|08-22-2010 08:09 AM|
This is an old thread, but he cam to use is a Comp 268 in a stock Pontiac like this, and is you get the pistons with 8 valve reliefs per piston, with the edge of the entire top of piston chamfered, that basically adds 16 cc's and lowers the CR a lot. Those pistons with a 400 .030" over will yield 9.6 CR which is a tad high. With 75 cc heads it may "just" make it. Or machine the piston a little to add 5 more CC then you're ok. I prefer to have 9.25 CR or lower with iron heads. After building many Pontiacs over 30 years time, they ping with any compression over 9.5 and iron heads, on pump gas.
|08-22-2010 08:06 AM|
the bigger cam bleeds off compression at idle or part throttle low rpm, but not at high rpm- those big cams "come in" above 3000 rpm, then the engine is detonating harder than ever, and actually doing some serious damage. That is not the solution, the solution is what you did, dish your pistons, which no doubt you gleaned from other posts elsewhere on the net, for a Pontiac. I run a 470 CID Pontiac with 80 CC heads, and dished the pistons 30 cc, they are TRW's flat tops- and I posted the results and pictures on the net of the parts. Back then everyone was pushing "quench" and 10:1 CR which will NOT WORK with today's gasoline and iron heads. I lowered the CR to 9:1 and the engine actually makes a lot more power, because now I can rev it and dial in 34 degrees total timing without ping.
the big cam fix doesn't work, it only kills the bottom end, and the engine will still ping above 3000 rpm
your 421 build sounds cool
|07-12-2006 01:19 PM|
Original topic is 3 1/2 years old
but we can still learn something.
|07-12-2006 12:24 PM|
You first stated that you wanted a mild cam to avoid detonation, but the opposite is true since the hotter cam bleeds off some of the initial compression since it keeps the valves open a little longer. A safe approach to compression is to have a total of 92cc's total combustion chamber and piston relief for regular gas. If you use one of the less expensive rebuild kits that have the cast pistons with the multi valve reliefs cast into them, the total "dish" of the piston will come out to about 12cc's. Take the 12 and add it to the 72 cc's that the 62 heads might be after a cleanup mill, and you have 84 cc's or about 9.2:1 compression. Premium fuel territory, but acceptable. The alternative would be to get a set of relatively inexpensive TRW forged pistons and have your machine shop cut a 20 cc dish into the top of the piston.
I just put together a 421 that was .060 over with a set of well worked 62 heads, and 26 cc dished pistons (bigger engine requires a little more dish). we installed the Summit cam and lifter kit and are really, really happy with it. the Summit cam visually looked better than several of the big name brands we have installed. This is in a HEAVY 1965 Pontiac 2+2 that weighed in at 4,418 pounds with driver at the drag strip scales. We run it on 87 Octane pump gas with no pinging. We participated in the High Performance Pontiac Magazine shoot-out on June 30th at the California Speedway POCI drags and the article should be coming out in a future issue.
|04-15-2006 08:12 PM|
if i were you id find a different set of heads and use those heads for another build.
those heads deserve to be in a pontiac!
i have used #16's on a 030 400 ci and finds that all you need is a really great ignition system. i ran a 308 dur cam with 515 lift and if i remember correctly 116 lope desperation. a MSD 6al with a 6k revv limiter performer rpm intake and a mighty demon 825. ran killer! i had it in a 68 tempest vert (which weighs the same as that cutlass) and doing 40mph, i could drop it into 2nd,sidestep the clutch and stay sideways for about 40 -50 yards.
until a rod bearing finally let go.
in short, if you want just a mild engine, use a set of 6x-4's a bit bigger combustion chamber and you can still put the 1.77 intake valve in.
then any cam you pick will not *PING*
|04-15-2006 06:37 PM|
Go to Performance Years
Wmarden - I hate to just send you to another board, but go to the tech forums (Street section) on www.performanceyears.com . You'll find 4 billion posts about cams for Pontiacs...and everything else Pontiac related for that matter. Oddly enought, the Summit grinds 2800, 2801, and 2802 are highly recommended for the average street car. I hate recommending the 2800 because it is SOOOO mild, so my recommendation would be the 2801 - what exactly is your compression ratio with those heads? Comp, Crane, and Crower all make great grinds for that motor. Somthing in the 214 intake duration range will probably work well for you.
|04-06-2006 11:04 AM|
I believe the poncho guy in Canada goes by SD Dave or maybe Dan . I'm told he's a wizard.
|11-05-2002 04:07 PM|
I'm not sure what you compression ratio is but, if it is high, then you want to go with a camshaft with a short Lobe Displacement Angle, this will keep the cylinder pressure down. also, you can clean up the combustion chambers so that there are no heat risers were detonation can propagate itself. Call comp cams help line and give them as much info as you can and they will hook you right up. I'm sure summit it good, but I have never used them, I've only used Comp Cams, or GM replacement cams, and have never had a problem.
Keep on roddin' <img src="graemlins/pimp.gif" border="0" alt="[pimp]" />
|11-05-2002 12:42 PM|
|chevyelc81||If you compare the specs of the summit cams to the edelbrock cams, you will find that summit makes cam shafts with the same specs as edelbrock, but for alot cheaper.|
|11-05-2002 11:25 AM|
|firstname.lastname@example.org||The Edelbrock Performer system is hard to beat and the Summit brand items are top notch. Either would be fine. Mail order either from Summit and shipping is free!|
|11-05-2002 11:12 AM|
|bullheimer||racer who built my chevelle used nothin but summit cams. i havent tried one yet and dont know who makes them. but their catologe has tons of specs to compare w/different brand name mfgrs. a good pontiac engine builer is up in abbotsford canada and has a web site but.... i forgot what it is he goes by his name or initials. lotta help aint i|
|11-05-2002 06:47 AM|
|drewhelm||Good all around Pontiac cam that would fit your needs is one from Summit. I belive the part number is SUM-K2800. This part number includes lifters and is 86.95. The cam itself is SUM-2800 for 57.69. I don't work for Summit, but will vouch for their products!|
|11-05-2002 01:11 AM|
Which cam should I go with for my pontiac? Ok let me give a few details. It is a pontiac 400 and will be rebuilt, but I need to start figuring out which parts to buy. SO I was looking at a few pages and decided to pick a cam.
The motor will be mostly stock with some overbore as needed. Heads will not have much work, they are 62 heads which have 72-75 cc combustion chambers. With those small chambers I feel I need a cam on the smaller side for street use to keep the detonation away as much as is possible. Plus to be honest, I am not much of a racer, I just want a decent streetable cam. The car it is going into is a 71 cutlass. So it is not going into a lightweight vehicle.
Edelbrock has one that looks good(the basic performer cam), but I could not get onto the comp cams website to do some comparison shopping. Plus I am also looking at budget here. I don't want to cut corners, but if I can save a few bucks for a comparable product, then of course I would go that way. I know enough to know I don't want a used cam because I wouldn't know what to look for or how to tell it was good. And besides the cam is perhaps the most important part of your motor.