~JOSHUA

iTrader: (92)

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

PREDATOR-Z

This is relative to DCR

GuitsBoy

iTrader: (4)

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.

~JOSHUA

iTrader: (92)

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.

GuitsBoy

iTrader: (4)

10 or more RWTq would not surprise me in the lower areas. Probably gain a bit up top too. I milled my heads .040 (10.85 c/r with 1cc reliefs) when I ported them. Even switching to a bigger cam and larger ports, my low end was noticeably firmer driving around town. Off idle torque might be about the same asn my stock headed f13 setup was, but anything above that is definately torquier now with a little more compression. If youve got the heads off, might as well mill a little bit, and maybe even clean up the heads yourself if you feel so inclined.

~JOSHUA

iTrader: (92)

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

dug

iTrader: (10)

The rule of thumb is 3-4% more power everywhere for each point of added compression.

~JOSHUA

iTrader: (92)

If have repeated that several times and that's stored away in memory. Someday when somebody asks I'll say " I remember hearing that somewhere."...

JakeFusion

iTrader: (7)

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."

PREDATOR-Z

I like what you say Jake

dug

iTrader: (10)

Yeah, when building a motor from ground up run as much compression possible. Its harder to do on a stock shortblock though. Pvc becomes a problem when milling the heads.

JakeFusion

iTrader: (7)

Sorry about your engine, Predator. Hope you get to build one mean sob on the new build.

~JOSHUA

iTrader: (92)

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.

JakeFusion

iTrader: (7)

What's the advertised numbers (.006") ?

~JOSHUA

iTrader: (92)

@.050 thanks

JakeFusion

iTrader: (7)

No, I'm asking what is the advertised number for your cam. You can't figure DCR from the .050" numbers that I'm aware of.

But, 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.

spazzyfry123

iTrader: (4)

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.

JakeFusion

iTrader: (7)

No, you leave some on the table with the larger cams for sure, but they are an economical way to increase power on an otherwise untouched motor. But a lot of times, people wonder why they don't make the power they want with their cam-only setups and that's why. That's also why a 224 cam with a tight LSA makes equivalent numbers to a 234 cam with a longer LSA--the DCR is so low on the 234, it doesn't matter that lots of air is getting into the engine that it doesn't produce nearly as an effecient setup in an otherwise stock engine.

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.

~JOSHUA

iTrader: (92)

Darn, I don't have the cam card so I don't have the numbers you're asking for. Good lesson though.

s346k

iTrader: (4)

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..