I have a '68 Etype with 4.2L engine, stock tranny, and a 3.54 rear end (all confirmed). Car is a series 1.5 in that it has twin Stombergs rather than triple SU's with crossover emmisions control. Car is 100% original except for Pertronix ignition and stainless steel exhaust. I'm running Yokohama YK520's which are 185/65R15's. Was running Michelin 185/HR15 XVS's. Tach and speedo appear to operate normally when checked against another vehicle road speed. Here's the issue. I seem to be running really high RPM's relative to road speed. Example - top gear at 40mph - 2500rpms, 60mph - 3700rpms', 80mph - 4800rpms, and a 5000rpm's running 84mph (all speeds without factory a/c running). Compared to spec seem to be running almost 1000rpm high in the upper speeds. Everything appears and sounds normal. Car is in tune, timing set at 10degree static and 0 degree strob at idle (800rpm) and 10 degrees at 1400rpm. I know the Yokos are a little smaller but had the same problem with the XVS's. What am I missing that is causing this car to run high revs vs. road speed?

Kirk Phillips
'68 Etype
'62 AH 3000 MkII

Submitted by NC19-03320J on Tue, 07/21/2009 - 09:54

These people are highly recommended by many people on the E-Type Lovers forum. They're in NM.
Give Margaret ( the owner ) a call at 505-766- 6661

Submitted by NC19-03320J on Tue, 07/21/2009 - 09:46

If you know someone with a shop tach of known accuracy you could check yours against that. I thought that perhaps the problem started when the Petronics was installed, which is not uncommon on cars with the later style tach, andif that had been the case there are several shops that can correct it. Duh- dumb me, actually they could correct what ever is wrong with the tach Petronics or not. Let me seasrch around and find contact information.

Submitted by ksphillips@pro… on Tue, 07/21/2009 - 09:22

Robert. Thanks. Just figured that out when I went out to look for the generator wires and they weren't there! Took another close look at the manual and saw that I was looking at the earlier engine. The manual indicates it is an impulse type tach. I am sure there is some method to test the accuracy. Can you advise?

Submitted by ksphillips@pro… on Tue, 07/21/2009 - 00:07

Thanks all. David Nock at British Car specialists referred me to a website www.angelfire.com/fl/procrastination/rear.html to calculate road speed based on RPM, rear axle ratio, gear ratio and tire size. The spec on the tires are 24.5" diameter (per Yokohama and by measuring the tire diameter), I verified that I have the stock 3.54 rear end and stock 4 speed gearbox. Based on this data, my road speed is higher than what the speedo is telling me. After reviewing manual, I will try the recommended test for the speedo to see if it is in spec (which I now doubt based on the other information) rather than relying on another car tracking my speed. The manual also says to test the rev counter to connect an AC voltmeter to the terminals at the rear of the right hand camshaft and assume one volt output per 100 rpms. I'll test that tomorrow. I highly recommend the website above as it is easy to use.

Submitted by SW03-47225 on Mon, 07/20/2009 - 23:40

Well, I believe in math far more than I believe in Jaguar gauges. The math says that you are getting 21 MPH/1000rpm for your tire size and gear ratio. Check your speedo with a GPS, it is far more accurate than checking against another car and better than an uncalibrated speedometer. I think that you will find that you tack is off.

Submitted by SW03-47225 on Mon, 07/20/2009 - 22:03

Well, I believe in math far more than I believe in Jaguar gauges. The math says that you are getting 21 MPH/1000rpm for your tire size and gear ratio. Check your speedo with a GPS, it is far more accurate than checking against another car and better than an uncalibrated speedometer. I think that you will find that you tack is off.

Submitted by rcmaury@bellso… on Mon, 07/20/2009 - 15:21

Although the tires are contributing to higher than normal revs, chances are if the tach is accurate, your diff has been changed out for a higher ratio. The car came with a 3.54 in the US market but there were other ratios that could be put in. Check and see what ratio you have in the back end.

Submitted by ksphillips@pro… on Mon, 07/20/2009 - 09:10

Bob:

Thanks for the reply. Yes, I had this problem before the pertronix and the tire change. I bought the car from the original owner in '04 and have checked everything out to make sure there were no other mods. It has always run high revs. I am thinking about taking a long road trip and so am just now trying to figure this out. I need to do a diagnostic on the tach to make sure it is accurate (appears to be) but if nothing else, my ear tells me that this engine is running way to high. My 6 cyl, 3 litre Healey in 4th running 60 is only turning around 2500 to 2700 rpm's. As I said, with the Michelin's it still ran high unfortunately I didn't record speed/rpm's before I changed out the tires.

Submitted by NC19-03320J on Sun, 07/19/2009 - 23:34

Kirt, I don't understand how you can have 10 degrees BTDC static timing and 0 at idle. A 185x65 is a "little bitty" tire compared to a 185x15 and that alone will raise engine RPM's at a given speed. Many people have had a problem with the later tach reading high after changing to the Petronics ignition. Did this problem exist before you installed the Petronics?
Bob