327 Rebuild

[Alexander D.]

Re: 327 Rebuild

The 327/300 max gross torque rating is 360 lb-ft @ 3200 and the 327/350 max gross rating is also 360, but at 3600. So the torque curves cross at about 3400 where both make the same power. Below 3400 the 300 HP engine makes more power at any RPM, and above this engine speed the 350 HP version makes more along with a about 1000 more useable revs.

The Special 300 HP configuration has the 300 HP torque/power curve below about 3500 and the 350 HP torque/power curb above 3500 along with 1000-1500 more useable revs, but idles exactly like an OE 300 HP engine, butter smooth, 500 in neutral with a manual trans @ 18-19" Hg or 450 in Drive with Powerglide at slightly lower vacuum.

The L-79 cam is responsible for the 350 HP engine's idle behavior, which is about 750 @ 14-15" Hg with a slight lope due to the significantly higher overlap than the 300 HP cam, and there is no way to disguise this. Back in the day most guys actually preferred the slight idle lope, but it won't pass judging, especially a PV!

Are torque and HP graphs for the L79 and L75 engines published somewhere? Where could we go to see these graphed for these and other C2 Corvette engines?

In '66 the 300hp engine became the base engine. It didn't have a specific letter code designation (that I'm aware of.) The base motor's quoted HP and torque ratings are the same as those for the previous L75 optional engine. This new base engine used a Holley instead of a Carter carb, and I believe the camshaft # was also changed. Were these the only changes? What effect did the changes have on the performance of the engine? Was the cam change made for emission control reasons? Why the change to a Holley carb?

[Duke W.]

Re: 327 Rebuild

SAE gross engine performance curves are in most year AMA specs that are part of the "vehicle information packages" you can download from the GM Heritage Web site.

I don't know why GM abandoned the AFB. Many, including the late Dale Pearman, consider the AFB to be a better carb. Dale used to refer to Holleys as a "packaged leak". Since both the AFB and Holley have near identical flow ratings, there is no difference in performance.

The 327/300 was optional and widely installed in other Chevrolet models, and I believe it retained the L-75 RPO number for those other model lines after it became standard in the Corvette for '66.

The '66 300 HP camshaft dated to '57 and was used on all hydraulic lifter Corvette engines through '66 except L-79. It was designed for the small port 283 heads that had a "balanced" E/I flow ratio of 0.75. The same lobe was used on both sides and .050" lifter rise duration was 196 degrees.

A new base engine cam was introduced in '67 that slightly reduced inlet duration while increasing exhaust duration (194/202) in recognition that the "big port" 327 heads had somewhat "unbalanced" E/I flow ratio of 0.65. In other words, the exhaust port was restrictive relative to the inlet port, so a few degrees more duration was added to the exhaust valve event by opening it earlier.

Dynamics were also improved by reducing jerk. When this cam was designed GM had learned a lot about valvetrain dynamics with the Optron system.

These low level detail changes may have slightly improved performance, but the SAE gross performance ratings were already quite "optimistic"; 330 lb-ft peak gross torque and 250 gross HP are more realistic. Back in that era engine performance numbers were heavily massaged by the marketing guys.

Given the performance similarity of the two cams, the '67-up cam is the replacement for '57-'66 hydraulic lifter engines except L-79.

Duke

[Alexander D.]

Re: 327 Rebuild

Thanks Duke for your informative response! Guys like you are what makes the NCRS and this forum such a valuable resource.
-Alex