Compression for boost

Performance mods and Advanced Stroker discussion.
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2manyrides
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Compression for boost

Post by 2manyrides »

Long time lurking, I have an 05’ Lj with the Banks turbo, A2W intercooler, AEM v2 progressive meth, Fastman 62mm TB, and a Greedy Pro Fec E-boost controller currently. I have been running 8psi daily for awhile. My transmission went out, so I’ve put the jeep into the garage and started on some mods that I’ve been planning. I have a ported Tuppy head. I did the port work, and no I do not have a flow bench. This is not my first rodeo either. I’ve done Oldsmobile heads on a 525ft# jet boat 455, and the last set of gt40 P heads I did dynoed 300hp at the wheelers with an e-cam and bolt on’s before tune. I did a a full on port and polish on the Tuppy head. The chambers where opened up to 60cc’s and then polished. Stock valves and I had the head shaved .010. I need to re-cc the head, but I believe this should put the chambers @ 58.6 cc’s or so. I’m installing JBA shorties and I’ve built a custom Y-pipe/head pipe 2.5’’ to the up pipe without the pre-cats. I’ll be running the mini cat cel fixer cats to hopefully get around the post cat o2’s. I’m at the point of picking the head gaskets. I’m thinking about raising the compression to 9.07:1 buy using a Cometic.40 gasket which would raise the DCR to 7.31. I could also entertaine going higher with a .30 head gasket. Just wondering what the consensus is on how much compression I could get away with running about 10# of boost. I run 93, I am intercooled and running AIT’s usually 100* less then engine temp (100-110*, with short rises to 125 during hard boosting ) I also have a progressive meth system. I mix my own 50/50 mix using VP fuels pure M1 and distilled water. My combustion chambers are polished, and either head gasket will improve quench. I also have aN AEM Wideband/failsafe, and Banks I-dash for real time monitoring. I currently have to much fuel if you ask me. Even without the meth, I see the Wideband hit richer then 11:1 during full throttle boosting. The banks tune is pretty fat and I’m hoping a better breathing head will actually lean it out some. I will also be installing a new fuel pump with a transplanted AEM 325 in it to insure the pump can keep up. The banks kit came with 35#hr injectors. Givin some of these factors. Do you think I’ll be ok bumping the compression up while raising the boost to 10? I’m pretty confident that I have the fuel to keep out of detonation. I will also be running ARP head bolts. Any input would be great. I need to make a decision soon and get the gasket ordered.
Cummins90
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Re: Compression for boost

Post by Cummins90 »

I can't speak for the banks system but my set up is a stock engine with 15psi out of a Mitsubishi S20g. Stock fuel pump with 36lbs injectors on 93 no meth. Intercooled with similar IAT to yours during the summer. Managed with an FIC. 5psi by 2k and pulls to redline. At full boost my AFRs are also 11:1.

Does the banks tune have timing retard?

The o2 defouler/spacer worked for my 96.
2manyrides
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Re: Compression for boost

Post by 2manyrides »

The banks system comes with a flashed tune via diablo in-tune sport with a tune from Banks. I have about 10k miles on the banks kit. The tune is ‘’canned’’ so I can’t change the peramiters. I do believe timing and fuel tables are all addressed in the original tune, but Banks states that the tune is for a stock engine with the banks kit. So any cam or displacement changes would require custom tuning. I’m prolly not going to push any harder then 10 psi on the stock engine. I’m just wondering if I’d be safe going from 8:8:1 to 9:1 while raising the boost. I truelly believe I have all the fuel I need with the meth kit being the wildcard safety factor. The banks kit uses a 2 bar map placed in a spacer just under the TB..
Glad to hear the mini cats worked for you. This system is a post pre-cat system. I put a brand newCatted Y-pipe on at the same time as the turbo system. The pre-cats are allready decaying, and from the looks of them it wouldn’t have been long before a chunk broke free and wiped out the turbo. I was never happy with the quality of the replacement Y anyways. The pipes where smaller then the factory, crinkled up bends, and the Y looked more like a T. I really feel it has cost me some power over the much nicer stock Y pipe that I replaced ( which had cat damage). My plan has always been to build a boost minded stroker to run 12#. I ported the head last year , and it was part of the stroker plan, but I’m going to put the stroker on hold for a bit and addresse some other things. I’ve got a rock jock to put in, need new drive shafts,need to build a cage, Raptor line the tub, the ported head, JBA header, hydro assist, new 42rle, and now the custom Y pipe. I have all or most of the parts and pieces for these projects (except the trans), but I might even do a Rubicrawler from Advanced adapters with the rock jock so I can still keep the 2:72:1 along with the 4:1. This is a big if, but hey. A good snow plow season and that’s all it takes! I’m figuring on being down for about a year. The jeep is paid off now and has mostly been a spare vehicle weekend wheeler I proubibly put 2k miles a year on it if I’m lucky.
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Re: Compression for boost

Post by Russ Pottenger »

As a general rule it’s better to lower the static compression ratio in order to raise the boost.
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Re: Compression for boost

Post by Cheromaniac »

You could either go old school with a lower compression and more boost, or go the more modern route with higher compression and less boost. The former will produce more turbo lag with a sluggish off boost response and a more dramatic onset of boost, while the latter will produce a more linear response. It all depends on what you want.
Your biggest enemy is going to be detonation, so attending to all the factors that'll predispose the engine to detonation (compression, boost, fueling, timing advance, cooling system upgrades, lowering IATs, using correct spark plugs) is key.
If the engine is basically stock with an 8.8:1 compression, I'd limit boost to 8psi. If you want to run 10psi, I'd suggest you lower the CR closer to 8.0. If you don't want to lower the SCR that far, you could run a longer duration cam than stock to bleed off some cylinder pressure.
2manyrides
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Re: Compression for boost

Post by 2manyrides »

So heads came back and I re cc’d them. All chambers except #3 came in dead on 59cc’s with #3 just a tab bigger at 59.4 cc’s, so where just going to call it 59. After looking through head gaskets and running #’s through the calculator I decided to go with the Cometic c5988-040. 4’’ bore with a compressed thickness of .040. This will land me right back on stock compression of 8.82:1 static, 7.23:1 DCR with a slight improvement on the quench to .061. The gasket came in today and it looks great. Now I need to find some time to get the old head of,new head on, and then check my pushrod length. This is only a head swap so nothing different about the block (98k miles never been overheated). So that’s .011 difference on the gasket, and .010 off the head. I’m assuming I will indeed need to change pushrods with that kind of difference .021. I have the comp cams 7704-1 pushrod length checker. My question now would be what’s the best method for checking the pushrod length on an engine that was not rebuild, and was running last in the beginning of December?
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Re: Compression for boost

Post by Russ Pottenger »

With the lifter on the heel of the camshaft you’ll want to adjust out your adjustable pushrod checking tool to zero lash.
Your new pushrod will need to be between .020 to .070 longer than that length to get to your desired preload.
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