Hi Walt,
Actually a large and complex topic...
The compression ratios you mentioned do seem low however, it is important to
differentiate between static and dynamic ratios....static is a calculated
number whereas dynamic reflects the compression ratio of the engine at full
efficiencies which incorporates all of the ramming and scavenging driven by
camshaft, porting and valving.
A competition head is designed (along with large overlap,high lift
camshafts) to "breath" (ram and scavenge) at high rpm. At this rpm, their
dynamic compression ratios are much higher than their static. However, the
cost to this is that the big valves, ports and camshafts rob the engine of
vacuum (as well as the length of compression cycle) at lower rpm. As a
result, you get no torque at the bottom end. In an extreme case, you get
dragsters that don't have enough power to idle below 2500 rpm but have
humongous power at 8000rpm.
For a street car, this is completely impractical....idle at the stop sign
will be very rough (if existent at all) and starting on an incline will be
difficult. If you have power brakes, you will loose them at lower rpm also.
As such, your biggest concern with going with a competition head is not the
compression ratio but rather, the valve and porting size.
I'd go with the FR head and mill it to the ratio that you want. Also, keep
in mind that doing all this without changing the camshaft is a waste. The
camshaft and head all work as one....having the engine ported and polished
but not having enough valve opening to accommodate full breathing won't
achieve your desired results.
Finally, keep the static Compression Ratio below 10:1...anything above that
is likely to give you detonation problems (though less likely on an ali
head) with the octane levels available to us today (and going forward).
Paul
-----Original Message-----
From: healeys-bounces@autox.team.net [mailto:healeys-bounces@autox.team.net]
On Behalf Of Walt Peterson
Sent: Wednesday, January 13, 2010 6:37 AM
To: Healeys@autox.team.net
Subject: [Healeys] Alloy Cylinder Head:
Not an expert at this, but what SCparts lists as "Fast Road" and
"Competetion" as far as their 3000 heads go, have a compression ratio of 7.5
and 8.5 respectivly & a mm increase in valve size. Doesn't sound like that
will get the max power out of the octane available in Western Pennsylvania.
We
want to add headers & go up a size in carbs: three 13/4 SUs. What to do? get
the FR head & get it milled or go with the competetion head, period?
Thanks,
Guys. I know some of you have been there before & your answers are always
interesting.
Walt
Healeys@autox.team.net
http://autox.team.net/mailman/listinfo/healeys
You are subscribed as britishcars@shaw.ca
http://www.team.net/archive
_______________________________________________
Support Team.Net http://www.team.net/donate.html
Healeys@autox.team.net
http://autox.team.net/mailman/listinfo/healeys
http://www.team.net/archive
|