The Stovebolt Site Forums The Shop Area Engine Shop 261 camshaft

My 261 engine seems to "pop" under load; are the camshafts for the 235 and the 261 the same or did I get a 235 camshaft when I should have had a 261?

ron

First the 235 and 261 camshafts are indeed different but niether installed in either engine should make it "POP" sounds like something else is goin on.

It looks like the duration and timing on the shafts are quite a bit different. The sound seems like an overlap of the intake into the exhaust. I don't know why this would happen under load and then taper off but this it what it sounds like. My homemade degree indicator seems like the valves are not opening and closing correctly. Guess I'll have to pull all the sheet metal and take it apart. Fun..........?

Intake and exhaust are supposed to overlap, otherwise you wouldn't be able to get any performance at all. It's not uncommon for there to be 30 or 40 degrees at least between the time the intake valve starts to open and the exhaust closes. On high performance cams, the overlap can be considerably more. As the idle speed increases, it's possible to run much more timing and lift. A stock cam will have approximately 240 to 250 degrees of duration, while a race-only cam might have over 300 degrees of duration and it would be totally unsuitable for street or low speed operation. As cam duration and lift increases, compression, port size, intake and exhaust manifolds, carburetion, and exhaust restriction must also be modified. The cam is in the middle of the flow- - - -both ends of the chain, intake and exhaust, have to match the center link, or no advantage is gained.
Jerry

and since we're on 235 cams <;-o, i need some advice on my '61block/'56head combo - i'm changing my solids to hydros, and going with a 700r - what would be a good, mid-price street cam, but with a tad of hop? i'm also planning on duals and headers, but not looking for the WRAP! of a hot rod. i'm seeing anywhere from $150-$500+, but for the $5+, it better be able to spoon with me. do these fit the bill?

from santucci
dur.050"/218* lift .460* lctr 110*
dur.050"/224* lift .470* lctr 110*
dur.050"/238* lift .480* lctr 110*

Popping under a load sounds like an intake valve is leaking, have you checked the settings?

Checked compression, all 120 or so, have correct camshaft. Added a washer under the ex valve springs to no effect. Next will try adding washer under int valve springs.

Double chek the spark plug wires. Possibly cracked distributor cap.

from santucci
dur.050"/218* lift .460* lctr 110*
dur.050"/224* lift .470* lctr 110*
dur.050"/238* lift .480* lctr 110*

Are those recommendations are for the stovebolt, or the Gen-3 230/250/292?

Stock 235 cam: 54.5° overlap
Stock 261 cam: 24.5° overlap

Just for everyones' information; After pulling the radiator, oil pan, timing cover and verifying timiing was correct, after changing all valve springs, doing everything possible to find the rattle I replaced the F'N MUFFLER!!!!!!!!!!!!!! There was a loose baffle that under certain exaust conditions gave off a deep rattling noise; I thought it was engine related...... I spent almost a week on this...... guess I didn't know as much as I thought.