Ello! I'm about to order some stuff from AMS and just wanted to confirm some info to do with the Cometic head gasket thickness I should order. My machinist says he'll probably take anywhere from 8 to 10 or even 12 thou from the block and heads. So I was thinking to get the 0.060 gasket which is 15 thou thicker than stock. This would then make the compression slightly lower than stock. However..... After doing some reading, using thicker head gaskets isn't generally a good way to go, and upping the compression slightly is also deemed a plus. So this would mean I could get a stock (or close to stock) head gasket. My question is, with say 10 thou (give or take) taken off the block and heads combined, what would my compression end up being using the 0.045 stock gasket, or even the 0.051? I can't see it changing much, but i'd be happy with 8.5 - 9.0:1. Ta! Ben PS, on a side note, anyone thinking of placing an order with AMS this week wanna go in with shipping? I'm getting mani's and a few other things so let me know
Ben the engine builder will set the piston to deck height clearance regardless of how much is taken off the heads and block. I used the 1.2mm cometic gaskets in my build and compression values are very close to stock values even after the block was decked. Higher compression values will make a more efficient running engine with better response off bottom but cylinder pressures will be higher with comparable boost pressure to a stock ratio engine which leads to higher chance of detonation. Less boost = less air= less top end power.
If you are doing a rebuild and not aiming more at big numbers, stick with stock thickness gaskets. Running the comp a little higher from the machining will make the engine a touch more responsive.
I made a few errors with my deck measurements. I though i ordered a .060 thou oversize. (large huh, im a dick) I ended up with .060 thou total thickness. (.015 thou oversize) Long story, but my fault i didint know what i was doing. Basically i told the builder to shave 40 thou off the head. If you add what he shaved off the block, say 6thou Thats 46 thou. I though i was being conservative and playing it safe by lowering the compression. But seeing as i have only .015 thou oversize. (instead of .060) And i have taken roughly 46 thou. That leaves me with... .031 thou too little. Which raised the compression. I also went .040 thou oversize pistons. Dont know how this effects compression. Soo factoring all this, what would my C/R be. Just so i got a heads up before i start running high boost. All this thou crap is annoying, i though something was up when the headgasket read 060 on it. Never thought of asking. GZ. Oh well. Its not like the car wrong run. right? right?? Ok i googled it, done some real fast math. Turns out i might raise my C/R anywhere from .5 to 1.5. Prob around the .7mark. No big deal, looking forward to the extra response. And yes increasing piston diameter does increase C/R. by the percenteage of displacement increase.
My piston increase will raise C/R by 2.3% To 8.6955:1. As for the gasket thickness. Im still working on that. But i would say i would be right around >< 9.2? If i took a stab at it. I need to find out the head volume.
Thanks everyone for the replys. I will be aiming for pretty decent numbers, i'm expecting at least 350 - 370rwkw, and hoping for 400+ I've basically done everything else to the car before the engine let go ie fueling etc, So the 20 year old engine with piss weak compression was the only thing holding it back at 300 odd rwkw. Ben
Yeah all this "thou" crap had me looking at measurement conversion charts and googling all over the place. I'm a metric boy dammit! Did the builder give you a funny look when you asked for 40 thou off? Or did you just sound like you knew what you were doing? Ben
Hey Rob, Yeah I read another post from late last year by JP who stated you had 0.045's on there. The question though is how much metal was taken off the block and heads? I'm guessing you probably don't have that info at hand lol. Plus your pistons are an extra 20 thou over what mine are so probably makes a difference there too. I'm sure I'll be able to find a comrpression calculator online somewhere, but i'll probably need the head volume like Zedzy. Should be able to get pretty close though. Ben
Yea he goes, 40 thou? thas 1 mm. OHH-KAY. Lol. Ah it happens, if thats my biggest mistake im sure the engine will still make plenty of power .
