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Discussion Starter · #1 ·
How much H.P. gain will a two point C.R increase be worth on a 351 street engine ([email protected] duration and .500 lift, single four and headers), with a present C.R of 8.5:1?

Thanks
 

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In the real world, we see from 15 to 18 HP per ratio .In your case, I would expect a 30 to 35 HP gain, because it is so low to start with. What is your cranking compression now?? this is a key point. If it is real low ( 150-160 ) you will realize a big improvement. JOE SHERMAN RACING
 

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Discussion Starter · #3 ·
I have not tested the cranking cylinder pressure but it is a 351C motor with closed chamber heads and .030 sealed power dished pistons. I was considering a set of Aus. cleveland heads with the small chambers but I think I would be money ahead ( and similar HP) to change pistons and use the "roush port stuffers" in the floors of my iron heads. The motor was built about 10 years ago but has been in storage.
 

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With that cam, you will probably pick up quite a bit.

I have heard bad things about port stuffers in a street engine. With a race engine, you tear it down every year or so, and so you can check the integrity of the stuffers, which are basically glued to the ports.

On a street motor, you might never see the inside of the ports... until your stuffers come off and lodge themselves in a valve or cylinder. They might be fine... but I don't like rolling dice with a couple thousand dollar motor.
 

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On 2006-11-11 08:53, 2800R wrote:
I have not tested the cranking cylinder pressure but it is a 351C motor with closed chamber heads and .030 sealed power dished pistons. I was considering a set of Aus. cleveland heads with the small chambers but I think I would be money ahead ( and similar HP) to change pistons and use the "roush port stuffers" in the floors of my iron heads. The motor was built about 10 years ago but has been in storage.
Aren't 351C closed chamber heads...the same size as aussie 2v heads? As far as the combustion chambers I mean. So changing heads won't do anything for you if you're using 4v closed chambers now. My advice is to do the piston thing =). If you meant the ports, I'd use the 4v heads with the closed chambers, then cam it for the usage you intend. Airspeed is not necessarily a function of port cc's ONLY! Cross section, and CAM SELECTION are far more important, and I personally know more than one guy with 4v cleveland heads that have sick street motors, and no loss of bottom end power.

My advice when it comes to clevelands, is to PM woody1. He knows where from he speaks!!

Cris
 

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On 2006-11-11 19:19, Is1BadFord wrote:
On 2006-11-11 08:53, 2800R wrote:
I have not tested the cranking cylinder pressure but it is a 351C motor with closed chamber heads and .030 sealed power dished pistons. I was considering a set of Aus. cleveland heads with the small chambers but I think I would be money ahead ( and similar HP) to change pistons and use the "roush port stuffers" in the floors of my iron heads. The motor was built about 10 years ago but has been in storage.
Aren't 351C closed chamber heads...the same size as aussie 2v heads? As far as the combustion chambers I mean. So changing heads won't do anything for you if you're using 4v closed chambers now. My advice is to do the piston thing =). If you meant the ports, I'd use the 4v heads with the closed chambers, then cam it for the usage you intend. Airspeed is not necessarily a function of port cc's ONLY! Cross section, and CAM SELECTION are far more important, and I personally know more than one guy with 4v cleveland heads that have sick street motors, and no loss of bottom end power.

My advice when it comes to clevelands, is to PM woody1. He knows where from he speaks!!

Cris
Cris,
I missed this little word the first trip through myself




You would be far better off in money spent/saved and the end result by using a set of FT pistons to up the CR.



IMHO I can't see that the stuffers are worth it, I can't imagine making the ports smaller will give me more power than I have now.

_________________
<font color="red">In Progress</font> 91 5spd GT with DSS built 408C 4V EFI Torker, solid flat cam
89 5spd Coupe, 351C EFI Spyder, AGS 4 suspension
80 LX 11:1 carb'd 351C w/GForce T5

<font size=-1>[ This Message was edited by: 1 Bad 88 GT on 11/12/06 11:59am ]</font>
 

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I would go to 11:1 compression. Big heads need big compression and big cam. The combo has got to be like a good soup, just the right about of everything in there! you have to build a motor to feed those big ol' heads. Cam and compression ratio should be matched up to what your going to flow through the heads. Seems to me that putting stuffers into the heads is going at it backwards, or putting the cart before the horse so to speak.
2 analogies in 1 post...hehe.
 

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If you have closed chamber heads now, then you most likely have more than 8.5:1 compression. Aussie heads won't help you.
 

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Discussion Starter · #9 ·
I m not refering to the Australian 2V iron heads. I was considering the CHI or AFD aluminum heads (made in Australia). They offer chamber sizes as small as 40cc's. The intake runners are about the size of the 2V iron heads but the roof of the runner is at the same height as the 4V iron head and flow as much air as a 4V iron head but at a higher velocity. There is even an Austrailian company that makes a push rod operated 4 valve head (two intake and two exhaust valves) for the cleveland engine but they are "very big bucks".
 

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On 2006-11-12 04:59, FATNFAST wrote:
I would go to 11:1 compression. Big heads need big compression and big cam. The combo has got to be like a good soup, just the right about of everything in there! you have to build a motor to feed those big ol' heads. Cam and compression ratio should be matched up to what your going to flow through the heads. Seems to me that putting stuffers into the heads is going at it backwards, or putting the cart before the horse so to speak.
2 analogies in 1 post...hehe.
Hmm...

If he goes with a big cam (assuming you mean big lift/duration with a relatively narrow LSA), on a 4v headed 351ci motor, he needs to be prepared for HIGH rpm, or for a dog of a motor in his intended rpm range. High compression WILL help this, however, but as he stated this is a street motor, with I'm assuming pump gas. I would limit compression to not much more than 10:1 with iron heads, especially since he should use a wider LSA cam with those bigger heads on his limited cubic inches, and that's going to keep cylinder pressure high, limiting static compression. A slightly narrower duration, wider LSA, and good sized lift will keep him happy on the street, with great usable power in his preferred rpm range. This will also help with piston to valve clearance. Again, big heads + big compression + big cam + 351ci = high rpm! Big heads + good compression + wide LSA narrower duration good lift cam + 351ci = great streetable power in a usable street rpm range. This was fords mistake on the boss 302 as well. Cam technology has come a LONG way in the last say, 35+ years.

If it were a 408 stroker, I'd agree with you completely, and I DO agree about the combination being incredibly important.

Hope this helps!

Cris
 

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Agreed Cris... my point being, I don't believe bigger heads will be the answer. Man, I tell ya it is hard not to cross that line of streetability! lol.
Keep always wanting more!
 
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