Corvair DiagramCorvair Photo
Corvair Center
home forum corvairs calendar links Corvair Podcast
California Corvairs
Clarks Corvair
Clarks Corvair
“CORSA"



Chevy Corvair License Plate
Chevy Corvair Chrome Wheel
Corvair Center Forum :  Corvair Center Phorum The fastest message board... ever.
Corvair Center 
RPM ranges - typical
Posted by: dedwinson ()
Date: October 11, 2020 05:33PM

For those who were following my thread earlier this year when I had a rod cap fail and totally trash my engine.........

I am totally rebuilt and as of today I was able to go on a ride around the neighborhood. WoooHooo.

With this rebuild I am noticing an increase in available "power". By that I mean it seems there is a lot more in the engine than what I had before the "incident" in June. Probably due to any one of a number of things that are different this time around.

So I have carbs all balanced and I have an idle about 800rpms. I'd like to shoot for lower, but I have the idle screws already to the point where they are barely even touching their respective tabs. this is way different than the old engine by the way.

My real questions are:
1.) How far off is the dash RPM gauge typically from the actual RPM. My dwell/tach showed 800 but the dash RPM showed roughly 1,000 maybe just a hair less. The engine "sound" suggested more like 800 as indicated by the dwell/tach.

2.) What is the typical RPM range you guys see at highway speed of say 65mph on a 4 speed manual? I haven't gotten this new build up to those speeds just yet, but my brief test at 55MPH looks like I am going to be at about 2,000 and 65MPG probably will push that to 2,500 - 3,000. I know it's not a fair comparison but my daily driver (automatic) hums at highway speeds at about 1,500 RPM

Just looking for other's experiences so i have a range to keep in mind.

Dave Edwinson
Doing Vairly well in Minnesota
'66 Vert 140HP




Options: ReplyQuote
Re: RPM ranges - typical
Posted by: acardon ()
Date: October 11, 2020 05:48PM

You can figure your RPM at a given speed by using the tire diameter and rear end ratio in this formula. Link to RPM Calculator. To get your rolling tire diameter, measure from the center of the axle to the ground and double it. This will compensate for the approximate squish of the tire.
Your daily driver probably has a overdrive to lower the RPM.

Don
Southlake, Texas
The Trunk Belongs in Front, Ask Any Elephant
Caution: Images in This Mirror May be Waterpumpers

Options: ReplyQuote
Re: RPM ranges - typical
Posted by: jjohnsonjo ()
Date: October 11, 2020 06:02PM

If you stock diameter tires and a 3:55 rear, the RPMs will be 50 x the speed. 50 x 60 = 3000

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


Options: ReplyQuote
Re: RPM ranges - typical
Posted by: MattNall ()
Date: October 11, 2020 06:12PM

To get the HOT idle speed lower retard the initial setting 2-3 degrees... NOTE: every engine is different!

MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

Options: ReplyQuote
Re: RPM ranges - typical
Posted by: dedwinson ()
Date: October 11, 2020 06:48PM

Matt - thanks for that timing suggestion! I was just absolutely delighted when I started it on my engine stand and it literally came alive with the stock timing. Didn’t think about removing some.

Thanks all for the calculations on RPM. What I am seeing is right about where the calculations suggest.

You guys are great.

Dave Edwinson
Doing Vairly well in Minnesota
'66 Vert 140HP




Options: ReplyQuote
Re: RPM ranges - typical
Posted by: RobertC ()
Date: October 11, 2020 06:55PM

You should be able to back off idle speed until engine dies.

Your linkage is not set up correctly or the throttle plate are off center and do not completely close.

RPM:

Tachs vary slightly. Tire diameter and axle ratio also are part of the equation.

3.55 = 60 mph at approximately 3000 rpm; 70 mph approx 3500 rpm.

Options: ReplyQuote
Re: RPM ranges - typical
Posted by: RobertC ()
Date: October 11, 2020 07:50PM

Ran some numbers for 3.27 axle ratio and 24 inch diameter tires.

2800 rpm = 61 mph

3300 rpm = 70 mph

Options: ReplyQuote
Re: RPM ranges - typical
Posted by: JimBrandberg ()
Date: October 12, 2020 07:45AM

Something isn't right with the linkage or something if it won't idle down.
If no idle circuit in the Secondaries can you feel suction when blocking them off with the palm of your hand?
I often disconnect the throttle rod from the crossover to take that out of the mix when troubleshooting. Hook the return spring to the rod hole.
A vacuum leak can make the idle wonky.
The engine speed on the highway can be disconcerting when compared to modern overdrive cars.
Glad to hear it has more power, a wonderful feeling indeed.
I'm more concerned with timing at high RPMs than low. Backing it off for idle purposes will reduce your power at other RPMs. AS much advance as it will bear without pinging is a good plan, within reason of course. Do you have a 140 distributor?

Jim Brandberg
Isanti, MN
CorvairRepair.com



Options: ReplyQuote
Re: RPM ranges - typical
Posted by: 66vairman ()
Date: October 12, 2020 09:21AM

As stated tire size and axle ratio impact RPM at a specified speed. When new the tachometers were accurate and some still are, others are not.

The "ideal" tire size is 825 revolutions per mile (that's measuring with the tire under weight on pavement). The original tires tended to be a little smaller than 825 revolutions per mile.

Anyway --- roll the car a couple tire revolutions (rear tire) and measure the inches and divide into inches in a mile and you have the rotations per mile. It's a mile a minute at 60MPH so rotations X axle ratio gives you the engine RPM at 60MPH.

Note the stock size tire is usually around 840 revolutions per mile. With 840 tire revolutions per mile --- about 3,000 RPM for a 3.55 axle and 2,800 RPM for a 3.27 axle at 60 MPH.

Yes we are used to new cars having low RPM at 60 MPH, but new engines are designed for high load at low RPM. The Corvair engine IS NOT. It will sing along at 3,000 to 3,500 RPM all day on the road without complaint.

Options: ReplyQuote
Re: RPM ranges - typical
Posted by: acardon ()
Date: October 12, 2020 09:56AM

How accurate is your speedometer is another variable? The speedometer needle is magnetically driven and wear and gummy can change it's accuracy. On the other hand, the odometer is direct drive and should show 1 mile for a measured mile on the interstate over a long distance. A single mile marker may not be exactly a mile but 10 or 20 mile markers are usually pretty close. Of course, there's also GPS.

Don
Southlake, Texas
The Trunk Belongs in Front, Ask Any Elephant
Caution: Images in This Mirror May be Waterpumpers

Options: ReplyQuote
Re: RPM ranges - typical
Posted by: dedwinson ()
Date: October 14, 2020 08:01PM

thanks for the notes


I knew speedos can be off and I used a speedometer app on my phone to get a more accurate speed and with the RPM calcs I am good there.

I will be playing with the idle ete. for a bit. but am on a short timeline as it goes into storage November 4th this year. Other RL stuff getting in the way at the moment.

postive side is it runs, drives and i have had it out just a bit for a gas run and testing. the move to storage is just a few miles away so some of these adjustments and a few other items will need to wait until spring.

the fact i made it this far is spectacular.

Dave Edwinson
Doing Vairly well in Minnesota
'66 Vert 140HP




Options: ReplyQuote
Re: RPM ranges - typical
Posted by: 66vairman ()
Date: October 15, 2020 06:24PM

A fellow in our club had a high idle problem.

I found the main link from the gas pedal linkage to the carburetor cross over shaft was keeping the carburetor levers from hitting the idle speed screw! I disconnected the main rod from the linkage cross over rod and the carburetors closed all the way and then the idle could be adjusted. Do make sure you leave the throttle return spring hooked up to the crossover rod while running the engine.

Not saying this is your issue, but it's an easy thing to miss.

Options: ReplyQuote


Sorry, only registered users may post in this forum.
This forum powered by Phorum.