engine taching a bit high?

[bowtieman55]

my '88 EB (351/C6) seems to be taching a bit high *in my opinion*

at 55 mph, it's taching right around 3300 rpm...is that about what I can expect?
TIA

[chuzie]

Need to know the gear you were in as well as the transmission type, preferably gear ratio, ratio of the rear end and the tire size to calculate that.

As for typical operation, I can't say.

[bowtieman55]

ok...C6 tranny, 32 inch BFGs, and assuming stock axle ratio (3.50?)

and I meant taching 3300 rpm at 55 mph in 3rd gear, of course

[blazinchuck]

that really seems high...if you were doing 65-70 maybe, but 55 I think is too high. How long you had the truck?

[chuzie]

You should be looking at 2050 RPM assuming your third gear on the C6 is 1.00.

Considering your current RPM, you are looking at a rear end ratio of 5.71 which I seriously doubt you have so there must be some other factor involved.

Can you pull more speed / gear data throughout the enitre range so we can look at them?

If the RPM were high at idle and other ranges I would have a few possibilities for you but if it is just in the high gears I will have to sit on it and think.

Are you carb or EFI?

[justin]

That doesn't sound right at all. He is EFI w/ an 88.

Are you sure that the gears haven't been swapped sometime in its past?

And I hate to ask such a silly question, but are you sure you're in 3rd, and not in 2nd.

[bowtieman55]

I've had the truck just over a week now, Chuck..

yep, I'm sure that I'm in 3rd gear:xyxthumbs:

definitely not sure about the gears, but when the P.O. got it, it had 235s on some aftermarket wheels, so I'm seriously doubting a gear swap.

[chuzie]

I really cant say this is engine related because it should be a direct relationship between the RPM and the tire rotation (gears in between of course).

If it had over drive I would suggest the linkage need adjusting to keep it stable in 3rd but that is not the case here.

If you try the other gears we can get an idea of what is going on with the transmission and the rear end to narrow this down.

Your transfer case is in 2WD too right?

The only possibilities here are the C6 has been reworked with different gears or you have a wacked out rear end gear ratio.

Thought is would not have a significant effect, measure your tires with a tape and confirm the height in case you are on a oddball rim.

[justin]

Vacuum modulator on tranny working correctly?

I seem to recall a few years back my C4 started acting funny like that....transmission didn't want to shift and I was taching really high....could only get up to about 50MPH tops, turned out to be the vacuum modulator.

[chuzie]

I had a similar issuite on my C4. Tried cleaning and adjusting it but ended up replaced the part in the end.

I think we really need to be positive we are in 3rd gear when this is happening. Perhaps running through the range manually will confirm it.

If he were in second gear he should be looking at about 3000 RPM which is closer to what was experienced.

However, if he had 4.11 in the rear and was in second he would be seeing 3465 RPM which is even closer to what was seen.

I think we need to verify the rear end ratio and confirm it was not kicking down to second gear.

Hmmmmmmmmmm :banghead: