Thread: Upgrade cam from 270H

Tweet

I have just built a 350 engine and it runs very strong with the Comp Cams 270H camshaft but I was hoping for a more aggressive sounding camshaft. The 270H sound pretty much stock. I was thinking of a 280H or the 292H.
The guy I had port, polish,gasket match, and put new valves and springs in my heads said that they were good to .510'' lift but I dont want to max my springs out with too big of a cam.
Does anybody have any idea of a cam that will sound better without over doing my springs? It doesnt have too be a Magnum Cam but I want a Comp Cam.

Do you want sound, or performance??? If it runs strong, why overcam the motor just to make it go rumpity-rump??? More cam means having to loosen up the converter, which builds extra heat when using a high stall converter on the street. The extra heat will also tend to wear out the tranny. If it sounds stock and runs strong, sounds like a good sleeper setup.

i agree with dave if your not going to go all the way dont go there. radical cams sound good but if your running strong put some good sounding exhaust on and cruise around smokem up turn alot of heads and keepem guessing whatcha got. big cams= more headache on the street and that means mo money,mo money, mo money. by the way if you want that thump thump pull off 3 spark plug wires to the untrained ear a multiple misfire sounds like a radical cam and at low rpm you get the same mileage and plug fouling

I would go the route of a good sounding exhaust system because that 270H cam with say a 40 or 50 series Flowmaster (not that I am stumping for Flowmaster but they're my preference) exhaust system should sound anything but stock. I put the 270H in my '67 Chevelle SS 396 when I rebuilt the engine. I had the 50 series (non-Delta Flow) mufflers WITH 2.5 " exhaust and 3.0" tailpipes mounted to Hooker Competition headers. I ran the Turbo 400 trans with shift kit and 2800 stall converter. That cam gave me all the power I needed and it had a great sounding lope to go along with the performance. Stick with the 270H, especially if you are running an automatic transmission.
That's my nickels worth...
Greg

I have a 400sb with a 270h cam,2.5" pipe(no crossover pipe) out the back with 40 series flowmasters. One of the best sounding cars I have had in awhile. Just recently I was screwing around and I took the Vacuum advance off of manifold vacuum and put in on ventury vacuum. Just to see if I could get a more consistant idle. Well, that made it sound even better at idle. Even after I turned the idle screw in to bring the RPM back up to where it was before. I have done this on other cars before and didnt notice much difference in the sound. The retarded timing at idle with the two cambers and tail pipes out the back is a killer combo.

"hoping for a more aggressive sounding camshaft"

I know what you mean, you want that "rumpety-rump" sound so you can impress the geeks at the local drive-in. I'm not dumping on you, I'm just saying that the geeks are the only ones who'll be impressed. With your near-stock idle, the savvy rodders will understand that you knew what you were doing when you chose that cam to complement your near-stock compression ratio. Moving to a cam with 280 or 290 degrees duration will move the intake closing point to later in the cycle and turn your motor into a real dog at lower r's. YOU MUST MATCH THE STATIC COMPRESSION RATIO TO THE INTAKE CLOSING POINT OF THE CAM to make a good running combination.

If you're going to insist on rumpety-rump, call up your favorite cam grinder and have them grind you a moderate-duration cam with an intake closing point that will complement your static compression ratio, but have them grind it on a narrower lobe displacement angle like 106 or 104 degrees.

I'm not sure how much my compression ratio is but I have Stock type dished 40 over pistons and about 58 to 60 cc combustion chambers.

Before you ever pick up the phone to order a cam, you must have all the vital information including your EXACT compression ratio. In order to figure that, you MUST know the EXACT combustion chamber volume, head gasket volume, deck volume, piston dish volume and cylinder volume. I'm simply assuming a stock compression ratio until you supply something different.

Pistons are dished .040 over
Block is stock deck height
Heads are 305H.O. w/60cc chambers Had machine shop cc them
Head gasket compresses to .039
I'm assuming the compression is around 10 to 10.5 to1 because the engine used to have 75 cc chambers with pretty low compression.

731.02 cc's in the cylinder
6.60 cc's in the deck, assuming 0.040" deck
60.00 cc's in the chamber
8.43 cc's in the gasket, assuming 4.100" gasket bore
12.00 cc's in the dish, just a wild-ass guess
Static c.r. is 818.05/87.03 = 9.40:1

Tech1, Great to hear from you! dcurtis try to plug your specs into this chart and see what you get:

http://home.tampabay.rr.com/1bking/c...calculator.htm

Tech1, when oh when will I get my engine running? I am coming to the end of my wife's Florida room project and hope to get the engine running this summer. My comment has to do with the Crane cam I have (Crane 10010, 194/204 @ 0.050" and 260/270 advertised duration with 0.398/0.420" lift, surely a mild cam BUT it is listed with a 104 degree lobe separation. Does the narrow lobe separation lead to a stock-like idle or some rump-rump? I can't wait until I can get this new room project out of the way to get the engine running, but with the 700R4 originally recommended by Tech1 it should run strong up to 4000 rpm and the OD should run OK at 2000. I just made a quick run to/from Charlottesville from Richmond on I-64 last night and the gearing in my Sunfire is almost exactly the same as my roadster with a 3.55 rear and 700R4 and running at 70 mph most of the way showed about 2500 rpm in the Sunfire so the cam in my 350 would be running at 2500 a lot of the time and that is where I would like plenty of PEP(!) to accelerate from 70 mph (occasionally). Man this building takes patience and lots of $ but I want to DRIVE the roadster! Good to hear from Tech1 again!

Don Shillady
Retired Scientist/teen rodder

HE! HE! HE!


~ Vegas ~

Hey Don, thanks for the kind words
As you know, the rumpety-rump is caused by the early opening of the intake valve on the exhaust stroke, so that spent gases are pushed back up the intake tract, thus disrupting the vacuum signal to the carb venturi. With any cam grind, it's necessary to start getting the intake valve open somewhere before TDC so that when the piston begins its descent, the intake will be open enough to begin passing mixture. The intake begins opening early and the exhaust valve closes late, somewhere after TDC to assist spent gas evacuation. This last little bit of exhaust slug also helps to "pull" the beginning of the intake slug past the intake valve. This period during which both valves are open a little is called the overlap period.

When you tighten up the lobe displacement angle, you are increasing this time during which both valves are open by opening the intake earlier and closing the exhaust later, thus increasing the overlap period. As the r's increase, you have less and less time for cylinder filling, so getting the intake open earlier and earlier is necessary as the cam timing gets wilder and you're moving your power band up in the rpm range. As with all things in life, there's a trade-off. Increased overlap is usually accompanied by increased duration, you must give up low-rpm power in favor of gaining mid-range and/or high-rpm power. It is possible, however, to grind a short duration cam using an early intake closing point for good low-rpm performance with a tight LDA of 104 or 106 degrees for a little rumpety-rump. I can't say how much rumpety you're going to get with that cam, but it should meet your performance expectations. Here's a link to the results of changing LDA and also changing advance/retard upon installation of the cam.
http://www.co.jyu.fi/~rax/lobe.htm

Once the motor is together, you can determine the best advance/retard position of the cam by doing a cylinder compression test, changing the cam position 2 degrees at a time to the retard and to the advance positions. The cam will tell you where it wants to be by reflecting the highest compression reading.

I've tried to nail down the cranking compression limit for any given pump gas, but everyone seems to have a different outlook on it. From what I've seen, a reading of 160-170 p.s.i. might be the limit for 93 octane pump gas before you experience knocking and you may have to fiddle with the ignition timing to get by.

Tech1, thanks for the good link AND the neat way to use pressure readings to optimize the timing, maybe I will get it ALL, a four speed to handle the narrow power band, low rpm torque, maybe a little stop light rump-rump AND over 20 mpg? We shall see.

Don Shillady
Retired Scientist/teen rodder