The 1966 L72 450 HP? 425 hp? Both?

[Bill M.]

Do you SHP cam gurus think that a revision in valve lash from 0.020I/0.024E to 0.024I/0.028E would result in 450 HP @ 6400 rpm dropping off to 425 HP @ 5600 rpm?

[Joel F.]

Re: The 1966 L72 450 HP? 425 hp? Both?

Bill,

I think that horsepower drop is actually attributed to simply "observing" the HP at a lower RPM. In other words, the "425" HP engine still made 450 at 6400, it was just officially observed at a lower RPM to make the car more insurable. A similar tactic was reportedly done with the L-88's so performance enthusiasts would choose the lower HP L-71's since on paper the looked like the strongest engine you could order.

Joel

[Duke W.]

Re: The 1966 L72 450 HP? 425 hp? Both?

The different lash specs might result in a slight difference in measureable power, but no where near 25 HP.

The rating reduction was a marketing decision.

The '65 L-78 lash specs are .020/.024", but were increased to .024/.028" for '66 with the same camshaft, and I recommend the tighter lash specs for all L-78/72/71/LS-6, all of which use the same lobes and indexing. The part number change for '67-up was due to the elimination of the rear journal groove on the '65-'66 camshafts.

Duke

[Bill M.]

Re: The 1966 L72 450 HP? 425 hp? Both?

The different lash specs might result in a slight difference in measureable power, but no where near 25 HP.

Duke

I know in the 283 days, Chevrolet recommended revising the lash from 0.012I/0.018E to 0.008I/0.018E for competition, but I've never seen any data on increased power for the tightened setting.

I dug this out of the archives:

"GM changed the specs on this and other cams and i would guess to get them to idle smoother. i can tell you from my work with these engines at the drag strip the cars have more top end power with the tighter setting"

Would the author (45134) of this quote care to add his $0.02? Got data? Specifically, mph.

[Clem Z.]

Re: The 1966 L72 450 HP? 425 hp? Both?

I know in the 283 days, Chevrolet recommended revising the lash from 0.012I/0.018E to 0.008I/0.018E for competition, but I've never seen any data on increased power for the tightened setting.

I dug this out of the archives:

"GM changed the specs on this and other cams and i would guess to get them to idle smoother. i can tell you from my work with these engines at the drag strip the cars have more top end power with the tighter setting"

Would the author (45134) of this quote care to add his $0.02? Got data? Specifically, mph.

this was a long time ago,40+ years, so i have no paper trail. also a lot of these cams were designed to be used with open headers

[Kenneth B.]

Re: The 1966 L72 450 HP? 425 hp? Both?

Do you SHP cam gurus think that a revision in valve lash from 0.020I/0.024E to 0.024I/0.028E would result in 450 HP @ 6400 rpm dropping off to 425 HP @ 5600 rpm?

Don't beleave anything GM listed as the HP. It was all PR & sales. Do you beleave the L-88 had only 430 HP. 70 LT-1 370 HP/ 70 Z-28 Camaro 360 & was the same engine. First 67's were 425 on the order sheets.
KEN

[Duke W.]

Re: The 1966 L72 450 HP? 425 hp? Both?

I know in the 283 days, Chevrolet recommended revising the lash from 0.012I/0.018E to 0.008I/0.018E for competition, but I've never seen any data on increased power for the tightened setting.

I dug this out of the archives:

"GM changed the specs on this and other cams and i would guess to get them to idle smoother. i can tell you from my work with these engines at the drag strip the cars have more top end power with the tighter setting"

Would the author (45134) of this quote care to add his $0.02? Got data? Specifically, mph.

I did not author the statement, but I can tell you the logic behind it. Loosening valve lash reduces effective overlap, which improves idle quality, but it also means the valves are yanked off and slammed back into the seat at higher than clearance ramp velocity, which will increase recession rate and also increases valvetrain noise, which is an indication of shock loading. Tightening lash increases overlap and effective duration, but the effects might be difficult to measure on cams with constant velocity clearance ramps. Some racing cams have constant acceleration clearance ramps, and they are much more lash sensitive in terms of power output. In fact the whole reason behind constant acceleration clearance ramps is "tunability", but racing only applications as they can be pretty tough on valves and seats.

GM specified clearances are usually based on the clearance ramp height times rocker ratio. For exmaple on the 30-30 cam drawing the specified clearance is .025" on both sides, which is based on the .017" clearance ramp height (which can clearly be seen from the lobe data) times 1.5.

However, all the published specs call for .030", which some feel was done to improve idle quality on FI engines because, for all practical purposes the 30-30 WAS a racing cam in 1964, and it had way more overlap than was usable for a good road engine with manifolds and mufflers, which is why these engines are known to have poor low end torque and idle quality.

The clearance issue is further complicated by rocker arm behavior, which is not constant ratio. My measurements of SB rocker arm behavior yielded 1.37:1 at the lash point and 1.44:1 at peak lift with about a 0.3" lobe, so my recommended lash specs are based on 1.37 times clearance ramp height, which is the MAXIMUM running clearance we want in the engine to be gentle on the valves. Clearance, particularly on the exhaust side, may close up during hard running, and that's okay. That's why mechanical lifter cams have generous clearance ramps.

I have some data sets on big block rocker ratio, but they they are not sufficiently consistent to make a final call. However ,so far it looks like BB rocker ratios may actually achieve very close to the OE 1.7:1 "spec" at max lift with a 0.3" lobe, but the lash point ratio is somewhat less, and my best current estimate is 1.63.

Based on the "spec" ratio of 1.7:1 and the .012" clerance ramp height, the computed clearance is .204 and .195 at 1.63, so that's why I like the .020/.024" specs that GM used for the L-78. The additional .004" for the exhaust valve is IMO an arbitrary addition to allow for exhaust valve stem expansion, which is not something they did for the 30-30 cam on 327s, which used the same lobe on both sides as does the BB SHP cam. IMO they should have used a taller ramp as was the case with the Duntov cam.

The Duntov cam clearance ramp heights are .008/.012. (Other than this the lobes are identical.) Multiply by 1.5 and you get the OE recommended .012./018"; mutiply by 1.37 and you get my recommended lash. The .008" on the inlet side was recommended for "weekend competition events" in the 283 era and became the OE spec for '63 engines to get a little more inlet duration. The Duntov cam was designed for 283s with small port heads and is not well matched to the 327 with big port heads, which is why I recommend the LT-1 cam to replace both Duntov and 30-30 cams on 327s.

If anyone would like to support this project by measuring BB rocker rocker ratio behavior, email me. It can be done on the car, but is easiest to do during engine assembly or disassembly. It's not hard, but is somewhat tedious. All that is required is two dial indicators and patience, because multiple data sets are required to ensure accuracy via consistency.

Duke

[Bill M.]

Re: The 1966 L72 450 HP? 425 hp? Both?

Duke:

Every time I read your posts I learn a little more about cam design. I have a long way to go! Thanks for taking all the time for the VERY detailed explanations.

It rubs me the wrong way that Chevrolet arbitrarily revised the ratings on production engines (I understand and agree with the L88 at 430, but why was it rated at all?) I guess my "pet" theory for the re-rating is on life support!

Thanks again!

[Duke W.]

Re: The 1966 L72 450 HP? 425 hp? Both?

The 1967 L-88 was not even legal to register for the road in the US - then and now - because it did not have a PCV system, which was/is required by federal law.

For 1968 GM actually made the L-88 road legal by adding a PCV system and sufficient exhaust emission controls to pass at least federal standards, but I'm not sure if it meet CA standards.

They still didn't want the engine to be used for normal road use, but making it street legal allowed drag racers to drive back and forth from the track.

As built by St. Louis with the OE manifolds and exhaust system I doubt if the L-88 made more peak power than the L-71, and it might have made even less - and it certainly was much weaker on the bottom end. That's because the restrictive exhaust system and high overlap cause HUGE pumping loss at high revs. The OE 2.5" exhaust system is pretty efficient on a massaged SHP/FI small block - about 3 psi backpressure, but exhaust backpressure increases with the square of flow, and since a good SHP big block pumps 30-40 percent more air and exhaust mass, exhaust backpressure increases to the 5-6 psi range, which is restrictive, and during the overlap period, that high pressure delays the start of the inlet stroke until well after TDC. With headers and open exhaust wave dynamics creates a depression in the cylinder before TDC, which gets the inlet process going earlier, which can improve VE by 10-15 percent over the OE exhaust system.

A good SHP/FI street engine will make peak VE of 90 percent or a little better- about the same as a base engine, but at higher revs. Now put a proper racing exhaust system on the SHP/FI engine - nothing else - and VE goes up to about 105 percent - a HUGE difference.

Blueprinted with a proper racing exhaust system - headers and open exhaust the L-88 made about 560 gross HP at 6500, which is how it was meant to be configured, but I bet it wouldn't break 350 SAE corrected RWHP in OE configuration, which is what a good L-71 will do.

Duke

[Michael H.]

Re: The 1966 L72 450 HP? 425 hp? Both?

The 1967 L-88 was not even legal to register for the road in the US - then and now - because it did not have a PCV system, which was/is required by federal law.

I know the 67 L88 didn't exactly meet emission standards but I don't think there was any restriction on registration for legal street use.
It may have been against fed regs to manufacture a car like that but it wasn't illegal to drive it on the street. Things were a little different in 1967.
I know of at least one 67 L88 that was a plant delivery and was driven home.

Even with factory exhaust, an L88 still makes more power than an L71.

[Bill M.]

Re: The 1966 L72 450 HP? 425 hp? Both?

Blueprinted with a proper racing exhaust system - headers and open exhaust the L-88 made about 560 gross HP at 6500, which is how it was meant to be configured, but I bet it wouldn't break 350 SAE corrected RWHP in OE configuration, which is what a good L-71 will do.

Duke

I can confirm the L-88 560 with tubular headers and low backpressure. I got to run the dyno at Tonawanda.

I would have agreed with the L-88 being severely restricted by the stock exhaust system until recently. I was shocked to see the times a dead stock L-88 ran at the Pure Stock Muscle Car Drag Race. These cars are not allowed ANY modifications right down to period-correct tires. He ran over 120 mph. That's about 500 SAE net at the clutch (assuming about 3500 lb. as tested.) More than one L-88 did this! I get maybe 425 RWHP.
These cars are torn down to verify internals.

[Bill M.]

Re: The 1966 L72 450 HP? 425 hp? Both?

Don't beleave anything GM listed as the HP. It was all PR & sales. Do you beleave the L-88 had only 430 HP. 70 LT-1 370 HP/ 70 Z-28 Camaro 360 & was the same engine. First 67's were 425 on the order sheets.
KEN

L-88...see above.

LT-1 difference may be exhaust. L-78 Corvette: 425 L-78 Camaro: 375

The early '67s being 425 seems to confirm the corporate limit of 425. I didn't know that. Do you suppose they had to increase it to 435 after they tested the L-88 and got 430?