Thread: Failing to reach 3000rpm

I just had to look it up.

The pulley sizes are 61mm & 49mm so a 12mm difference in diameter. If you measure the size of the harmonic balancer you can calculate the difference.

But i think that is a waste of time. It shouldnt tick at max rpm regardless

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I hope I'm not making this up.... but here goes:

If you went from 49mm alternator pulley = 153mm circumference
to 61mm alternator pulley = 191mm circumference
= 25% increase in circumference = 25% decrease in rotation per crank rotation....

3000rpm - 25% = 2250
4000rpm - 25% = 3000

Now someone tell me I didn't make that up!

Originally Posted by Judo

I hope I'm not making this up.... but here goes:

If you went from 49mm alternator pulley = 153mm circumference
to 61mm alternator pulley = 191mm circumference
= 25% increase in circumference = 25% decrease in rotation per crank rotation....

3000rpm - 25% = 2250
4000rpm - 25% = 3000

Now someone tell me I didn't make that up!

Sounds reasonable Justin, add to that probably has wrong length serp belt as well.

Serp belt length has no influence on rpm diffefence, only pulley diameter.

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Originally Posted by SSmith

Serp belt length has no influence on rpm diffefence, only pulley diameter.

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Correct but i was just putting it out there for him.

the serpentine belt i have is correct for the pulley i have. already checked it a while back. but wasnt aware that putting different size alternator pulley to original may put your tacho out.

i cannot remember what pulley size my original alternator was. at the time of changing it i wasnt aware of the different sizes, although when i put new serpentine belt on later i made sure it was right size for the pulley i had and not for VIN.

maybe the first belt i had was wrong size.

thanks for mentioning though

problem is, my idle is fine. and cruising rpm seems fine. maybe smaller numbers=smaller % difference so less noticeable

just wondering. what do the push rod seats look like in the block? is it an item which could bend easily or would it just break? reason im asking is as i mentioned before that guy trying to force it when i was doing clearances. would it be possible before it slipped out from under rocker arm that it bent the seat down on the camshaft a bit so that when it is at the up position its not fully opening the valve? or is that not something which is possible?

Pushrod is (by design IIRC) the weak link, it will bend before anything more difficult to get at breaks. Cant hurt to check it, pull the pushrod out and one next to it and play spot the difference, but id expect the idle to also be rough if that was the case. Did the tappet clearance set correctly after the mishap?