ez2cdave

For a long time, I have heard people asking "what compression are you going to run ?", when selecting a camshaft for a motor . . .

I have been thinking about this and have come to believe that is the "WRONG" question to ASK . . . I feel that the cam profile, in order to truly be "optimum", should REQUIRE certain things instead of compromising . . .

Here's what I think should be supplied when ASKING for a cam suggestion,

(1) Intended Usage
(2) Displacement - Bore & Stroke
(3) Head Type & Flow Numbers
(4) Maximum RPM Desired
(5) Type of Transmission & Ratio's in each gear
(6) Vehicle Weight
(7) Exhaust System - Tube Size(s) and CFM, if known
(8) Fuel Type & Octane Rating

THEN:

The "CAM GURU" Spec's out your cam and TELLS you the following:

(1) Specs for your Camshaft
(2) Compression Ratio Range you will NEED to run with the cam
(3) Ring & Pinion Ratio you NEED to run with the cam in your vehicle

PLEASE add in to this thread and feel free to comment !

Dave

GTO1_OHIO

iTrader: (3)

In specing out the 427 build that I'm doing right now ERL came up with the initial setup for the motor to run at 11.5:1 with the heads that I currently have. I then went to Ed Curtis gave him the info on my full setup and had him grind a custom cam for it. Once I got the cam specs I gave them to ERL who then had to change the pistons that had planned on using so that with this cam I would still be at 11.5:1

...so I think your pretty close on this.

ez2cdave

Let me be sure I get this straight . . .

You were ALREADY at a Static CR of 11.5 with the pistons you already had chosen for the motor. Yet, when the custom cam was ground, the Engine Builder had to CHANGE the pistons, so that you would STILL be at 11.5 CR.

Why . . . ???

(1) Piston to Valve clearance ?

(2) Dynamic Compression issues ?

(3) Static Compression change ? (personally, I doubt this one)

(4) "Mystery Factor" ?

I hope the PATRICK G. and others will comment here . . .

Thanks to everyone who posts !

Dave

GTO1_OHIO

iTrader: (3)

Yes ... the cam was ground to try to see how far I can push the 61cc AFR 205s that I had Tony port last year on the 427. The lift we went with to help with the top side RPMS caused the change to the pistons.

Patrick G

iTrader: (14)

A good engine builder will spec a combo to achieve the goals of their customer.

Often, the customer chooses their own parts which may not be ideal for the intended use. Taking all the listed mods, the engine specs, and the intended use, a seasoned engine builder will spec a cam that will bring maximum performance with the least amount of compromises.

A rule of thumb is, choose all your components based around achieving your goals, but choose the camshaft and pushrods last. You don't choose the heads to match the cam you've chosen, you don't mill the heads to match the pushrods you've chosen. You don't choose the converter to match your cam. You don't choose the turbo or intake manifold to match your cam. You choose the cam after everything else is decided.

And for online courtesy, please don't type in all caps or in bold if you're not mad. It gives the appearance that you're pissed off.

ez2cdave

Patrick,

Sorry about the BOLD print . . .

I figured it would be best to choose the Compression Ratio last, since the camshaft profile and valve timing events would, necessarily, change the dynamic compression within the cylinders.

Since the cam would be custom and optimized for the bore, stroke, and displacement, it seems logical that the Static Compression ratio would then be matched to the optimized camshaft's compression requirements, via custom pistons, if necessary.

That way the cam could be "maxed-out" for performance/efficiency and the compression "dialed-in" to complement it.

Thoughts ???

Dave

Irish350

iTrader: (14)

I had hke build my bottom end and ed @ ati spec my cam around the premise that id be stuck @ ~11.1cr due to only having access to crappy az gas (for my dd)

sometimes there are other factors that dictate what can/cant be done, and luckily there can be some 'reverse engineering' done to make sure itll all work okay

TurboS10

iTrader: (12)

You need all the info you listed.
Because, as Pat said, people invariably have at least on part that's a miss match.
Usually two or three if they are clueless on parts selection.

And yes, you have to know CR to pick the cam.
IVC determines DCR based on SCR.

examples
9-1 w/ a Giant cam don't normally got together. But add 25psi......
OR - A 22x on a 110+4 - not a good idea w/12-CR.

And you need to know what converter they are willing to run too.
Lots of people aren't willing to drop the coin on a converter to support the HP they want.

ez2cdave

My line of thinking is that the "required" Compression Ratio would be based the the "needs" of a given camshaft design, rather than designing the camshaft around "commercially available" Compression Ratio's.

It seems to make more sense to me that the camshaft should FIRST be optimized to give the desired performance characteristics in the motor, based on Bore, Stroke, Head Flow, Gears, Weight, RPM Range, etc.

THEN, the Compression Ratio would be "fine-tuned", via Custom Pistons, to the necessary level, without having to "compromise" the Camshaft Specs, by "locking it in" to a predetermined FIXED value for CR that was chosen beforehand.

Dave

racer7088

The camshaft and intake and heads will end up trapping a certain amount of VE and the better the setup the higher the average VE will be in general in the powerband you run in.

The static compression and the best average VE along with the type of fuel you are using will tell you how much compression you can use without risking detonation regularly.

The worse your cam and intake and head combo is at raising VE the higher a compression you can run. The better or the higher an average VE you create the lower your static compression will be.

ez2cdave

Hmm . . . That is interesting, it almost "sounds" backwards, but I think I understand the principle . . .

Are you saying that on a more efficient motor, although a LOWER SCR would be required, the DCR would be unchanged or even INCREASE, due to the superior VE of the motor ?

I understand what Volumetric Efficiency is, but how is VE calculated on a motor that is being "pieced together", rather than determining VE on an assembled motor ?

Thanks,

Dave

racer7088

You can't calculate true VE you can estimate by experience though or look at it on the dyno.

We have had engines where we took compression out and gained power and could run full timing again.

KCS

iTrader: (20)

The DCR isn't dependent on VE so it stays the same. Its the maximum cylinder pressure (and all that entails) that will increase as you trap and compress more air mass.

racer7088

Exactly.

ez2cdave

Wow - great information guys!

Is it "better" to run less spark advance and higher compression or more spark advance and lower compression ?

Is there a "standard" or "rule of thumb" for spark advance vs CR ?

What is considered "full timing" in a N/A Street motor without power adders ?

Dave

ez2cdave

I'm in the "component selection phase" of a motor for a 99% street-driven 2000 Pontiac Firebird Formula and this is extremely interesting to me.

Can anyone give me an "educated guess" about VE on my combination ?

Rough maximum timing advance?

CR - Increased or Decreased - 91 Octane Pump Gas ?

427cid / 7.0L

LS2 Block - DARTON Dry Sleeves - 4.125" Bore
COMETIC "PHUZION" Gaskets - 051" Compressed Height
WISECO "LS Series" Pistons #K395X130 - 10.9:1 CR (Not 100% chosen-More CR Info Please ?)
F.A.S.T. - LSXr 102mm Intake - Unported
102mm Throttle Body
KOOKS Long-Tube Headers ( 1.875" - 2.00" Primaries / 3.5" Collector, Dual High-Flow Cats, Full 3" Magnaflow Cat-Back Exhaust System

CAMSHAFT : Will Be A CUSTOM Hydraulic-Roller . . . Specs to be determined !!!

West Coast Cylinder Heads - CNC Stage 2 L92/LS3 Heads ( Flow Numbers Below )

INTAKE - 2.160" Valve / EXHAUST 1.600" Valve / 28" of H20 w. a 4.155" Test Bore

LIFT 0.100 0.200 0.300 0 .400 0.500 0.600 0.650 0.700

INT _75.4 _160.0 _226.6 _280.4 _323.0 _353.7 _360.6 _367.6

EXH _57.4 _117.6 _182.4 _222.5 _240.1 _249.0 _251.4 _254.3

http://www.proheads.com/WCCH%20L92.html

Weight - Approx. 3850lb w/Driver
4L80E Transmission - 2.48 1st Gear, 1.48 2nd Gear, 1.00 3rd Gear, .75 4th Gear
YANK 3400 RPM Lock-Up Converter
3.54 Ring & Pinion ( Strange S60 Dana rear )

Thanks, in advance !

Dave

racer7088

I cant but I think 11 to 1 will work fine for what you are doing. I think the L92 heads will run with more timing than some other LSx stuff. Maybe one of teh tuner guys can chime in.

You want to run the appropriate amount of timing without getting knock which results in eth most power and economy in general.

TurboS10

iTrader: (12)

It's buried in here
https://ls1tech.com/forums/generatio...b-c-d-e-f.html

ez2cdave

Ouch . . . 63 pages !

Looks like I have some more reading to do . . . LOL !

Thanks for the link !

Dave

87silverbullet

iTrader: (11)

Get in touch with WKMCD on here. He has a 427 with rect port heads and Shawn at Virginia speed pretty much spec'd everything and the timing is very conservative and the cam is nothing compared to what everybody would think.