Time for a little true/false session

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Voodoo Rufus
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Time for a little true/false session

Postby Voodoo Rufus » Tue Sep 06, 2005 5:24 pm

"synchros outlast the rest (of the transmission) if properly engineered"

^^ huh? I thought you can wear synchros out in time, even if you shift nicely.

"if you don't run the revs up, the in-cylinder temps never get really high, so the carbon doesn't get burned off, it builds up and eventually fouls the valves" - with respect to running the engine to redline.

"My motor had 350,000km on it when we were in there, you could still see the cross hatches on the bores. I redline it all the time" - running a BMW 325ix.

I was under the impression that you can vastly decrease engine life when running higher revs frequently as the loads go up with the square of the engine speed. Also, I've never run an engine above 2/3 redline. Also, the combustion temp is still the same for given weather conditions, no matter the rpm.

Help me out here please?
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Postby nourdmrolnmt1 » Tue Sep 06, 2005 5:41 pm

true/false. depends on tranny.

false, just need the engine to warm up to optimal operating temp

wtf you want to know?

true/false depends on engine
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Postby Voodoo Rufus » Tue Sep 06, 2005 9:22 pm

I want to know if it is possible to wear out synchros in a modern manual transmission, assuming the driver knows what he's doing, and lets the revs fall before downshift, or doesn't slam the tranny in an upshift.

I also want to know if you can safely redline the engine once in a while without decreasing the life of the engine significantly. Not something I'd do everyday, but I'm curious. The redline is there for a reason. Is carbon buildup that big of an issue in modern engines as well?
"I am the right hand of vengeance, and the boot that is going to kick your sorry arse all the

way back to Earth. I am death incarnate, and the last living thing you are ever going to see. God sent me."



Main system: Sony TA-FB9, 1980's era Klipschorns.

Small system: GR AV-1 clones completed! :cwm21:
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Sound_Miata
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Postby Sound_Miata » Wed Sep 07, 2005 7:21 am

redline on the tach or redline as in fuel cut? most EFI and a number of non EFI cars have rev limiters to prevent ACCELERATION revs (meaning it doesn't take in to account shifting from 5th to 2nd on the highway or anything).

it is possible to get engine up to full temp and never burn the carbon off...whether "redlining" helps with that, I can't say, haven't done enough study to prove either way

yes you can redline frequently and not have any issues...but it depends on the motor. besides, what is "significant wear" mean to you?

yes I have seen 200k mile (not k) motors with crosshatch still in place, not a big deal if well maintained.

Carbon is an issue on every car, fuel burns, leaves deposits, thats just life until we find a different fuel method...but better/increased burns and all that will make things better.

"Also, the combustion temp is still the same for given weather conditions, no matter the rpm."

Well, ignition perhaps, but the exhaust temp can vary wildly. EGT's (exhaust gas temps) are a function of your fuel quantity and ignition timing. depending on tune you can get over a 400 degree F range based on any number of factors

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Postby zac.carlson » Wed Sep 07, 2005 7:53 am

Voodoo Rufus wrote:[...] and lets the revs fall before downshift [...]


What do you mean by letting the rev's fall before a downshift? You should always rev-match (heel and toe downshifting is an example while downshifting and braking) while downshifting. I can't think of a case where you'd rev-match by letting the rev's fall.

When downshifting, you need to bring the engine speed (rev's) up to the same speed as your transmission for the given gear. Say your car does 3k rpm at 30 mph in 3rd and 4k rpm at 30 in 2nd. You need to bring your engine speed up to 4krpm while shifting from 3rd to 2nd.

...unless you like to use your clutch as a brake. And no, letting the rev's fall and dumping the clutch in a downshift is not engine braking. Letting off the gas is engine braking - the compression of the engine naturally slows the revs, in turn slowing your car when the clutch is fully engaged.
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Postby LIVE!Auto » Wed Sep 07, 2005 10:33 am

Voodoo Rufus wrote:"synchros outlast the rest (of the transmission) if properly engineered"

^^ huh? I thought you can wear synchros out in time, even if you shift nicely.


The synchros are the only wear items in a transmission. The gears basically don't wear. Synchros will wear out faster than the rest of the transmission, especially if they're abused.
There are a few cases where a certain bearings can wear out before the synchros, but this is uncommon. The Getrag 282 design, bought and built by GM is an example of this. If the synchros are treated gently, they can out live the differential carrier bearings, which are tapered rollers.

"if you don't run the revs up, the in-cylinder temps never get really high, so the carbon doesn't get burned off, it builds up and eventually fouls the valves" - with respect to running the engine to redline.


The added stresses of running an engine hard CAN break up carbon deposits and keep the engine cleaner. This is the origin of the "Italian tune up" (from a dead stop, flat foot it through three or four gears).
This is also why GM has more trouble than they ought to with Northstar engines. When they were being developed, the engines were run HARD all the time, to be sure that they were as robust as possible. This led to the use of extra deep hone patterns in the cylinder walls to keep the rings oiled at high RPM. When the engine never sees high RPM, as when driven by the typically Caddy driver, the extra oil deposits carbon on the rings, and after about 100K miles, the rings get carboned up. If the engines saw redline a couple times a week, they'd be fine until the timing chains wore out at 200K.

"My motor had 350,000km on it when we were in there, you could still see the cross hatches on the bores. I redline it all the time" - running a BMW 325ix.


I had the top end off of the 3.1 pushrod V6 in my '90 Pontiac 6000 at 175K miles and it had hone pattern still visible with no cylinder ridge. Depends on use, metallurgy, initial hone pattern, etc. For instance, the iron liners in GM aluminum blocks are centrifugally cast, which gives them a denser, harder grain structure than the same alloy of iron in a gravity casting process. This translates to longer wearing cylinders.

I was under the impression that you can vastly decrease engine life when running higher revs frequently as the loads go up with the square of the engine speed. Also, I've never run an engine above 2/3 redline. Also, the combustion temp is still the same for given weather conditions, no matter the rpm.


Running an engine hard DOES decrease it's life span. Porsche research found that from an engine wear standpoint, 1 mile of race driving is like 1,000 miles of sedate street driving.
However, today's engines are so good that it basically doesn't matter. As long as your engine is stock, you probably won't meaningfully decrease its life span by running it hard.

Cylinder temperatures can vary a LOT, especially with turbo engines. The turbo adds heat, the temperature of the gas increases with compression, etc. Imagine starting with 30 degree air or 90 degree air... which is going to give the highest cylinder temperatures?
As was mentioned above, EGT's can vary widely based on how the engine is tuned.
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Voodoo Rufus
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Postby Voodoo Rufus » Wed Sep 07, 2005 1:39 pm

zac.carlson wrote:What do you mean by letting the rev's fall before a downshift? You should always rev-match (heel and toe downshifting is an example while downshifting and braking) while downshifting. I can't think of a case where you'd rev-match by letting the rev's fall.

When downshifting, you need to bring the engine speed (rev's) up to the same speed as your transmission for the given gear. Say your car does 3k rpm at 30 mph in 3rd and 4k rpm at 30 in 2nd. You need to bring your engine speed up to 4krpm while shifting from 3rd to 2nd.

...unless you like to use your clutch as a brake. And no, letting the rev's fall and dumping the clutch in a downshift is not engine braking. Letting off the gas is engine braking - the compression of the engine naturally slows the revs, in turn slowing your car when the clutch is fully engaged.


You are correct, and that is what I meant to say. I typically double clutch when downshifting to rev match, or downshift as quickly as possible before the revs drop without forcing the transmission into the next gear.

Live!Auto, so basicly revving high will decrease engine life, but engines are so well engineered these days it is not a very big deal? I would call a good engine life span 300K miles. My '88 Honda Accord has 294K and still runs nicely. I have heard of the "cylinder ridge" effect, if an engine is not revved high much at all and then if a person tries to rev hard, the connecting rod stretches more than it normally did and things break. Right? Congrats on your first post too. Nice one.
"I am the right hand of vengeance, and the boot that is going to kick your sorry arse all the

way back to Earth. I am death incarnate, and the last living thing you are ever going to see. God sent me."



Main system: Sony TA-FB9, 1980's era Klipschorns.

Small system: GR AV-1 clones completed! :cwm21:

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