CR and Cam Choice
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
CR and Cam Choice
Can someone simply 
explain to me how compression ratio affects cam choice. Is it so high that some needs to be bled off with overlap/duration
I have a BBC with 10.35 CR. I currently have a Comp XE274H now and want to to go to a Lunati 60201 (256/262 213/219 .515"/.530" 112/108) to make the car more street friendly for my wife. I keep hearing with my compression I need "more cam"
explain to me how compression ratio affects cam choice. Is it so high that some needs to be bled off with overlap/durationI have a BBC with 10.35 CR. I currently have a Comp XE274H now and want to to go to a Lunati 60201 (256/262 213/219 .515"/.530" 112/108) to make the car more street friendly for my wife. I keep hearing with my compression I need "more cam"
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
The amount of overlap determines how much pressure bleeds off.
Your present cam is 230/236 110 the Lunati you posted is 213/219
That is a radical change and your compression is too high for it. This crane cam will tone it down.
http://www.cranecams.com/?show=brows...tType=camshaft
Your present cam is 230/236 110 the Lunati you posted is 213/219
That is a radical change and your compression is too high for it. This crane cam will tone it down.
http://www.cranecams.com/?show=brows...tType=camshaft
Last edited by gkull; Jun 9, 2010 at 04:55 PM.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
The amount of overlap determines how much pressure bleeds off.
Your present cam is 230/236 110 the Lunati you posted is 213/219
That is a radical change and you compression is too high for it. This crane cam will tone it down.
http://www.cranecams.com/?show=brows...tType=camshaft
Your present cam is 230/236 110 the Lunati you posted is 213/219
That is a radical change and you compression is too high for it. This crane cam will tone it down.
http://www.cranecams.com/?show=brows...tType=camshaft
Yeah that cam is one step down and will run fine my problem is what problem are you having with the current cam ? That cam should run fine for anyone to run around town with 230/236 is mild for a big block.
Tell us what it runs like and what your wife doesn't like about it and maybe you don't even have to swap cams.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Thanks for the suggestions, but the Crane cam recommended usable RPM range is 2000-5800. I'm not using a stall converter (well I am now- but it's coming out). Would this be a drivablity issue, again this car needs to be very street friendly, don't want to go to neutral at stop lights etc.
Again not clear on the compression issue...what is it about my current compression that makes the Lunati 6020 not a good choice (I guess I need to know how that will affect engine performance and drivability)???
So I need a cam with more duration to bleed off the compresson somewhat?
Mike
Again not clear on the compression issue...what is it about my current compression that makes the Lunati 6020 not a good choice (I guess I need to know how that will affect engine performance and drivability)???
So I need a cam with more duration to bleed off the compresson somewhat?
Mike
Last edited by Hvymtlc5; Jun 11, 2010 at 09:37 AM.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
This is one example of why it's not worth wasting your time posting.
230/236 is actually a mild cam if the lobe sepperation is higher than 110 and the valve events are altered. But people gravitate to the all show and no go BS comp cams line up of XE cams. Only later in real life find out that they suck.
Another part of the problem is owners that don't understand tuning and the whole package of parts relationship.
Then I suggest dropping 8 degrees on the intake cam. Very major change especially since the lobe center also went to 112
230/236 is actually a mild cam if the lobe sepperation is higher than 110 and the valve events are altered. But people gravitate to the all show and no go BS comp cams line up of XE cams. Only later in real life find out that they suck.
Another part of the problem is owners that don't understand tuning and the whole package of parts relationship.
Then I suggest dropping 8 degrees on the intake cam. Very major change especially since the lobe center also went to 112
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
Thanks for the suggestions, but the Crane cam recommended usable RPM range is 2000-5800. I'm not using a stall converter (well I am now- but it's coming out). Would this be a drivablity issue, again this car needs to be very street friendly, don't want to go to neutral at stop lights etc.
Again not clear on the compression issue...what is it about my current compression that makes the Lunati 6020 not a good choice (I guess I need to know how that will affect engine performance and drivability)???
So I need a cam with more duration to bleed off the compresson somewhat?
Mike
Again not clear on the compression issue...what is it about my current compression that makes the Lunati 6020 not a good choice (I guess I need to know how that will affect engine performance and drivability)???
So I need a cam with more duration to bleed off the compresson somewhat?
Mike
He's also stating, the car will need a high stall speed converter, gearing, carb tuning and distributor tuning just to drive it!!!
I have learned over many years of screwing around that if you want a screamer/track car then go for it
BUT If you need a cruise night, cruise down the highway, take it to work kind of car then stay conservative!!!10 to 1 compression is nice if that's what you really have....I assume you need to run 91+octane and may have a ping every now & then... A mild cam wild be a nice improvement. I would keep it under 230 duration @.050
IMO If you go any high your car will be a dog off idle unless you want too start making all those chages we talked about...PS Anything over .500 lift you cannot use stock rocker arms, long slots are needed
Le Mans Master
Joined: Apr 2005
Posts: 6,359
Likes: 428
From: Seattle Area WA
If you want a driveable car then why not use a GM grind cam, or something along the lines that GKull suggested, a wider LSA with slower ramp rates to reduce your dynamic compression and non of the comp xe BS to deal with.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
FWIW...
We have an old school Comp 270h in my dads 9.8-1 461" and it has a nice idle, you can still hear a little lope...but not at all choppy. It makes power right off idle with the Performer intake and is very docile, torquey....it's in a 4200lb 69' Grand Prix with a 4sp and the car is a dream to drive....at least for a cruiser...cam has been in that motor 20 yrs.
Ofcourse there is much better cams out today that make way more power but if you wanted something mild, can run stock converter and exh manifold etc, I think the 270h is still a good one... We have never had any pinging in this one running 36* total but do not run vacuum advance.
We have an old school Comp 270h in my dads 9.8-1 461" and it has a nice idle, you can still hear a little lope...but not at all choppy. It makes power right off idle with the Performer intake and is very docile, torquey....it's in a 4200lb 69' Grand Prix with a 4sp and the car is a dream to drive....at least for a cruiser...cam has been in that motor 20 yrs.
Ofcourse there is much better cams out today that make way more power but if you wanted something mild, can run stock converter and exh manifold etc, I think the 270h is still a good one... We have never had any pinging in this one running 36* total but do not run vacuum advance.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
This is one example of why it's not worth wasting your time posting.
Another part of the problem is owners that don't understand tuning and the whole package of parts relationship.
Another part of the problem is owners that don't understand tuning and the whole package of parts relationship.
I started building this motor/cam based upon C3 group advice 4 or 5 years ago and then got side tracked with my C5. Now I basically want to go back to something very conservative without the race crap, like stall converters, and lopey cams, and big gas sucking carbs.
I need to buy a new cam package and want to get it right this time. Tired of working on an engine that has not left the garage due to hodge podge of upgrades that don't work well together.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Right off of the crane cam card what does this say:
GOOD IDLE, PRIMARILY USED IN UP TO 350 HP NEAR-STOCK ENGINES FOR MILD PERFORMANCE APPLICATIONS
Right off of the Comp cam 274 card what does this say:
Hydraulic-High performance street, very strong mid-range, with headers & 2500+ stall.
There is a point when you go down on duration where you have to look at what is called dynamic & static compression ratio. You find these calculators on the web. Remember these are not 100% true. They are rough est. They don't account for intake or combustions chamber design. You might come across a person called David Vizzard, well I rate him right in there with the Branch Davidians, David Koresh
GOOD IDLE, PRIMARILY USED IN UP TO 350 HP NEAR-STOCK ENGINES FOR MILD PERFORMANCE APPLICATIONS
Right off of the Comp cam 274 card what does this say:
Hydraulic-High performance street, very strong mid-range, with headers & 2500+ stall.
There is a point when you go down on duration where you have to look at what is called dynamic & static compression ratio. You find these calculators on the web. Remember these are not 100% true. They are rough est. They don't account for intake or combustions chamber design. You might come across a person called David Vizzard, well I rate him right in there with the Branch Davidians, David Koresh
Race Director
Joined: Jan 2000
Posts: 13,018
Likes: 2,262
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
George....you're typing too fast!
Remember...overlap occurs when there is NO compression going on..both valves are open. It doesn't do anything for cylinder pressure until you take into account *possible* cylinder filling advantages from scavenging effect.
If you have 10.3 compression...I'd be scared to dropping to a 213*@.050 cam on a 110 LSA. You will have big time cylinder pressure for sure. Without some timing controls..it's going to be tough.
If you are going *wife friendly* then the wider LSA will help a lot. I'd use the larger cam with a 112-114 LSA. That will close intake later, improve vacuum, idle better and still pull well on the top end.
Lift is always good....so don't give away power with a .500" lift cam.
JIM
Remember...overlap occurs when there is NO compression going on..both valves are open. It doesn't do anything for cylinder pressure until you take into account *possible* cylinder filling advantages from scavenging effect.
If you have 10.3 compression...I'd be scared to dropping to a 213*@.050 cam on a 110 LSA. You will have big time cylinder pressure for sure. Without some timing controls..it's going to be tough.
If you are going *wife friendly* then the wider LSA will help a lot. I'd use the larger cam with a 112-114 LSA. That will close intake later, improve vacuum, idle better and still pull well on the top end.
Lift is always good....so don't give away power with a .500" lift cam.
JIM
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Right off of the crane cam card what does this say:
GOOD IDLE, PRIMARILY USED IN UP TO 350 HP NEAR-STOCK ENGINES FOR MILD PERFORMANCE APPLICATIONS
GOOD IDLE, PRIMARILY USED IN UP TO 350 HP NEAR-STOCK ENGINES FOR MILD PERFORMANCE APPLICATIONS
This cam stuff is just something I can read and read and not quite understand because I don't have any practicle experience with it - so I appreciate evryone trying to help and suggest.
I had a 7162 Edelbrock in there to begin with and wiped a lobe on start up...so I went to the XE274 (with performance in mind) ..that goal has since changed
After forum comments checking on CR...everything I can find shows flat top notched pistons and closed chambered 101cc heads yielding about 9.5CR. I know the head were milled 10K, we have bigger valves, not sure on the block decking. Using a standad felpro 1037 gaskets at .039...so maybe I'm not at 10.3 like the machine shop said.
Last edited by Hvymtlc5; Jun 11, 2010 at 04:00 PM.
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
George....you're typing too fast!
Remember...overlap occurs when there is NO compression going on..both valves are open. It doesn't do anything for cylinder pressure until you take into account *possible* cylinder filling advantages from scavenging effect.
If you have 10.3 compression...I'd be scared to dropping to a 213*@.050 cam on a 110 LSA. You will have big time cylinder pressure for sure. Without some timing controls..it's going to be tough.
If you are going *wife friendly* then the wider LSA will help a lot. I'd use the larger cam with a 112-114 LSA. That will close intake later, improve vacuum, idle better and still pull well on the top end.
Lift is always good....so don't give away power with a .500" lift cam.
JIM
Remember...overlap occurs when there is NO compression going on..both valves are open. It doesn't do anything for cylinder pressure until you take into account *possible* cylinder filling advantages from scavenging effect.
If you have 10.3 compression...I'd be scared to dropping to a 213*@.050 cam on a 110 LSA. You will have big time cylinder pressure for sure. Without some timing controls..it's going to be tough.
If you are going *wife friendly* then the wider LSA will help a lot. I'd use the larger cam with a 112-114 LSA. That will close intake later, improve vacuum, idle better and still pull well on the top end.
Lift is always good....so don't give away power with a .500" lift cam.
JIM
if you go over .500 you will need to change your rockers! Even though a cheap set can be picked up for around $200
Le Mans Master
Joined: Apr 2005
Posts: 6,359
Likes: 428
From: Seattle Area WA
Your fine with a stock converter, that just means the powerband starts to come on at that RPM, you'll have a relatively smooth idle that will not effect your off idle starts with a stock converter.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Hugie82 - his cam in the motor in the first post is over .500 so his springs and rockers will be fine for a milder cam.
Last edited by gkull; Jun 11, 2010 at 07:51 PM.
Race Director
Joined: Jan 2000
Posts: 13,018
Likes: 2,262
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
.500 is NOT a dead set number. I've run well over that with stock rockers and no issues. You just have to check clearance. You can also get the stamped long slots from GM or others that work great.
I've run a .600/.620 cam with stock rockers. Now they did crack eventually.....but they lived a long time. Long slots would have lasted forever.
JIM
I've run a .600/.620 cam with stock rockers. Now they did crack eventually.....but they lived a long time. Long slots would have lasted forever.
JIM
Burning Brakes
Joined: Apr 2003
Posts: 844
Likes: 0
From: Central KY
Wife Friendly?
I have to ask - why not leave the cam alone and see about quieting down the exhaust sound if you can...? My wife looks right past the nice paint and classic lines of my car and goes right to "it smells funny inside and it is so obnoxious..." Whose carpets have not been soaked a few times and had to air dry? As for obnoxious - alrighty then! It runs like a big block should for the street and that is high praise...
Bottom line I am inferring here that your wife would likely not want to drive or be in that thing even if it had a Northstar Caddy motor swapped in... Enjoy YOUR car !!!
Bottom line I am inferring here that your wife would likely not want to drive or be in that thing even if it had a Northstar Caddy motor swapped in... Enjoy YOUR car !!!
