Air_Adam

I know this is a pretty broad question... but would a compression ratio between 9.0 - 9.5 be too low for a Comp XE274? A few people have told me it would be, and would be a tuning nightmare because of that, and others have told me it will be fine.

Its for a street/strip 350:

Heads 64cc chambers, 195cc intake
Edelbrock RPM intake
Holley 750 VS
Comp XE274 cam
Full headers and exhaust, cutout for the track
MSD ignition

The reason the CR is so low is because I'm using my existing shortblock, which when I got it, had dished pistons (12cc dish) and I don't want to swap them out.

Will this cam work alright, or would I be smart to go smaller?

305sbc

iTrader: (3)

For aluminum heads that's too low IMO.
the good news is that it's really not hard to tune at all and you'll be able to run 87 octane.

84z28350

i got a xe274 sitting here and i have the choice if either running it with my 416 heads which is about 10.5:1 CR or my 882s with about 8.5:1 CR.

I think ill run it with the 416s and see what happens but i guess i would be screwed if it detonates if i cant run a low CR with this cam...

Air_Adam

Does anyone know anything about Lunati VooDoo cams? I've been looking at P/N 60102 for awhile now, and I think it might be better suited to my engine combo.

Specs:

Advertised duration: 262*/268* I/E

Duration @ .050": 219*/227* I/E

Lift w/ 1.5 rocker: .468"/.489" I/E

LSA: 112

I don't see anything about recc'd compression ratios for any of these cams, but this one seems like it would be much better matched to my engine...?

84zeddd

I have used the crane powermax 2050 in three engines now and
still love it.It has an rpm range of 1800-5600. I run a corvette
conveter and it pulls strong from idle to 6000 no problem.

grumpyvette

heres an older thread that gives the basics

theres a few things that you need to understand

(1)on pump gas youll tend to get into detonation range when the engines hot at about 8.5:1 DYNAMIC cpr, now using aluminum heads and a knock sensor may allow that dynamic cpr to work but its best to stay at or slightly under 8:1 dynamic cpr, especially with iron heads
(2) the higher the dynamic cpr the more efficiently the engine can produce power
(3)your cams durration and LSA will to a large extent determine the rpm range the engine runs in effectively.




its obvious some of the guys on this site need to understand the differance between static and dynamic compression ratios, and thats understandable as its a difficult concept to grasp at first
but you need to understand it before selecting a combos components

the differance between STATIC COMPRESSION RATIO AND DYNAMIC COMPRESSION RATIO is where the piston is in the cylinder when the valves close and the piston can accually start compressing the REMAINING VOLUUM IN THE CYLINDER VS the STATIC COMPRESSION THAT ASSUMES THE PISTON STARTS COMPRESSING THE INSTANT IT LEAVES BOTTOM DEAD CENTER AND STARTS UPWARD ON THE COMPRESSION STROKE!

let me try and explain, [color:"red"] the short version is that the PISTON COMPRESSES NOTHING untill BOTH VALVES ARE CLOSED, .......thats the only compression ratio that matters,.... since its the only compression ratio the engine ever sees.[/color]

static compression is simply the differance between the cylinder volume at BOTTOM DEAD CENTER(BDC) and its compressed volume at TOP DEAD CENTER (TDC), into the combustion chambers,... dynamic compression takes into account that on the pistons upward compression stroke the valves have not yet closed and nothing gets compressed by the piston untill they do, that of course depends on the cam and rockers, pistons and connecting rods, the cylinder voluum, the rod/stroke ratio, ETC.,used, in the combo, and the rpm levels to some extent[color:"green"] BTW, ALUMINUM HEADS can usually operate at a higher dynamic compression simply because ALUMINUM releases heat to the coolant much faster than iron, its the lower heat levels that remain in the cylinder that help prevent detonation..when you increase the dynamic compression the heat levels in the heads combustion chamber rise , the differance in the RATE heat leaves the cylinder allows a slightly higher dynamic compression level from aluminum before the same HEAT levels are REACHED & MAINTAINED in the combustion chambers [/color]

heres a calculator for static cpr, which you need to figure first, (below) and that youll want the final distance from the piston at TDC to the cylinder heads to fall in the .037-.044 range to increase SQUISH/QUENCH to help prevent detonation and speed the flame front burn speed

http://cochise.uia.net/pkelley2/crc.htm

let me point out a few things
first look at this chart

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

then lets assume your 350 sbc engine has a static compression ratio of 11:1 but youve installed this cam

http://dab7.cranecams.com/SpecCard/D...1=Display+Card

looking at the cam specs we see that the effective stroke is not the 3.48" that the static compression ratio is measured from ,at BDC, BUT from about 2.6 inches from tdc where the valves close as the piston moves upward, so your true working compression is closer to 8.1:1 NOT 11:1

heres a longer more detailed explanation and access to the software to figure dynamic cpr with the cam your useing in your engine

http://cochise.uia.net/pkelley2/DynamicCR.html

http://www.diamondracing.net/cocalc.htm

heres some differant calculators

http://www.kb-silvolite.com/calc.php?action=comp2

http://www.wallaceracing.com/dynamic-cr.php

http://www.smokemup.com/auto_math/compression_ratio.php

http://not2fast.wryday.com/turbo/com...pressure.shtml
average the results

keep in mind that you can easilly run a stattic compressio of 11:1 with aluminum heads if you keep the cam timing in a range so that the DYNAMIC COMPRESSION is CLOSE TO 8:1
take the time to understand the concept,it VERY IMPORTANT

read this

http://www.diamondracing.net/cocalc.htm



http://victorylibrary.com/mopar/cam-tech.htm

http://victorylibrary.com/mopar/otto-c.htm

http://victorylibrary.com/mopar/cam-tech-c.htm

http://kb-silvolite.com/article.php?acti...3117842f4eb4c49

http://victorylibrary.com/mopar/rod-tech-c.htm

http://victorylibrary.com/mopar/piston_position-c.htm

http://www.iskycams.com/techtips.html#2003

http://victorylibrary.com/mopar/rod-tech.htm

http://www.zhome.com/ZCMnL/PICS/deto...etonation.html

http://www.chevytalk.org/threads/showfla...true#Post397334

Sonix

iTrader: (1)

air adam, I am planning on using a voodoo 60103, which lunati compares directly to a compxe268 (favorably to lunati obviously, but it's actually between a 268 and 274 in terms of specs...).
I think the 60102 is more comparable to the comp262, which is kinda far from your planned 274... Sure you want to go that "low" ? Anyway, i'd recommend the 60103 for you.

84Z, i'd go with the 416's, and make sure to polish the combustion chamber well, maybe even get it ceramic coated... And make sure you don't forget quench, 10.5 is a tad higher then what I'll be shooting for, but close....

Air_Adam

Thanks alot Grumpy, thats exactly what I needed to know!

I knew about dynamic compression before, and how it was more important, but just never knew how to figure it out or how to use it. Thanks alot man!

F-BIRD'88

You have to take it all with a grain of salt. the difference in power output between 9:1 and 10:1 is 4% * or less*
usually less.

Use a felpro .015" shim head gasket to max your actual cr.
should be 9.49:1 with the shim gasket. (stock bore 350)

As long as you'll be using a mild converter and some gear it will work well.
if you're that worried about it install the cam with 2-3 extra deg of advance to maximize cranking cylinder pressure.
103-104 intake C/L insted of 106 intake C/L as per cam card. the amont of top end power you will give up for the extra low end torque and cylinder pressure will be less than if you wimped out and went with a smaller cam.
it will be about 5 hp
As for "tuning" it will tune exactly the same as if the motor had 10:1.
Will want a modified ignition advacne curve with more initial advance at idle. try 18-20deg at idle 34-36 total @3000.

Dynamic cr theory is all fine and good for paper tigers and armchair engine builders but practical experience in the real world is where the rubber meets the road.

This cam will work fine for ya.
Combine it with a 2800stall, 3.73-4.10 rear gearing and don;t forget the advance curve mods.
A 4.5" power valve will work fne for this on a holley.

You could put thsi cam in a 7:1 compression motor and it would want the same "tune". Output will be less than if 10:1 but the it would not be a "nightmare" to tune at all.
Some people just don;t know how to tune a motor.

you'll be just fine at 9-9.5:1.

F-BIRD'88

"et me try and explain, [color:"red"] the short version is that the PISTON COMPRESSES NOTHING untill BOTH VALVES ARE CLOSED, .......thats the only compression ratio that matters,.... since its the only compression ratio the engine ever sees.[/color]"

This is not true.

When the motor is turning at cranking speed it is true.
When the motor is spining at high rpm the air is moving toward and into the cyilinder while the piston is rising.
this ram tuning extra air filling results in compression as the piston raises against it because the pulse pressure wave ariving at the cylinder thru the intake port is higher than the pressure of the raising piston. This cannot be calcultated by simple dynamic cr therory and math based on piston position and valve closing point BTC because it totoaly ignors pulse wave cylinder filling @ speed.

Must use the math capability of a program like Dynomation to predict the actual engine output.

F-BIRD'88

If you are willing to "tune" your motor to the track you can find lots of performance.

Install the cam as per the cam card. do a compression test and note the reading. Now retard the cam to straight up
110-110 cls take a reading

now advance the cam to 100 intake cl and take a cranking compression reading. (6deg more advanced than cam card)
if it is more cranking pressure, start there and go to the track.
Then retard the cam 2 deg at a time and note the performance difference ( make no other changes)
You'll find that the motor wants,*** what it wants***, for max performance, not what the cam card says or what Dynamic cr theory, or any other theory dictates.
usually the best intake cl will be 100 to 109deg BTC for a SBC with an automatic.
You're looking to find the best actual average torque output that the car will handle over the quarter mile.

Only track testing will show this.

The therioritical power difference between this motor with 9:1 and 10:1 compression will result in a MPH difference of 1.3MPH in the quarter mile (at best). Finding the optimum cam intake centerline point ( cam advance) that the **car** wants for maximum performance at the track will more than make up for this.
Much more.

Air_Adam

Converter isnt an issue, as its a 4spd car, not an automatic

I think this engine will be right around 9.4-9.5 CR with these heads and the gaskets I'm going to use. I'll just advance the XE274 a bit, maybe 4* or so and see how it runs, and tune it from there. Thats what a few local guys suggested I try with this cam.

Confuzed1

iTrader: (4)

That was quite a long thread to see if a Comp XE274 cam will work with a 9.3:1 compression ratio. I think it will since that's exactly the compression ratio and cam I'm running .....but I installed it "straight up".

But I will agree that tuning seems to be a headache (but always is with me) - but I've gotten through alot this past year, and the engine's running better than ever!

grumpyvette

F-BIRD'88
brings up a factor or point in this discussion that I thought was understood,simply that

DYNAMIC compression ratio means exactly what it says, DYNAMIC MEANS CHANGING
we can CURRENTLY ,
only set up the engine to match one rpm range effectively at this time,(no VERIABLE VALVE TIMING) but the engine can be very effectively tuned to match the ideal parramiters over a fairly wide rpm range thru careful component sellection.
as the engine rpms increase the ram tuneing or inertial energy in the runners tends to increase as the collum of air speeds up and the inertia of the air moving increases,thus the cylinders can fill more effectively,as that coluum of air tends to resist stopping is forward rush toward the intake valve as the valve closes and may even cause a slight increase in the voluum of air entering the cylinder when it does open, as it compresses slightly at the restriction,before the valve opens, up to the point where the time the valves remain open begins to get shorter than the air flow requires to completely fill the cylinder. this is comonly refered to as the rpm curve and cylinder volumeric efficiency
look at this dyno
you can be fairly sure that the peak volumetric efficiency was reached in the 5000-5500rpm band as thats where the cylinders getting filled completely starts to loose to the increasely shorter time available to fill the cylinders as the rpms build and the time gets shorter for the coluum of air to enter the cylinders.
notice that below that point,(1000rpm-5000rpm)(the air in the ports was gaining in inertia and the cylinder fill was increasing so the tq tended to go up. but above that point power continues to increase only due to the increased number of ever less effective power strokes as the voluumetric efficiency falls off.
btw the same basic thing is happening on the exhaust side, as the engine speeds increase the exhaust gases exiting the cylinder tend to scavage or form a slight negative pressure wave at the exhaust valve IF the header primairy length and dia. are correctly sized to the compression and cam timing are also well matched.
this is why VERIABLE VALVE TIMING has such a good POTENTIAL performance future, it will allow the engine to more effectivelt ram tune the cylinders over a much broader rpm range increaseing efficiency

Air_Adam

Its not so much MAX power that I'm worried about with this, but low rpm driveability. I know what over-cammed engines drive like at low rpm, and I just don't want that. I know it will probably lose some low end power anyway, just because of the size of the cam, but I don't want a low rpm dog.

I think this cam should be alright though.

Cnonfused1 - aren't you using a 400" mill? That would make some difference I would think, since I'm using just a 350?

Confuzed1

iTrader: (4)

If what you mean by "make some difference" is the fact it's considered mild by some's standards on a 400, well maybe. I'd think the principles are the same only more cubes.

- Not to mention, that cam has been recommended by alot of people that have 350's on this board. I think it would do really well on "just a 350" on the street.