Xander Posted March 27, 2005 Share Posted March 27, 2005 Recently I have build an engine for my turbo project. I have a f54 block with a p90 head. The pistons are flattop but they have a squish pad on top. Now I never thought that would be a problem but just now I used this calculator http://www.csgnetwork.com/compcalc.html and came up with a ratio of about 1:11. This is on a turbo car with 7 PSI of boost. The car drives great but I already had to replace the headgasket twice . they where cheap gasket though. I am now going to install a felpro with ARP studs. I hope this will hold at least 10 psi. I am using an intercooler. Using a roaddyno I guesstimated I am currently at 214hp@5600 and 224lbsft @4100. both measured at the wheels. I would like to acheive 300 hp at the crank. Should I sell this block as a high compression NA block and build a regular block or shall I just hope and see if the block will hold 10 psi. choices, choices. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted March 27, 2005 Share Posted March 27, 2005 Whoa... one problem with that calculation, you're calculating the compression after adding the turbo boost, which isn't really accurate. Forgive me that I dont know all the technical jargon to explain this, but it has something to do with the difference between static compression and dynamic compression. For instance... I did some math a long time ago with my P90/F54/dish top setup running 14psi of boost and it gave me a compression ratio of like 13 or 14:1 something really high like that, which should be WAY too high to run on street gas... yet I run 91 octane for the last two years with no problems. I don't see how an F54 with flattops and P90 head is anywhere near 11:1 CR so I'm assuming you are figuring this with the added boost... at any rate, you're probaby more at a mid 8:1 CR which is fine on street gas and will do fine up to 10psi of boost as long as you have the fuel and deal with ignition timing. I had a F54 block/flattop piston/N42 head combo which gave me about a 10.3:1 CR... and the N42 is a bit higher compression than the P90 so that 11:1 just can't be right. Quote Link to comment Share on other sites More sharing options...
Doug71zt Posted March 27, 2005 Share Posted March 27, 2005 Euro N/A pistons had a raised quench area on them, so technically they aren't flat-tops, they have raised domes, cutting the chamber area and raising compression. I think you are going to have problems keeping headgaskets in that thing, as it is probably knocking under boost. Your engine is trying to tell you something - the compression is way too high. If you manage to get a HG that stands up, the pistons will be next. Any L-series head isn't a good enough design to run boost at 11:1 static compression. I can't think of any engine that would do this, off the top of my head. I would get the compression down to around 8.0-8.5:1 with new pistons, unless you want to run alcohol... Doug Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted March 27, 2005 Share Posted March 27, 2005 oh snap okay heheh now I feel stupid... he said flat top pistons I thought he meant the regular flat top pistons . my bad... well at least it makes sense how you're getting 11:1 static CR and yeah I agree I dont really think there's a way you're going to run that on a boost L28 but yes it would make a great high compression N/A motor especially since it *ARGUABLY* has the best L head. Either way that motor should be a great performer even off the boost. Quote Link to comment Share on other sites More sharing options...
Xander Posted March 27, 2005 Author Share Posted March 27, 2005 thanks guys for your info but get this. I AM STUPID!!! I miscalculated the volume of the squishpad by a factor of 10. I am "only" at about 1:9.7. stil very high for a turbo block. I was amazed at the low end performance and now I know why . I am stil going to try this CR with a mild 7 psi of boost. I think with a good gasket and the ARP studs it should hold. And if not I will just sell it as a high compression NA block. And build me a regular dished piston turbo block. Is there a better way to break in the headgasket? I nomally just slap it on and retorque it after a good heat cycle. Quote Link to comment Share on other sites More sharing options...
frank280zx Posted March 27, 2005 Share Posted March 27, 2005 Well a 2mm HKS gasket or home built ( with help og our machinist friend ) Anyway im pretty glad with the qench area in my NA f54 'flat' top piston combo with n42 shaved head LOL getting me in the 1:10.9 / 1:11 like we just discovered in our phone call, but for boost ... hmm well on teh other hand do 3.1 ltr stroker guys manage to run upto 2 bar of pressure ????? given the have prolly installed some forged pistons.. ohh well i say a 2mm HG would do it as said it will bring you down to ~1:8.9/ 1:9 witch would work as you take care of the 'extra raised pad effect' lets see how long this Nissan gasket will hold up ! Or yet better you finally get to mess with your water injection, lucky YOU! Quote Link to comment Share on other sites More sharing options...
BleachZee Posted March 29, 2005 Share Posted March 29, 2005 Can anyone explain exactly why a higher compression motor can't run the same boost as a lower compression motor? Like was said, when you pump in all that forced induction you really do have greater compression... Lets stay more along the lines of what we might actually do with the L28 Turbo. I've got an 82 turbo and 83 non-turbo engine. I'm really considering using the flat top L28 bottom with the P90 and turbo setup. What really is the boost limit running 8.8:1 compression? If it is 10psi then why can you run 20psi on a motor with 7.5:1 compression and have twice the power output? If I just make sure the fuel mixture is correct won't that be enough? My goal is 240-270whp. Would running the stock turbo at 12psi on the 8.8:1 engine be about the same as a T3/04 turbo at maybe 15-17psi on the 7.5:1 stock turbo engine? Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted March 29, 2005 Share Posted March 29, 2005 Would running the stock turbo at 12psi on the 8.8:1 engine be about the same as a T3/04 turbo at maybe 15-17psi on the 7.5:1 stock turbo engine? In a word, no 12psi on a T3 at 8.8:1 CR probably get you like 250-270 at the wheels, 15-17psi on a T3/04 on stock F54 would be more than 300rwhp, maybe even like 350?. Adding boost is NOT exactly the same as adding compression, they are comparable in some ways, but you would get much less total power from a big increase in compression than if you just turned up the boost... while at the same time you would get closer to detonating with the increased CR. I WISH I knew the right way to explain why this is... I can picture it in my head but can't think of the words to use other than to say it's the differences between static CR and dynamic CR. The only real advantage to upping compression is having the more responsive motor off boost, which is why I imagine there are people who do the flat top pistons on a stock turbo block setup... the stock turbo motor is a real dog off boost. with high enough octane and enough timing retard, you COULD run boost on a really high CR motor... there was an article in Sport Compact Car where turboed the stock motor on a Toyota matrix, which had like mid 10:1 compression. However, they did end up blowing that motor up a couple issues later I think they were running some low boost number too like 7 or 10psi. Another example of my personal experience that shows why it really WOULDN"T be all that worth it to go with the higher CR.... back when I had my N/A motor, dynoed it with the stock motor but with headers and free flow exhaust, free flow intake, big bore TB... and got like 135 to the rear wheels... that was at the stock compression like mid 8:1, then went with a N42 and flat top pistons and went up to like 10.3:, tested on the same dyno and only got up to 160rwhp, that was also with a hotter cam when I went back, so the CR bump wasn't even worth the whole increase. Quote Link to comment Share on other sites More sharing options...
BleachZee Posted March 29, 2005 Share Posted March 29, 2005 Thanks for the info. You have convinced me to go that route. I'll stick with the stock internals and just go for the larger turbo. Quote Link to comment Share on other sites More sharing options...
Forrest Posted March 29, 2005 Share Posted March 29, 2005 Can anyone explain exactly why a higher compression motor can't run the same boost as a lower compression motor? Like was said, when you pump in all that forced induction you really do have greater compression... http://www.amazon.com/exec/obidos/tg/detail/-/0837601606/qid=1112117655/sr=8-1/ref=pd_csp_1/102-6169500-8340957?v=glance&s=books&n=507846 Quote Link to comment Share on other sites More sharing options...
BleachZee Posted March 29, 2005 Share Posted March 29, 2005 Ya, that's the book mentioned in the sticky at the top of this forum. I think I'll get a copy. Quote Link to comment Share on other sites More sharing options...
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