L88 Horsepower

[Everett Ogilvie]

Re: L88 Horsepower

Dick and Jon, as a reference, the '64 Engineering graph indicates the engine makes about 10 - 15 more HP between 6000 and 6500, depending on which exhaust system is in place. Recall that this test engine had earlier heads with smaller exhaust valves and slightly less cam, so later L88 engines may make more HP at higher rpm. Dick, I ran into a staggered jetting issue on my L88 recently when jetting it for altitude; the front jets were "reversed" (the big jet was where the small one should have been and vice versa, meaning the rich cylinder(s) were actually lean and the lean cylinder(s) were actually rich. After correcting the pattern and leaning all 4 symmetrically for my high altitude, the throttle response is awesome...

[Mark #28455]

Re: L88 Horsepower

The original carbs also have the mixture distribution tabs on the booster venturis to help the lean cylinders. Only the 3418 and 4296 carbs are commonly available.The 4296 carb I bought as a replacement from Holley 20 years ago had the boosters in the wrong places - the ones with the tabs should be diagonal from each other, not on the same side as my "restoration" carb.The 3418 is essentially identical to the 4054 except lacking a PCV nipple. The boss is there, you just have to drill it.

The Chevy Power manual from the early to mid 1970's listed their recommendation for jetting -it's boxed up now, I just moved to Orlando and can't find it, but I recall it was two steps leaner on the primary side and four steps leaner on the secondary than Holly's stock sizes (but still stagger jetted). Once I made that change, the plugs were a perfect light tan and the engine response was much better.

[Michael H.]

Re: L88 Horsepower

Mark,

Going strictly from memory, the HD book recommended jetting was as follows for the original 3418. LF #78, RF #74, LR #76, RR#78. I think (but I'm not 100% sure) the jet sizes increased slightly for the 4055.

There was also some confusion on the jetting in either the Holley book or the GM parts book but I don't remember which. One of these listed the left side jets for the right and right for left. I'll dig out my old Holley book and check it out.

[Everett Ogilvie]

Re: L88 Horsepower

From the '69 Service Manual; LF 82, RF 78, LR 80, RR 82. From the Power Book; LF 80, RF 76, LR 76, RR 78. Note that the Power Book is leaner by 2 sizes on the primaries, but by 4 sizes on the secondaries. The mixture distribution tabs on these carbs are on the LF and RR boost venturis, and these are also the corners that have larger jets (when comparing LF to RF, and when comparing LR to RR). It is cylinders 4 and 5 that lean out at WOT, which the mixture distribution tabs and staggered jets are supposed to take care of. One other item of note - the 1st design '69 carb (same carb as the '68 carb) is the 4054 which has vacuum secondaries and only one accelerator pump, on the primary side. The 2nd design '69 carb is the 4296 which has mechanical secondaries and two accelerator pumps - this carb dumps lots more fuel in, earlier.

[Michael H.]

Re: L88 Horsepower

The jet numbers that I posted are correct for the 3418A, which is a carburetor that may not have been used for long in the early weeks of L-88 production in 67. The mid production change to the 3418-1A used much richer jetting, exactly the same as the 4054 and 4296 as you quoted.

I dug out the old Holley book and found that I had the jet numbers correct for the original 3418A.

The larger jetting in the 3418-1A was used along with larger air bleeds so the increase in fuel delivery wasn't exactly linear throughout the RPM range. That L-88 jetting issue was the subject of much debate and testing at GM in early 1967.

[Duke W.]

Re: L88 Horsepower

Traditionally observed lab dyno data is corrected to "standard sea level conditions", which is 29.92"/60F dry air.

Reworking the heads with at least a pocket port/port match and three angle valve job will probably yield at least another 50 HP and raise the power peak to somewhere near 6500.

It's not clear if GM's 560@6400 was with the standard plant head machining or if the engine had been blueprinted to include porting and machining to acheive maximum flow and minimum quench distance and maximum compression ratio.

Being as how the L-88 cam was designed with very high overlap to harness header generated wave dynamics, the OE manifolds and OE exhaust system will probably cost about 100 HP. Any racing cam designed for headers and open exhaust will take a HUGE hit with manifolds and mufflers. That's why racing cams are not recommended for street engines.

As an interesting comparsion when Dave McDufford had his '65 L-79 tested on a lab dyno it made 356 HP @ 6400 and 338 lb-ft@4400 Mods included a LT-1 cam. Sportsman rods, and a nicely set of pocket ported 461s with a measured CR close to 10.35:1 on all cylinders. Everything else is OE. A few pulls were made with a set of dyno headers and the best power run was 369@6300, and best peak peak torque 358@4500. The LT-1 cam was designed with manifolds and has only a moderate response to headers. On the car with mufflers the percent difference between manifolds and headers will probably narrow.

As a further comparison I have data on a stock rebuild 327/340 and it only made 294 HP @ 5000 and then fell off. This leads me to the conclusion that the actual claimed SAE gross ratings from the era could only have been achieved with blueprinting and head work.

Dave just earned a Top Flight award.

Duke

[Dick W.]

Re: L88 Horsepower

Most of us old "pharts" realize that the hp ratings of old were without any parasitic loads i.e. generator, alternator, etc. The numbers were loaded in most cases. The notable exception being the L/88 and the ZL-1. The name of the game was who could make the most advertised hp. Horsepower sold cars. It did not cost but a few dollars more to make a high performance carburated engine that it did a base engine. Sold for quite a bit more though. Warranty costs I am sure went up quite a bit also.

[Everett Ogilvie]

Re: L88 Horsepower

Doug's post above indicates Traco got 560 HP testing the motor "right out of the car", with headers I believe - this may indicate stock L88 factory configuration (no blueprinting, portng etc.). His post goes on to say after the Traco rework the result was 640 HP (this surely included blueprinting, porting, etc).

The intent of my original post was to point out the difference between Corvette exhaust manifolds and headers on an early L88-style engine. This difference was about 50-55 HP, a smaller difference than most of us imagined. The book that featured this dyno test was making the point that if you have to use exhaust manifolds instead of headers, use Corvette exhaust manifolds as they actually flow quite well.

Duke, if the L88 makes 510 (or more) SAE Gross HP with exhaust manifolds and tail pipes (as indicated in the graph), what might the SAE Net and rear wheel figures be?

[Duke W.]

Re: L88 Horsepower

Don't forget that in the car any difference in power between manifolds and headers on the dyno will be exacerbated by by the vehicle exhaust system and mufflers. The OE 2.5" system is not too bad for a 350 HP SB and probably generates only about 3 psi back pressure. Increasing exhaust flow by about 50 percent is going to more than DOUBLE backpressure. (backpressure increases with the square of flow rate). In addition to the pumping loss from this additional backpressure, it will negate much of the wave dynamics action from headers, so manifolds and mufflers in the car is a double whammy!

As a "rule of thumb" SAE net is usually estimated at about 80 percent of gross. This takes into account the difference in standard correction air density (about 4.5 percent), and the effect of front end accessories and the vehicle exhaust system. A second rule of thumb is to multiply SAE net by .85 (on manual transmission cars) to account for drivetrain and tire loss ("SAE corrected RWHP" uses the same air density value as SAE net flywheel output).

Conversely SAE net can be estimated by dividing rear wheel output by .85 and SAE gross can be estimated by dividing net by .80 to estimate gross.

These "rules of thumb" are not carved in stone or derived from the primodial muck of phyics like Einstein's Theories of Relatively. They are EMPIRICAL estimates - just a ballpark average of observed data for many different engine configurations.

The 0.8 ratio of SAE net to gross is probably reasonable for a typical SB both medium performance and SHP configurations. I expect it would be lower for all BBs due to their greater exhaust system restriction, and even greater for a L-88 with manifolds due to their incompatibility with the high overlap racing cam.

Hopefully Jon (and Dave McDuffored) will run their cars on a chassis dyno, so we can get some comparative data and see what the actual conversion factors are for these two engines.

You can get an idea of the range of net/gross ratings by computing the ratios for 1971, which were dual rated:

Base 210/270 = .78
LT-1 275/330 = .83
LS-5 285/365 = .78
LS-6 325/425 = .76

For 1972 the ratings were further reduced to 200/255/270 for the first, and I am not aware that there was any difference in the internal configurations of these engines, but there were some changes to "tuning" to meet emission standards. The ratios of '72 net/'71 gross degraded to .74/.77/.74 respectively.

To further complicate manners, I think the standard air density correction for SAE net changed at some point since the early seventies, with later air density being a little higher than it was in the early seventies, so these early seventies engines might be a few percent higher if they were rerated using today's standard SAE net air density.

Another complicating is that most modern EFI engines have cold air induction so actual induction air is close to actual ambient air density. With most carbureted engines the air density is a function of underhood air temp, which can vary widely running down the road. These early emission controlled engines also had heat stoves, but on some configurations they were bypassed at WOT and either drew in engine compartment air or with some configurations, actual ambient air.

Complicated? Confusing? You bet, but that's the way it is!

Duke

[Jon S.]

Re: L88 Horsepower

At some point, I plan to put my car on a chassis dyno, but it's not real high on the priority list right now. If I can get around to it, I'll post the results.