SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: kachina 22 on March 01, 2014, 07:28:36 PM
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Hey guy, doing a new build and using flat top pistons with bevel, what gasket should I use to get most compression?
Thx
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sorry new to this conversation , but what heads are you running . and id be more worried about if the valves are going to touch the pistons!
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Are oval port, cam has 632 lift, block is gen 4 heads are gen 5
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Closed chamber, but think with that piston only making 9:5.1 or 9
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You sure Mk4 block and Gen 5 heads and not the other way around? Typically closed chamber ovals are Mk4.
No offesnse... but it kinda sounds like you have the makings of a possible mis-matched combo there. Closed chamber heads are gonna typically run in the 106cc range (some as low as 100... some as much as 109)... but assuming 106, even if the block has been "zero decked" (which I do not know with the information given), but assuming best case scenario it has, you're still only going to be 9:1 at best. Assuming the heads have been whacked down to let's say 97cc's... you're still in a best case scenario of about 9.4:1 with a .039 gasket. Reguardless... any flat tappet camshaft that is going to get you in the .630 lift range is going to be awful big for a 9:1 oval port deal.
I know this doesn't exactly answer your question as far as what gasket to run... but hopefully at least keep you from going in a direction you may not be super stoked with.
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Thx for responds, measured piston at tdc and measured .065 below deck. Heads are ported intake and exhaust with larger exhaust valves off rectangle heads I had, don't think heads were shaved.
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I'm sure the block is a 1973, I'll see if I can find gen for heads off the cast
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Thx for responds, measured piston at tdc and measured .065 below deck. Heads are ported intake and exhaust with larger exhaust valves off rectangle heads I had, don't think heads were shaved.
At .065 in the hole... no matter what gasket you run, you are going to be well under 9:1 with ANY closed chamber head. I'm thinking you may want to rethink this combo, or at least do some machine work to the block and heads to try to get the compression up... but even then, like I said before, any flat tappet camshaft that is going to have roughly .630 lift is going to be a bit too big duration-wise for anything in the 9:1 range... and really too big for anything under 9:1.
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Cam is hydraulic roller, was just told the pistons are .020 in the hole, will that help
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.020 is much better than .060... but still not good as far as getting you up in your 9+:1 compression range. being a hydraulic roller and in the .630 range is better as far as what duration would be needed to support that lift... but still has got to be at least 300 degrees advertised... which is still a lot for a 9:1 oval port.
Rather than keep trying to guess what you have going on here and make suggestions based on guesses... what CC's are you heads (not a guess, but an actual measurement)? What are the specs on the cam (duration, separation, etc...)? What piston do you have (brand and part number)? And then, what is this thing going in?
I know your original question was just concerning a head gasket choice, but a couple things are not adding up as far as what your putting together, one of which, depending on what pistons, may be a possible piston to valve issue.
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Cam, 255@50 exhaust 263@50 lift 632 110 separation head Cc's 101, .027 in hole with bevel flat top piston. Have oversize exhaust valves and ported oval heads closed chamber and 30 over
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With all that... using a FelPro 1037 (4.37x.039), which is going to be about as thin a conventional gasket as you're gonna get, you still are looking at 8.73:1 compression.
IF... you deck the block to "zero deck", meaning the pistons flush with the deck... you'll get up to 9.16:1...
IF... you shave the heads down to 97cc without decking the block... you'll get 9:1 (which will take about .025 off the surface to get there)
IF... you do BOTH, then you'll get up to about 9.45:1
Now... keep something in mind... shaving that much (.052) may exacerbate an already possible problem of piston to valve interference. The TRW pistons are not exactly large lift/duration friendly... add in larger diameter intake valves (2.19 or 2.250) and you have a recipe for a serious problem. If your at 255@.050 with 110 separation... I'd imagine you're gonna be opening @.050 somewhere about 20 BTDC... I'd be super careful as you could have piston to valve issues.... I cannot stress this point enough. Also, I have no idea what you're trying to build, and who, or why that camshaft was selected... but just based on what is given... I will say again, that is an awful big of a cam, not only lift, but duration for an oval port 454 with anything less than AT LEAST 10:1 compression... preferably, 10.5:1 or better... not even considering what the application may be, which depending on drive/impeller/hull/EXHAUST/etc... may be even worse. Not trying to piss on your parade bro... but just tryig to give you a heads up before you end up being not very happy with what you have going on there.
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What more info do you want? Do you know what CD's are on bevel pistons are?
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Cc,s on heads are 101
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What more info do you want? Do you know what CD's are on bevel pistons are?
I really kinda do not need any more information. I realize that your head cc is 101... I get the notion that you're running TRW flat tops wit the 2 valve reilefs that are .030 over and .027 in the hole.... based on all that, I gave you a "where you are now" with the thinnest commonly available head gasket (Felpro 1037)... which would be, in your current configuration land you at 8.73:1 compression. I gave you three possible ways to try to get your compression up, or at least closer, to where you said you wanted it.
My concerns were involving the camshaft selection and/or piston to valve issues, also due mostly to the cam selection. Also, the concern of the camshaft not matching the application. Based on the information given, excluding all other factors that do play a large role in camshaft selection and how well/poorly your engine runs, just the simple fact of you engine combination (cubic inches/heads/compression), I believe it to be a less than ideal combination at best. As far as selction a cam that would be the best choice... then hull type, exhaust type, drive, impeller all also have a role into selecting a camshaft suited to the application and what would work best given an engine combination. Nothing in boat should be consider independent of all the other factors (drive/hull/engine)... they all play a role together to make it work to it's best potential given a desired use. Trying to do otherwise typically nets less than desired results.
Just my 2 cents.
But if you wish to disregard all of that... and just simply want to know what would be the best gasket to run... then the simplest answer would be: FelPro #1037... that will get you the highest compression.
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Your information is greatly appreciated' yes running TRW piston with bevel' the boat NASA dominator pump running AA impellar. Boat is a machina 22.5ft semi deep V, but bottom of hull is round, not Vat the back. Not try in to set up a race boat, just trying to get a perspective of where I'm at. I'll look up that head gasket info, was told to get a .020 shim and would get me to 9:1. What's or thoughts?
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Bad pick but u get the idea
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Your information is greatly appreciated' yes running TRW piston with bevel' the boat NASA dominator pump running AA impellar. Boat is a machina 22.5ft semi deep V, but bottom of hull is round, not Vat the back. Not try in to set up a race boat, just trying to get a perspective of where I'm at. I'll look up that head gasket info, was told to get a .020 shim and would get me to 9:1. What's or thoughts?
The steel shim is like .022 thick, and yes they do work, I mean GM used them waaaaay back in the day, but the downside is they are very particular to surface finish and flatness. Often times, they will leak when replaced, even if the originals didn't... and they'll leak if the head or block surface is too rough... composite gaskets are much more forgiving. I run the MLS gaskets a lot on the engines I build, but then I know exactly what Ra finish I am working with and know that the heads and blocks are dead flat after I run them through my surfacer... for guys not going through that trouble... the composite is a safe bet as possible water leaks outside, and more importantly, inside the engine really can be a bummer. As far as the shim gasket goes though, .022 gasket will get you pretty much at 9:1.
Now seeing the type of boat, and the AA impealler, and I'd imagine you are going to run some sort of wet exhaust manifold setup... now I'm not concerned that the cam may be too big... I am quite positive it is way too big. It will not work within the rpm range of the impeller you have, it will not jive with that big and heavy of boat, and it for sure will have reversion problems. About 64-67 degrees of overlap is about all you can get away with on log and low center rise wet exhaust manifold setups... and even then, that is more of a hotrod-ish deal where you're gonna be idling 1k+... add in the large duration and healthy lift that is going to put the rpm range outside where that AA impeller is going to hold it back to... and the oval heads, lower compression, etc... and it just is not a good match.
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Exhaust, only topped out at 4300rpms before it blew before, difference now is the pistons that are beveled, only 30 over now apposed to 60 before and head gasket was .046
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Ok... neat.
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I don't understand with all the knowledge being provided to you. Why are you married to that camshaft? You can save yourself a lot of money and disappointment by listening and taking the advise from those that have been there done that on this site. JMO
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Not married to it, in fact I wiil change it out later' just trying to get it going and taking every body advise. The problem is mula!!
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Not married to it, in fact I wiil change it out later' just trying to get it going and taking every body advise. The problem is mula!!
I here you on that one. It always comes down to money. The lotto would come in handy.
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Bringing back the past;-, was thinking Whipple blower to compisate for lowcompression, what's your thoughts??
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After reading through the history of this thread I've got to say that a supercharger would help with the compression issues, but you would still have to change the cam to get a better combination of parts.
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Your probably right, also changing the impeller to an A to allow the cam to work better, only topping out at 3500rpms