What limits upper RPM in my LS5
Since now is the time of year for upgrades, I have been thinking about how to improve a symptom I have had for the twenty some odd years I have had my car, but I can’t seem to get my head around it.
I have been reading grumpyvette’s posts and links trying to understand what is going on with my 1972 C3 LS5. As I rev up through each gear, after about 5200 RPM I start to feel myself not getting pushed back into the seat anymore. There is no miss, so probably no valve float causing the power drop off.
Any ideas on how far up the RPM range my oval ported 454 should pull.
I rebuilt the engine in 1992 pretty much “stealth†stock but I added a hydraulic Crane cam with a little more lift (515-510) and duration (216-228). I used TRW 25.7cc domes and triple ground valves in the stock open chamber 3999241 heads (105 cc chambers). It has been a while since the rebuild so I lost track of a few things like deck height and gasket thickness, but I figure around 10.8 compression ratio. I woke things up some more with a 800 CFM double pumper under an LS6 air cleaner, MSD 6A ignition, and 3.7 gear.
I have some bigger exhaust pipes and free flowing mufflers, but I got a feeling the stock intake and exhaust manifolds are holding back the breathing over 5000 RPM. Not much room under the hood for any other intake. Room for headers seems a bit of a reach as well, but I read the vette big block exhaust manifolds were actually pretty good.
Thanks to grumpyvette for all of the links and information to wake up a top end. I hope I have read them all. I have downloaded many calculators from those links and the one from Comp Cam shows a similar cam is a good choice with only a minimal TQ or HP improvement available with a cam only swap; heads, headers, and intake is another story.
Don’t get me wrong. All is very good. The rebuilt M20 actually squawked forth gear until I added the 275R60 ET Street Radials last year. Now is the time of year for upgrades, but if I changed the cam or cam timing to slide up and/or increase the power curve, I am not so sure things would work out the way I expect. I might still “run out of breath†with the stock iron.
I kinda like the fact that I can spend an afternoon to put back all the stuff I upgraded and someone needs to open up my matching number engine to find anything non-stock. So changing the hood for a better intake and heads doesn’t feel right.
I am just wondering what does someone with a similar set up see after 5000 RPM; what is limiting my RPM; can I do something that can increase higher end breathing capability with my stock top end, or should I?
Thanks in advance for your opinions
For any engine to rev freely and pull well in the upper rpm ranges the heads need to breath, the cam timing needs to allow the ports to flow in the upper rpm ranges and the intake can,t be a restriction at the flow levels, necesassary to proviide a effective cylinder fill of the fuel/air mix, and youll need to control the valve train... ok step one, you need to verify everything is functioning correctly in your current combo, and that you really do have a 10-10.5:1 cpr
and the intake and cam you have CURRENTLY installed was sellected and designed for low and mid rpm torque,and more than likely your valve springs are old and need replacing,and the stock intake is not ideal for the power production above 5000rpm, in fact over about 5000 rpm its damn restrictive.
Id strongly suggest swapping to a cam with a 220-230 duration and an oval port single plane intake if your really looking to spin the 454 up, AND USE NITROUS IN THE FUTURE, but youll generally want to stay under about 6300-rpm (especialy if you sellect a hydrolic cam) and youll need to verify clearances and spring bind, personally ID sellect a mild solid lifter, or hydrolic cam with between about 225-230 durration, and under about .530 lift to avoid problems with clearances )and your cam sellection is critical so call crower and crane and erson for advice, but be sure to tell them exactly what your expecing to do, and the swap to the single plain oval port intake has a huge effect on how the cam will be able to breath so they form a matched combo, what goes in needs to exit so headers and a low restriction exhaust are necessary to getting the full benefits, and theres potential for hood clearance issues so check that out carefully before buying parts, but youll notice a distince improvement and a noticably faster increase in engine rpms if you do that.
Id also expect a noticable increase in useable hp if you went that route
a cam like this will noticable increase the breathing potential and give a very noticably lopey idle but still be street drivable in a 10.5:1 cpr 454 (Id sellect ROADES LIFTERs ) and this cam below
http://www.rhoadslifters.com/Pages/OriginalFT.html
http://www.crower.com/catalogsearch/res ... 06&x=0&y=0
intake like these
http://www.holley.com/7620.asp
http://www.midwestmotorsportsinc.com/order_part.php?item=53037&line=MWM
http://www.digitalcorvettes.com/forums/showthread.php?t=84157&highlight=beehive
you might want to know that the EDELBROCK OVAL PORT ,RPM AIRGAP DUAL PLANE intake is a better choice IF your NOT going to use nitrous, but a POOR choice if nitrous is used.
but the stock cast iron intake is much more restrictive that either choice
Thanks for the feedback grumpyvette.
OK, valve springs need to be on the top of the list and an easy upgrade. I do know they are still stock. I just read your post on how to change them in the car. The 3/8 inch cotton rope thing is way cool. The beehive springs are a great idea as well.
Do I understand the Rhoades Lifters? It looks like the rhoads lifters will reduce the duration and lift at the lower RPMs and after 3500 RPM it reverts back to deliver the design intent of the cam. Does idle and low end response mimic a smaller cam? This would seem to flatten out/improve the lower end torque curve right?
I am with you so far, but I may not be committed to the single plane manifold because of the needed hood swap. I did fire up the CamQuest6 program you recommended in another post. I may not be able to get exact results, but all things being equal, the program may be a good indicator of proportions of change. The manifold and cam you picked out really wakes up the numbers, especially if I add headers. The torque curve is lengthened and the added horsepower is intriguing, but I am still hung up on the hood swap.
I think I am going to call some manufacturers and get some opinions in the forum of which intake manifolds will fit under the hood. I can then get multiple recommendations on the best cam for the biggest intake. I can then, as you have pointed out in another post, get the averages of the manufacturers recommended specs and pick the closest cam to the averages.
I may not be able to get to the added duration and lift you picked out, but if I can increase the intake size, more of each might make sense, especially with the Roads Lifters.
This may be off the wall, but IF (I know its a big if) the beehive springs help out and I can get the RPM up a bit, adding a small shot of Nitrous would seem to do everything torque and HP wise an intake, cam, and headers would do without the hood swap.
I did go with forged pistons during the re-build. If I limit the shot to 75-100HP, do you see any concerns with my existing engine set up? Assuming I learn what it takes to do it right, what is a save level of Nitrous?
Thanks in advance for your opinions
generally you would NOT sellect a single plane intake UNTILL the cam durration was in the 235-240 range at .050 lift as the dual plane intake generally give better average torque over the cams rpm band, BUT a DUAL plane intake is generaly a poor choice for NITROUS USE, with its unequal length runners and dual plenums, and you mentioned useing NITROUS.
PLUS the oval port big block heads with the reasonably long runners of the single plane intake work reasonably well from about 3000rpm-6500rpm, with your reasonably high cpr and 3.73 rear gears youll funcion just fine with the single plane intake on or off the nitrous, and the slight loss of off idle torque may even be a plus in that youll have a lower tendency to have the tires go up in smoke before the cars up and moving when the power curve hits as you gain rpms.
a decent single plane intake on a higher compression ratio oval port BBC rarely has a problem getting enought low rpm torque but a dual plane frequently has a problem suppling air flow above about 5500rpm where the single plane intake accells, remember your complaint was that at about 5500rpm the current combo died?
ALL I can say is IVE tried BOTH ROUTEs in the past and in YOUR PARTICULAR application I think the good single plane and the other components I mentioned previously are the correct parts to get where your trying to go with YOUR application, given the manual trans, 3.73 rear gear, higher than stock compression and that may run nitrous a 454 with oval port heads
Mr. Grumpyvette Sir
Did you notice how you got elevated to sir status ?
You wrote, “the slight loss of off idle torque may even be a plus in that you’ll have a lower tendency to have the tires go up in smokeâ€.
Thanks for bringing the practical side into the thinking process. If I can’t hook up, it is wasted torque anyway and boy would I be disappointed if the RPM’s didn’t increase that much. Softening up the launch is not a bad thing; actually I am not only worried about the tires but the rest of the drive train holding together as well.
I have all new Spicers and bearings from the flywheel back along with a rebuilt tranny, posi and trailing arms, but applying too much shock loaded torque to my drive train has always been a concern. When I added the ET Street Radials last winter I put in a 2500-2600 pound clutch plate and an organic disc. I calculated the clutch should be good for 450-470 ft pounds of torque. In case the drag radials got too much bite, I wanted the clutch to slip before anything broke. I usually get a little bark out of the big tires, but I did noticeably slip the clutch once already. I think I am on a nice safe edge there.
With my minor bump from stock on the existing set up I have told you about, I think I now have around 450 ft# of flywheel torque and around 310 rear wheel horsepower, which is about 15% over stock. Assuming my original estimates are cool, I am now thinking the new stuff we have talked about will get me about a 30-50ftlb bump in peak flywheel torque and maybe 40-50 more peak rear wheel HP from what I have. Assuming I can find headers to fit, is my assessment above close to what you have seen in the past?
With the added bump in power, I am thinking I need to bump up my clutch disc as well. For my app, what clutch setup would be the best all around of what you have used in the past?
I can’t tell you how much a guy like me appreciates a guy like you taking the time to share your thoughts and experiences. All I can say is thank you very much
http://www.crower.com/catalogsearch/result/?q=01206&x=0&y=0
(crower cams phone numbers on the lower spec tag info bar)
if your really looking to boost the upper rpm power a cam like this (which is one step hotter than the cam I orriginally sellected) will produce noticably more hp/tq than your current cam or the slightly milder cam I suggested first. but I was hesitant to go quite that far because its going to drop the off idle tq a bit more than IDEAL for a car thats used mostly on the street,and a very noticable change in the useable RPM band your currently used too having with the current engine combo. but its a better match to the rest of the parts and takes more advantage of the single plane intakes potential,PLEASE,!!
if theres anyone reading this with the computer skills and a software program to post both the orriginal engine combo and this cam with the other mods listed I think it would be very helpful to post a comparison in the tq curves.
EITHER cam will work IF all the other mods are done, the first cam will be slightly more STREET FRIENDLY, but the second choice will provide a more noticable increase in hp/tq and be more fun to play with
the choices
http://www.crower.com/catalogsearch/res ... 06&x=0&y=0
the safe (orriginal)choice above( a noticable boost in hp but still a decent street cam, and your very unlikely to have any problems)
http://www.crower.com/catalogsearch/res ... 06&x=0&y=0
the garranteed higher PEAK hp choice with the potential to be a mild P.I.T.A. but probably the better choice IF the cars used mostly as a weekend toy and surely better if your looking to maximize the gains
now your choice depends on what YOU want most, increased hp with a slight loss in streetability or give up some potential peak hp . you either play it safe and go with the milder option, or you go for the max reasonable power, I really can,t make that choice for YOU, but if its used mostly as daily transportation, go the safer route, with the milder cam, youll still find a nice boost, if its mostly a TOY Id sure go for the wilder durration choice.
why not call crower and talk to thier tech guys and ask them for thier input on the choice between thier two cams?
but either choice with the rhodes lifters will work better as a dual use cam
http://www.rhoadslifters.com/Pages/OriginalFT.html
because the lifters bleed off some effective lift & durration at idle-about 2000 rpm, making either cam easier to live with on the street
with EITHER CAM CHOICE youll need to check ALL clearances and probably replace the old valve springs to maximize the results
Since now is the time of year for upgrades, I have been thinking about how to improve a symptom I have had for the twenty some odd years I have had my car, but I can’t seem to get my head around it.
I have been reading grumpyvette’s posts and links trying to understand what is going on with my 1972 C3 LS5. As I rev up through each gear, after about 5200 RPM I start to feel myself not getting pushed back into the seat anymore. There is no miss, so probably no valve float causing the power drop off.
Any ideas on how far up the RPM range my oval ported 454 should pull.
I rebuilt the engine in 1992 pretty much “stealth†stock but I added a hydraulic Crane cam with a little more lift (515-510) and duration (216-228). I used TRW 25.7cc domes and triple ground valves in the stock open chamber 3999241 heads (105 cc chambers). It has been a while since the rebuild so I lost track of a few things like deck height and gasket thickness, but I figure around 10.8 compression ratio. I woke things up some more with a 800 CFM double pumper under an LS6 air cleaner, MSD 6A ignition, and 3.7 gear.
I have some bigger exhaust pipes and free flowing mufflers, but I got a feeling the stock intake and exhaust manifolds are holding back the breathing over 5000 RPM. Not much room under the hood for any other intake. Room for headers seems a bit of a reach as well, but I read the vette big block exhaust manifolds were actually pretty good.
Thanks to grumpyvette for all of the links and information to wake up a top end. I hope I have read them all. I have downloaded many calculators from those links and the one from Comp Cam shows a similar cam is a good choice with only a minimal TQ or HP improvement available with a cam only swap; heads, headers, and intake is another story.
Don’t get me wrong. All is very good. The rebuilt M20 actually squawked forth gear until I added the 275R60 ET Street Radials last year. Now is the time of year for upgrades, but if I changed the cam or cam timing to slide up and/or increase the power curve, I am not so sure things would work out the way I expect. I might still “run out of breath†with the stock iron.
I kinda like the fact that I can spend an afternoon to put back all the stuff I upgraded and someone needs to open up my matching number engine to find anything non-stock. So changing the hood for a better intake and heads doesn’t feel right.
I am just wondering what does someone with a similar set up see after 5000 RPM; what is limiting my RPM; can I do something that can increase higher end breathing capability with my stock top end, or should I?
Thanks in advance for your opinions
For any engine to rev freely and pull well in the upper rpm ranges the heads need to breath, the cam timing needs to allow the ports to flow in the upper rpm ranges and the intake can,t be a restriction at the flow levels, necesassary to proviide a effective cylinder fill of the fuel/air mix, and youll need to control the valve train... ok step one, you need to verify everything is functioning correctly in your current combo, and that you really do have a 10-10.5:1 cpr
and the intake and cam you have CURRENTLY installed was sellected and designed for low and mid rpm torque,and more than likely your valve springs are old and need replacing,and the stock intake is not ideal for the power production above 5000rpm, in fact over about 5000 rpm its damn restrictive.
Id strongly suggest swapping to a cam with a 220-230 duration and an oval port single plane intake if your really looking to spin the 454 up, AND USE NITROUS IN THE FUTURE, but youll generally want to stay under about 6300-rpm (especialy if you sellect a hydrolic cam) and youll need to verify clearances and spring bind, personally ID sellect a mild solid lifter, or hydrolic cam with between about 225-230 durration, and under about .530 lift to avoid problems with clearances )and your cam sellection is critical so call crower and crane and erson for advice, but be sure to tell them exactly what your expecing to do, and the swap to the single plain oval port intake has a huge effect on how the cam will be able to breath so they form a matched combo, what goes in needs to exit so headers and a low restriction exhaust are necessary to getting the full benefits, and theres potential for hood clearance issues so check that out carefully before buying parts, but youll notice a distince improvement and a noticably faster increase in engine rpms if you do that.
Id also expect a noticable increase in useable hp if you went that route
a cam like this will noticable increase the breathing potential and give a very noticably lopey idle but still be street drivable in a 10.5:1 cpr 454 (Id sellect ROADES LIFTERs ) and this cam below
http://www.rhoadslifters.com/Pages/OriginalFT.html
http://www.crower.com/catalogsearch/res ... 06&x=0&y=0
intake like these
http://www.holley.com/7620.asp
http://www.midwestmotorsportsinc.com/order_part.php?item=53037&line=MWM
http://www.digitalcorvettes.com/forums/showthread.php?t=84157&highlight=beehive
you might want to know that the EDELBROCK OVAL PORT ,RPM AIRGAP DUAL PLANE intake is a better choice IF your NOT going to use nitrous, but a POOR choice if nitrous is used.
but the stock cast iron intake is much more restrictive that either choice
Thanks for the feedback grumpyvette.
OK, valve springs need to be on the top of the list and an easy upgrade. I do know they are still stock. I just read your post on how to change them in the car. The 3/8 inch cotton rope thing is way cool. The beehive springs are a great idea as well.
Do I understand the Rhoades Lifters? It looks like the rhoads lifters will reduce the duration and lift at the lower RPMs and after 3500 RPM it reverts back to deliver the design intent of the cam. Does idle and low end response mimic a smaller cam? This would seem to flatten out/improve the lower end torque curve right?
I am with you so far, but I may not be committed to the single plane manifold because of the needed hood swap. I did fire up the CamQuest6 program you recommended in another post. I may not be able to get exact results, but all things being equal, the program may be a good indicator of proportions of change. The manifold and cam you picked out really wakes up the numbers, especially if I add headers. The torque curve is lengthened and the added horsepower is intriguing, but I am still hung up on the hood swap.
I think I am going to call some manufacturers and get some opinions in the forum of which intake manifolds will fit under the hood. I can then get multiple recommendations on the best cam for the biggest intake. I can then, as you have pointed out in another post, get the averages of the manufacturers recommended specs and pick the closest cam to the averages.
I may not be able to get to the added duration and lift you picked out, but if I can increase the intake size, more of each might make sense, especially with the Roads Lifters.
This may be off the wall, but IF (I know its a big if) the beehive springs help out and I can get the RPM up a bit, adding a small shot of Nitrous would seem to do everything torque and HP wise an intake, cam, and headers would do without the hood swap.
I did go with forged pistons during the re-build. If I limit the shot to 75-100HP, do you see any concerns with my existing engine set up? Assuming I learn what it takes to do it right, what is a save level of Nitrous?
Thanks in advance for your opinions
generally you would NOT sellect a single plane intake UNTILL the cam durration was in the 235-240 range at .050 lift as the dual plane intake generally give better average torque over the cams rpm band, BUT a DUAL plane intake is generaly a poor choice for NITROUS USE, with its unequal length runners and dual plenums, and you mentioned useing NITROUS.
PLUS the oval port big block heads with the reasonably long runners of the single plane intake work reasonably well from about 3000rpm-6500rpm, with your reasonably high cpr and 3.73 rear gears youll funcion just fine with the single plane intake on or off the nitrous, and the slight loss of off idle torque may even be a plus in that youll have a lower tendency to have the tires go up in smoke before the cars up and moving when the power curve hits as you gain rpms.
a decent single plane intake on a higher compression ratio oval port BBC rarely has a problem getting enought low rpm torque but a dual plane frequently has a problem suppling air flow above about 5500rpm where the single plane intake accells, remember your complaint was that at about 5500rpm the current combo died?
ALL I can say is IVE tried BOTH ROUTEs in the past and in YOUR PARTICULAR application I think the good single plane and the other components I mentioned previously are the correct parts to get where your trying to go with YOUR application, given the manual trans, 3.73 rear gear, higher than stock compression and that may run nitrous a 454 with oval port heads
Mr. Grumpyvette Sir
Did you notice how you got elevated to sir status ?
You wrote, “the slight loss of off idle torque may even be a plus in that you’ll have a lower tendency to have the tires go up in smokeâ€.
Thanks for bringing the practical side into the thinking process. If I can’t hook up, it is wasted torque anyway and boy would I be disappointed if the RPM’s didn’t increase that much. Softening up the launch is not a bad thing; actually I am not only worried about the tires but the rest of the drive train holding together as well.
I have all new Spicers and bearings from the flywheel back along with a rebuilt tranny, posi and trailing arms, but applying too much shock loaded torque to my drive train has always been a concern. When I added the ET Street Radials last winter I put in a 2500-2600 pound clutch plate and an organic disc. I calculated the clutch should be good for 450-470 ft pounds of torque. In case the drag radials got too much bite, I wanted the clutch to slip before anything broke. I usually get a little bark out of the big tires, but I did noticeably slip the clutch once already. I think I am on a nice safe edge there.
With my minor bump from stock on the existing set up I have told you about, I think I now have around 450 ft# of flywheel torque and around 310 rear wheel horsepower, which is about 15% over stock. Assuming my original estimates are cool, I am now thinking the new stuff we have talked about will get me about a 30-50ftlb bump in peak flywheel torque and maybe 40-50 more peak rear wheel HP from what I have. Assuming I can find headers to fit, is my assessment above close to what you have seen in the past?
With the added bump in power, I am thinking I need to bump up my clutch disc as well. For my app, what clutch setup would be the best all around of what you have used in the past?
I can’t tell you how much a guy like me appreciates a guy like you taking the time to share your thoughts and experiences. All I can say is thank you very much
http://www.crower.com/catalogsearch/result/?q=01206&x=0&y=0
(crower cams phone numbers on the lower spec tag info bar)
if your really looking to boost the upper rpm power a cam like this (which is one step hotter than the cam I orriginally sellected) will produce noticably more hp/tq than your current cam or the slightly milder cam I suggested first. but I was hesitant to go quite that far because its going to drop the off idle tq a bit more than IDEAL for a car thats used mostly on the street,and a very noticable change in the useable RPM band your currently used too having with the current engine combo. but its a better match to the rest of the parts and takes more advantage of the single plane intakes potential,PLEASE,!!
if theres anyone reading this with the computer skills and a software program to post both the orriginal engine combo and this cam with the other mods listed I think it would be very helpful to post a comparison in the tq curves.
EITHER cam will work IF all the other mods are done, the first cam will be slightly more STREET FRIENDLY, but the second choice will provide a more noticable increase in hp/tq and be more fun to play with
the choices
http://www.crower.com/catalogsearch/res ... 06&x=0&y=0
the safe (orriginal)choice above( a noticable boost in hp but still a decent street cam, and your very unlikely to have any problems)
http://www.crower.com/catalogsearch/res ... 06&x=0&y=0
the garranteed higher PEAK hp choice with the potential to be a mild P.I.T.A. but probably the better choice IF the cars used mostly as a weekend toy and surely better if your looking to maximize the gains
now your choice depends on what YOU want most, increased hp with a slight loss in streetability or give up some potential peak hp . you either play it safe and go with the milder option, or you go for the max reasonable power, I really can,t make that choice for YOU, but if its used mostly as daily transportation, go the safer route, with the milder cam, youll still find a nice boost, if its mostly a TOY Id sure go for the wilder durration choice.
why not call crower and talk to thier tech guys and ask them for thier input on the choice between thier two cams?
but either choice with the rhodes lifters will work better as a dual use cam
http://www.rhoadslifters.com/Pages/OriginalFT.html
because the lifters bleed off some effective lift & durration at idle-about 2000 rpm, making either cam easier to live with on the street
with EITHER CAM CHOICE youll need to check ALL clearances and probably replace the old valve springs to maximize the results