cylinder head flow alky vs diesel January 25, 2010 02:28PM
Do alky cylinder heads need to flow as much air as diesel to make equal power ?
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Re: cylinder head flow alky vs diesel January 25, 2010 03:29PM
It depends (how do you like that answer?). Regardless of diesel or alcohol, power comes down to air mass flow. A general rule of thumb for either fuel, is 10HP per lb/min of air flow. So an 1100HP super farm needs about 110lb/min airflow, which is the upper right side of a 3" turbo compressor map. A 2300HP LSS alcohol needs about 230lb/min airflow, just what a 2-into-1 3 turbo combo gives. The key difference between alcohol and diesel is the fuel flow. The specific fuel consumption for alcohol is more than twice the diesel.

So, for the same displacement, both fuels can make the same power provided the air mass flow is equivalent. But typically the diesel makes that power at a lower RPM. So the same power at a lower RPM means a higher cylinder pressure at the same mass flow, which is why you see diesels blow apart and alcohol motors less stressed.

If you are building either one, and have no limit on turbos, build the head to flow the most air, keeping in mind the basic rules for cross section and velocities. If you are talking a limited turbo class, then you need to put some math behind the head flow. Bigger is not always better for a limited turbo class. A head that flows a lot of air needs different cam timing than one that flows less. Head and cam need to be matched....tattoo that in your brain. Most 466 hot farm motors with 260cfm head flow are making the same power as 640 cube 380cfm super farm motors. The 466 builds that power 1000 RPM higher than the SF motor.
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Re: cylinder head flow alky vs diesel January 26, 2010 05:27AM
One thought - everything else equal (which I know it never really is) since the alky head has to flow all that fuel along with the air doesn't it need to be a little larger than the diesel head that only has to flow air & a little bit of water...

On the 466 vs 640 I'm having trouble working out the less cfm ports on less cubes = same power. On one hand it makes sense because the 466 has smaller buckets to fill, but on the other hand since it's at higher RPM's it has less time to fill it which raises the required flow rate...
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Re: cylinder head flow alky vs diesel January 26, 2010 05:56AM
I have heard this that in a limited turbo application size is not always bigger beter. how do you match a cam to a head? and does opening the exhaust have a negative effect? what should a superfarm head flow with a non slotted 3x3
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