compression ratio help please
06-10-2008, 03:25 PM
#1
compression ratio help please ok, I dont know a whole lot about figuring it out so i thought id ask. Ill give you as much info as i know. Im getting a 350 built at the moment. Its a late model roller chevy 350, they had to go .040 over on it to clean it up. They will be using coated dished pistons. They are p/n H423ncp. Thats a coated -10cc dished hyp. sealed power piston. I am going to run vortec heads with the hot cam kit that comes with the 1.6 rockers. I was told i needed to get the dished pistons to keep from detonating with pump gas and the iron 64cc heads. Just wanted to see about what the compression ratio would be with just a regular head gasket. Im not boosting the engine, just driving it daily. The engine shop did not tell me this, this was from another source. Would i be better off with some 58cc darts or torquer style heads? I say that because by looking at the charts for that piston, it seems that with the 64cc heads, it would put me at 8.7 cr. This seems a little low to me, but i really dont want to risk it detonating. The pistons are already installed so i really dont have a choice as far as that is concerned.
06-10-2008, 03:57 PM
#3
well a decent running daily driven small block is the goal. Im just wanting to know what the ~ compression ratio will be with those pistons and the 64cc vortec heads. So basically what Im asking is would it be a better choice to order something with a 58cc. The vortecs are a 64cc. I dont know exactly what is figured into those "charts" for the pistons. I mean i know it says 8.7 with a 64cc head, but is that with the gasket thickness figured in etc etc.. Like I said my plan was just to order the vortec heads, hot cam kit and slap it on the motor. I was looking for around 300 or so at the wheels. Any more is just icing on the cake. I dont want to have to worry about it detonating. This will be a daily driver and I dont want to "over" do it. If Getting a set of world sr torquers or similar with a 58cc would keep me safe on the street with premium gas and offer better power than say the vortecs, then i would go that route. I just dont honestly know which is the better route. 
06-10-2008, 04:06 PM
#4
http://www.straightline-perf.com/crcalculator.html
you have to know the amount of piston to deck clearance you have, or how much the shop decked the block to give a good flat surface to drop the heads on, this will have the most affect on your static compression ratio and cause the need for the dished piston.
I used 0.041" for head gasket thickness, a 0" piston to deck clearance yields an 11.16:1 CR, a 0.020" clearance = 10.6:1, both for flat top pistons. Using a -10cc (dished) piston for the .020" clearance @ 10.6:1 brings the CR down to 9.49:1. So the numbers in general seem to jive along with the recommendation for the dished pistons. but the info you need to know from who did the block work is piston to deck clearance.
http://www.chevyhiperformance.com/te...ide/index.html
you have to know the amount of piston to deck clearance you have, or how much the shop decked the block to give a good flat surface to drop the heads on, this will have the most affect on your static compression ratio and cause the need for the dished piston.
I used 0.041" for head gasket thickness, a 0" piston to deck clearance yields an 11.16:1 CR, a 0.020" clearance = 10.6:1, both for flat top pistons. Using a -10cc (dished) piston for the .020" clearance @ 10.6:1 brings the CR down to 9.49:1. So the numbers in general seem to jive along with the recommendation for the dished pistons. but the info you need to know from who did the block work is piston to deck clearance.
http://www.chevyhiperformance.com/te...ide/index.html
06-10-2008, 04:15 PM
#6
cool, ill call tomorrow and ask them. What if the block didnt need it and its still the standard height? I know the "305" heads with the 58cc would put me at about 9.5. I think they make a set of 58cc torquer heads that may do the job.
Last edited by redbandit98; 06-10-2008 at 05:24 PM.
06-11-2008, 01:02 PM
#7
i called, they said it was a standard deck height. I told them to go ahead and install some flat tops instead. The pistons spec out at about 9.4cr with 64cc heads. I think im going to just order the vortec heads and put a hot cam in it. That is unless somone has a similar cam that they think would do better. i was looking at a comp xr282hr. I think it may be a tad too bit though. Ive been told that the vortec heads are pretty good, but too much cam will negatively affect them. I dont know if thats too much cam or not.
here is link to cam specs of comp cam
http://www.compcams.com/Cam_Specs/Ca...?csid=189&sb=2
that or the hot cam, that is the question.
here is link to cam specs of comp cam
http://www.compcams.com/Cam_Specs/Ca...?csid=189&sb=2
that or the hot cam, that is the question.
Last edited by redbandit98; 06-11-2008 at 01:09 PM.
