we have got the A coupe done and she runs real good. We are using a 355 sbc(I know,I know) with a comp cams 270 magnum bumpstick(270 duration,.470 lift,224 degrees duration at .050, 110 centerline) We have 1.5 full race roller rockers now. We would like a LITTLE more rumpity-rump without changing the cam. Would 1.6 rockers on the exhaust side make much of a difference. Anyone with experience with this? Other motor info: 11 to 1 pistons, 74 cc heads (motor is probably about 10 to 1 with these heads). dual 500 carbs with progressive linkage. 4 speed transmission. thanks
the car is plenty loud..would like just a little more choppity-chop. Sounds great when it is cold-mellows out a little after warming up good.
Putting a 1.6 rocker on the exhaust will only change how far the exhaust valve opens. (you'll have .501 lift then) Duration and lobe seperation will affect the cackle factor. The High Energy 268H would probably have suited you better. Try running it really really rich
I don't think you'd see much if any change in the rumptiness of the cam by changing rockers. Granted, there's a bit more of a 'window' to let fuel/air in and burnt gasses out, but the basics won't have changed that much - especially at idle. My understanding as to why rumpty cams go rumpity rump is that during the overlap some of the fresh air/fuel mixture is shoved back up into the intake. The next intake stroke drags that in as well as shoving part of the mix up into the intake manifold as before and the cycle repeats until it settles out at a particular 'rich' setting and what you end up with is an overly rich mixture at idle. Rumpty can be improved by use of a manual choke, but don't use it to go too rich or the old hands will be able to tell what you're doing. Not that I've ever used the choke to make a stock cam sound bigger ... and you didn't hear it from me....
Seat timing/duration will not change, but duration @.050 lift will increase slightly due to the quicker acceleration rate of the valve. You will see an increase of about 2-4 degrees @.050 depending on how aggressive the lobe design is when going from 1.5 to 1.65 rockers, so going to 1.6s should at least add 2 degrees @.050. Not enough to make much difference in how the engine sounds. If you really want more 'rumpity rump' grab a new cam with about 10 degrees more duration @.050. ----- Bart -----
LS-1 engines have 1.7 roller rockers stock from the factory. I have heard about some folks using them on other SBCs but don't have any experience with that application myself. I have some rockers and mounting rails that I bought off ebay, you could probably get a set there as well and try 'em out...
Actually, changing to a 1.6 rocker does increase duration by about 3 degrees. I don't think that would make much difference at all in the sound. Why did you go with such a small cam in the first place??? With that much static compression and that small of a cam I'd be VERY concerned about detonation!!!
actually this is a very good "street cam". Good gas mileage,doesnt run too rich at low RPM, good bottom end power as well as midrange. NO detonation whatsoever. It was recommended by the comp cams techie as well as being similar to what G.M. used in the factory 350-350H.P. engine in the late 60s. A static compression of about 10 to 1 is really not too high-like I said we are using the early 70s heads with 74 ccs,not the 64 ccs the motor came with.
I had a 270-H cam in my old bomber with a small block Chevy. Ran almost as good as the stock 350 cam it replaced in my 305! Not impressed at all with square pattern cheap-o cams for street cars running stock heads and an exhaust sytem (although 'square pattern' cams will work okay on race motors with open headers and port work). I put Comp Cams Magnum roller tip rocker arms on my engine, using the 1.52:1 rockers on the intake side, and 1.60:1 rockers on the exhaust side, as you are contemplating. (A buddy of mine worked at a speed shop and sold me a mixed set so I could have the 1.52 intake and 1.60 exhaust rockers!) By running the 1.60 rockers on the exhaust, you are creating the effect of a dual pattern cam by giving the exhaust side a little more lift and duration, but it really didn't help the 270-H in my car...it ran better with a TRW 274 cam (dual pattern) with the split ratio rockers. To get a more "lumpity" sound, you may have to sacrifice bottom end grunt...either by buying a bigger cam, retarding your cam timing a few dgrees, or retarding your ignition timing a bit and backing the idle speed screw down a little. I wouldn't recommend any of those steps if your goal is just to play the "Hot For Teacher" intro as you idle through the lot...but you can do it if the 'sound' is your primary goal! Buy a dual pattern cam with an intake duration @ .050" of about 230 degrees, and you'll hear it! It also pull very hard through the midrange! That'd give ya the sound, at the cost of some off-idle grunt. (Not a big issue in a lightweight car with steep gears, though!)
[ QUOTE ] 11 to 1 pistons [/ QUOTE ] [ QUOTE ] A static compression of about 10 to 1 is really not too high [/ QUOTE ] Ok... 10 to 1 is not too much for that cam. 11 to 1 is. Which is it??? They are NOT 11 to 1 pistons if they make your compression 10 to one! then they are 10 to 1 pistons!!! What are the pistons you're running? Flat tops? domes? How big? what's the deck height? Head gasket thickness? For a dual 4, 4 speed car that still seems like a pretty small cam to me...
Thanks for an answer from someone with experience with this cam. Actually the cam does have a definite "lump" and iI am happy with how it runs. It is a highboy style 31 steel coupe,so it doesnt weigh much. It has a muncie with 3.40 gears in the rear so there is plenty of "grunt". I guess I was just looking for the easy way for more(the kid in me). Anyway we are not going to use any of the cheesy ways mentioned for more rumpity-rump. It has lake style headers and old style chambered mufflers so there is plenty noise . The heads do have the big valves,but have not been ported. I guess we better leave well enough alone...
Yea you are right..I should not have called them 11 to 1 pistons. Actually they are .100 dome "pop up" pistons which give you 11 to 1 with 64 cc heads. Sorry, my mistake. I dont know the exact amount,but I am guessing about 10 to 1 with our heads.The heads have only been cut about .010 and we are using the felpro composite gasket(about .035 to .040 thick).thanks , Dale
i think you made good choices in the selection of your parts for that car, granted the 270 doesnt have the pro gas idle to it, but it is a very good cam in a light car. while being very streetable. C9 eluded to what causes the rumpity t rump in the engine, so you can see why changing the exhaust rockers would not change the sound of the cam. most guys build their engines with a particuler sound in mind, rather than performance, and are usually dissapointed in the performance on the street of their engine. do to the fact of to large of a cam,kudos to you i think you did good. i did some quick math and with your combo,depending on valve reliefs, you should be at 9.5-9.7 to one with a standard issue small block head gasket
I bought parts to build much the same setup, but was told to use a cam with more overlap else I'd be detonating if i floored it at lower revs. I then bought a 280H comp cams bump stick, which says DON'T use 1.6:1 rockers. When****embled and running the motor was too smooth and streetable for my liking. Ran great, nice power, but idled comfortably and smoothly at 700rpm. Pull a plug cap off and it'll sound great, choppy and lumpy as*****, then set the idle too low so you have to keep blipping the throttle to keep her running
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