more cam less ignition knock?

[fuzz]

So Im talking to a machine shop about using my Vortec heads on a street motor [sbc] and told him I would use a dish piston to lower the compression in order to reduce ignition knock caused by todays crappy gas. He said I would only have about 10 to 1 with flat top pistons. I said right and I don't want to buy race gas for the high compression. He said it depends on the cam i use. He said a stock profile cam will give me problems but if I run moore lift and duration, like a Comp Cam 268 I shouldn't have an ignition knock problem.
He didn't have time to go into details on the phone so if any of you can explain this to me please do cuz it don't make sense to me....[P

 

[Sam_Fear]

My guess: The valves will close later, meaning your piston will be slightly higher on the stroke before compression begins, therefore you'll have a little less compression.

 

[Gastrick]

Cams with a lot of overlap tend to bleed off some of the compression. That's probably what he was talking about. Disclaimer: I'm not an expert on this subject but have a little experience, just enough to be dangerous.

 

[26Troadster]

sam and gas are right but when the rpm's get higher then the comp will be higher too, and may still need better gas. i have a low comp (8-5 to 1) dish and open chamber with a longer overlap, when the r's are up it runs great, but don't stand behind it at ilde it will burn your eyes and another thing i have fought with is it will eat up spark plugs, r-44 and 45's last about a month the r46's last about two, and i'm running a hei with it.

 

[hotrod preacher]

a 350 with flat-tops and 64 cc chambers (like camel humps or vortecs) will be between 9.5:1 to 10:1 compression, a bigger cam, 280 comp or bigger will have some overlap and bleed off the cylinder presssure -compression ratio is still the same, but the psi in the cylinder will be slightly lowered, which helps eliminate the crappy gas issue...i seriously doubt a 268 cam would be large enough to bleed off much though, my brother is running a 10:1 460 ford with a 274 extreme energy comp cam and runs premium with very little issues

BTW-if you put vortecs on a 400 sbc and run flattop pistons it gets the ratio closer to 11:1

 

[Toad]

Saw a graph in a car mag once showing the 'overlap' of the intake and exhaust valves closing and opening and it made it clear to me how it worked and what the cam specs actually meant!

Toad

 

[Sam_Fear]

Like this?

 

[fuzz]

After thinking about this more I don't think the cam could have much affect on ignition knock. The cam won't affect compression at the compression stroke cuz both valves will be closed. May have to call the guy back and ask him to explain his point of view. Ignition knock, or preignition is the result of the fuel in the combustion chamber burning too soon. Crazy as it sounds low octane gas burns faster than high octane gas.The vortec heads themselves may help the problem simply by the better combustion chamber design and true flat tops, with no valve reliefs may help cuz there would be fewer sharp edges to cause hot spots that might preignite the fuel. I don't think any body makes true flat tops. .[S
My concern is the lousy gas at the pumps and I don't want to run high priced race gas in my street only rat rods. I think if I clean up any rough edges in the combustion chambers and run dish pistons to keep compression down to say 8 to 1 I will be able to run a smooth idle cam and pump gas on the 383 I'm building and still have plenty of power on the street. Another possible way to run higher compression and pump gas might be to use some kind of ignition retard but that could get complicated cuz then I'd have to deal with some sort of computer and I hate computers. Of course most rodders think they need to run as much timing as possible to make horsepower but too much timing leads to ignition knock..

 

[railroad]

After thinking about this more I don't think the cam could have much affect on ignition knock. The cam won't affect compression at the compression stroke cuz both valves will be closed. May have to call the guy back and ask him to explain his point of view. Ignition knock, or preignition is the result of the fuel in the combustion chamber burning too soon. Crazy as it sounds low octane gas burns faster than high octane gas.The vortec heads themselves may help the problem simply by the better combustion chamber design and true flat tops, with no valve reliefs may help cuz there would be fewer sharp edges to cause hot spots that might preignite the fuel. I don't think any body makes true flat tops. .[S
My concern is the lousy gas at the pumps and I don't want to run high priced race gas in my street only rat rods. I think if I clean up any rough edges in the combustion chambers and run dish pistons to keep compression down to say 8 to 1 I will be able to run a smooth idle cam and pump gas on the 383 I'm building and still have plenty of power on the street. Another possible way to run higher compression and pump gas might be to use some kind of ignition retard but that could get complicated cuz then I'd have to deal with some sort of computer and I hate computers. Of course most rodders think they need to run as much timing as possible to make horsepower but too much timing leads to ignition knock..

It does not sound like you are going to be red lining the engine at every red light, so I do not think race gas with a 268 cam is going to be a problem. The advise your mechanic gave you is good. Do a google on static and dynamic compression ratio. Have the engine and cam specs available to plug in the values. I think you will be able to run 10-1 with no problems, especially for your lack of desire for ultimate hp, timing adjustment will allow you to enjoy the abliity to step up the plate, if you desire. Not something so easy with a cam or compression upgrade. The poor gas we have is not due to octane ratings, although most would like more options, it is due to the ethanol. With a little ethanol protective additive and some regular use, most evils can be held in check. The 268 cam will be a lot more enjoyable in a street rod.

 

[hotrod preacher]

you could run 9:1 or so with a stock dish-type piston and be ok and be a little better off than 8:1
i have read somewhere where one guy claimed the better chamber of the vortec allowed him to run 10:1 with no issues...but i wouldn't swear to it
i say get it in the 9.5:1 range or below and you'd not have anything to worry about -and you could always run premium gas...


and to those discussing cam specs....there's more info there than i'll EVER figure out...but it is fascinating stuff especially how 2 cams can have the same lift and duration and sound different and perform different

 

[Racer]

Alot of the late model Chevys run 9.5/1 and do it on 87 octane. Run the flat tops and the Vortec heads with a thicker head gasket, (Felpro makes a large selection and drop the compression that way. Always run as much compression as your fuel and cylinder head design will allow.