Thread: Cam specs correlating to cylinder pressure

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This is a reply to Richard (Techinspector). You wanted to know what cylinder pressures I had with the current cam.

OK, I have now done a compression test on my 454. Pressures are 175-180 psi, this is with a cold engine, plugs out, throttle wired open. I couldn't do it at running temp, as I would have no fingers left by now! I guess pressures might increase a little when properly warmed up. I have Total Seal gapless second rings.

Static CR we reckoned to about 9.5 - 9.8:1

Cam Specs again are:
Lunati hydraulic roller 50247. 218/226@.050", 112 lobe centres, .534/.544" lift.
In Opens: 1 BTDC In Closes: 37 ABDC
Ex Opens:49 BBDC Ex Closes: -3 ATDC

How does all this sound to you, Richard? Would the Comp Cams 230/236@.050", 112 lc, mid .500 lift work with this setup, or would cranking pressure be too low?

Thanks again, Chris

no .i have run more can than that with that cr in a big block . but i not richard

Are you experiencing any audible detonation (ping)??. What fuel are you using. This is an interesting question. If go longer on the cam and delay the intake closing point, you will lower the cranking pressure. The question is, will the cylinders fill better at speed with the valve open longer and make up for the decreased pressure??

Thanks guys!
We can get Super Unleaded (98 octane) here, which is what I use. Also, I have the Edelbrock aluminum heads, so don't seem to have any pinking issues.
It sounds like I could safely step up to the slightly bigger cam - I have a pretty good intake (Eelbrock Perf RPM, Holley flowed to 890cfm) and exhaust (4-into-1 1 7/8" long primaries into twin 3" system), so maybe I'll "suck it & see"!
Chris

Carb is basically a 750 double-pumper, choke horn milled off, entry passages smoothed and flowed, butterflies and shafts thinned, etc. I was told it would flow up to 18% better than standard, which I worked out to 885cfm. Does that sound reasonable, or am I missing something?? Work was done by Chuck Nuytten outside Dallas. He has a good reputation from a number of people I have talked to, even with the historic racing people over here.
Chris

Yeah, Denny,
the idea was to keep the "snappy" response of the 750 carb with its strong signal, but enable it to flow better than the off-the -shelf version, if needed.
Chris

Denny, I understand that you can streamline the carb and increase the airflow through it just like you can ports in the head. ("choke horn milled off, entry passages smoothed and flowed, butterflies and shafts thinned, etc.") What you are referring to is when I took exception to the impossibility that someone could "jet up" or "jet down", in other words change the flow capabilities of the carb simply by changing jets. Every time I see that in print, I go ballistic and do my best to bring a grain of reality to the situation. I DON'T CARE WHAT ANYBODY SAYS, IT IS NOT POSSIBLE TO CHANGE THE FLOW CHARACTERISTICS OF A CARBURETOR SIMPLY BY CHANGING JETS OR METERING RODS. THAT'S JUST STUPID.

A 750 is a 750 and will remain a 750 until someone does surgery on the mechanical parts of the carb.

Thought about using one of these ?

Originally Posted by southerner

Thought about using one of these ?

Thats the best single item you can use on the top of a carb to help increase flow and the only time a velocity stack does any good is with one of those under it.A carb/hood scoop attached to the carb,does not come into use till about 90 mph and will do more harm than good on a street car facing forward and facing backwards its worse,draws air out into the low pressure behind the scoop and fights the carb ,forward it disrupts the carb signal untill about 90 then it creats a ram air effect increasing carb flow from pressure.It works to a point under 90 but the air builds and releases pressure confusing the carb signal .I have graph that shows ram air /cold air improvements over non ram /under the hood air.I think its like 2 percent at 65mph.But the graph covers none of the air flow variables from bumper to grill and hood variables.
Put your hand out the window on the freewayat 60 and feel the air pressure and how it changes constantly ,blowin harder then softer,this is what confuses the carb at the lower speeds,put your hand out the window at 90 plus and this disruption is not present ,just one constant force.JUst my take on it from what I have read and seen posted,just like all aftermarkett parts ,they have there specific use ,stray outside that intended use and all bets are off,performance usually drops.

Originally Posted by DennyW

Yes, those work, and I also use the different base plates to. This one works good for that dead spot. The venturi picks up the speed for better, and quicker flow.

Went to the site..............Theres a heck of a lot to choose from.. Now then, just to pick up on your carburetor knowledge, without causing to much uproar. Why did you pick that particular one ? And does it come from the shop like that, or did you modify it ? What type of manifold does it work best with ? And what size carb ?, I think I see it is a 4150, 4160 square flange Holley type. What else ?, what is the effective rev range that it works with or does it effectively broaden torue all around ?

tuning with spacers...
site of Barry Grant has a nice article on this:

http://www.gnetworks.com/v4files/bar...h%20images.pdf

Originally Posted by DennyW

Thanks for the link. That is pretty much what I was trying to explain.

that's why I understood what you were trying to explain...
kinda...hey, I've seen this before and not in another life

Nice model 4150 Holley in the background. What do those fibre baseplates cost and how thick are they ? I might just be able to get that on under our limited stockcar rules.

Originally Posted by DennyW

I can't find my other source since I had to redo my hard drive. Heres one place, and the price is comparable. If you gain horse power with a spacer, it stands to reason you increase cfm flow.

CFM to the engine after the carb or laminar flow improvement is the term,I think.This is just my opinion no actuall tests here on my end.If you have turbulent flow under the carb which disrupts the smooth flow of air ,you will see less hp ,if you install a spacer and improve this turbulent flow and make it more smooth you will see more hp,not sure if the cfm thru the carb improves enuff to go over 100 percent efficiency,stands to reason that you just improved the flow and now your getting 95 instead of 90 percent efficiency.? sounds logical ,..but so did Hitler to the germans soo I am not sure. But it sure sounds like good logic. .