Joedatsun Posted February 28, 2006 Share Posted February 28, 2006 The bottom end of my turbo motor is currently being torn down and evaluated by the person who will be doing the machine work and assembly. Depending on what he finds, I was planning on doing a stock rebuild with the addition of forged pistons, leaving the CR close to stock. My RWHP goal is in the 300-350 range. It was suggested to me that another option would be swapping out the 280xzt rods and replacing them with 9mm 240z rods and forged pistons which Dave at AZC sells. I believe that this would in effect up the CR to approx. 8.4-1 to 8.5-1. I was told that this would make the motor less of a dog off-boost and would be able to reach my HP goals using less boost. I plan on using my SDS to control everything. My question is this: does it make more sense to go with the Stock CR which would leave a larger margin of error when tuning etc. or is it over conservative considering the superiority of todays stand-alone (SDS) EFI systems? All opinions welcome! Thanks Joe Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted February 28, 2006 Share Posted February 28, 2006 Great question. I wish I had the answer for you. I will be watching this!! You might want to state what turbo you plan on using. Quote Link to comment Share on other sites More sharing options...
Joedatsun Posted February 28, 2006 Author Share Posted February 28, 2006 I was thinking a T3/TO4E for the turbo. Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted March 1, 2006 Share Posted March 1, 2006 I may be missing some info. but if you put 240 rods on a stock L28 crank with a P90 head you get a 9.16:1 CR and if you do a 20 thou. over bore you get 9.28:1 CR. There also seems to be a positive deck height of 2.730mm (i don't understand how thats possible with the given CR) (stupid engine calculator, or maybe just stupid calculator operator). So Im guessing that the forged pistons have a different pin height in order to get an 8.5:1 CR? I'll be building a 8.02:1 CR turbo motor: 20thou over flatop piston block, 2mm hks head gasket and P90 ... Im hoping that 8:1 is "zippy" enough off boost but still affords me enough slop in the tuning so I dont blow up my engine. If I want a little more CR I can take out the 2mm head gasket and use a stock one and get 8.6:1 ... but I'm not sure if thats to high. arnt forged pistons just insurance against detonation? and if you dont detonate, do you need them? So I guess I don't have any new info for you... just more questions... sorry! Quote Link to comment Share on other sites More sharing options...
240zwannabe Posted March 1, 2006 Share Posted March 1, 2006 i figure the less chance of detonation the better....it's your motor you pick Quote Link to comment Share on other sites More sharing options...
mario_82_ZXT Posted March 1, 2006 Share Posted March 1, 2006 I may be missing some info. but if you put 240 rods on a stock L28 crank with a P90 head you get a 9.16:1 CR and if you do a 20 thou. over bore you get 9.28:1 CR. There also seems to be a positive deck height of 2.730mm (i don't understand how thats possible with the given CR) (stupid engine calculator, or maybe just stupid calculator operator). So Im guessing that the forged pistons have a different pin height in order to get an 8.5:1 CR? The custom AZ zcar pistons compensate for this. You are left with a better stroke and bore ratio. 8.5:1 cr turbo's seem to have a nice sweet spot. Almost the same compression as a NA L28 and turbo to boot. They are awesome for power and torque and with standalone engine management, you could easily keep the detonation at bay. This or turboing a flat top block and P90 is going to be my route for when I build another turbo car. Mario Quote Link to comment Share on other sites More sharing options...
zguy36 Posted March 1, 2006 Share Posted March 1, 2006 Comparing compression ratios, you need to look at the causes of detonation. Detonation happens when the cylinder pressure are too high. So lets say that with your combustoin chamber, you can have X amount of pressure in the cylinder before detonation. With lower compression, you can have a larger mass of air (since it isn't compressed as much) than you can have with a higher compression. The tradeoff is that higher compression is more efficient, making better use of the air and fuel that is in the chamber. Finding the sweet spot of optimizing max mass to max efficiency is what you are looking for. In my opinion, you aren't making drastic enough changes to your compression ratio to have any negative effect on maximum power. If you want better off boost response, go with the higher of the two ratios. Quote Link to comment Share on other sites More sharing options...
pjo046 Posted March 1, 2006 Share Posted March 1, 2006 I am going with a 8.5:1 CR on my build. It's nice to have good of-boost characteristics as well! Seeing you are going with a standalone, you have much better controll when tuning the engine compared to those running the stock system, so I would say the standalone makes up for the increased risk of detonation when running this higher CR. Quote Link to comment Share on other sites More sharing options...
ihatejoefitz Posted March 1, 2006 Share Posted March 1, 2006 300+ hp has been made on engines straight out of the junk yard, but if you've got the money to spend thats awesome. Quote Link to comment Share on other sites More sharing options...
Guest motorheadeb Posted March 6, 2006 Share Posted March 6, 2006 If the pistons are really close to the perimeter the squish may help suppress detonation more than a lower CR so the higher deck height combination could yield more power with more compression (go figure). Quote Link to comment Share on other sites More sharing options...
AK-Z Posted March 6, 2006 Share Posted March 6, 2006 I say get higher flowing injectors than what you need, then you can lower fuels presure and/or adjust injection duration. Quote Link to comment Share on other sites More sharing options...
Lurch2461 Posted March 6, 2006 Share Posted March 6, 2006 I am currently building a replacement engine for my Skyline. It now has an L20ET on board. My new engine is an F54 block, P90 head, stock L28ET cam (thanks to another member of this forum) 1.0mm oversize L28 Flat Top Pistons, L28 rods & crank. It will have about 8.7:1 compression, 460cc injectors, will be boosted by a VG30 BB Turbo and I'm looking for similar HP and expect it with less than 1 Bar of boost, running on 100 octane ULP fuel. ECU is going to be stock except for Microtech piggy back to control the fuel & boost issues. The engine guru's in this country say it's the only way to go. My only question is re TB size. Cheers, Dennis in Oz Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted March 6, 2006 Share Posted March 6, 2006 Engine gurus in Aus say a microtech piggyback is the only way to go? Are you kidding me? Re t/b, do what you want, its turbo, wont' have particularly good response in any case, and you can fiddle with the actuation a tad anyway. Dave Quote Link to comment Share on other sites More sharing options...
Lurch2461 Posted March 6, 2006 Share Posted March 6, 2006 Sorry! maybe I should have put the ECU piggyback comment in another thread or equally on another forum. Dave, I thought you were an intelligent being. Maybe you should join the likes on SAU cause they show similar intelligence. You take this out of context ....... I guess your a woman! Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted March 6, 2006 Share Posted March 6, 2006 Right well being an aussie i'll assume you're taking the piss. I try not to let on I'm intelligent. Glad to hear I succeeded. Dave Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.