big bore short stroke
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
What's important imo is that port velocity is limited to sonic speed. Once airspeed hits sonic, it won't flow air in any faster. Basically your engine runs out of air at that point.
That point is different depending on the amount of air it inhales, and the piston speed.
So, as a shorter rod, long stroke moves the piston faster and moves more air, it also limits the amount of rpm's an engine can reach, since you run into sonic airspeed. A longer rod/short stroke moves that point upward.
This is of course with the same heads. If you open up runners and the resistance to air speed is less, then you could reach the same top engine speed with that bigger engine, BUT... as you increase your runner, your low end will suffer just as much as mounting a too large runner head on an engine does because air speed gets lower at low rpm.
So, what is ideal ?
IMO (!) if you stay at the same top end speed, but you can pull more rpm's with that short stroke motor, use a shorter rear axle, you'll get the same low rpm wheel torque (which is what makes the car move) and you get in the high speed a lot faster....
If you mount a short rear end with a long stroke motor it will run not as fast (lower top end)...
AND the long rod, short rod has another advantage : minimized dragresistance and lower side forces working on the piston ATDC.
That point is different depending on the amount of air it inhales, and the piston speed.
So, as a shorter rod, long stroke moves the piston faster and moves more air, it also limits the amount of rpm's an engine can reach, since you run into sonic airspeed. A longer rod/short stroke moves that point upward.
This is of course with the same heads. If you open up runners and the resistance to air speed is less, then you could reach the same top engine speed with that bigger engine, BUT... as you increase your runner, your low end will suffer just as much as mounting a too large runner head on an engine does because air speed gets lower at low rpm.
So, what is ideal ?
IMO (!) if you stay at the same top end speed, but you can pull more rpm's with that short stroke motor, use a shorter rear axle, you'll get the same low rpm wheel torque (which is what makes the car move) and you get in the high speed a lot faster....
If you mount a short rear end with a long stroke motor it will run not as fast (lower top end)...
AND the long rod, short rod has another advantage : minimized dragresistance and lower side forces working on the piston ATDC.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
Dave and Dal at Woodburn...3-24-2013 Chevelles wheelies.. - YouTube
The bigger bore shorter stroke engine is faster. The long stroke smaller bore engine makes more power.
The bigger bore shorter stroke engine is faster. The long stroke smaller bore engine makes more power.
Melting Slicks
Joined: Mar 2002
Posts: 2,164
Likes: 7
From: Beringen
There are several variables to mach speed. Both pressure and temperature change what speed mach is reached. For example, Nascar engines run with restrictor plates need larger ports because the conditions in the ports and manifold caused by the restrictors lower the mach speed. It is generally accepted that .6 mach is the point at which most ports will start to stall out (flow stops increasing) in a NA engine. I have achieved .75 in ports of heads I designed that flow very smoothly and have a lack of turbulance. I've seen ports with a lot of turbulence not be able to achieve .6 mach. Again, a lot of variables involved here.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
Many things can cause turbulence. For instance older style heads with a tight short side radius turns tend to have the air tumble around the turn instead of following it like it would in modern port with a longer, gentler turning short side radius. Mismatched seat and valve shapes can cause turbulence. A lot of what restricts airflow is turbulence. You want smooth transition of shape. Port shapes that allow for matching the air speed on the roof of the port closer to that on the floor and the short side radius will have less air sheer and flow smoother. List goes on!
