Should I use a L88 flywheel on a built L72?

[Joe R.]

I have the original block to my car. I'm thinking about using another 427 block (not from a Corvette) and building it for max HP using GM parts. In a recent thread about Pure Stock Muscle Car drags, Duke mentioned using the lightweight L88 flywheel. It's GM 3856579. I searched the archives a little. The L88 flywheel (also used on L78) weighs about 15 pounds, the "regular" 427 flywheel weighs more like 25 pounds. So the L88 flywheel helps the engine spool up quicker. What is the disadvantage? A rougher idle, maybe? Would I need a different starter and bell housing?

Thanks,
Joe

[Domenic T.]

Re: Should I use a L88 flywheel on a built L72?

Joe,
I used aluminum flywheels in my small blocks and the only drawbacks I found were that you will use more clutch in normal driving and the lighter flywheel has a harder time breaking larger tires loose at the line.

DOM

[Ken A.]

Re: Should I use a L88 flywheel on a built L72?

I would not use an L-88 flywheel on a street car, there is no advantage.

[Joe R.]

Re: Should I use a L88 flywheel on a built L72?

Hi Ken,

Could you please elaborate on why it is not a good idea?

Thanks,
Joe

[Joe R.]

Re: Should I use a L88 flywheel on a built L72?

Hi DOM,

Does it help RPMs build up more quickly? Would it increase HP slightly?

Joe

[Duke W.]

Re: Should I use a L88 flywheel on a built L72?

It depends on how you use the car. If you want maximum performance and are willing to accept some compromise in normal driving, then the L-88 flywheel is a good bet, but getting the car going from a dead stop might a a bit more difficult, especially if the axle ratio is numerically lower than 3.70.

The L-88 flywheel is a bolt-on part for a small block since it's 12.75" diameter and accepts at 10.5" clutch, but since all other big blocks other than L-78/L-88/ZL-1 used a 14" flywheel and 11" clutch, other parts will have to be changed, which will include the bellhousing and starter motor.

I know one person with a '65 "327 LT-1" (CR trans) who has run both 4.11 and 3.36 axles. Getting the car going from a dead stop in normal driving with the 4.11 is not much different than with the original 33 pound lump. It's a little trickier with the 3.36, but he lives in the Flatland, so doesn't have to deal with too many hill starts.

A lighter flywheel reduces total engine rotational inertia. It requires power to accelerate the engine's rotational inertia through the gears, so the lower the rotational inertia, the less the power required to spin up the engine, and the more power goes to the rear wheels. But the difference is slight, so it's really only worthwhile if you plan to do some serious racing.

Duke

[Domenic T.]

Re: Should I use a L88 flywheel on a built L72?

Joe,
I loved it, yes the RPM's came quicker and it did gain them faster. One other factor to deal with is the weight of a car as to what weight range flywheel is needed.


I did cut my 427 wheel down a few pounds as I know the tire size I will use won't be to hard to break loose at the line. After all we are talking GENTLEMAN RACING. I loved the sound of the engine with the lighter flywheel and it did accelerate faster, but not so good off the line.
The BB is a monster and I think you will like the lighter flywheel.

DOM

[Ken A.]

Re: Should I use a L88 flywheel on a built L72?

Hi Ken,

Could you please elaborate on why it is not a good idea?

Thanks,
Joe

It was designed for road racing, not street or drag racing.

[Joe R.]

Re: Should I use a L88 flywheel on a built L72?

Hey Duke,

I was hoping you would chime in. I don't plan to have the car judged in this configuration. I would mostly do a little street racing, perhaps a trip to the drag strip would be in order. I was planning to use a 4:11 rear end. I was also thinking about stroking the engine. With that much torque on tap, I can't imagine that taking off from a stop would be all that difficult. How many horsepower do you expect might be gained?

Joe

[Michael H.]

Re: Should I use a L88 flywheel on a built L72?

I have the original block to my car. I'm thinking about using another 427 block (not from a Corvette) and building it for max HP using GM parts. In a recent thread about Pure Stock Muscle Car drags, Duke mentioned using the lightweight L88 flywheel. It's GM 3856579. I searched the archives a little. The L88 flywheel (also used on L78) weighs about 15 pounds, the "regular" 427 flywheel weighs more like 25 pounds. So the L88 flywheel helps the engine spool up quicker. What is the disadvantage? A rougher idle, maybe? Would I need a different starter and bell housing?

Thanks,
Joe

Joe,

When I rebuilt the engine in my 425 HP 66 30 years ago, I installed the 15# flywheel and related parts. (clutch, clutch housing, starter nose etc) I would do it again in a second. I really like it.

Did the same for one of my FI 63 around the same time.

All 65 396 cars came with the lightweight flywheel with small diameter clutch assy and they didn't drive any different than a 66 425 HP with the heavy large diameter wheel.

[Carl N.]

Re: Should I use a L88 flywheel on a built L72?

Joe, I drive my L78 on the street almost weekly. Stock tune with stock clutch, flywheel, throw out bearing and 4.11 rear. The only major upgrade other than 215/70/15 rear tires (at 26psi) is old school Hurst Competition Plus Shifter. Compared to L72 & L79's it is a bit quicker thru 1st and 2nd. Wife feels the clutch is stiffer than the L79 she drove for years. I'm in the hill country of Texas after 40+ years in the flat lands of the Panhandle of Texas and have no issues with the car as long as I avoid heavy traffic in Austin. It can hold its own against most street cars from light to light as long as I'm careful with launch and 2nd gear transition. I don't see a problem with this set up on the street, but see no real advantage over the stock L72 setup.

[Jim L.]

Re: Should I use a L88 flywheel on a built L72?

I've used an aluminum flywheel on the street and would do so again. Pulling away from a start is, well, trivial. Engine revs like a sprint car motor.

The light flywheel makes sense, in my opinion, if any of the following are true:

1. Vehicle weight is very light
2. Overall 1st gear ratio (trans X diff) is relatively steep (Say 2.2 x 4.11 or 2.52 x 3.70)
3. Engine low end torque is significant (Pretty much any 427 qualifies.....)

Our car which uses the aluminum flywheel is the one in my avatar. It satisfies condition #1, weighing 2460 lbs, but it violates condition #2 having a long legged 1st gear (2.2 * 3.55). Still, it is a very driveable vehicle.... no special footwork is required to get it moving.

Jim

[Joe R.]

Re: Should I use a L88 flywheel on a built L72?

Mike,

Thanks for the info! Your testimonial is very helpful!!! Do you know the GM part numbers for the L78 / L88 starter and bell housing?

Joe

[Carl N.]

Re: Should I use a L88 flywheel on a built L72?

starter 1107352 bell housing 3788419 the bell housing from the L72 should work fine - per the Chevrolet P&A Guide 30B dated October 1972 3788419 was sold for use on 64-68 Corvette with 327 and 65-66 Corvette (396,427) w/Sp. H/Per - see page 0-21 section 0.683

[Duke W.]

Re: Should I use a L88 flywheel on a built L72?

Given your intended use, the light flywheel is a good bet. Steady state power is the same, but the engine will deliver a little more power to the rear wheels while accelerating. I don't have a good feel for the amount. It's probably no more than a few horsepower, and will be greater the lower the gear, and the difference might be detectable on a chassis dyno in back to back tests with both the heavy and light flywheels.

If I knew the rotational inertia of each (which I don't) I could calculate the power required to accelerate either at any given rate of angular acceleration.

One advantage is that when you let off the throttle to shift, revs will drop off faster, so you can make quick, smooth shifts... ever driven and old 911? They have very little flywheel effect (and somewhat of an on/off clutch), which makes them a little hard to get going from a dead stop, even with their short gears, but you get used to it, and they like to be shifted very quickly.

Duke