670 headed engine only made 611hp and 545lb/ft of torque on the SF902 dyno we use for all our testing. If you run those numbers through most HP by ET/MPH/Weight calculators you come up with a much higher hp number than the engine actually produced, but the key was it had a 5000ish stall and 5.13 gears (31" tall tire) which perfectly complimented the power curve the engine had, it took years of tweaking and tuning to get it there, but it shows you that getting the entire drive-train combination right can and will make a huge difference on performance at the track.

Without a doubt for a street/strip car it's easier to get the longer stroke engines to work well at the track as they are much more forgiving on gearing and converter stall, but having way too much low end torque won't do you much good if you can't get it to the ground either, so choose wisely, personally I feel the 4.250" stroke is plenty for most of these combo's.

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