Forum Member
Posted 13 December 2013 - 01:17 PM
Hi,
I'm not sure if this has been covered or not but I wondered if I was to have both a H and a L stamped 161 heads for example, and I had the L head machined to raise the CR
to the same of the H head, what are the differences in terms of performance or does this cancel each other out as they become the same?
Maybe another way to ask this is, bigger chamber with high compression or small chamber with high compression. Which is better and why?
Cheers,
Adam.
Forum Fixture
Posted 13 December 2013 - 04:05 PM
Wasting cash making a low comp head the same as the high comp version. Although swirl in the chamber may be different if you know how to 'massage' it. The highest you can get the comp for the fuel you use the better.
Oh My, Don't you post alot
Posted 13 December 2013 - 06:16 PM
You'd be taking some metal off the head face to get a 179/186/202 head to the same chamber size as a 149-161-173H head.
Find Johnno's red 6 build thread on this site for your answers.
_Bluejinx202_
Posted 13 December 2013 - 07:30 PM
Forum Member
Posted 18 December 2013 - 12:51 PM
Can I get some more feedback on this as I'm hearing conflicting views from forum users and head machining shops.
Cheers.
_oldjohnno_
Posted 18 December 2013 - 05:30 PM
I'm not going to write an essay on this (I've wasted a helluva lot of time on that here lately, time that could have been better spent elsewhere..) but simply put squish/quench is a sometimes necessary evil and if you can manage without it you should. For a Holden six that's going to spend a lot of time at low rpms (say <5000 - 6000rpms) you'll probably need a small chamber and a fair bit of squish area. With higher rpms you'll make more power with less squish area, so at high rpms if you can get enough CR with an open chamber that's what I'd do.
Forum Member
Posted 19 December 2013 - 10:23 AM
Thanks Johhno I'll keep that in mind.
_glennhailstone_
Posted 10 October 2015 - 05:29 PM
Can somebody please explain the difference between a low comp head and a high comp head. And which one is better in general for most HP gain.
My 9 port head is low comp, but its had a lot of work done to it. But would I be better off with a high comp head.
Also should I stick with my yella terra type 9 port, (depending on what is said about it being a low comp) or go to 12 port. And where the 12 ports L and H
Thanks
Edited by glennhailstone, 10 October 2015 - 05:34 PM.
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Posted 10 October 2015 - 06:02 PM
Its not that complicated really...
Good power can be made with an open chamber head, but it will inevitably need shaving to get comp matched to cam. I am running open chamber head and it works well. If the cam has really long duration, and widish LSAs I would use closed chamber head as more comp will be bled off due to the late closing intake valve. If things are tightened up with duration and LSA you can probably still run open chamber and work well.
Some argue that the open chamber heads are easier to get flowing well due to less valve shrouding- but with the right work the closed chamber can flow the same. I cannot be sure this is true though, just what I have heard- not seen any evidence.
_oldjohnno_
Posted 10 October 2015 - 09:50 PM
Unless you are using no more than a very mild cam you won't have much choice but to run the closed chamber head in order to get a suitable compression ratio. Insufficient compression for the cam absolutely kills performance.
You have to machine a lot off an open chamber head to make it work and that has a negative impact on the strength of the head and also valve-to-block clearance so it's easier all round to start with a small chamber head.
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Posted 10 October 2015 - 10:11 PM
Yes the 12 port does come in Low & High compression, most High comp heads were fitted to the 2850 engines & the fuel injected 3300 motors.
_glennhailstone_
Posted 11 October 2015 - 05:22 AM
Ok thanks, is there a photo so I can see the difference between open chamber and closed, because I still have no idea.
I assume closed chamber is high comp and closed is low comp.
Thanks
Edited by glennhailstone, 11 October 2015 - 05:34 AM.
_oldjohnno_
Posted 11 October 2015 - 06:17 AM
The very first thread in this section is Bombers excellent Holden Six Cylinder Head 101. You'll find photos on the first page.
_oldjohnno_
Posted 11 October 2015 - 06:20 AM
Yes the 12 port does come in Low & High compression, most High comp heads were fitted to the 2850 engines & the fuel injected 3300 motors.
Actually the injected 3300 was large chamber though it did have some subtle changes to the ports and seats. AFAIK only the 2850 got the little chamber.
Forum Fixture
Posted 11 October 2015 - 06:42 AM
_oldjohnno_
Posted 11 October 2015 - 06:59 AM
To put it simply if you want to make dcoe's work the way they were designed to then you need a 12 port head. This could be a true 12 port or a 9 port with a 12 port conversion.
These 2 options require different manifolds so pick the the head type to suit your existing manifold.
Dcoe's should run one barrel per cylinder. Each carb is really 2 carbs in the one body that share a common float bowl and spindle.
Fortunately on a street car you'll have no restrictions on head design that some classes of racing having to work within.
I know where you're coming from but I'm not sure it's so black and white as that. Yes, a 12 port arrangement makes more sense but the reality is that it's a lot harder to get 200cfm+ from a 12 port than it is from a 9 port. The 12 ports can be made to run very very well but usually if the absolute maximum in peak hp is the goal then it's easier to get the flow from a 9 port. Having said that I'd have no hesitation in running a 12 port on a street/strip motor. The other thing to keep in mind is that the OP already has 9 port manifolds.
DCOES should run one barrel per cylinder..
It might be more accurate to say they were designed to run one cylinder at a time per barrel, which sorta-kinda fits the 9 port setup. But I agree, the triple DCOE setup on a 9 port is ugly and not ideal but you have to weigh this up against the flow advantage of the 9 port. I'm sure that with some development work a 3 throat arrangement (eg. HSRs or even VMs) could be made to beat the triple DCOES on a 9 port.
Just remember that if you do go 12 port it's gonna need more throat area and more cam. And I've never seen a 9 to 12 port conversion that worked.
Edited by oldjohnno, 11 October 2015 - 07:01 AM.
_glennhailstone_
Posted 11 October 2015 - 07:25 AM
Ok thanks, bottom line.. 9 port or 12 for my 3.3L blue motor XU1 cam with 3 x 45 webers.
_glennhailstone_
Posted 11 October 2015 - 07:43 AM
Just reading through the 101 by Bomber now. So can't I use my stud mounted roller rockers? What other work needs doing to get it up to scratch, I'll do some digging through "How to build a HP 6". But any quick tips?
Thanks
_glennhailstone_
Posted 11 October 2015 - 09:25 AM
_oldjohnno_
Posted 11 October 2015 - 11:03 AM
But is 10:1 enough? I don't know what cam you are using but street/strip motors more typically want over 11:1.
_glennhailstone_
Posted 11 October 2015 - 03:44 PM
10:1 is the most I can go with NZ pump gas.
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Posted 11 October 2015 - 04:23 PM
10:1 is the most I can go with NZ pump gas.
We have explained this before, but the comp ratio on its own is irrelevant... Bigger cams with later intake valve closing angles will bleed off compression and allow you to run more comp. You need to consider the cam intake valve closing angle as well as the static comp. Some cars will run fine on 98 with 12:1 if the cam is bleeding off enough dynamic comp.
You will throw good money after bad if you dont research your build properly, and try to learn as much as you can about engines before making important decisions.
_oldjohnno_
Posted 11 October 2015 - 05:25 PM
10:1 is the most I can go with NZ pump gas.
Ah Glenn, sometimes I wonder if your woolly friends are the source of your engine advice...
_glennhailstone_
Posted 11 October 2015 - 05:50 PM
Shots fired! Hahaha, the guy who is building the engine builds the HQ race car motors. He also is the first guy to run sub 5sec 1/4mile in NZ history, I think he knows a thing or two. 
Anyway who cares, lets keep this thread about Low Vs High comp heads.
Edited by glennhailstone, 11 October 2015 - 06:01 PM.
Shed tinkerer
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Posted 11 October 2015 - 05:58 PM
Shots fired! Hahaha, the guy who is building the engine builds the HQ race car motors. He also is the first guy to run sub 5sec 1/4mile in NZ history, I think he knows a thing or two.
Good luck with your build mate- sounds like you have it under control.
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