Changing head gasket to lower comp

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67 Nova
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Changing head gasket to lower comp

Post by 67 Nova » Mon Oct 15, 2018 11:40 am

Ok here's the deal. I currently have a .030 427 that is 12.2 to compression, CC piston and heads. That is with a .039 thick gasket. Now I can look at running a .060 or even as much as .080 thick gaskets.

How would that effect possible performance and running of this engine? Like to run a more "street-friendly" deal for pump gas in my street car.

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Re: Changing head gasket to lower comp

Post by engineguyBill » Mon Oct 15, 2018 11:46 am

You will need to do the math to determine whether or not the additional gasket thickness will help your situation. I don't think the additional .040" of gasket thickness is going to make much difference in compression and it will affect your squish as well. It might be time for you to invest in flat-top pistons in order to make the car behave on the street.
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67 Nova
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Re: Changing head gasket to lower comp

Post by 67 Nova » Mon Oct 15, 2018 12:02 pm

Thanks Bill. I use this as a guide:

http://www.csgnetwork.com/compcalc.html

At .080 with everything I can put in as accurate as can be, I can get it down to 10.6. Motor is fresh and not fired yet. So going the route to take it all back out, unassemble, machine and back again isn't really what I want to do for a couple years. Other cars before that. It's really just a street cruiser now. It has been a strong runner. SFT cam, "215" Gm oval heads and strip dom intake. At 3600 it's been 10.70 NA and 9.50's on a 200 hit. Nothing fancy or expensive, all GM stuff. Be cheaper, and probably better to do heads and not pistons?? Going to a 119/121 head over the 101cc heads I have now.

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Re: Changing head gasket to lower comp

Post by mag2555 » Mon Oct 15, 2018 12:11 pm

It sounds like you have iron heads but you did not post that info?
If you want to do this right and still get efficient burn and maybe not have much of a powe loss then ditch the thicker head gasket idea and do the more labor intensive thing and put valve chamfers into each side of the Bores to drop the compression.

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Re: Changing head gasket to lower comp

Post by redliner » Mon Oct 15, 2018 12:42 pm

mag2555 wrote:
Mon Oct 15, 2018 12:11 pm
It sounds like you have iron heads but you did not post that info?
If you want to do this right and still get efficient burn and maybe not have much of a powe loss then ditch the thicker head gasket idea and do the more labor intensive thing and put valve chamfers into each side of the Bores to drop the compression.
Thicker gaskets will make quench worse and prolly not solve anything. For a pumpgas motor, quench is everthing!

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Re: Changing head gasket to lower comp

Post by 67 Nova » Mon Oct 15, 2018 12:44 pm

Yes OEM cast iron 215 oval port heads.

Would like to look into the chamfer idea??? I believe there "usually" on the intake valve side of the head, which also help flow to a degree. Never thought of that.

Just this motor being fresh, and engine bay being all painted and chromed, less chance pulling and reinstalling engine.

Thanks

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Re: Changing head gasket to lower comp

Post by mag2555 » Mon Oct 15, 2018 1:00 pm

It can be done with the motor assembled and in the car!

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Re: Changing head gasket to lower comp

Post by Newold1 » Tue Oct 16, 2018 2:28 pm

I would suggest you keep it simple and kill two birds with one stone. Get a hold of a nice set of oval port iron heads or a newer set of aftermarket inexpensive aluminum oval port heads that will have 115cc to 118cc combustion chambers versus the 109cc that your 215 chambers are. Those 215 heads are not great heads anyway and if you use the 115-118cc chamber better flowing oval port heads with .051 head gaskets your compression ratio will drop down in the 10.5 to 11.0 to 1 range which will work fine with pump gas and a good tune. These replacement heads are available used and in the aftermarket for somewhere between $600 and $1000 per pair ready to run and that would be a better solution than trying to "mickey mouse" what you have now and get you better running heads on your engine. JMHO

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Re: Changing head gasket to lower comp

Post by novadude » Tue Oct 16, 2018 4:11 pm

Newold1 wrote:
Tue Oct 16, 2018 2:28 pm
versus the 109cc that your 215 chambers are.
Aren't 215s a little closer to 101-104cc?

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Re: Changing head gasket to lower comp

Post by Newold1 » Tue Oct 16, 2018 6:33 pm

My reference materials say 109 cc but who knows exactly, sometimes the printed data is wrong.

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Re: Changing head gasket to lower comp

Post by 67 Nova » Wed Oct 17, 2018 9:08 am

Newold1 wrote:
Tue Oct 16, 2018 6:33 pm
My reference materials say 109 cc but who knows exactly, sometimes the printed data is wrong.
Ya, I've been bite a few time to what print says to what is actually in my hands. I have 106cc on my records as they were cc'ed when flowed.

I have been talk with * about a set of 290 Flotek ovals. There pictures make them look more od a big "radius-corner" sq port. Being Canadian, the heads will be in the $2000+ range. Gaskets honestly will be $3-400 range as at these thicknesses, MLS is the only option.

Car is just going together now. Fuel system, wire to the dist/coil, PRIME, and fire.

I have access to a VERY high-end chassis dyno. Give RWHP, CHP and 1/4 mile ET and MPH. Probably get it on there and see where this set-up is. Do a cranking compression test, then see where I go from there.

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Re: Changing head gasket to lower comp

Post by Newold1 » Wed Oct 17, 2018 9:51 am

Are you saying that when Canadians buy parts like this from just across the border in the US you have to pay those ridiculous higher prices because of duties, tariffs, taxes/ !! That is ridiculous! So much for NAFTA! or the new supposed agreements! You guys are being ripped off by your own government and private sellers here in the US are loosing sales for individuals like you who need the properly priced performance parts! This type of crap really pisses me of !! :x

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Re: Changing head gasket to lower comp

Post by 67 Nova » Wed Oct 17, 2018 10:05 am

Newold1 wrote:
Wed Oct 17, 2018 9:51 am
Are you saying that when Canadians buy parts like this from just across the border in the US you have to pay those ridiculous higher prices because of duties, tariffs, taxes/ !! That is ridiculous! So much for NAFTA! or the new supposed agreements! You guys are being ripped off by your own government and private sellers here in the US are loosing sales for individuals like you who need the properly priced performance parts! This type of crap really pisses me of !! :x
Actually it's not that much duties/tariffs. So say when I look at something, lets say say, new or used, and I paid them man $1,500.00 in US funds. I usually ship to Buffalo/Niagara Falls facility due to the whole shipping over the boarder, brokerage fees scandal that is, and I'm only 1 hr and 15 mins from my place I ship too. So $1,500.00 US, i pay mu place usually $5-$10 for handling receiving storing till I get there. Now I declare $1,500.00 at the boarder, and go in to pay.

Custom, take my receipt and check if it's "too good a deal" and looks for comps. If they agree on the $1,500.00, they convert that into CND $$$, "about" 33% exchange now, so 1500 X 1.33 is now $1,995.00 CND. Now I pay the tax on the 1995 @ 13%. So that would be at the cashier $259.35. So for me, the $1500.00 US $$ heads ends up costing me about $2,270.00 directly out of the bank account. PLUS time and fuel.....

Only once in my "young" 52 yrs has the border "adjusted" my receipt saying that was too good a deal, and added about $300.00 CND to the price to pay some extra taxes.

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Re: Changing head gasket to lower comp

Post by Frankshaft » Wed Oct 17, 2018 10:12 am

67 Nova wrote:
Mon Oct 15, 2018 12:02 pm
Thanks Bill. I use this as a guide:

http://www.csgnetwork.com/compcalc.html

At .080 with everything I can put in as accurate as can be, I can get it down to 10.6. Motor is fresh and not fired yet. So going the route to take it all back out, unassemble, machine and back again isn't really what I want to do for a couple years. Other cars before that. It's really just a street cruiser now. It has been a strong runner. SFT cam, "215" Gm oval heads and strip dom intake. At 3600 it's been 10.70 NA and 9.50's on a 200 hit. Nothing fancy or expensive, all GM stuff. Be cheaper, and probably better to do heads and not pistons?? Going to a 119/121 head over the 101cc heads I have now.
Are you going to spray it still? I personally think it will be fine. It will run. Is it ideal, no, but you know that already. For a nos deal, the big quench distance is actually a positive. It will likely want a little more timing. The other thing to think about, part of my Common Sense and Simple Logic approach to things, look at a 440 mopar. Even with a 0 deck, flat top piston, if your using a 906, 346, 452, etc factory head, basically every single stock casting with the exception of the one year only 915 closed chamber heads, they all have around .120 of recess in the quench side of the head, + the gasket thickness. They run just fine. Built many dozens of them over the years.

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Re: Changing head gasket to lower comp

Post by 67 Nova » Wed Oct 17, 2018 11:17 am

Frank, honestly I doubt it will be back to running nitrous. It's going to be a driver with AC, overdrive, PW etc. The guy I bought this car off ROLLING said with the PG and EFI alum headed 502 him in it was just over 2800LBS. So lets say I will be around 3000lbs. This exact combo should still be low 10's and maybe even high 9. I do want to take the SFT cam out and get a roller cam that's designed after 1999. Comp doesn't even have a record of the grind it so old. Looking either tight last SR or a hyd roller.

BUT

Always be nice to have that nitrous there if needed. Although with no roll bar that might not be a great idea.

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