stockA4

My li'l engine has a delayed intake valve closing event. This directly affects the cranking compression, which affects all kinds of things, like that feeling of the engine "coming on the cam" when you get the car in the right gear at speed.

I think about it like the engine is filling up with a larger and denser charge as the revs climb.

The cylinders are not completely filled at low engine speeds because the intake valve has not sealed until the piston has made its way partway up the bore.

this throws away some of the engines effective displacement until engine speed sufficiently increases.

if you know how to accurately calculate your dynamic compression you will find your effective stroke.

Multiply you effective stroke by your bore and...

you will find how many cubes the engine actually smooshes and explodes starting out, before the engine speeds increases enough to effectively force the cylinder to use more of its total swept volume.

You can see how this is like you are removing displacement from the engine.

in some load conditions this may affect the economy of your engine.

​​​​
if everything balances out you'll have your ideal compromise of economy and power from your chosen cid!

stockA4

I wanted to add that my LS1 is only using about 268ci of its 346ci at low speed with the ivc set at 64.5 @ 0.006

that's still earlier than the stock cam I think.

Anyone have good #s for the stock stuff @ 0.006 ?

​​​​

G Atsma

Why are you running such an odd cam?

stockA4

What's wrong with closing the intake valve at 64.5 abdc @ 0.006 On a stock compression LS1 ? That's 39 degrees @ 0.050 with this cam if the timing chain isn't too loose!

Darth_V8r

iTrader: (4)

I was sorta trying to figure this post out. If it was a question, an attempt to teach, etc. something is off based on your description of how the engine behaves and your description of the cam.

A cam with a 050 IVC of 39 degrees should peak early and fall off fast. If you're getting almost a turbo lag or delayed power from that cam, the cam is about six degrees retarded or has a ton of overlap or you have very low compression.

Also the idea of you "using 268 out of your 346 CI" is kinda off. Air continues to enter the cylinder after BDC due to air momentum. Typically in a 346, 43-45 degrees ABDC is considered the ideal IVC with the ideal 006 IVC being 69-72 depending on ramp rates.

Earlier IVC tends to make higher cylinder pressures resulting in good bottom end torque and great throttle response. Can also contribute to spark knock.

Virtually every engine has a IVC later than BDC. Even stock cams. The Ls1 stock cam has a very large ramp rate, so the 006 IVC is around 82 degrees.

ddnspider

iTrader: (26)

This HAS to be KT under a different username.

stockA4

Thanks for the interest. I was just throwing some ideas around to help visualize what takes place inside the engine.

​​​​​​ the LS1 engine I mentioned doesn't really exhibit the characteristics of a late intake closing I was describing. I'll get to that in a moment.

That otherwise stock LS1 with the 39@ 0.050 ivc makes power everywhere and has perfect street manners. Full specs are 218/224 113+3. Car has gone 12.1@114.5mph shifting at 6500 at full weight with the stock converter still in place.


now my l'il 4.8L is a whole other animal... that one really needs to rev more to create good cylinder pressure.

​​​​​​it's got a 212/218 115+0

the ivc on this cam is 41@0.050 and 68@0.006!

That's only 218ci actually being sealed up by the valves and smooshed by the pistons at low speed. Trust me there is no noticeable ramming effect in that engine until about 3000rpm

I almost put a 98-00 cam in that engine. An ivc of 82@0.006?

​​​​​this engine also has perfect driveability and marvellous economy with a stock converter and 2.73 gears behind it, however on the highway shifting into fourth gear too early can pull the engine down and out its power band.
​​​

​​​​​

01ssreda4

iTrader: (96)

Put the pipe down. Drugs are bad mmmk?

JoeNova

You beat me to it.

stockA4

Ok someone check my math for dynamic compression using the closing figure of 82@0.006 for the stock LS1 cam.

​​​​​98-00 LS1 closing at 82 would be 6.85 dynamic

my 9.5:1 4.8 closing at 68@0.006 would be 7.30 dynamic

my 10.1:1 LS1 closing at 64.5@ 0.006 would be 8.1

​is that accurate? The 4.8 runs perfect on 87, I've only ever used 93 in the LS1. I bet it would have been fine on regular when it was stock though lol

ddnspider

iTrader: (26)

G Atsma

Darth_V8r

iTrader: (4)

sentences are too short. and he's asking math questions, not answering them

01ssreda4

iTrader: (96)

Puff puff pass dude. Quit triple hitting.

ChopperDoc

I'm pretty sure I just spent a few IQ points trying to make sense of all this...

I HIGHLY doubt you are at 7.3 DCR and on 87, or your timing is seriously retarded, which is probably why your cam is running like ***. Hell, STOCK engines knock like crazy on 87, on STOCK tunes.

stockA4

can someone confirm that my math is correct? I'm trying to understand how the closing figures at 0.006 and 0.050 can affect the potential octane requirement of the engine for a given displacement and compression ratio. Both of my engines are cam only and in good running condition with quality wideband tuning with HP tuners.

Darth_V8r

iTrader: (4)

yes

7.4 close enough

yes

As to your 3000 RPM comment on cylinder fill.... yes, that's true. That's always true. Where your engine makes peak torque is where it gets the best cylinder fill. On an SL1, that's almost always around 4800 RPM.

What's your end game? You looking for something to change? Just trying to understand what's going on? Looking to run lower grade fuel?

stockA4

thank you for confirming my math. I'm just trying to understand what's going on haha.

that really late intake closing of the early LS1 cam has me wondering how far I can push the static compression while lowering the dynamic compression with a delayed intake closing before running into trouble.

I've got an LQ4 short block that I'm building to replace the 4.8 with and I was hoping to gleam something from here that will help me with that. The engine needs pistons and I want flat tops. I also want to try the 706 heads on a +4" bore while I wait for LS3 top ends to get cheaper. I believe I need to get the dynamic compression somewhere in between my 4.8 (7.4) and my 5.7 (8.1) so it can be happy on pump gas. I don't want to build a detonation prone dog either though but if it makes the same power as a stock LS1 at a lower rpm with more tourqe with parts I already have I'll be a happy camper.

sooooo....

flat top 370ci
stock castech marked 706 heads 61.5cc
I'm getting around 11.5:1 static there

Add a stock 98-00 cam closing around 82@0.006

I'm getting about 7.75 dynamic?

Is the world going to end if I do this?

​​​​​

Darth_V8r

iTrader: (4)

No, the world won't end by any means, but the DCR on the stock 98-00 ls1 cm is misleading. There's a thing called "ramp rates", which is the difference in degrees between 006 and 050 lift. "normal" for most aftermarket cams is around 51-54 degrees. The 98-00 LS1 cam has something like 89 degree ramp rates. The reason this is important is while it has low DCR, it behaves like a cam that has an early IVC, because it does at 050 lift.

You would really do a lot better to spring a few hundred for a mild performance cam from cam motion or other vendor. Aim for about 45 degree IVC at 050 and about 72 degrees at 006. With 11.5:1 compression, that will put you around 8.5-8.6 dynamic compression, which will make great midrange power and carry quite nicely. Something like 228/234-114+3 will do quite nicely with a 6.0 and 706 heads.

If you're trying to stick with a stock cam, then grab the 02 LS6 cam, but keep compression to 11.0:1.

Enigma540