What compression should I be running with a 274h Extream Energy Comp Cam with .487/.490'' lift.... I was thinking of putting what was orginaly in my motor back in, 11:1, would that be too much. I have a 1970 350/350hp motor and a 4speed. I want to run the highest compression posible with pump gass to get the most power out of the engine i can. Advice :confused: :confused: :cheers:

 

There's a fine line between what's "ok" and what's "too high" for pump gas. If you didn't have any problems with 11:1 before you "probably" won't now, especially with the larger cam. I'd shoot for a little less cr just to be safe though. The XE series were designed to work with moderate cr's and I don't think a 1/2 point or so lower cr will significantly affect your power. My 327 has SpeedPro pop ups that they say are clones of the factory 11:1 pistons. According to their figures, I'm at 10.35 cr with a .039 gasket and 64cc chambers. I've never had a problem with pump gas with the XE-274 or when it had a milder cam.

 

Are they 11:1 pistons with a bigger gasket or are they 10.35:1 pistons you have in your motor?? how does gasket size effect CR?? How do I know which gasket to choose? :confused: :seeya :chevy

 

any please help me

 

Cory, they are the same pistons that GM rated at 11:1 with the stock steel shim gasket. The thicker gasket increases the combustion chamber volume, reducing cr. Keep in mind that I'm working with a 327 so the effects will be a little different than with a 350. You can play with this calculator to see the difference the thickness can make. http://www.angelfire.com/fl/procrastination/motor.html

 

hey are those pistons cast or forged, the speed pros

 

Cory,
Please reread this thread.Answer is there.
http://forums.corvetteforum.com/zerothread?id=253643

 

forged. They're listed in Jegs catalog along with other brands and types. The only domed 350 pistons I saw were more like 11:1, way too much for your application and also more expensive. For a street 350, I'd use flat tops. I don't think you'd be giving up much power, especially if they allowed you to run better ignition timing. Typically, the four valve relief flat tops for 350's give about 9.7 with 64cc heads. The 2 valve relief flat tops are a little over 10:1 with the same heads but are more expensive also.

 

i have the same style 11.1 speed pro pistons in my 350 with a .039 gasket and world products sportsman2 72cc heads and can run plus(medium grade) in it and not detonate.your quench has alot to do with it too.

 

I agree with VetteRodder, but I'd change "probably won't" to "probably will".

Oh, well, we can't expected to always agree, right?

CR is really on a mathmatical calculation of swept volume compared to compressed volume and should be used as a kind of barometer/yardstick.

What's really important is actual cylinder pressure.

Example: Take a Pro Stock mechanical roller and stuff it in your 11.0:1 engine and with pump gas the thing will never ping.

Why: because there's so much duration and overlap in the cam that your engine will never create enough cylinder pressure. Anyway, by the time you get the revs any where near high enough to begin making some serious cylinder pressure, the engine will explode.

It'll drive like a dog to boot, both before and after the mechanical difficulties it encountered..

Now take the same engine (repaired of course) and stuff in a flat tappet hydraulic cam out of a 305 SB Chevy. Guess what, well you already know.

So my thoughts: 11.0:1 with a 274 is gonna ping and detonate. The 274 has pretty short (comparatively speaking) duration specs and will allow a lot of cylinder pressure; that's what it was designed to do. It was designed for engines with a less CR.

Changing cams is a real PIA so I'd suggest you call CompCams first. I'll bet $2.00 on what they say.

Let us know.

Jake

 

Cory: Compression is a good thing but on a street motor a full point of compression is only worth about 4% more power, so a 1/2 point is approx. 2%. On a 400 HP motor that is 16 and 8 HP respectively, is that worth the chance of detonanting or having to run high octane fuel. I would go 1/2 point less then what you think you can probably get away with, gas can vary from pump to pump anyway. On a daily driver you do not want to have to worry about it.

 

My 350 has stock flattop pistins. In 73 it came with 76cc heads and 8.5 comp. It now has 64cc heads that are milled .015. The stock specs for the typical older factory 350's using 64cc heads have 10.25 comp. So with my flattops and milled 64cc heads I figure im closer to 10.5 to 1 at least. Anyway, for the record I use the XE268 which is a cam size smaller than the 274. I have never had a problem with detonation using premium gas. I have full spark advance too. Never had to retard anything to combat ping. I would venture to say that 11 to 1 with the 274 would not have excessive detonation problems provided you stay with premium gas. I base this suggestion on my own expierence with my 350 and its setup. Like the others have said though, Just cause a pistion is listed as 11to1, thats not the full equasion. Final cylinder pressure is the important factor. Cylinder pressure is effected by bore, stroke, cc voulme, and head gasket thickness.

The xe274 wont start making good power till about 3000rpm. It will put out good from 3000 to 6000rpm. Make sure the rest of the stuff is up to snuff so you dont have to worry about pushing the redline. Comp Cams lists a much wider power band then I have for that cam. Believe me though, it wont start really commin in till around 3k.


[Modified by Jvette73, 7:59 AM 3/24/2002]

 

You can read my sig for engine specs. My engine will literally blow the tires off at anything over 2K rpm. I've revved it out to 6500 but I believe it signs off at like 6 thou really. If I leave the line at anything over 2K I lose time in wheel spin, seems happier to launch (street tires, mind you) at 1800. I personally would put the pwr band for the XE274 at 1800-6 thousand rpm, with lotsa power hitting at 2200. Just my seat-of-the-pants experience. :cheers:

 

The XE274 in my 327 behaves about the same as Daves.