onephatz Posted October 29, 2004 Share Posted October 29, 2004 ok here is my delema. I have a complete motor setup from a '82 turbo that is currently taken apart. I was wanting to go back in with flat top pistons with my p-90 head giving me 8.5:1 compression. BUT I don't have any flattop piston motors at the moment and I dont' have the time to rebuild my dished piston motor with flattop pistons. I want to shoot for ~400hp and wanted to know how much harder it's going to be for me to achieve this with 7.4:1 compression like on a stock turbo motor vs a motor with 8.5:1 compression ratio? is it going to make a significant difference or only a nickels worth? jesse Quote Link to comment Share on other sites More sharing options...
Guest Zcarsmakemyheadhurt Posted October 29, 2004 Share Posted October 29, 2004 Depends on your set up? Do you have a turbo capable of producing 400HP as well as a injector that can support that horsepower? Do you have a cam for this powerhouse that requires higher compression? Most turbo cars make more power with a larger area in the cylinder to full with air and fuel. Higher compression wouldn't hurt but may not help much in trying to break the 400hp mark. Let us know. Quote Link to comment Share on other sites More sharing options...
onephatz Posted October 29, 2004 Author Share Posted October 29, 2004 here is the setup that I'll be running when it's all together L28 p-90 head pistons ??? felpro 1mm headgasket twin t-25's (good for ~400hp)--->don't ask.... compcams 280/280 (still up for debate) non egr intake w/ 60mm tb front mount intercooler dsm 450cc injectors msd 6btm ignition megasquirt efi Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 30, 2004 Share Posted October 30, 2004 Well I am by no means in the 400hp area, but I have run both a 7.4:1 and a 8.3:1 setup back to back. (the rest was the same essentially) I really like the 8.3:1 setup for daily driving, autocross, etc. It really is a huge difference in power delivery and feels much faster even though the potential peak power is less. If you are looking for big dyno number, or are racing an a manner that you can keep your engine in a narrow powerband, then the low CR setup will be fine. If you want all around drivability and area under the curve, and are willing to sacrifice that peak HP number, than the 8.5:1 setup would be my choice. Of course at higher CR's the importance of a good intercooler, fuel control, timing control, fuel delivery, etc. plays a larger role. Quote Link to comment Share on other sites More sharing options...
Brad-ManQ45 Posted October 30, 2004 Share Posted October 30, 2004 What Drax said.... Quote Link to comment Share on other sites More sharing options...
onephatz Posted October 30, 2004 Author Share Posted October 30, 2004 sounds good. my car is no daily driver, it is only and i mean only a weekend warrior. my intent is to hand out beat downs on guys who think their cars are fast I want all my power to be top end where i'll be shifting thru. thanks for the info jesse Quote Link to comment Share on other sites More sharing options...
GrayZee Posted October 30, 2004 Share Posted October 30, 2004 One thing I like about the 7.4CR is something I found out one day that I accidentally left the headlights on and killed the battery. I decided to give it a push-start and pushed the car about all of two feet and popped the clutch. The engine fired up instantly!! I do have another F54 engine with flattop pistons, after reading Drax's post I'm tempted to drop that in my car and try it out. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 30, 2004 Share Posted October 30, 2004 Seems to me on a very basic level if there is less compression you can add more boost, and you get more power out of the boost than out of the compression. If lag isn't an issue, it seems that lower compression would be the easy choice. At least that's what all my turbo'd buddies tell me. Quote Link to comment Share on other sites More sharing options...
Sean73 Posted October 30, 2004 Share Posted October 30, 2004 I am running a f54/P90 l28ET with flattop P79 pistons. The CR is actually 8.8:1 (not 8.5:1 -- the pistons have a little deck height that causes this miscalculation). Overall I am very happy with my setup. I only have about 500 miles on my motor, and have taken it up to about 12 psi without any issues. The nice thing about the higher CR is driveability off boost. Of course I am being very conservative with boost, because there is not a lot of experience out there on this motor setup. After the break-in, I plan on taking it to the dyno for some real data, and hooking up some kind of aftermarket knock sensor. Sean 73 240Z Turbo L28ET, T3/T04E, NPR IC, Z31 ECCS Quote Link to comment Share on other sites More sharing options...
pallnet Posted October 31, 2004 Share Posted October 31, 2004 I just finished putting together the same engine. I'm running SDS with 460cc injectors but only have it set at 5 pounds of boost at the moment. Well to be honest with you, at the moment she is asleep in the driveway waiting on a 6 puck clutch and 10 pound flywheel setup. The first time forrest and I took her out for tuneing. We realized that the stock turbo clutch just will not hold the torque. The problem starts to show right before I get into boost. Yes, the car is a beast even out of boost. The throttle responce is insane. I really have no clue how long the pistons or rings will be able to hold up. I imagine that as long as the fuel and timing is right and I keep the charge cold enough, it should last long enough to hurt the feelings of a few V8 owners out there. I have ARP head studs and am useing a victor HG. I know I will play hell if I have to change it later on cause, I coppercoat both sides before install. I don't want to have to change a HG on this setup just for a small coolent leak. I did notice that this engine likes to run below a certain temp. A bit over that and she does not sound right. I may install a backup electric fan for "just in case" the main electric fan fails on me. One on each side of the radiator. Quote Link to comment Share on other sites More sharing options...
Clifton Posted October 31, 2004 Share Posted October 31, 2004 Making 400 rwhp is probably easier with a 7.4 cr because you can run more boost with less chance of knock. You will need to run around 20 psi to do it though and have a turbo that can flow the air for it. My L28 is a little under 7.4 due to a little unshrouding in the head and makes a little over 400rwhp... My 7M motor is around 7.8. Off boost power and throttle response is fine even at almost 2600lbs. Anything above 4K and 18 psi (max on 91 oct) it will spin the 275's in 2nd from a roll. Quote Link to comment Share on other sites More sharing options...
fl327 Posted November 1, 2004 Share Posted November 1, 2004 7.4 is good I think, from what Ive seen the flattop turbo setups work pretty good up until 300hp mark, and at that point ive seen four degrees too much timing blow rings lands OFF QUICK!. Swapped in 7.4cr pistons and rods and corrected timing, and my buddy has like 50 low 12second passes, traction being the only issue left... The factory turbo motor was born that way, and LIKES TO BOOST. Quote Link to comment Share on other sites More sharing options...
Guest Cronic Posted November 1, 2004 Share Posted November 1, 2004 Did I read that right? two t25 turbos? lmao.... Those turbos are garbage. For any motor, you will outflow both of them very quickly. I do not see them working for your high rpm (above 5000rpm) needs. Quote Link to comment Share on other sites More sharing options...
onephatz Posted November 1, 2004 Author Share Posted November 1, 2004 Did I read that right? two t25 turbos? lmao.... Those turbos are garbage. For any motor' date=' you will outflow both of them very quickly. I do not see them working for your high rpm (above 5000rpm) needs.[/quote'] yea I heard all the hype. I know they aren't much but they are going to do me for now. eventually I will put either 14b's or 16g's on as an upgrade. right now it's more of a novelty than a practical. I would much rather have the simplicity of a single t3/t4e but that isn't the direction I am going right now. if you don't like t-25's what else would you recommend for a twin setup. and let me stress this IS going to be a twin setup. i'm not trying to be difficult I just have an idea of what I want to do and will continue with it till I get fedup with my stupid ideas and go the easy route. any input you could render in this situation would be great thank jesse Quote Link to comment Share on other sites More sharing options...
280zinsanity Posted November 1, 2004 Share Posted November 1, 2004 Did I read that right? two t25 turbos? lmao.... Those turbos are garbage. For any motor' date=' you will outflow both of them very quickly. I do not see them working for your high rpm (above 5000rpm) needs.[/quote'] yea I heard all the hype. I know they aren't much but they are going to do me for now. eventually I will put either 14b's or 16g's on as an upgrade. right now it's more of a novelty than a practical. I would much rather have the simplicity of a single t3/t4e but that isn't the direction I am going right now. if you don't like t-25's what else would you recommend for a twin setup. and let me stress this IS going to be a twin setup. i'm not trying to be difficult I just have an idea of what I want to do and will continue with it till I get fedup with my stupid ideas and go the easy route. any input you could render in this situation would be great thank jesse I got your back chief! Just wait until you finish the project with good results. You will have done more with your car than alot of people. So dont worry about them haters And, I would do twin super 16g's DSM guys sell those off really quick to go for a bigger turbo. Quote Link to comment Share on other sites More sharing options...
Pyro Posted November 1, 2004 Share Posted November 1, 2004 Try 2 T3's. Those turbos litter the junk yards and can be had for cheap! Then you could go for 500 hp! You should also get a "turbo cam" (no overlap please). The compcam 280 is not designed for a turbo. You need a cam with 280 on the intake and 260 on the exhaust with no overlap. Quote Link to comment Share on other sites More sharing options...
Guest Zcarsmakemyheadhurt Posted November 1, 2004 Share Posted November 1, 2004 Two T3's in .48 housings would be exhaust flow effective.... Quote Link to comment Share on other sites More sharing options...
onephatz Posted November 2, 2004 Author Share Posted November 2, 2004 Try 2 T3's. Those turbos litter the junk yards and can be had for cheap! Then you could go for 500 hp! You should also get a "turbo cam" (no overlap please). The compcam 280 is not designed for a turbo. You need a cam with 280 on the intake and 260 on the exhaust with no overlap. ok t3's...... what car does the t3 come on with a a/r .48 exhaust. maybe i can find some of those cheaper. maybe a late 80's early 90's chrysler's? about the cam, I get my info from a engine dynamics book that say's: " blowing down is the process by which a supercharger forces mixture into the cylinder and expells spent exhaust gases there by fully scavenging the hot exhaust gases. If carried to extremes, it is obviously wasteful of mixture and to that extent inefficient. The benefits obtained from the complete scavenge of the head and absence of hot remnants are howere enormous in relation to the extra power obtained. For its successful application blowing-down requires an early opening inlet valve -say 40 or more degrees btdc to give time to allow the high pressure mixture to enter at high rpm. the exhaust valve remains open for a corresponding period after tdc. because of the positive high pressure, there is no doubt about the direction of mixture flow, nor the fact that the cylinders are thoroughly scavenged and left full of uncontaminated charge." -phillip smith 1967 true this book was written in the sixtys but I still think it has many truths that can be applied even today. if things have drasticlly change since then, let me know. jesse Quote Link to comment Share on other sites More sharing options...
Pyro Posted November 2, 2004 Share Posted November 2, 2004 Turbos are different than superchargers. As you know, superchargers get energized from a belt drive and turbos get power from the exhaust. So it makes sense that they would have different cam requirements. And turbo don't like to scavenge. The reason is the pressure in the turbo exhaust manifold can be as high as 35 psi! As compared to the exhaust on a supercharged engine which is like 4 or 5 psi when exiting through headers. So if there is ANY overlap on a turbo it causes exhaust to enter the intake. Therefore, bigger intake duration and smaller exhaust duration is the best setup. The small exhaust duration helps keep the "zero" overlap and increases exhaust velocity, just what a turbo likes. I'm pretty sure Talons and Eclipes use the T3 with a 0.48 housing. Quote Link to comment Share on other sites More sharing options...
Clifton Posted November 2, 2004 Share Posted November 2, 2004 Chryslers do use .48's but they have a different downpipe with the wastegate intergrated. I "think" Volvos and maybe Saabs used a .48 too. A supercharged engine would have very low exhaust back pressure so. On most turbo setups there will be alot more exhaust manifold pressure than intake manifold pressure unless one was to run a very large turbine wheel with a larger a/r. There would be exhaust wanting to flow back into the combustion. Quote Link to comment Share on other sites More sharing options...
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