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Author Topic:   motor combo question
posted January 03, 2004 11:58 PM
Ok, scenerio is. I have a 355 with 11-1 pistons with 305 heads been shaved a little. Running mwm #8 cam.

I understand some run 305 heads to make cheap power, I'm at a claimer track, that has history of alot of claiming.

Am I better off leaving the 305 heads on or putting a set of 76cc heads on and letting it breath a little better?

I understand its a very vague question, but I'm just trying to understand the reasoning behind each method.

thanks in advance....

posted January 04, 2004 09:17 AM
I say leave the 305 heads on. IMCA hobby type it sounds like?

posted January 04, 2004 09:58 AM
If you put on 76cc heads your comp will go way down.Leave the 305 heads on.

posted January 04, 2004 10:21 AM
I totally understand about the compression, but where/when is the trade off a benefit.

I mean to put on a set of decent heads, ie. 487x, camelback, or even 200cc heads a benefit.

I keep hearing the scenerio about 305 heads choking off a motor, and not letting it breath. So if you run an open chamber head, to be able to get the same amount of power you have to run a 13-1 piston?

Someone educate me....


posted January 04, 2004 11:35 AM
How did the engine run ?What kind of rpm do you run ?

posted January 04, 2004 01:39 PM

i would run the better flowing heads. try double humps. with 64cc chambers your not gonna loose alot of compression but you will gain power. you can angle mill the humps and get them down to 58cc or so like what your 305 heads are.


posted January 05, 2004 08:45 AM
My 601 heads were 52cc with 1.94" and 1.6" valves and .010" milling. The difference between 52cc and 76cc would be about 2.5 compression points. So, my 11.2 motor would drop to around 9. That's about 8%-10% power EVERYWHERE across the power band. Putting on the heads that would flow better over 5500rpm really would just make you slower. They'd pull hard for the last 100 ft of the straight, but you'd be buried off the turn where the high comp. and small runners make more torque.

If you spend significant time over 6000rpm, then the bigger heads make sense. Under 6000, jack the compression through the roof!!

The best compromise is the 'double hump' or vortec style heads and milled for as much compression as you can afford.

posted January 05, 2004 02:27 PM
Originally posted by Raz_900:

Thanks Raz, thats they types of "technical" answers I am looking for. I understand the idea behind raising compression with 58cc heads, but I've been shunned by professional motor builders as why I would constrict my motor with less flow.

The 305 head way of building motors is what I've been around so thats all I know, I am just trying to learn and better understand what best suits my needs for now and in the future.

Just trying to learn....

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