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Paul Workman · 02-28-2012

Just following along here...

Seems to be a lot of pushrod SBC experience being projected onto a DOHC platform; a platform that was tested by alternating it from max rpm to max torque continuously for over a week before being torn down for inspection. It was deemed ready to go again!

I believe it was in the book "Heart of the Beast" where it was mentioned that the motor was good to about 8000 rpm, and above that oiling was the problem and lubrication-related failures of varous kinds, e.g., cams seizing, chains breaking ensued, as I recall. (However, Fastlane was running his LT5 to the mid 9000s in the quarter, but I never saw anything written about what all mods were done to that motor to address the rpm issue - if any.)

The (approx) 7200 factory limiter setting was chosen as a practical limit because 1) the power curve had peaked around 6400 (stock motor) and was tailing off past practical use, but 2) the main concern was the periphery components - alternator, PS pump, etc. (The AC compressor is shut off by the ECM above 4xxx rpm)

Serious performance is in the offing after porting and especially porting with cams, and cutting it off at 7200 is apparently premature for most properly modded LT5 motors (or certainly the case with the "FBI" modded motors at least).

Cases in point:

Y'all have seen Pete's video where he hits 129.x mph in 3rd gear. (The video done at Great Lakes Dragway showing a couple burnouts and 3 runs) Considering 120 mph is achieved at 7200 in third gear for stock tire sizes, Pete would have to set his limiter north of 7700 rpm to hit 129 mph.

Then there's the 10.x second big inch LT5s running way north of 7200+++ at WOT

After porting my LT5, it was still pulling hard when the danged limiter would cut me off; I hit the limiter many times. I, as well as Dom and others had to bump the limiters to at least 7600 to accomodate (in my case) a 7200 shift point, AND allow some headroom as I hit the 7200 rpm mark way short of the finish line.

Something else...

Some have questioned if their rev limiter was working at all because they had observed the tach sweeping up to or past 8000 without hitting a limiter. I can tell you that before I recalibrated my tach the needle would sweep off the end of the scale before hitting 7200 rpm! It is apparently quite common for these tachs to go out of calibration over time - mine was reading 20% high, in fact (resulting in running off the scale before hitting 7200 actual rpm).

So, I suspect there are some that have been cutting their LT5s waay short of their potential because of a combo of unwarranted concern for reving too the stock limiter setting, and because they're looking at a tach that may be reading significantly higher than the actual rpm. That is too bad, because it is robbing them of the biggest gift the DOHC LT5 has to offer...RPM!

(I observed this very phenomenon when I let a friend of mine drive my Z while I rode shotgun. He was short-shifting it at about 5800 rpm! I asked him why he wasn't riding it out, and his response was he was going by the sound and what he knew from "regular" V8s. So, I wonder how many Z drivers there are out there that are instinctively shifting their Zs according to patterns long etched into their minds by experience with pushrod putters!?

AND, here is a classic comparison between a pushrod LS1 and a stock LT5. Notice what happens to the gap every time the C5 driver has to shift. You'll get a really good view of the beauty of the LT5's reving)

Fuel cut-off is the method used by the limiter. The result of hitting the limiter is the engine will pop and sputter until the rpm is reduced. I don't know why anyone would attempt to hold the rpm against the limiter intentionally - that can't be a good idea (on so many levels). But, bumping into the limiter occasionally is apparently not going to hurt anything: after all, the limit of the stock motor itself is somewhere more like 8000 rpm. (excluding belt-driven accessories - I'd be more concerned about that Chinese-made replacement alternator than anything.

)

A shift light, e.g, the Raptor that can be hidden behind the vent grill, would be "thee" way to know when to shift without bumping the limiter - AND the lights are much more accurate than the tach, due to the tach's inertia lagging (especially in lower gears).

Uz guys afraid of going above 6500 rpm either have lace on your panties, or have "pushrod characteristics" embedded in your brains, I recon!

Let that LT5 stretch it's legs and show ya what she'll really do!

P.

02-28-2012	#71
Paul Workman Join Date: Nov 2007 Location: Squires (near Ava MO in the Mark Twain N'tl Forest) - Missouri Posts: 6,493	Re: Poll: Who has ever hit the rev limiter? Just following along here... Seems to be a lot of pushrod SBC experience being projected onto a DOHC platform; a platform that was tested by alternating it from max rpm to max torque continuously for over a week before being torn down for inspection. It was deemed ready to go again! I believe it was in the book "Heart of the Beast" where it was mentioned that the motor was good to about 8000 rpm, and above that oiling was the problem and lubrication-related failures of varous kinds, e.g., cams seizing, chains breaking ensued, as I recall. (However, Fastlane was running his LT5 to the mid 9000s in the quarter, but I never saw anything written about what all mods were done to that motor to address the rpm issue - if any.) The (approx) 7200 factory limiter setting was chosen as a practical limit because 1) the power curve had peaked around 6400 (stock motor) and was tailing off past practical use, but 2) the main concern was the periphery components - alternator, PS pump, etc. (The AC compressor is shut off by the ECM above 4xxx rpm) Serious performance is in the offing after porting and especially porting with cams, and cutting it off at 7200 is apparently premature for most properly modded LT5 motors (or certainly the case with the "FBI" modded motors at least). Cases in point: Y'all have seen Pete's video where he hits 129.x mph in 3rd gear. (The video done at Great Lakes Dragway showing a couple burnouts and 3 runs) Considering 120 mph is achieved at 7200 in third gear for stock tire sizes, Pete would have to set his limiter north of 7700 rpm to hit 129 mph. Then there's the 10.x second big inch LT5s running way north of 7200+++ at WOT After porting my LT5, it was still pulling hard when the danged limiter would cut me off; I hit the limiter many times. I, as well as Dom and others had to bump the limiters to at least 7600 to accomodate (in my case) a 7200 shift point, AND allow some headroom as I hit the 7200 rpm mark way short of the finish line. Something else... Some have questioned if their rev limiter was working at all because they had observed the tach sweeping up to or past 8000 without hitting a limiter. I can tell you that before I recalibrated my tach the needle would sweep off the end of the scale before hitting 7200 rpm! It is apparently quite common for these tachs to go out of calibration over time - mine was reading 20% high, in fact (resulting in running off the scale before hitting 7200 actual rpm). So, I suspect there are some that have been cutting their LT5s waay short of their potential because of a combo of unwarranted concern for reving too the stock limiter setting, and because they're looking at a tach that may be reading significantly higher than the actual rpm. That is too bad, because it is robbing them of the biggest gift the DOHC LT5 has to offer...RPM! (I observed this very phenomenon when I let a friend of mine drive my Z while I rode shotgun. He was short-shifting it at about 5800 rpm! I asked him why he wasn't riding it out, and his response was he was going by the sound and what he knew from "regular" V8s. So, I wonder how many Z drivers there are out there that are instinctively shifting their Zs according to patterns long etched into their minds by experience with pushrod putters!? AND, here is a classic comparison between a pushrod LS1 and a stock LT5. Notice what happens to the gap every time the C5 driver has to shift. You'll get a really good view of the beauty of the LT5's reving) Fuel cut-off is the method used by the limiter. The result of hitting the limiter is the engine will pop and sputter until the rpm is reduced. I don't know why anyone would attempt to hold the rpm against the limiter intentionally - that can't be a good idea (on so many levels). But, bumping into the limiter occasionally is apparently not going to hurt anything: after all, the limit of the stock motor itself is somewhere more like 8000 rpm. (excluding belt-driven accessories - I'd be more concerned about that Chinese-made replacement alternator than anything.) A shift light, e.g, the Raptor that can be hidden behind the vent grill, would be "thee" way to know when to shift without bumping the limiter - AND the lights are much more accurate than the tach, due to the tach's inertia lagging (especially in lower gears). Uz guys afraid of going above 6500 rpm either have lace on your panties, or have "pushrod characteristics" embedded in your brains, I recon! Let that LT5 stretch it's legs and show ya what she'll really do! P. __________________ Good carz, good food, good friendz = the best of timez! 90 #1202 "FBI" top end ported & relieved Cam timing by "Pete the Greek" Sans secondaries Chip & dyno tuning by Haibeck Automotive SW headers, X-pipe, MF muffs Former Secretary, ZR-1 Net Registry Last edited by Paul Workman; 02-28-2012 at 09:03 AM.