I believe I am getting a good deal on a 451 with 12.5 compression. The down side is that is has 12.5:1 compression. What would be the best/cost effective way to lower the compression to 10.5:1-ish so I can run it on pump gas ? Here is the specs I have: 12.5:1 compression steel 440 Crank LY rods, Ross 14.00 CC dome Pistons, balanced rotating assembly, 452 heads, extreme energy cam 510 lift. Thanks !
You could probably reduce the amount of advance on the distributor to reduce the chance of detonation and obviously make sure you buy the highest octane gas you can find in your area. MD
Won't be nearly enough. The Mopars are very intolerant of compression with pump gas. You have a few options but some will be counter intuitive to best results. Just shimming the head up higher will help reduce SCR but also increases dead quench space.. ie; hurts mixture motion away from the quench pads. One aspect helps while the other further hurts on an already too large combustion space that's prone to end gas detonation issues. Head swap towards larger chambers. Mill the domes(this depends on the piston crowns design). Install a cam that snuffs out some of the dynamic compression ratio. Do none of the above and just do an E85 carb conversion. This engine will LOVE the extra octane and will run a bit cooler. Just be sure to run a fuel pump and lines that are tolerant of the ethanol. IF.. you were to combine a thicker MLS or shim style head gasket.. with a cam swap to snuff out some DCR.. retard the ignition timing a bit.. run a cooler t-stat combined with a water wetter agent.. run shorter rear gears(this helps reduce the time the engine needs to accelerate and further wards off heat spikes/end gas detonation..plus you get back some of the acceleration you killed off by lowering cylinder pressure/power output).. and find some good premium fuel?.. you could probably get by so long as you don't EVER lug the motor in too tall a gear. PS.. you may also get by with a 50/50 premium/E85 blend too. Don't forget to install a killer coil too. Spark voltage is extremely important as cylinder pressures go up and E85 mixes are much denser and can use the extra energy to pop things off correctly.
Since the engine is not installed yet, why not get it on an engine stand, pull the heads, flip it over, pull the pan, pull the rods and pistons and if the cylinders and crank look good install a set of lower compression pistons and fresh rings. I would be temped to pull the crank , hone the cylinders, and reinstall the crank with fresh bearings and seals but at that point we're pretty much looking at a rebuild. MD
Thank you guys for the input. Dude; I`ve thought of replacing the pistons and having the assembly rebalanced. Just trying to get some ideas before I tear into it. Last time I was left unsupervised I bought an engine I can`t even run on the street without aviation fuel. Groberts, So the E85 and compression of my motor will get along ? I need to go look if I can get some local, haven`t noticed any around here but haven`t looked. Will milling my pistons (assuming there is enough material) have the same negative affect on the quench as running a thicker gasket ?
I pulled the engine apart and did the math on it all. My 12.5:1 engine is in fact a 11.16:1 engine. From what I have read 7.5 cc is worth a point of compression so if I have the dome milled off the pistons that should put me pretty close to 9.5 (correct?) so that makes me happy. Also my ported heads are kind of a joke but all well. Thanks again for the input.
Milling domes off pistons is not something I'm familiar with so I'm going to punt. Probably time to discuss this with a professional engine builder. Good luck! MD
yeah.. I didn't think the numbers added up with only 14cc domes on that motor. Also keep in mind that many/most newer piston domes are molded into the crowns in such a way that they are hollow underneath and you cannot get back that whole 14cc's of dome volume without milling right through them. Not to mention flycutting pistons isn't exactly cheap to do in the first place. Plus you'll throw the cranks balance requirement off in the process. Snowballs right over your wallet real quick. You should post the chassis and drivetrain combo if you want deeper opinions on what's required to avoid detonation. You already have the fact that the 451's are lower deck 440's and have much lighter reciprocating masses to equal quicker rev's. Combine that with some shorter rear gears and tuning for premium fuel and you should be just fine. Really great motors.. although the domes can be a PITA on the street.. I'd build one in a heartbeat if I was into that brand. Stroker version would be really nice too. You could also invest into an MSD programmable 6AL ignion box to control timing lead based on engine load. ie; loaded car.. hills.. hotter weather. They can be run with any old Chevy MAP sensor tee'd into manifold vacuum. Just need a cheap laptop and Bob's your uncle. Could also tune it for AVgas or race fuel and have a little extra fun to the tune of about 25-30 horsepower on a some rare occasions if you wanted too. PS. don;t go too big on the exhaust system or you'll kill the pipes scavenging effect and get a nasty case of EGR related issues in that big combustion chamber. 3" max size on the collectors and 2.5" intermediate pipe sizing will get the job done up to about 500+ horsepower and keep exhaust gas velocity higher(crisper throttle snap as a result of more torque). And a dual plane manifold would help to reduce intake reversion while giving more torque down low to avoid bogging/loading the motor. Especially helpful to tame a too big cam choice and squash detonation issues.
Right on, The pistons are actually solid underneath so right now the thinnest spot is the valve reliefs and after having the domes milled off the thinnest spot will still be at the valve reliefs so I`m fairly confident that wont be an issue. A friend of mine builds airplane parts and has access to alot of neat stuff. So if the machine shop wants too much to mill them he can do it for pretty much nothing. I realize I`ll need to get it balanced but I would with buying new pistons just the same. The 6al sounds pretty neat I`m gonna check that out. The car is a `68 4 spd charger, I haven`t checked the gearing but I`m probably going to end up putting a gear vendors or a t56 in it with 3.73s. I`m going to assume it has 3.25s or 3.50 in it now. I just put 2.5 exhaust on it, got tired of listening to the oil burning 400 rattle through the open headers the car came with. I`m sure my neighbors are glad I did that.
Cool.. I love those cars.. one of my life-long favorites. Slightly heavy'ish cars.. higher SCR likes more gear to avoid temp spikes/higher end gas contamination in the combustion chambers that come's with slightly slower rev's/acceleration(such as heading up long grades in high gear). The 4 speed helps out a ton in that regard but I'd still lean towards adding another .3-.5 into the rear gear if you have a 3.23. I think a 3.55 would be the bare minimum depending on your cam choice/engines powerband. Into the low 4.xx's would be a killer rear gear setup with a bit more cam and the t-56's double OD gear would settle you right down into conservation mode on the highways. Can't remember exact ratio but look for the one's with really tall final OD since they have shorter first gears. I've ridden in several powerful 3.73 rear gear taller OD T56 combos and they are pretty close to a best of all worlds type deal. And yeah.. always much cheaper to lighten a cranks counterweights than adding heavy metal. Another thing you could do is just sell the used pistons for whatever you could get to help offset the cost of some new flat tops. Really.. after you factor in all the hassle and hidden costs of modifying what you have, it's not much more money for some good quality hyper's(not sure of your power levels but they can handle well over 400-500 horsepower but the tuneup needs to be more striclty maintained) or cheaper forged type deals(which might be even lighter than your existing pieces may be after the flycutting.. another little bonus as your crank gets even lighter/helps extend margins). Then again.. you'll need fresh rings.. might as well look at the bores, bearings, and rod bores.. and it quickly leads right back to the snowballed wallets thing I mentioned. lol Here's a nice little fancy quick calculator that's fairly accurate to plan a build. Can be optimistically off a hair due to not factoring in the exact head gasket bore volume. http://www.eaglerod.com/ Or the more techy one is here. Easy to lose another half hour of my day doing engine design every time I visit this site. John's math and coding skills are highly accurate compared to much more expensive and detailed engine building software. http://www.wallaceracing.com/cr_test2.php PS. If you end up getting the programmable MSD box.. feel free to PM me and I can tell you how to cross the pins over on the GM MAP sensor. Google helped me along several times already.. very easy to find the pin layout info. Reason for all that hubbub is that MSD is tricky.. they designed their "special MAP sensor for use with this ignition box harness" with a slightly different pin layout to get more of your money. You can also graft a GM connector into the box's MAP wiring too. Just be aware of the PIN layout differences and the OEM stuff will work perfectly vs their special "MAP sensor sold separately". Good luck and have fun with it.
