I will be rebuilding my first Pontiac motor, a 68 400. I've done several Chevy's but am not familiar with the Pontiacs.
The stock compression ratio is supposed to be 10.75:1. I would like to reduce it to around 10:1 by using different pistons.
Are the factory pistons flat topped? If they are, do they make dished pistons?
Should I even worry? The customer uses premium anyway and doesn't have a problem with that. But, at the moment, the cranking cylinder pressure is very low, about 50 PSI and I am worried with a fresh rebuild the pressures will be too high. I am also planning on using a new cam with the factory grind. Is today's 91 octane enough for this motor?
Pontiac varies the compression ratio by changing the combustion chamber size. What is the number that is cast on top of the centre ports of the heads on the engine?
The stock pistons are flat-tops with small valve reliefs.
Kenny (PDUDE) at Kens Speed and Machine specializes in Pontiacs and can get pretty good pricing on dished pistons, not cheap by any stretch though. http://www.angelfire.com/fl4/pontiacdude428/
Most factory pistons are thick enough to mill a little out to create a dish though or even mill some off the entire top since they are flat tops with reliefs.
But I'd cc the chambers and check piston to deck ht first, chances are it's lower than 10.75 already. You might could find some .060 head gaskets as well. Other option is change the heads to one with a bigger chamber. Check the right side bar on Kens web site, there's a list of head numbers with cc's there.
Wanting a Custom fit in an off the rack world.
I don't have time for a job, I just need the money.
10:1 is still too high for 93 octane. We always place SCR at 9.3:1 to allow for a "bad batch" (gas) or 91 (93 not available in ALL places).
The Icon forgings (FKA Keith Black) are quite good for the price. They offer a "dished" piston for 400. Unfortunately, the dish isn't quite "big" enough to get the compression all the way "down". With the 72 CC heads (advertised by Pontiac at 10.75:1), the SCR is 9.9:1. The head of the piston is thick enough to enlarge the dish to the necessary size.
The Speed Pro L2262F-xxx is a very good forging as well. A bit heavier than the Icon, but perhaps the TOUGHEST piston available. Plenty of "meat" in the head to turn a "simple dish" (3" in diameter, .100" deep will do the trick). The dish can also be milled in a "D" shape to mirror the chamber shape. This costs more to machine, but takes full advantage of the quench pad and chamber shape.
Since you're new to Pontiacs, I have two "pearls of wisdom". CHANGE THE RODS! Use a good forging and dump the cast rods. There are several suppiers and a few different configurations. The RPM 5140 ("Tomahawk") rod is the most economical at about $250-275 a set. The Eagle H-beam is the best-priced "premium" rod at just under $500 for a set. ANY forged rod (except the '50s-'60s production "rubber rods") is better than ANY cast rod.
The other "pearl" is to get Jim Hand's "How to Build Max-performance Pontiac V8s" published by SA Designs. This is the only current study of the ol' Injun available. Even if it isn't an exact "match" to your performance goals, there is a TON of relatively new information and a "history" chapter to get you "up to speed" on the "quirks" of the Pontiac. As a Chevy-oriented builder, no doubt you've heard a WHOLE LOT about the Pontiac, most of it based strictly on "myth and superstition". Contrary to "popular" belief, the Pontiac was the most sophisticated engine GM made during the muscle car era. It took Hemi to "knock it off the top" as the world's "premier" race engine. Chevy never managed that position.
As with any engine, zero deck height and optimized piston to head distance is important for detonation control and that certainly applies to the Pontiac. I'd recommend strongly against having the pistons down the hole, machining off the entire piston head, and/or using head gaskets that are more than .045" thick, as anything over .060" clearance between the head and the piston increases detonation sensitivity rapidly. Combustion efficiency will also suffer. If a dish must be machined in the piston, match the dish to the chamber shape as Mr. P-Body said; the extra cost shouldn't be that much.
Pontiac combustion chambers are fully machined so the CR is more accurate and consistent than SBCs and only need the sharp edges rolled. Pontiac heads do have a quirk that makes them more sensitive to detonation; they have the spark plug closer to the intake valve, which means that the flame front moves from that side to the exhaust side, leaving the last part of the mixture to be burnt (end gases)right beside the exhaust valve. The exhaust valve raises the temp of the end gases, increasing the detonation sensitivity, and the hotter the exhaust valve gets the more it increases the likelyhood of detonation.
To help with that, make sure the exhaust system is as free flowing as possible. That will reduce the exhaust duration needed on the cam and will keep the exhaust valve cooler by keeping it on the seat longer. Also, make sure the cooling system is working as designed; the separator plate behind the water pump needs to have the right clearance to pump the coolant properly and there are several threads about that.
I like to use a cam that has a fairly tight LSA (110-112) to bleed off a little more cylinder pressure at the lower end while concentrating the mid and upper-range power. A wide LSA tends to take power from the middle and add it to the low and high rpm range. That has two consequences; it increases cylinder pressure at the low rpm, which we're actually trying to reduce, and requires the engine to rev higher to make up for the drop in mid-range power. Pontiacs are not high revvers so using a cam with an LSA of more than 112 spreads the power range out too much and reduces the famous Pontiac mid-range torque without adding horsepower. The Pontiac factory grinds usually had more than a 112 degree LSA and were great cams at the time; a newer grind will work better IMHO.
The higher the static compression ratio, the more valve timing overlap is needed to reduce the cranking pressure and keep detonation under control, and a smaller LSA increases overlap, as does increasing the cam duration. Matching the cam selection to the rest of the engine is important.
Tuning is also important: an overly rich or lean mixture will elevate the exhaust valve temp, again raising its temp and increasing detonation sensitivity, as will ignition timing that is too far advanced or retarded. Use vacuum advance to keep the engine clean on light throttle and of course to increase fuel economy.
Proper selection and matching of parts will result in an engine that makes very good power on pump gas, keeps the engine temp down and gets decent fuel economy.
All good info here. Seems funny as I just went through all of this very same stuff with the motor from the ole Bird. Mine is all done now and ready to install and I can not wait! You guys all offer some great technical info here as it all pretty much matches up with my friend in MD who owns a high end race engine shop.
I think if your building to stock HP or a little better, the cast rods are fine if you resize the bearing journals and use ARP bolts. That is my opinion only.
I have a set of dished pistons which I removed from my 68 400 with #16's. The CR that I calculated is 8.3:1 The dish was 31cc's. I am going to flat tops with a CR of 9.6-9.8:1 depending on where my heads cc at.
4300 feet is about 12.7 psi without temp correction. That's about 15% less pressure; not absolutely sure that translates to a corresponding reduction in effective compression ratio.
