Is it bad raising cam duration without raising compression ratio?
Is it bad raising cam duration without raising compression ratio? I've been reading some advanced engine manuals and one guy said " The biggest mistake hot-rodders make in changing the cam is that they raise duration without increasing the compression ratio." He was referring to milling the heads or different pistions. I know it's meant to combat the loss of cylinder pressure because of the extra time the valve is open, but is it really an issue?
I've got a cam only (236/236 .581.. 112) and I don't have plans to do the heads anytime soon because I want to add a blower in the future. I want the compression no higher than it is now.
Suppose I didn't port my heads, but just milled them to raise compression by .5 - What would that gain?
Any thoughts on the duration/compression issue? -Joshua
I've got a cam only (236/236 .581.. 112) and I don't have plans to do the heads anytime soon because I want to add a blower in the future. I want the compression no higher than it is now.
Suppose I didn't port my heads, but just milled them to raise compression by .5 - What would that gain?
Any thoughts on the duration/compression issue? -Joshua
Imagine a car running right on the edge of pinging with a small cam and a 10:1 C/R. Now lets say you increase the cam duration a bit. The car will of course make more power even at the same c/r, however the low end will be a little soft. Now with the extra duration bleeding off cylinder pressure, you can increase the c/r quite a bit before you get back to being right on the edge of ping. This will go a long way in boosting your low end torque. Without bumping up the c/r, youre simply leaving a lot on the table.
Originally Posted by GuitsBoy
Imagine a car running right on the edge of pinging with a small cam and a 10:1 C/R. Now lets say you increase the cam duration a bit. The car will of course make more power even at the same c/r, however the low end will be a little soft. Now with the extra duration bleeding off cylinder pressure, you can increase the c/r quite a bit before you get back to being right on the edge of ping. This will go a long way in boosting your low end torque.
Originally Posted by FIREHAWK#608
You've painted the picture nicely. I wonder how much I'd gain under the curve if I only raised the compression (no porting) .5 or a full point.
Yeah, I'm going to leave the heads the way they are for now. I want a D1 'charger so bad. 
I'm going to use that with a high duration wider LSA (114 or 115) cam and run it till I need to uprade to large chamber (Escalade) heads, with more boost and forged pistons/rods. -Joshua
I'm going to use that with a high duration wider LSA (114 or 115) cam and run it till I need to uprade to large chamber (Escalade) heads, with more boost and forged pistons/rods. -Joshua Trending Topics
Originally Posted by dug
The rule of thumb is 3-4% more power everywhere for each point of added compression.
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Ah, thank God this is coming up more recently on the forum. My biggest gripe isn't with the shadetree mechanic who installs a 230+ duration cam in a stock block, but it's people who buy or build 400+ cid long-blocks and run 11.0:1 compression with 240+ duration cams, when they should be over 12.0:1+ specific to the ICL of the camshaft. Giving up a lot of power just because they have "big cubes."
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From: BFE
Originally Posted by JakeFusion
Ah, thank God this is coming up more recently on the forum. My biggest gripe isn't with the shadetree mechanic who installs a 230+ duration cam in a stock block, but it's people who buy or build 400+ cid long-blocks and run 11.0:1 compression with 240+ duration cams, when they should be over 12.0:1+ specific to the ICL of the camshaft. Giving up a lot of power just because they have "big cubes."
Suppose I wasn't going to supercharge later this year and I was going to keep this cam and bump up the C/R. What would you run on this motor if ptv clearance wan't an issue.
I believe this cam is on a 108 centerline with no advance ground in. 236/236-.581, .581-112
Just curious.
I believe this cam is on a 108 centerline with no advance ground in. 236/236-.581, .581-112
Just curious.
Originally Posted by FIREHAWK#608
Suppose I wasn't going to supercharge later this year and I was going to keep this cam and bump up the C/R. What would you run on this motor if ptv clearance wan't an issue.
I believe this cam is on a 108 centerline with no advance ground in. 236/236-.581, .581-112
Just curious.
I believe this cam is on a 108 centerline with no advance ground in. 236/236-.581, .581-112
Just curious.
Originally Posted by FIREHAWK#608
@.050 thanksBut, to estimate based on XE lobe profile:
You'd need to run around a 61cc head to get a DCR around ~8.5:1 with a .045" gasket, no flycutting, and .007 piston out of the hole.
Originally Posted by JakeFusion
Ah, thank God this is coming up more recently on the forum. My biggest gripe isn't with the shadetree mechanic who installs a 230+ duration cam in a stock block, but it's people who buy or build 400+ cid long-blocks and run 11.0:1 compression with 240+ duration cams, when they should be over 12.0:1+ specific to the ICL of the camshaft. Giving up a lot of power just because they have "big cubes."
So are you saying no one should put, say, the TR TRAK cam (231/234 .640/.590 112 LSA) in an untouched motor? I ask this because I have been planning on putting this cam along with upgraded springs later on in my SS with nothing done to the motor at all.
Originally Posted by spazzyfry123
So are you saying no one should put, say, the TR TRAK cam (231/234 .640/.590 112 LSA) in an untouched motor? I ask this because I have been planning on putting this cam along with upgraded springs later on in my SS with nothing done to the motor at all.
With a fully built engine, it's stupid to keep compression down near stock, because the camshafts are much larger and the discplacement is a lot bigger thereby pushing the DCR way down below an acceptable level.
if that cam is on a 108 icl, it HAS to have 4* of duration ground into it. that is where you get 112 lsa...
one reason people might keep compression down in a ground up build is to maintain the use of pump gas. not everyone wants to run cam 2 race fuel..
one reason people might keep compression down in a ground up build is to maintain the use of pump gas. not everyone wants to run cam 2 race fuel..