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Wondering if anyone can add in other examples of low comp engines and what the maximum compression ratio was for Australian Standard grade 87 Octane fuel into the 70's up to 1980-ish? Also chasing some piston data (see end of post.
What I can add is the 149 low comp and later L21 (161/2600 low comp) engines were respectively 7.7:1 and 8.2:1 and these were simply done by using the large chamber 179/186/202 heads on the 149 and later 161 bottom end which was the same as the high comp bottom end. The 161L was higher simply as it had a larger bore.
Come around to HQ, GMH didn't do this with the 173 - the low comp engine didn't use the same pistons as the high comp engine, they had a dish in them. The 173L still used the 202 head but used dished pistons to keep compression at 8.3:1. Obviously they didn't want to exceed 8.3:1 for standard grade fuel.
186L in HT-HG was 7.8:1 and HQ onwards 202L was also 7.8:1, both courtesy of dished pistons (173HC and 202 HC also had dished pistons, just not as big a dish as the LC version).
253LC was 8:1 also courtesy of dished pistons. In Bedford it was 7.8:1.
5.0LC (Bedford): big piston dish, 7.8:1 compression.
138 used dome top pistons to achieve 9.2:1, and the low compl just used the 186/202 head for 7.8:1.
The only specs I've been able to find on piston dishes (and domes) are from the HX Engineering Technical Specs. Unfortunately the 4.2 raised to 9.4:1 in HX so it doesn't have the low comp 253 dish size:
130HC: dome top, cc unknown.
138HC and LC: dome top pistons, cc unknown.
149/161: same pistons for LC as HC (flat top).
173LC: 3.71cc
173HC: 1.92cc
179: flat top.
186LC: ??cc
186HC: flat top
202LC: 19.89cc
202HC: 2.36cc
253LC: ??cc
253HC: flat top.
308 (HT-HQ): dish, cc unknown.
4.2HC in HX (9.4:1): -3.34cc (dome).
5.0HC (HJ to mid HZ): 9.25cc.
5.0LC (Bedford): ??
The 292 in C20/C30 etc was 7.3:1, revised to 7.25:1 and ended as 8:1 before being replaced in the later 70's by a 350 V8 (once ADR27A became applicable to commercial vehicles like these trucks, Landcruisers etc).
I did find reference in the release of the 350 V8 in C20 and C30 in 1979 that the engine's compression was 8.5:1 and it was suitable for Standard Grade fuel. That is the highest compression ratio I can find in GMH literature fro Standard Grade fuel operation. |
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I was having a further think on the 173 and why GMH dished the pistons for both engines. As you can see they are very small dishes in the pistons for the 173's. The Low comp was 8.3:1 with the 202 head and a 3.71cc piston dish. The high comp 173 was 9.4:1 with the 149/161 high comp style heads and 1.92cc dish. Clearly GMH didn't want to go over 8.3:1 in the red 6 with 87 Octane fuel or 9.4:1 for 97 octane fuel with that tiny little HQ 6cyl camshaft - the dynamic compression with that cam must have been far too high to allow any higher compression:
HQ 6cyl cam at (I think) 4 thou lift:
Inlet: 15 45 240
Exhaust: 50 10 240
Overlap 25deg
You'd think that with a larger camshaft in either engine (as fitted to the late LC GTR with 2850S engine, HT-HG regular 186 (not Trimatic) or the 202S as was to be fitted to the stillborn HQ GTS coupe) that these engines could have run flat top pistons for something like 9.6:1 - the 202HC with flat tops would have had 2.36cc less combustion chamber and the 173HC 1.92cc less. The LJ XU1 was happy with 10.3:1 (+/-0.5) but with a significantly bigger cam. Funnily enough the original GMH specifications for the XU1 state it was designed for 98 Octane fuel, whereas all the other Super fuel intended engines were 97 Octane. The Octane rating is missing (as in blank) for HT-HG GTS350 10.25:1 and not even there for LC XU1.
S engine, HT-HG 186 (and HX to HZ ADR27A auto engine cam which was close) cam:
Inlet: 23 53 256 (ADR27A auto 29 59 268)
Exhaust: 58 18 256 (ADR27A auto 64 24 268)
Overlap: 41deg (ADR27A auto 53)
LJ XU1 cam:
Inlet: 44 80 304 (was 38 86 304 prior to LJ release)
Exhaust: 83 40 304 (was 77 46 304)
Overlap: 84deg |
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Originally Posted by: HK1837  I was having a further think on the 173 and why GMH dished the pistons for both engines. As you can see they are very small dishes in the pistons for the 173's. The Low comp was 8.3:1 with the 202 head and a 3.71cc piston dish. The high comp 173 was 9.4:1 with the 149/161 high comp style heads and 1.92cc dish. Clearly GMH didn't want to go over 8.3:1 in the red 6 with 87 Octane fuel or 9.4:1 for 97 octane fuel with that tiny little HQ 6cyl camshaft - the dynamic compression with that cam must have been far too high to allow any higher compression:
HQ 6cyl cam at (I think) 4 thou lift:
Inlet: 15 45 240
Exhaust: 50 10 240
Overlap 25deg
You'd think that with a larger camshaft in either engine (as fitted to the late LC GTR with 2850S engine, HT-HG regular 186 (not Trimatic) or the 202S as was to be fitted to the stillborn HQ GTS coupe) that these engines could have run flat top pistons for something like 9.6:1 - the 202HC with flat tops would have had 2.36cc less combustion chamber and the 173HC 1.92cc less. The LJ XU1 was happy with 10.3:1 (+/-0.5) but with a significantly bigger cam. Funnily enough the original GMH specifications for the XU1 state it was designed for 98 Octane fuel, whereas all the other Super fuel intended engines were 97 Octane. The Octane rating is missing (as in blank) for HT-HG GTS350 10.25:1 and not even there for LC XU1.
S engine, HT-HG 186 (and HX to HZ ADR27A auto engine cam which was close) cam:
Inlet: 23 53 256 (ADR27A auto 29 59 268)
Exhaust: 58 18 256 (ADR27A auto 64 24 268)
Overlap: 41deg (ADR27A auto 53)
LJ XU1 cam:
Inlet: 44 80 304 (was 38 86 304 prior to LJ release)
Exhaust: 83 40 304 (was 77 46 304)
Overlap: 84deg Yep a low comp 173 with 8.3:1 on 87 octane that sounds high for such fuel ! Look at all the rest of the Holden's that ran close to that ! did they have to run super ? like 1159cc 4cyl HB LC-J Torana was 8.5:1 and LC-J 1.6L was 8.5:1 Did they run Super or standard fuel ? With the HT-G GTS350 4SP at 10,25:1 and Ford 351 XY GT being 11:1 and XA GT with dot heads came down to 10,7:1 and our 97 octane did the trick ? or did it truly ? did they have to back off a bit of spark timing advance with some 97 octane fuels ? as not all maybe the same value in reality maybe ? look at how gutless some of the test were of such cars in the test back in the days and dyno figures i have seen of back them days. In the USA such as the 350 with 10.25:1 ran what fuel ? maybe our 97 octane ? but the next up 350 with more compression was not sent to Aus, why our fuel was not good enough to run such ? but why did the Ford 351C 4V ok to run 97 Octane ? or were they de tuned to run such as 97 or does the bigger cam just brake the ice with our 97 ? Our 351C in XW GT-HO P2 Is fine to run 97 octane and it has more higher lift cam than the XY GT-HO. Did they use 97 octane in Bathust etc or higher ? as the Bathurst cam in the GT-HO had a higher lift than both P2 and P3. maybe even the Holden HK GTS327 ran more lift as well, as well as compression ? as who would run 8.75:1 in 327 racing ?
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US fuel in the time we are talking was 100 octane for the good stuff and 91-92ish for regular. That is why GMH specified 97 Octane Super fuel for the 8.5:1 HQ and 2nd type GTS327, 8.75:1 307, 9:1 GTS327 and HT-HG GTS350 auto engines as they wouldn't run properly on the below 90 Octane Standard fuel in Australia. It is also why all of those engines except the 1st GTS327 ran far more advance than the same engines did in the USA - they all actually used the HK GTS327 spec. distributor in GMH vehicles. It is also probably why GMH gave the 307 10hp more than it was rated in the USA, as it was using regular fuel and much less advance in the USA.
You weren't allowed to change compression in series production, so you ran what they were standard within manufacturing tolerances. HK GTS327 at 8.75:1 is actually an error, it's just over 9:1. Chevrolet actually publish the identical sealed engine in 1969 as 9:1. The 327 has to be higher than 8.75:1, remember the 1968 307 uses the identical heads, identical stroke and flat top pistons with the same size valve notches and it is 8.75:1. So the 327 with 1/8" bigger bore has to be higher compression.
The Fords you are talking about with the really high compression ran much larger solid cams than the GMH engines did, so their dynamic compression would be suitable for 97 Super. Whereas the 10.25:1 manual HT-HG engines ran the 307's GP hydraulic cam, those 350's are in reality 10:1 or just over and are rounded up to 10.25. That'd be getting close to enough with that cam on 97 with cast iron heads. We'd never have been able to run the lumpy top 11:1 or higher L46 or LT1 engines on 97, although the LT1 350 (1970 Z28) did have a really lumpy solid cam, the L46 in the 1969-70 Corvette was a hydraulic cam but bigger cam than the GP hydraulic cam that was used in the L48 1969-70 350 (HT-HG manual). |
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