RPM for highway cruising

I knew my speedometer was off, so today, when I went out for a drive, I brought along my GSP. Looks like my speedometer is off by approximately 9%.
At a corrected 70 mph my tach shows 3800 rpm, and at 75 mph it shows 4000 rpm. Is this considered too high of a cruising speed for our engines? My IM will do a lot of cruising and most will be in the 70-75 mph range.

That's about right for a stock 1971 and up transaxle; we always pretty much doubled the tach for the mph. It's terrible for a true Freeway Flyer unless your tire radius is considerably less than stock.

The engine, even a good stocker, will work well in that rev range, but air cooled engines need to have the rpm varied frequently during long drives, don't keep a constant speed and rpm for long periods.

A well built engine will rev freely past 7,000 and can live well at 6,000. We have an engined designed by AJ, with some of his parts and pieces installed, that takes 8,000 on nearly every shift and is running strong after three years.

Luck,

TC

Good thing cruise control isnt a standard option on these cars.

Ron, This has been talked about a lot here---you can see more if you will do a search.

You are seeing 4,000 at 75 MPH . At 75 I'm doing 3,200.
At 4,000 RPMs I'm doing 88 MPH.

What are your specs? Mine are:

Tire height, 25"
R/P 4.120
4th gear .821

Mine was set up for long highway drives but it's still plenty peppy around town. I don't drive around in town in 3rd as 4th is plenty strong and doesn't lug the engine at all.

I wonder what your specs are. Also, maybe you aren't getting a real accurate speed reading?

What is your highway MPG?

Jack, are you running a 195/65/15 rear tire?

Larry, my tires are just the 165 X 15s. I don't have the clearance at the rear fenders to run larger tires.

I presently have 185/65-15s (24.5" diameter).

When I first owned the car I had some work done on the tranny, but I've forgotten what I had done. I do know that first through to third are on the short side and there is a big gap between third and fourth.

My speeds were checked with a GPS, not the speedometer.

I haven't driven the car enough to do a mpg check.

Ron--I envy you being able to run the larger tires. My space between the edge of the fender and the tire is so small that I can't get my hand in there to was the rear tires--I actually have to roll the car a little bit to wash the top part of the tire. It never rubs though.

That's not a big complaint though---everything is really a-ok.

I'm running the same size that Ron is, but I have less than 0.25" between the tire and the fender. My speedo reads high because of the lower profile, but I like the stickiness.

--"but air cooled engines need to have the rpm varied frequently during long drives, don't keep a constant speed and rpm for long periods."

Um, why?

Yeah, that's a new one on me, too.

A few, maybe more than a few really, I became friends with one of the head mechanics at Kurtzman's VW and Cutters. He had a lot of 'ol skool and pre-skool knowledge and always cautioned me not to drive my Ghia at constant speed for long periods of time. Didn't make sense to me, but he explained that the VW aircooled engine made maximum horsepower, reached maximum cooling, got maximum gas mileage, created maximum oil pressure, and maintained maximum operating temp all at different RPMs. And the engine was very suceptable to temp changes which could alter everything about the engine including the actual size of it.

He said that by occasionally varying the speed that I drove, I would have a better chance of hitting one or more of these optimum RPMs for a certain length of time and temp, mileage, oil pressure, etc. would improve.

If the constant speed was wrong for one or more of the crucial readings, say for cooling,the the engine would heat up, the oil pressure would then suffer, the engine size would change, the engine may start running leaner, making it hotter still, lowering the oil pressure even more, and so on.

Might be bullshit, might be cautionary or just folktale, but it's what he swore by.

I still do what he said and still get REALLY great service out of my stock/mild engines. I mean CRAZY good service, considering.

Anyway, just saying.

Oh......

OK.

I guess......

Who am I to be a disbeliever of an Old Skool kinda guy.

But wait a minute....

I'm an Old Skool kinda guy, too, and ya know what?

I don't believe it!!

Yeah, the engine does all that stuff at different RPMs, but so does every other engine out there, air and water cooled, so....BALONEY!

I'm with Cory: Drive it like you stole it!

Oh, and Ron O....an answer to your original question:

I tend to run between 70 and 80 on turnpikes (if you're on I95 in the South and not doing at least 80, you get run over).

Running 16 inch rims and I think I have about 24.5" diameter on the outside of the tires (at least it sounds familiar for a 225R16X50).

The tach usually runs between 3,250 (about 68) and 4K (about 78), and most often between 3,500 and 4K (sometimes blipping to 4,200 if I'm on the New Hampshire Turnpike on a Friday afternoon and just trying to keep up with traffic).

On highways it seems to like running above 3,250 but with the external cooler it never goes above 200F no matter what, unless I stop for a light or something and then climbs to 205F and stops until I move again and then returns to 200. All checked with a Dipstick thermometer.

This help?

gn

Gordo,

If you're ever up this way again, I'll introduce you to Jim, you'll honestly like him. You can tell him yourself that you think he's bullshitting.

I'd love to be there for that . . .

Does Jim have any views on adding MMO now and then ? :-)

At 125 MPH @ 6700 rpm

My front end feels light at anything over 80 mph. At 125 mph my IM would join the speedster air force. :-)

Jack, my 185/65 tire is VERY close to the body on my passenger side. We're talking millimeters here. There's a fair bit of room on the driver's side.

"At 125 MPH AT 6700 rpm"

Really?

4900 @ 100

"Does Jim have any views on adding MMO now and then ? :-) "

You aren't suggesting that we're no longer in the era of top end lubricants are you ? ! ? A nice slow measure of Mystery oil, a drop or two every few seconds at high vacuum, a table spoon of Cod Liver Oil for the driver . . . and all's right with the world . . . of 1937.

What am I gonna do with my glass bottle AMPCO set-up now?

Around here, we have Jimbo, Mike Banner, Bill Cline, and one other guy in Plymouth who's name slips me, they ALL have these weird and wonderful solutions to problems that no longer exist. The debate still rages over the proper length of warm-up time, Bill subscribes to the "length of time that it takes to roll a joint" theory. It'll be kind of a shame when they're gone. We lost Don Berry a couple of years ago, he was another Pre-Skool guy who knew loads of stuff about how to "correct" the faults in VW engines.

It's a ton of fun to listen to these guys.

Joe--I have wondered about some top speeds for Speedsters.

My chart shows 143 MPH in 4th gear doing 6,500 rpms. I would never do that in the car myself. The fastest I've gone was on the way to Morro Bay last spring and went to 90 a couple of times. It was smooth and not light feeling in the front but cheatin' death ain't my style.

I wonder how fast others have gone in a Speedster?

This is NOT a highjack but it's at least more thread drift, I guess.

At 8000 RPM I'm doing 171.33 MPH. Of course I haven't personally driven it that fast, I'll leave that to "the stig", I'm just sayin' . . . .

David---what in the world is your redline? Your valves work at 8k?


Damned impressive.

Our fan belt turns inside out at 8 grand. Nearly every time.

I forget exactly what John Steele said to me about RPMs and such for my engine, but he allowed as how it would red line at 6,000 easy, and would take more for short times, and all of this in the first three gears. In 4th, he said something about how it would go faster than I would have nerve to try. And I think he is right about that. I may have topped 90 a time or two on the intersate chasing Cory home, in the rain, and that would have been around 4500. The car is light at the front, and the notion of joining the Speedster Air Force is just about right on. I am not sure where the lift-off point would be. It will probably do 120, but I'm not sure I'd feel very happy about it. Maybe after three bags of sand in w/ the battery, and a spoiler at the front. That might be OK. I'm supposed to have a freeway-flier and I run 185-15 Kumos that just barely fit inside the body at the back. Doubling the tach reading just about gives the right speed. 3,000 is a bit above 60 mph, I'm pretty sure.

PS: Merry Christmas to all!!

Hmmmmm. 2500 at 55 for me, 5K is 110. 70 is about 31-3200. 80 is about 35-3600. 125 is I don't know, was too bust driving to look, maybe Cory knows!!!! Stock gears, 3.44 final, 195/60 rears and 185/65 fronts.

No liftoff tendencies noted, even at 125. Spyders ran 138 on the Mulsanne straight on 4" wide bias tires, back in the day. Redline in mine, 6500, is good for 143, and no, I will never try it!

I just did a search (transmission ratios) and came across one of my old posts from 2004. In the post I mentioned the numbers for my tranny: R&P is 4.125, and 4th is 0.82. Now I know!!
Using a gear ratio calculator, my engine should be turning 3230 rpm at 70 mph. Using a GPS I found that my engine was turning 3800 rpm at 69 mph.
This can't be right.
That's a difference of over 500 rpm.
Could my Porsche 914 tach be out that much?

Typo - 5700 rpm not 6700.
At 105 mph the car really squats down. There still more in the car to go faster. It was a bit hot that day so I had to watch the oil temp.

I've decided I'm going to buy a cheap photo tach, to see how accurate my VDO tach is.

My friend Chris took the first photo, back when I still had the 9,800 rpm chip in the MSD box. I took the chip out after putting the car on the track for the first time, and replaced it with a 6,000 rpm chip.
I have a 3.88:1 Bus fourth, and an 0.89:1 ring and pinion.

I asked Jim sartwell about the rpms and cooling thing once, and he said folks want to have the best of both worlds with a "Freeway Flyer."
Making the transaxle that's supposed to be mated to the 1641 into a FF involves taking out the fourth gear of the Beetle box and installing the late Ghia fourth -- so the engine doesn't have to work as hard to maintain the rpms ... at the expense of fewer rpms from the fan.

Sounds like your Jim is a lot like my Jim, well meaning but . . .

Ron, you can get a cheap one at an RC airplane shop. They should be less than $30. You can put a couple pieces of black tape on the crank pulley, you need to simulate a propeller.

Using a FF trans ? Just add a power pulley at the crankshaft to drive a welded & balanced cooling fan a bit faster. ~ALan

I always thought a power pulley would spin the fan slower, using less horsepower on the fan.

You use a smaller fan belt that is adjusted close to the upper pulley hub, this turns the fan faster.

I just realized I don't need to buy a photo tach. When I plug my fuel injection ECU into my laptop it shows the rpm.

Ron, I remember when my son was racing his Eclipse he did much of the ECU tailoring with his laptop, but when he was at the track he could do a lot to the ECU (touch-ups and fine tuning) with a Palm Pilot in his pocket. Saved a lot of lugging the laptop around (or worrying about where it is at the track).

gn