big-phil Posted April 12, 2006 Share Posted April 12, 2006 is a stock turbo 7.4-1? and a n/a 8.3-1? I'm searching and this is what I found. I want to set up my P90 head for my N/A turbo. Its a 78 so I guess I have dished pistons? so I don't want to just put the head on I think my compression would be to low? Quote Link to comment Share on other sites More sharing options...
EZ-E Posted April 12, 2006 Share Posted April 12, 2006 Mine is 150 across the board, with HSK one millimeter head gasket. Quote Link to comment Share on other sites More sharing options...
Brad-ManQ45 Posted April 12, 2006 Share Posted April 12, 2006 Your combustion chamber is 44.6cc's on the N47 engine. The P90 (and P79) combustion chamber is 53.6cc's You're welcome to do the math, but with the stock pistons, I believe you would be well below 7.4:1 with your stock pistons.... The NA ZX has flat top pistons and an 8.3:1 CR, and the turbo has a small dish... Quote Link to comment Share on other sites More sharing options...
big-phil Posted April 12, 2006 Author Share Posted April 12, 2006 So what CR do I want to be at for the turbo? I'm taking the P90 to a head shop and need to know how much to mill the head so I will be at what ever a good turbo CR is. I know that if I just put it on my block with my dished pistons it will be a real low CR. So what CR do I tell the guy I want to be at so he can mill the P90? I know I probly need to be lower than what I'm at now but how much???? Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted April 12, 2006 Share Posted April 12, 2006 Bradman, 7.4:1 is with P90 and dished pistons, ala stock L28ET. http://www.ozdat.com/ozdatonline/enginedesign/ Knock yourself out. Dave Quote Link to comment Share on other sites More sharing options...
big-phil Posted April 12, 2006 Author Share Posted April 12, 2006 someone please shoot me, I took the head to the shop today. He said he just needs to know how much to take off. I can not figure it out. I have dished pistons, how much do I take off the P90 to get a 8.5:1 cr:confused: :confused: Quote Link to comment Share on other sites More sharing options...
blake culp Posted April 12, 2006 Share Posted April 12, 2006 Easy there killer, before you go shaving your head you need to remember that the cam is up there. What im getting at is you are going to have slack in your timing chain, not a good thing. Im not sure how much you are going to shave off but if you are wanting to up the compression im guessing you are going to shave a fair amount. However if you want to get crazy with it, you can shave your head .08in then shim the towers back up .08in and install valves .08in longer All that is good for just over a point of compression and stock chain tension and stock valve train geometry. Its a pretty big pain in the a@@ but its a trick lots of NA z guys use to increase their compression. I actually did the shave and it worked well, but that was way back before i saw the light and went turbo. I believe the complete write up on how to do the head shave with the tower shims and longer valves is on brian littles zcar garage web page. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted April 12, 2006 Share Posted April 12, 2006 11cc's worth according to the above mentioned calculator. He is a head guy, he should be capable to CCing the head from 53.6 stock down to 42.6. Dave Quote Link to comment Share on other sites More sharing options...
Clifton Posted April 13, 2006 Share Posted April 13, 2006 The dish will be the same as the turbo pistons. Only difference is the ring lands. Your C/R with the P90 on your 78 block will be the same as an L28ET. I run less than 7.4:1. Quote Link to comment Share on other sites More sharing options...
BayAreaZT Posted April 13, 2006 Share Posted April 13, 2006 Here is the mentioned website to shave .080. Scroll down towards the middle of the page. http://www.geocities.com/zgarage2001/head.html Quote Link to comment Share on other sites More sharing options...
big-phil Posted April 13, 2006 Author Share Posted April 13, 2006 The dish will be the same as the turbo pistons. Only difference is the ring lands. Your C/R with the P90 on your 78 block will be the same as an L28ET. I run less than 7.4:1. Thank you clifton, so I don't need to take anything off. I have cam tower shims now on my n/a head. I do not know if the P90 is straight. what if I took off just the thickness of the shim? so as to just sort of resurface it, then put the shims on when I install the P90? Or should I just take it to the head shop to make sure its straight and if it is just put it on my motor? Quote Link to comment Share on other sites More sharing options...
big-phil Posted April 13, 2006 Author Share Posted April 13, 2006 I don't want to put in longer valves!! so I guss I might just get it resurfaced? Quote Link to comment Share on other sites More sharing options...
blake culp Posted April 13, 2006 Share Posted April 13, 2006 You could try shaving it, and then putting in the tower shims. But without putting in the longer valves you get different wipe patterns on the rocker arms and loose some lift because the cam is now farther away from the rocker arm by whatever thickness your shims are. but if you have a spare head....youre not really losing much if it doesnt work out, right? If you have a garage and an engine puller it might be easier to just find a NA motor with flat tops (i got one for free a while back), put the unshaved p90 on it and then you would have a solid, un-molested/shimmed setup running at the stock NA compression ratio. My furst turbo setup i ran a turbo on a NA block and head, Made it about 6 months before i blew it up (stuck wastegate/boost controler....kaboom) but it was a blast to drive with a lot more off boost power. The motor i have in my car now is a complete turbo block and head and you can definatley tell its slower off the line, kinda sad to take a step down. Quote Link to comment Share on other sites More sharing options...
Clifton Posted April 13, 2006 Share Posted April 13, 2006 Put a straight edge on it or have a machine shop check it. If it's out just a little I would not shave it. If it's warped and you cut the bottom you'll have to cut the top too. If you do surface it and just take a little off, you can run it without shims. Quote Link to comment Share on other sites More sharing options...
Brad-ManQ45 Posted April 14, 2006 Share Posted April 14, 2006 thehelix112: That is a great utility - if I had all the information to use in it. I didn't say that the turbo CR was 8.3 - I said both the NA and turbo heads had the same cc's and that the NA had flat tops and an 8.3:1 compression ratio. Doing the quick math there is a 9cc difference in chamber volume, not sure about piston dish differences. I have an '83ZXT and am well aware of the low 7.4 CR - bane of my existence in traffic If indeed his dish is the same, then he would be the same as the ZXT. I guess I would want to ask if he is going to go to a T3/T4 turbo before answering what compression hje wants to run, also how much booast and HP goals. With a stock turbo, I'd stay at 7.4. anyway and not go higher. A larger compressor section will reduce heat tremendously and allow closer to his current CR with saftey at the same boost levels. Quote Link to comment Share on other sites More sharing options...
DAW Posted April 14, 2006 Share Posted April 14, 2006 The US L28's (na) from '75-'80 were 8.3:1 with dished pistons. The '81-'83 flat-top piston (na) L28's were 8.8:1. If you have 0.020" cut from your P90 you will increase the cr some and still be able to deal with the timing chain slack without resorting to cam tower shims, etc. If you are looking to gain more than .25-.30 of cr then you need to consider changing pistons. DAW Quote Link to comment Share on other sites More sharing options...
big-phil Posted April 15, 2006 Author Share Posted April 15, 2006 wow thanks for all the info guys! Yes I do want to go to a larger turbo, t3/t4? Holset?? I am now thinking I may not put the head on untill I get the bigger turbo, and an ECU that will handle more boost. My goals are above 300hp. I have some turbo Supra 440 injectors. If I can get the money for a MSnS, and a larger turbo. Then I could put the P90 on and run the 7.4:1 cr but have the boost and fuel to make it feel right. I would like to run closer to 20 lbs of boost? From what I've read that might be TOO much. But after seeing all of these videos of Z's with 300-400hp all I can say is I WANT THAT. So I guess my goal is more like 350-400?? Quote Link to comment Share on other sites More sharing options...
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