Grumpy, et-al, valve lift question.

[HICKL]

Does anyone have any flow bench data on stock open chamber SBC heads (77 vette motor). I am trying to determine optimum lift for my cam and don't have a flow bench. I know there is a lot of factors involved, but this would help.

[Pyro]

I guess the 77 vette would still be using the 882 heads. In the chevyhiperformance.com head flow data base the numbers of the 882's look like this.

 

0.050": I=39 cfm, E=34 cfm

0.100": I=70 cfm, E=58 cfm

0.200": I=125 cfm, E=108 cfm

0.300": I=175 cfm, E=135 cfm

0.400": I=204 cfm, E=141 cfm

0.500": I=205 cfm, E=142 cfm

0.600": I=206 cfm, E=142 cfm

 

1.94/1.50 valves, 76 cc chambers, 151 cc intake ports.

 

A little over 0.400" lift is ideal. Lifting the valve to 0.500" on these heads will eat up power. Lift a valve quickly up to 0.420" and hold it there for awhile and these heads can make 400 hp (204 cfm x 2 = 408 hp). I have read engine building stories that used a head like this with a low lift cam but with long duration (300 degrees) and made good power (400 plus).

 

It looks like more duration on the exhaust would also help. And don't forget to increase the compression if a long duration cam is used.

[HICKL]

Awesome, thanks. My machine shop guy put in a 467 lift 306 duration cam that he claimed would work really well. we'll see!

 

Thanks Again.

Jeff

[Pyro]

That cam seems about right. However, you will need at least 10:1 cr to get power out of a cam that big. 10.5 would be even better. Also you should set your ignition timing at 20 inital and 40 total and don't use the vacuum advance. You will also need some hp springs to keep the valves from floating at hp rpms.

[Pyro]

That cam seems about right. However, you will need at least 10:1 cr to get power out of a cam that big. 10.5 would be even better. Also you should set your ignition timing at 20 inital and 40 total and don't use the vacuum advance. You will also need some hp springs to keep the valves from floating at hp rpms.

[HICKL]

I am running flat tops but don't think I will get 10.5:1. Well see how she runs. Why no vacuum advance? Cuz the initial will be so high?

[Pyro]

That is right, a lot of inital timing will already be in the ignition curve. If you try to run the stock inital timing (8 to 10 degrees) with a cam that big, it will cause the headers will glow red at idle and the off idle responce will be very weak.

 

Furthermore, those old style heads are not what we call "fast burn heads" so more timing is required to start the burn sooner which will allow the combustion process more time to complete. Typically, the new modern heads (fast burn design) only need about 34 degrees of total timing.

 

With 0.030" over flat top pistons, 350 crank, and 76 cc heads, the compression will be about 9.25:1. If you use a 0.020" thick steel head gasket and shave the head 0.020" you should be able to get the compression up to 9.9:1.

 

I would strongly recommend trying to increase the compression or use a smaller cam, like a comp magnum cam 270 or 280.

[HICKL]

I had a 465/290 cam in it and it seemed very mild. No lope at all, had decent torque but not much on top. I have the motor pretty much ready to drop in again so I will probably go on and see what she does with this one. I have some 0-60 times and will be curious to see if I gain or loose. I appreciate all your input.

 

Jeff

[Pyro]

Wow, really?? My comp cam 292 is a huge fender shaker with a lot of top end (pulls to 7000 rpm). And not much vacuum at idle or toque below 3000 rpm. I guess it all depends on the duration at 0.050" lift. The comp cam 292 has 244 degrees duration at 0.050, the 280 has 230, and the 270 has 224.

 

Did your machine shop guy give you anymore cam specs? duration at 0.050 or lobe center line?

 

If the last cam you had (290) was mild then we might already be talking about the same thing. That is.... "using a smaller cam than a 306 duration". Cam manufactures rate the advertised durations differently. Some at 0.006" lift, some at 0.004, and other lifts which can make huge differences in the advertised duration of a cam (numbered name on the cam, 290, 306, etc). The duration at 0.050" is the main thing that determines how the cam is going to act. With your step-up (9.25:1 cr), a cam in the area of 230 degrees at 0.050" would work well. And I get the feeling that your new cam will be about that size. When I read that you were going to use a 306 duration , I was thinking 250+ degrees of duration at 0.050" which would be a very very big street/strip cam.

 

And, like I wrote earlier. High performance valve springs are very important. I have made that mistake in the past. Weak springs will kill top end power. Please don't try to save money on springs. Good luck with your project.

[HICKL]

You are absolutely correct, my old 290 cam had a .050 duration of 224. I'm not sure on the 306 but I don't think it's much more. The old fart that built it calls it an "old 350 horse Vette cam". Says it's his favorite cam for the type of motor setup I have. We'll see if he knows his stuff

 

Thanks again for all your input

Jeff