bore vs. stroke

[wannabebuilder]

why is it that master engine builders prefer more stroke versus more bore or a combination of both more stroke and more bore? which set up makes the most power? does it matter on the engine?

[Bob In Ct]

HP = (TORQUE * RPM) / 5252

Longer stroke engines make more torque, shorter stroke engines make more RPM. Both Torque and RPM make HorsePower. Builders are always looking for the best blend of both for their application. Drag racers may want longer stroke engines while road racers may want an engine that can go all day at high RPM.

Adding stroke without increasing the deck height has its own set of problems. I'll leave that for the engine guys out there.

It's all a compromise.

Bob

[quickjack]

The Engine Masters builds are always running more compression than a useable engine would with pump gas, and the longer stroke gives a more favorable dwell time at TDC to help prevent detonation.
Those engines are not built for real world conditions.

[Woodz428]

A big reason for the smaller bores, even in the new car engines, is for a quicker and more efficient burn. There is only so much time for a flame front to move across a bore and the larger it is the less gets fully burned. In emmissions as well as HP, unburned hydrocarbons are wasted ones. I think it was last year that The big block build, I think by Kaase, used a reject FRRP 460 block and had the cylinders sleeved down just to help promote a more complete combustion. Since they are restricted to engine size, the current technology leans toward the smallest bore/longest stroke that is practical.

[Dennis Kelley]

The reason for the bigger stroke than bore is that the engine master challenge only measure horsepower and torque up to 6500 rpm I believe. Its designed to limit the rpms by the rules set forth. If they measured horsepower up to 8000 rpm I believe you would see more shorter strokes but maybe not. The engine builders want to make as much HP/Tq through the rpm range and it would seem that bigger stroke small bore does the trick up to the specified RPM. Just my opinion.

later
dennis

[Barry_R]

Lots of variables enter into this decision. Airflow is crucial - and a larger bore will permit larger valves - hence better breathing. Since my FE entries are inline valve engines, they are inherently airflow limited - so I use the biggest bore feasable. Guys with better airflow combinations such as canted valve Clevelands will go for less bore and more stroke. The added stroke seems to help with low end and midrange torque - I thinks its a function of more distance traveled under combustion pressure per cycle - an advantage that dwindles at higher RPM - and an advantage that I unfortunately give up while trying to get the top end power I need to compete.

Contrary to some postings, these motors are in no way simple "dyno queens". Although the tuning may be on the ragged edge as we try to get the most points possible, the basic combinations are generally top shelf, and the engines would be pretty darn nice to run on the street. All I'd need to do is back the timing off by 2-3 degrees and "maybe" use a less aggressive cam (depends on your tolerance for spring replacement).

As an example, my 10.5:1 434 inch FE made 678HP at 6500RPM, and 587 TQ. All on 91 octane. It has a steel crank, Carrillo rods, Genesis block, CNC ported Blue Thunder heads, and a T&D rocker system. Nothing is gonna break at 6500RPM unless I screwed up somewhere. The attrition you see in the contest is usually tuning driven - rings, bearings, pistons and plugs getting hurt as the guys lean them out in search of the last possible score number. In your Cobra you would most likely drive that thing forever without pushing it that hard - dynos are usually tougher on parts than any racetrack.

[Anthony]

Originally Posted by quickjack

The Engine Masters builds are always running more compression than a useable engine would with pump gas, and the longer stroke gives a more favorable dwell time at TDC to help prevent detonation.

A longer stroke will decrease dwell time, which will hurt power, and if you are able to run higher octane/ i.e. more compression than a short stroke, high dwell, then maybe it would be a wash.

A while ago, HotRod did a comparison of two BBC's, same displacement, but different bore/stroke comnbinations, and using the same heads, intake, headers, cam, etc., even I think keeping the stroke/rod ratio similar, and they didn't find a measurable difference in torque/hp. However, a bigger bore will allow bigger valves, but that wasn't the point of their test. Keep everything else the same.