We are in the process of getting parts for our 354 Hemi. We would love to get 400 horsepower out of it but we are stumped as far as what C/R to run. As the motor sits now, it has forged 11:1 pistons that we had hoped to be able to use, but we are not so sure now. Would it ping on premium pump gas at 11:1? If so, what compression ratio should we go with?
My 392 has 10.25:1 with aluminum heads and has never had a pinging problem (in a pretty heavy '51 Ford). I would suspect 11:1 with aluminum heads might work fine with an appropriate duration cam. I am sure others can give a more thorough, better supported answer.
My 331 (bored .060" ) (in the coupe in my avatar) is very close to 11:1 compression. Some background on that statement... It started out with 10.5:1 Ross forged pistons, then I hurt a cylinder (dropped stock valve) and ended up having the block decked and using a thinner steel shim head gasket. Also the '54 331 heads (100cc before) now have larger stainless valves. So I'd say it's at least 10.75:1, maybe even 11:1 now. My machinist said that with the amount he decked the block, it was going to be very close to 11:1, but I never bothered to re-CC the heads, or run the compression calculator. Anyway, I run 93 octane pump premium, and I don't have any pinging problems. I do have a pretty big cam with a lot of overlap that's probably bleeding off some compression. I run a Muncie 4-speed, and heavy billet steel flywheel. I think you will probably be fine. If you are still worried, you can buy thicker MLS head gaskets from Cometic to bring the compression ratio down a bit. Really nice gaskets, and much cheaper than new pistons.
Thanks for replying. We'll keep this in mind, and yes we'd much rather buy thicker gaskets than new pistons!
A "rule-of-thumb" is you can run 1 point higher C/R than a wedge engine with the same C/R on the same octane level.
timing is also a big variable here. trying to run 32 degrees on junk gas most likely won't work. 28 on premium will most likely be fine. just play with it and make the necessary adjustments.
400hp is doable. As to the c/r, until you actually do the math you will never know what is in there.....just sayin... .
What transmission, rear end, and how are you going to drive it? Chugging around at 2000 > 3000 rpm is going to be asking for detonation with high compression and a "small" cam. I think you'll need a pretty serious cam and related parts to reach that HP goal and rpm. And intake, and exhaust I'd be talking to someone like CamKing over on speed talk. My arm chair expectation is 11-1 is too high on pump gas with a streetable cam and iron heads with a typical mechanical ignition curve. 11-1 pistons for a gen II 426 look like this - http://camtecs.typepad.com/.a/6a00d83451eb1c69e2019b035ac492970d-800wi That much dome makes for crummy combustion characteristics ( slow flame travel, even with centrally mounted spark plug) requiring extra advance, exacerbating tendency for detonation. Chrysler set the Street 426 at 10.5 compression, in the 60s when real high octane gas was available.
We're probably going with a Muncie 4-speed, M22 most likely. Rear end is a 3.50 true trac from Strange Engineering. The car is pretty light though, I've heard numbers ranging from 2800 to 3000 lbs. We probably will drive it most on weekends and to shows, but it won't be a trailer queen. I have heard from many people that 400 horses from a 354 is very doable. We aren't going to buy anything until we calculate the compression when we get the motor apart, but if we have to go with the thick head gaskets, will that present any problems?
...a 354 is an EarlyHemi NOT a gen II (LateHemi) Pistons for the same c/r will not look the same or interact the same with the heads.... .
i second 73 rrs comment. until you cc it then you are just guessing. who knows what you will find after you check it. you might be surprised. it might be low.
We'll have to call somebody but I'm pretty sure Forged true is no longer in business so not sure what to do there.
If you haven't completed the rotating ***embly,balancing still has to be done. cc what you have with the Pistons you have and get a real number. If need be,any machine shop with a piston fixture can cut the domes on a lathe to achieve the compression you need. This is a easy process. Then ballance and build the rest of the motor. Tire size,gear ratio,transmission,induction and intended use play on a cam selection. For a few dollars more,a custom grind won't break the bank. Hemis are never cheep to do so this is one place you shouldn't skimp. I have done business with jones cams and bullet cams, both real helpful and knowledgable. Comp could even grind something for you but you need to know what specs you want.the tech line is going to sell you a shelf grind less than optimum. Cylinder pressure is key,make a plan do the math and build it so there are no surprises. It's worth the work Good luck, Gary
Thank you for this response. I can ***ure you we won't skimp on anything that goes into this project, just daydreaming right now until we finish the garage. Hope to be posting a build thread soon though, but until then thanks to everyone for responding!
With a hemi just roll the piston to the top fill the cylinder with 90w using a cc burette. If you are just ***embling seal the piston to the wall with a little grease.
You really need to have the piston 'down' in the hole a specific distance so that you can actually measure the dome volume, either a positive or negative. We normally use a ½" down-fill position. Mine is a Pyrex 250ml, I think it is a '#4' but not positive. A burette stand is also needed. .
I understand the concept but I'm having trouble picturing this. Do you have any pictures or maybe a more in-depth explanation?
In order to calculate static compression ratios you need to account for all of the volume in a sealed cylinder. This includes chamber volume, head gasket volume and cylinder volume and dome volume. Let me try to explain the basic procedure to find dome volume..... We can easily calculate the 'swept' volume of the cylinder, or portion thereof. Now, to get the dome volume you need to: •***emble one piston-rod and install in block with crank in place. •find TDC •rotate the crank so that the piston drops 'x' amount. (any depth that will allow the piston to drop below the deck) •then fill the cylinder for a direct reading of the volume above the piston. Now, compare the 'found/measured' volume to the 'calculated' volume (for a cylinder that is 'x' tall) and you will have 'dome' volume, either a positive or negative number. If the cylinder volume is calculated to be 100cc and your downfill volume measures 80cc then the piston dome is 20cc. Negative number says you have a dished piston. I have little doubt that there is a you-tube video with someone showing the procedure but since I am a self-confessed techno-tard I won't try to find one. Need more help? Just ask.
Thank you for those instructions. I'll run them by dad as well and I'll contact you if we need help when the time comes.
So, looking at a calculator like this one: Engine Compression Ratio (CR) Calculator http://www.csgnetwork.com/compcalc.html The dome volume has to be considered in terms of how "far" it protrudes into the chamber? They have a final step where the portion of the dome that is above the deck is called out. Can you share the best way of arriving at that?
============== Post 23 describes it pretty well. Mahle video starting 4:53 or so here for domed piston -
I looked at the video ...I knew that there would be one... My issue with the video is that the worker-bee measured from the deck to the top edge of the piston with a scale rather than using a dial indicator from TDC which is more accurate and also accounts for any of the piston shoulder that might sit above the deck. Whether or not the dome extends into the chamber is a function of the amount of c/r and the chamber shape/design (volume). The volume of the dome is found when doing the down-fill; different engines at different c/r will require checking at different depths. The relatively low c/r in some Hemi engines will still have, what the manufacturers call, a dome, even though it may not have a positive deck height. I guess that some of this is semantics. Way back in the old days, just after we discovered fire, I had a 12.5:1 426 Hemi and those domes damn near filled the entire combustion chamber, but that is/was due in part to the design of the head/chamber. The early head is a bit different. As you can imagine, each head will have a different relationship to the 'dome' on the piston at any give c/r. Pistons for a wedge engine will have a 'pop-up'; is it a 'dome'? Then consider the 348-409...... Hope that this helps, if not we'll try again. .
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