engine heads mismatched... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

berencam · 03-04-2008, 12:34 PM

the block is a 350 (original 1970 block) but the heads are off a 80-85 305 with 53 cc chambers (head # 14014416) is this going to cause issues with anything? also the p.o. told me the cam was 'too big' for the motor. Now i firstly question the validity of his statement, but if that is the case, im assuming he means the lopes have to much lift?

i posted earlier with freeze plug issuse, i replaced both the freeze plug under the starter and the one under the drivers motor mount.(just for the guys that helped me on that thread) but i wanted to touch back on it. I had asked the question about the vacuum thing on the t400 auto trannys, and was told it needed hooked into the intakes vacuum hole. The problem here is i thought thats where the brake boosters vacuum line goes. can i cut that line and add a T, or is there a second vacuum spot on the intake i dont see.

Gray Ghost · 03-04-2008, 01:33 PM

The 305 heads will give you more compression...could require premium gas to run without pinging. I prefer 350 heads on the 350's. I have a set of 993's on mine...not too expensive but flow very well. They are 76cc chambers iirc.

JimKshortstep4x4 · 03-04-2008, 03:29 PM

On your vacuum line for the transmission, you can get a manifold "Tee" that has the two nipples that you need for the hookup. Here is a picture of the "Tee" that connects the tranny line and has the 3/8" nipple for the brake vacuum hose, ( brake vacuum hose is not attached in the pic).

Jim

Longhorn Man · 03-04-2008, 03:39 PM

I had 305 heads on a 350 and they totally sucked. Had to run 94 octain, performance sucked, the engine wouldn't rev pars 3500 no matter what I did to it...

berencam · 03-05-2008, 09:36 AM

since it will have alot more compression would it be recommended to tighten the head bolts more than spec? i bought the expensive gaskets so hopefully i wont blow the gasket, and i don't intend on leaving the 305 heads on for long just as a temp means of getting it running.

joedoozer · 03-05-2008, 10:08 AM

Never over torque the head bolts, that will only cause problems. The compression won't be that high. Maybe 10.5 to 1 at the most. You will need to run some octane booster or the expensive gas. 305 heads also have small valves, and I believe smaller intake runners. So they don't flow very well at higher rpms. But with a nice dual plane manifold, and don't put to huge of a carb on it. It should be a decent combination.

berencam · 03-05-2008, 11:57 AM

recommended carb cfm with this combination then?

03-04-2008, 12:34 PM	#1
berencam Registered User Join Date: Aug 2007 Location: branson mo Posts: 933	engine heads mismatched... the block is a 350 (original 1970 block) but the heads are off a 80-85 305 with 53 cc chambers (head # 14014416) is this going to cause issues with anything? also the p.o. told me the cam was 'too big' for the motor. Now i firstly question the validity of his statement, but if that is the case, im assuming he means the lopes have to much lift? i posted earlier with freeze plug issuse, i replaced both the freeze plug under the starter and the one under the drivers motor mount.(just for the guys that helped me on that thread) but i wanted to touch back on it. I had asked the question about the vacuum thing on the t400 auto trannys, and was told it needed hooked into the intakes vacuum hole. The problem here is i thought thats where the brake boosters vacuum line goes. can i cut that line and add a T, or is there a second vacuum spot on the intake i dont see. __________________ 69 chevy c10 350 3 speed manual wood bed build thread here r.i.p. 70 c10 auto 350 build here

03-04-2008, 01:33 PM	#2
Gray Ghost Senior Member Join Date: Sep 2004 Location: Terrell, TX Posts: 1,967	Re: engine heads mismatched... The 305 heads will give you more compression...could require premium gas to run without pinging. I prefer 350 heads on the 350's. I have a set of 993's on mine...not too expensive but flow very well. They are 76cc chambers iirc. __________________ Kelly '05 GMC Sierra SLE Z71 Bone stock except for new bed rail caps.

03-04-2008, 03:29 PM	#3
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Re: engine heads mismatched... On your vacuum line for the transmission, you can get a manifold "Tee" that has the two nipples that you need for the hookup. Here is a picture of the "Tee" that connects the tranny line and has the 3/8" nipple for the brake vacuum hose, ( brake vacuum hose is not attached in the pic). Jim Attached Images __________________ 1971 Shortbox step side 4x4, 350 sbc, 3:07 rear end 1965 Impala SS 400 sbc, Muncie rock crusher 1966 Impala SS 396 bbc, TH 400 1969 El Camino, 350 sbc, TH 350 1971 Snowplow built on a Blazer frame 1972 GMC Short bed, stepside, TH 400, 427 BBC

03-04-2008, 03:39 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: engine heads mismatched... I had 305 heads on a 350 and they totally sucked. Had to run 94 octain, performance sucked, the engine wouldn't rev pars 3500 no matter what I did to it...

03-05-2008, 09:36 AM	#5
berencam Registered User Join Date: Aug 2007 Location: branson mo Posts: 933	Re: engine heads mismatched... since it will have alot more compression would it be recommended to tighten the head bolts more than spec? i bought the expensive gaskets so hopefully i wont blow the gasket, and i don't intend on leaving the 305 heads on for long just as a temp means of getting it running. __________________ 69 chevy c10 350 3 speed manual wood bed build thread here r.i.p. 70 c10 auto 350 build here

03-05-2008, 10:08 AM	#6
joedoozer is Certifiable Join Date: Feb 2008 Location: Hinesville, Georgia Posts: 160	Re: engine heads mismatched... Never over torque the head bolts, that will only cause problems. The compression won't be that high. Maybe 10.5 to 1 at the most. You will need to run some octane booster or the expensive gas. 305 heads also have small valves, and I believe smaller intake runners. So they don't flow very well at higher rpms. But with a nice dual plane manifold, and don't put to huge of a carb on it. It should be a decent combination. Last edited by joedoozer; 03-05-2008 at 10:09 AM.

03-05-2008, 11:57 AM	#7
berencam Registered User Join Date: Aug 2007 Location: branson mo Posts: 933	Re: engine heads mismatched... recommended carb cfm with this combination then? __________________ 69 chevy c10 350 3 speed manual wood bed build thread here r.i.p. 70 c10 auto 350 build here

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