Author Topic: 1970 XH 400 with # 13 heads, good or bad?  (Read 5236 times)

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Offline Mr. P-Body

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Re: 1970 XH 400 with # 13 heads, good or bad?
« Reply #15 on: December 31, 2009, 09:24:39 AM »


   Please, take "steps" to reduce your compression. If not, it will be a steady "diet" of mixing race gas and pump gas to avoid detonation. THERE IS NO FREE LUNCH! If your static compression is above 10:1, it WILL detonate on 93.

   The approach we take here (CVMS) is to reduce the compression to a manageable point (9.5:1 max with iron heads) and use a camshaft designed for the purpose of making power with lower compression. The only cams we've found to do that in a consistent and superior manner are the Comp XE series.  But don't use one UNLESS you reduce compression!  It will compound any detonation issues.

   As for the carb, the 7040240 is a 1970 Buick. It is not 800 CFM. They (800s) first appeared for the '73 455SD.  It's rated at 730 CFM. That being said, it's still a good carb.  It has been rumored the '71 455HO carb was the first "800", but according to Cliff Ruggles, it PERFORMS like the 800s up to the flow limit. Something to do with the boosters is different than any other Q-Jet.

Jim

Offline rad400

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Re: 1970 XH 400 with # 13 heads, good or bad?
« Reply #16 on: December 31, 2009, 01:15:30 PM »
I have #12s crower 60210 cam 229/239 @.50 .481 lift/.500 108 straight up. Heads shaved twice, never cced. No pinging since I fixed the blown head gaskets. Torker II holley 750 vac. 10" 3500 stall 3:73:1 rear Turbo 400 trans with shift kit. More testing when the weather warms up. 93 octane. 406 not deck stock rods and crank. no porting on heads. roller tip rockers, TRW slugs.
Conrad
79 Trans am 400 t400 3500 stall #12 heads holley 750 vac carb. Torker II intake 3:73:1 gears 12 bolt rear.

Offline jphillips3333

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Re: 1970 XH 400 with # 13 heads, good or bad?
« Reply #17 on: December 31, 2009, 08:33:26 PM »
I'm running #13s, a 3.42, Performer RPM and a Holley 750 ... comparable to Conrad and BrianC.
   
  Over a year now and I run it hard - I think the heads cc'd at 75.  Florida has a 93 Octane ... I'd calculated a comp. ratio of about 9.6:1

If you run the Edelbrock Aluminum heads, you'd probably get away with a little more compression ... bore/cam/dish/gasket and other things considered.
John

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Offline 80 Formula

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Re: 1970 XH 400 with # 13 heads, good or bad?
« Reply #18 on: December 31, 2009, 09:11:37 PM »
Thanks for the all your input on this, it looks like I need to take the heads off and check everything out, I talked to the owner, and he said that it would only ping going up a grade in third gear. He also has now idea if it has flat top, or dished pistons it when it was rebuilt.

I have yet to take it out for a real drive yet to see what it really does.

So if I take the heads to a machine shop, do I tell them to cc the heads? if so, to what? Or do I just buy new pistons and rods and then I can hammer on it at the strip?

Thanks again for all your help.
96 Impalla SS
94 TA
87 944 Turbo
80 Formula 301
80 Z-28
79 TA WS6 400
68 RS
67 Bird
67 El Camino

Offline jphillips3333

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Re: 1970 XH 400 with # 13 heads, good or bad?
« Reply #19 on: January 01, 2010, 08:54:32 AM »
You have to remove material in the head in order to increase the volume in the chamber which in turn reduces the compression in the engine. 
The other option is you run dished pistons - TRWs/Speed Pro L2359s have like a 7 cc valve relief.  With a 72cc head @ .030 over, you'd need a 12cc dish to reduce your compression to ~9.5:1 per Jim's recommendations.

So, you need a shop that knows what they're doing ...

Try SD performance - they're in WA state which is probably easier to get to/ship to for you ...
http://www.sdperformance.com/calculators.php
John

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Offline 80 Formula

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Re: 1970 XH 400 with # 13 heads, good or bad?
« Reply #20 on: January 01, 2010, 12:43:30 PM »
Jim, your are right about the q-jet #7040240, it not a 800 cfm, the first year of the 800 is 71.
Jphillips, that is the info I needed, thanks.

Thanks
Mike
96 Impalla SS
94 TA
87 944 Turbo
80 Formula 301
80 Z-28
79 TA WS6 400
68 RS
67 Bird
67 El Camino