How Do Flywheels Affect Engine Performance

[Duke W.]

Re: How Can Flywheels Affect Engine Performance

In racing where hundreths or even thousanths of a second make a difference light weight flywheels can make a difference.

For cars that primarily prowl the streets and highways the OE or equivalent flywheel is the way to go. Combine a big cam with a low inertia flywheel and too tall gears and you have a receipe for a cantankerous driver.

I considered replacing the 30 pound OE flywheel in my SWC (mechanical lifter cam, CR four-speed, 3.08 axle) with the 18 pound L-88 flywheel, but decided against it as it's already tough to get rolling and achieve decent clutch life.

Duke

[bruce11495]

Re: How Can Flywheels Affect Engine Performance

Duke...You have the right combo, you need to lose the 3.08s though. I think the lowest you could go and still have decent clutch life would be 3.55s. If there was a ratio in the 3.4 area it would be the rock bottom I would think.....

[Duke W.]

Re: How Can Flywheels Affect Engine Performance

I like the 3.08s. In a 30-40 MPH rolling start you just go to first gear instead of second, and the speed in the first three gears at 6500 is 76, 102, and 127. First, second, and third are about the same overall as second, third, and fourth with a 4.11.

Clutch life was an issue when I lived in Seattle in the early SWC days. The last clutch was in good shape with 65K miles, but that was AFTER Seattle.

The other nice thing about the SHP/CR/3.08 combination is lazy 80 MPH cruising at 3100 revs, 22 MPG on the highway, and 150+ top speed, but they are not for those who judge car performance by 0-60 times.

Duke

[Jon #40768]

Re: few of us are at a circle track

I agree that a heavy flywheel will affect engine breaking. But for a street car, there are better ways to improve a cars acceleration.

[Jon #40768]

Re: to 3:08 ,or to 3:70, that is the question

Duke, I am in agreement with everthing you said, but the difference between my friends bb with 3:08s and my bb with 3:70s is amazing. The 3:70s are a wonderful all around gear for a bb car. I get 18 mpg on the freeway (notice I said freeway not town) and can still burn all the rubber you want in third gear at a standing start.

[Roger Legge]

Re: How Can Flywheels Affect Engine Performance

I have a '68 L79/M21. I determined it had 3.36 rear gears, so I assumed it was an M20. Should have known better by the RPM drops between gears. I put a clutch in it and bought a GM Perf Parts flywheel that was quite a bit lighter than the stock one. Pulled it apart, found the M21, went ahead and put it back together with the lighter flywheel. Definite difference getting moving from a stop, but you get used to it. Hindsight, I should have had the stock one reground and used it instead of the lighter one. I have no idea why anyone would couple a 2.20 first gear with a 3.36 final drive, but the car has A/C and a gentleman emailed me and said something about a default of 3.36 for '68 L79s with A/C.

Roger
#36316

[Duke W.]

Re: How Can Flywheels Affect Engine Performance

A glance at the NCRS Spec Guide power train chart indicates that '68 L-79 was available with M-20/3.36 std. or opt. 3.55; or M-21/3.70 std. or opt. 4.11. These combinations are the same with or without A/C, so it sounds like your car has a non-OE powertrain combination.

Duke

[Duke W.]

Re: to 3:08 ,or to 3:70, that is the question

The old four-speeds create a dilemma. When I ordered my SWC I had a choice of six axle ratios from 3.08 to 4.56 (3.70 was standard). The 3.08/4.56 extremes yielded the equivalent of a five-speed with no first, or a five speed with no fifth.

Since I knew I would spend a lot more time at high speed cruise and hot lapping race tracks than stop light or drag strip hole shots, I decided to go with the former extreme.

Duke

[Clem Z.]

rear gears and A/C

GM would limit the rear gear sometime because they did not want the A/C compressor turning high RPMs for long periods of time.

[Jon #40768]

Re: Your car only has to please you

I enjoy reading you comments. By the way what does SWC stand for? I am sure I should know.
Jon
ps my friend with the bb 3:08s always won his class in autocrossing. Maybe he was just a good driver.

[Duke W.]

Re: Your car only has to please you

"Split Window Coupe"

Gearing doesn't have a big impact in autocross. If you can run a course without having to shift, you've considerably reduced the chances of making a mistake because you can devote your concentration to throttle and steering control.

Duke

[Jon #40768]

Re: Your right

I dont think Floyd ever shifted. The 3:08s with a 427 worked well in an autocross. That is the only place he beat me and my 3:70s and 427. One of the most embaressing things that ever happened to me was when we were comming down hwy 152 from a corvette event. The on ramp from 152 to hwy 101 is the most beutiful in california. It is a banked sweeping left turn leading onto a downhill straight onramp. When we approached the onramp, I told my wife that Floyd was going to nail it coming off of the curve, and I slid down into 3rd gear. When I saw the front of his car raise from the acceleration I nailed it and went by him so fast that I almost ran into the back of his car. Needless to say, my wife was not happy (but I had bragging rights). Sorry I rambled.

The moral; this may not be the differance between 3:08s and 3:70s, but it is the diffenerce between 3rd and forth gear.

The length of this thread is surprising. Almost as long as the right side big block hood prop.
Jon