If it is original the engine number will tell you, but by now most LC engines would have been rebuilt as high comp. Most LC engines were prefixed QM but a few HM, ZM etc would have existed, possibly even an XQM or WM.

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If we all had the same (good) taste, who would buy all the Fords?

Hi Gyro,
if your engine is non original there are two ways to tell if a engine is high or low compression.
1. do a compression test, high compression should be about 150 PSI if in good condition.
2. look at your cylinder head, see if it is H or L at the back, should be L for 202 and 186, H for 138, 161 173 & maybe 179.
If it is a HR head or earlier head the H or L will be at the front of the head.
HK to HZ it is at the back.
Blue engine heads it is back at the front again.
H or L on a head does not necessarily mean that it is high or low compression, it depends on other factors also, like head deck height, cubic capacity and piston dish.

Mike

Purpul Police

The H & L code only really works on the smaller engines. It only really means small or large chamber, not hi or lo-compression

To explain: If we break the Holden 6-cyl range into 2 groups of 3 & ignore the tiny 130 & 138 Red motors, we get 149, 161 & 173 in the smaller group & 179, 186 & 202 in the larger group.

An H coded head is the standard fitting on the hi-comp versions of the smaller motors & if an L coded head is fitted it then becomes a lo-comp version. Simple so far, H=hi-comp & L=lo-comp.

However, the L coded head was the standard fitting for all hi-comp versions of the larger range. There was no lo-comp 179 made so that leaves the lo-comp 186 & 202, which were made using dished pistons & retained the L head. So all 3 larger engines got the L head regardless of whether they were lo or hi-comp.

The exceptions are the XU-1 engines. These had H heads on 186 or 202 motors making them extra hi-comp, which they were at 10 & 10.3:1.

Dr Terry

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When calculating a car restoration budget, be as accurate as you can & then double the final figure. It will be closer to the truth.

H all,
Good explanation DR Terry.
I might just add that the XU-1 heads apparently were a compromise to fit in with Holden's production line machining practices.

what I mean is, that their small combustion chamber was not the most efficient for gas flow, remember the old Hemi Valiants!

Hemi meant hemispherical combustion chamber, this was the most efficient at that time, and probably still is.

Holden just grabbed a 161 head ( small chamber ) and stuck larger valves and a other few minor changes, this produced the higher compression required.
Not quite that simple as they were cast a little different too.

If they had been more serious about supporting motor raceing they would have used the larger combustion chamber with flat top pistons ( for 202 ) and or machined up to .020 off the bore top.

But of course this would not have fitted their machine shop practices as all blocks were the same height from 149 to 202,
including XU-1 engines, and it appears that they were not about to change the machine settings for small batches of performance engines to come through.
Shorter blocks could also have led to push rod length problems too, the XU-1 was already using the shorter ones anyway.

All this means if you are building a 179, 186 or 202 engine and you are not trying to keep to originallity, use the larger chambers.
The only reason that I will be using the smaller chamber is to keep my car original.

Mike

Purpul Police

quote:

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Originally posted by Dr Terry
The valves were only slightly cantered

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Canted, Terry. Unless you mean they were trying to run away?

Hi Mike.

I didn't mean it wouldn't make much difference, I meant it wouldn't make 'enough' difference.

A standard 186 has a comp ratio of 9.2 & a standard 179 is 8.8. I can't see .020" off the deck height raising the comp ratio from 9.2 to 10 or more.

On another point, Dave Bennett (Dyno Dave of Perfectune, Yella Terra etc.) has proved over many years that you don't necessarily need high compression to get high performance. If you get one of his Yella Terra large chamber heads & fit it to a stock 202 you get a drop in compression but a good improvement in performance.

Dr Terry

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When calculating a car restoration budget, be as accurate as you can & then double the final figure. It will be closer to the truth.

quote:

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Originally posted by DCOE
Hi Terry, I noticed that you rarely address posts that say you made a mistake. Is this hubris or oversight? It would be helpful to your audience to acknowledge errors rather than ignoring them to prevent perpetuation of the usual myths which accompany technical forums. I am sure that some of the usual errors are irksome to you on a personal level, so let's not have a new generation of Bozos discussing "cantered" valves.

Now, ignoring galloping valves, As far as SC v LC heads go, let's just say that a seasoned head porter has much more scope to improve upon an SC than they have with a large chamber head. I doubt you will find anyone who knows what they're doing (including Dyno Dave Bennett) telling you to bring them a large chamber to work over instead of a small chamber.

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Mr Weber

I'm sorry if I've offended you in some way, essential especially with my spelling mistakes, but you've missed my point. I'm sorry if you feel offended, 'canted' it is.

Compression isn't the be-all & end-all of engine performance, head flow is more important. The gain made by un-shrouding valves will be more than what is lost in the drop in compression due to the removal of that material.

'Pumping losses' are more than considerable in a street motor & high-compression is only of any use if you have the hi-octane fuel & tuning to match. But, again you are correct, a hi-compression head will yield more horsepower than a low comp head, as long as everything else falls into place.

Dr Terry

_________________________________________

When calculating a car restoration budget, be as accurate as you can & then double the final figure. It will be closer to the truth.

Steady boys,
we all make mistakes, and I do also.

When considering heads and power of course all the variables must be considered, and mostly they are not!

Fuel octane ratio alone has changed a couple of times over the period since these heads were made.

I agree that Dave Bennet could have made the last of the Yella Terra Heads better than he did, but he obviously he had financial considerations when making low volume exchange heads at the end.
I bought one of the last cast iron heads, don't think they do them now.
I was told it had a slightly lower compression than my 186 would have had, ( so looks like this was really a 202 head anyway ).
It was suggested by staff that this head was ideal for gas, this was clearly bad advice as gas should have had a higher compression ratio.

But this is a myth that has persisted to justify compromising compression for the early unleaded engines, of course modern unleaded vechicles now have higher compressions again, but old habits die hard and myths are created!
Gas engines should have a minimum of 10:1 to get the best from them.

Small chamber heads have a limit to the combustion chamber material that can be safely removed, so I cannot see them being better than a large chamber head as once you chamfer the edges of a large chamber head it woud be more clear for gass flow, but of course compression ratio would have to be watched closely.

When building a whole engine there is far more scope to change compression ratio anyway.

Mike



Purpul Police

P.S. While the Valiant Hemi 6 was a good performer, it wasn't a true 'hemi' in design terms. The valves were only slightly cantered & they weren't crossflow. A BBC or Ford Cleveland were probably more 'hemi' than the Chrysler 6. I think that Chrysler Aust were just trying to cash in on the fame of the U.S. Hemi V8, which was a true 'hemi'.

OT- but if you want a real "Hemi" head, have a look down the inlet ports of a Fiat DOHC head. You'll see daylight through the exhaust ports. A true crossflow head. And thanks to all posters on this thread. No drivel from the gelati brigade.
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One pair of hands working does more than a thousand pairs praying.