Head Specs

midnight sun · March 01, 2013, 06:24:06 PM

With all other specs being the same on a set of heads, what effect does the exhaust runner volume have? Looking at a comparable set of heads but one has a 75cc exhaust runner volume and the other a 60 cc exhaust runner volume. Thanks in advance.

kb426 · March 01, 2013, 08:41:19 PM

You might add more info so the intended usage is known.

wayne petty · March 01, 2013, 08:42:45 PM

smaller exhaust port volume equals more low end torque as the scavenging effect is better.. but will limit the high RPM operation in comparison to the larger exhaust port volume

larger port volume allows higher / increased RPM range.

at least in my way of thinking...

there has to be velocity in the exhaust stream to properly scavenge the last of the burned gasses from the chamber as the intake valve starts to open as the exhaust valve is just about closed...

inertia as the exhaust stream is accelerated by the pressure wave as the exhaust valve opens... then the piston accelerates to the middle of the stroke.. then slows as the piston approaches TDC... this slow down of the piston speed with the exhaust flow racing away causes a low pressure that extracts the last of the burned gasses ..

has anybody ever notices the nasty rap when open headers are run without a collector extension as the exhaust flow is pulled away from the back of the closed valve.. then the flow stops and reverts back slamming into the closed valve... making a horrible sounding slapping noise...

the guy across the street has a little 2.2 honda civic that was cranking out 265 HP... but it had a terrible rapping noise from the exhaust. even though he had long exhaust..

i had him get a friend at a muffler shop to make an insert at the collector flange joint.. this brought the horsepower up 10 to 275 on the chassis dyno... and ended the terrible rapping noise from the exhaust...

this allowed the 4 into 1 to scavenge the cylinders... then expand into the collector... but the length of the tuning tube kept the exhaust flowing out of the collector at a higher speed to reduce pressure in the collector... also acted like a antireversion device..

i had been thinking of this device for a LONG time.. i was sitting at cyclone headers lobby one day and overheard a conversation about their anti reversion headers they had made for 440-3 motorhomes... their first test resulted in 33MPG.. but melted the pistons.. and they could not chance that somebody would install a set without retuning the carb before engine damage .. them blame them for the damage..

midnight sun · March 01, 2013, 08:51:44 PM

Quote from: "kb426"You might add more info so the intended usage is known.

Street Rod. SBC 327 .030 over. 9.2 Compression, Currently has stock iron heads (double hump). Going to aluminum heads. Comparable heads have 180cc Intake port volume, 64cc Combustion Chamber volume, intake valve 2.02, exhaust valve 1.6. Cam is hydraulic, .420 intake lift and .443 exhaust, 112 lobe separation, Edelbrock Performer intake with 650 Edelbrock Carb.

midnight sun · March 02, 2013, 04:31:23 PM

Quote from: wayne pettysmaller exhaust port volume equals more low end torque as the scavenging effect is better.. but will limit the high RPM operation in comparison to the larger exhaust port volume

larger port volume allows higher / increased RPM range.

at least in my way of thinking...

there has to be velocity in the exhaust stream to properly scavenge the last of the burned gasses from the chamber as the intake valve starts to open as the exhaust valve is just about closed...

inertia as the exhaust stream is accelerated by the pressure wave as the exhaust valve opens... then the piston accelerates to the middle of the stroke.. then slows as the piston approaches TDC... this slow down of the piston speed with the exhaust flow racing away causes a low pressure that extracts the last of the burned gasses ..

Okay thanks for the insight Wayne

midnight sun · March 02, 2013, 04:32:17 PM

smaller exhaust port volume equals more low end torque as the scavenging effect is better.. but will limit the high RPM operation in comparison to the larger exhaust port volume

larger port volume allows higher / increased RPM range.

at least in my way of thinking...

there has to be velocity in the exhaust stream to properly scavenge the last of the burned gasses from the chamber as the intake valve starts to open as the exhaust valve is just about closed...

inertia as the exhaust stream is accelerated by the pressure wave as the exhaust valve opens... then the piston accelerates to the middle of the stroke.. then slows as the piston approaches TDC... this slow down of the piston speed with the exhaust flow racing away causes a low pressure that extracts the last of the burned gasses ..

Okay thanks for the insight Wayne

unklian · March 02, 2013, 04:43:41 PM

I would go with the smaller ones.

chimp koose · March 02, 2013, 09:41:59 PM

What are the flow specs for the heads you are looking at ? A good cyl head will have exhaust flow at or near 80% of intake flow. Your cam is biased to the exh. so a bit lower flow % wise to the intake would be ok with that cam. Do you have the heads and cam yet? if you are still looking at heads ,take a look at what EQ (engine quest ) has to offer.