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I did the calculations on my motor being built, and I've come up with the following.
My builder is telling me that this setup with -15cc Pistons is going to be 10.25:1. That's not what 5 liter eater or myself got. We got the same numbers. He's extremely reluctant to build the motor and wanted to go with -20 to -22 Pistons and wants me to assure him this motor won't see pump gas+meth. It's an iron 370 with PRC 227 heads that were cut down when I lifted them on the 347 when the meth failed. So that's where the 58.6cc head info is coming from. Car has a Novi 2500(YSI) with a 2.85 pulley. Probably in the 850-900 area.
This isn't my first rodeo. But I also don't build engines for a living. I had a 402 in my C6 that was probably closer to 11.1:1 with the milled LS3 heads and Novi 1500. Car made 850/800 on that tiny blower on pump and meth. It's still living a few owners later and beat on like a rag doll. But I've also blown my share of motors.
I do plan on running E85 most likely. That, or gulf 116+ meth if I don't change my current fuel system. But for cruising, not racing, I'd like to stick to pump + meth.
Should I run the -15 or -20 Pistons?
Last edited by winters97gt; 04-03-2016 at 01:21 AM.
My engine, although different, is exactly 9.9* with -15 pistons and both the builder and myself agreed it's perfect for my use. Running a YSI at near 20lbs on 94 and meth. The motor has met all expectations !!
I got just slightly under 10:1 also. Even if it was 10.2, I can't see that being hugely bad. Especially on E85. All else equal, you might not be able to run quite as much spark advance with the slightly higher CR. With the kind of octane you're going to be using, I wouldn't worry about it.
Your math is correct regarding the compression ratio, your builder is wrong. I would be really reluctant to use a builder that cannot come up with correct static compression.
I like to see compression right around 9.8-10.2 for what you are doing and you seem to be right where you need to be. The thing that concerns me is the zero deck height for a couple reasons..
1. Neither Diamond or Wiseco make a off the shelf -15cc piston for the 3.622" stroke with a 4.035 bore that I can see, so im assuming this is a custom piston? You will need to ensure that the block is decked and measured before you order them if you want a true "0" deck because you will need to order them with the correct wrist pin location/height.
2. I think that building the engine with a "0" deck is silly if you are using a .051" gasket. Having the engine built with the piston .010" out of the hole will give you a much better quench. That or use a gasket that is .010" thinner.
I would also opt for upgraded tool steel wrist pins right away.
I get an even lower DCR than that. Does your DCR calculator use intake duration or intake valve closing angle? I'm assuming the cam uses Comp lobes. Not sure of the lobe style, so that may be where I'm off.
Your math is correct regarding the compression ratio, your builder is wrong. I would be really reluctant to use a builder that cannot come up with correct static compression.
I like to see compression right around 9.8-10.2 for what you are doing and you seem to be right where you need to be. The thing that concerns me is the zero deck height for a couple reasons..
1. Neither Diamond or Wiseco make a off the shelf -15cc piston for the 3.622" stroke with a 4.035 bore that I can see, so im assuming this is a custom piston? You will need to ensure that the block is decked and measured before you order them if you want a true "0" deck because you will need to order them with the correct wrist pin location/height.
2. I think that building the engine with a "0" deck is silly if you are using a .051" gasket. Having the engine built with the piston .010" out of the hole will give you a much better quench. That or use a gasket that is .010" thinner.
I would also opt for upgraded tool steel wrist pins right away.
Just my $.02
Thanks.
1. Yes, it's a custom piston being cut by Wiseco. My builder wanted to go -20cc and I didn't. The car is all top end, it's gutless down low.
2. The zero deck height was just a base reference since I wasn't sure. Many guys here run his motors, including guys like Flash, etc.
I do have the steel wrist pins going in. I'm backing off the 2.85 pulley and going with a 3.125 on my Novi 2500. Going to keep the power at 775-800. Then I'll put the 2.85 back on for the fall when it cools back down to race with a single M15 nozzle. Should be around 900rwhp give or take.
Yes, but are you at that compression on 23-25psi? That's what my Novi 2500 makes on this setup on the last 370 setup. This is the same.
My old set up with an Old billet wheel standard cover s480 made 837whp/741tq @ 5.8psi. I didn't get my new turbo setup retuned yet but I will be running a direct port methanol kit along with E85 for anything over 10psi. You should be fine as long as you have the proper fueling and octane even at 25+psi. Not sure if you heard of extreme tuners from Greece but they actually had an Evo (4G63) de-stroked and running 17:5.1 compression with a GTX55 C16 fuel I believe. Made insane power.
I got just slightly under 10:1 also. Even if it was 10.2, I can't see that being hugely bad. Especially on E85. All else equal, you might not be able to run quite as much spark advance with the slightly higher CR. With the kind of octane you're going to be using, I wouldn't worry about it.
Based upon LXL lobes with the given info and .006 closing point
I'm curious how the 58.6 heads will do. I'm thinking less timing. I've decided to run a 3.125 pulley on the car for now and take the 2.85 off. I'll put the 2.85 back on it when the air cools back down and see if I can't crack off some 9.50's.
04-03-2016, 01:19 AM
Compression Ratio Gurus
