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Offline Odge

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Stock heads restrictive?
« on: September 26, 2007, 01:38:27 PM »
I have read that the stock heads on a cb750 are restrictive. Was this done by Honda to keep power down and increase reliability? Or is the restriction designed to enhance torque and real world usability? I am thinking about all this because I was thinking about getting some porting work done and wondered about potential downsides to changing the original design.
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Offline Soos

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Re: Stock heads restrictive?
« Reply #1 on: September 26, 2007, 01:47:45 PM »
Personally, I think they are restrictive due to designs considered "standard" at the time.
By the time they made the 650(1979) they went with a fully hemi head.
But there are improvements that are able to be realized through porting the intake and exhaust side of the head even on the fully hemi-heads.

But... i dunno. possible it was due to limited casting manufacturers.

l8r

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Offline MRieck

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Re: Stock heads restrictive?
« Reply #2 on: September 26, 2007, 02:33:08 PM »
Not after you do this.... ;)
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #3 on: September 26, 2007, 02:36:12 PM »
Who did that for you mate ? ;D ;D ;D ;D ;D ;D ;D ;D

Sam ;)
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #4 on: September 26, 2007, 02:52:19 PM »
Quote from: SamCR750 on September 26, 2007, 02:36:12 PM
Who did that for you mate ? ;D ;D ;D ;D ;D ;D ;D ;D

Sam ;)
Some clown....I forget her name. :D ;D
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #5 on: September 26, 2007, 03:16:03 PM »
 ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D

Sam. :D :D :D
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Offline eurban

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Re: Stock heads restrictive?
« Reply #6 on: September 26, 2007, 03:27:32 PM »
Cylinder head flow work will give you gains in the upper range of RPM band.  It will have some negative impact on low RPM power depending on the extent of the flow work and the skills of the one doing the work.  I am also curious to hear from those performing and running flowed/ported engines as to the up and down sides to these engines on the street.  Often times these mods are combined with bigger piston bores and higher CRs which can offset lower RPM power losses.
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Offline RRRToolSolutions

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Re: Stock heads restrictive?
« Reply #7 on: September 26, 2007, 04:13:02 PM »
Mine is definitely a whole different animal after 7,500 rpm. With the cam, added 100cc, and the higher compressions - I doubt I've lost anything below that. I can pull from corners at 2,000 rpm with ease and roll-ons are great.

I ran with and without the CR's and I do feel a little softer mid-rang when compared to the stock carbs jetted and fitted with K&N's, but man o man - the top hit is worth every dime the CR's set you back.

My 02.

Gordon
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #8 on: September 26, 2007, 04:28:56 PM »
Quote from: eurban on September 26, 2007, 03:27:32 PM
Cylinder head flow work will give you gains in the upper range of RPM band.  It will have some negative impact on low RPM power depending on the extent of the flow work and the skills of the one doing the work.  I am also curious to hear from those performing and running flowed/ported engines as to the up and down sides to these engines on the street.  Often times these mods are combined with bigger piston bores and higher CRs which can offset lower RPM power losses.
Plus if you use cams that focus on lift rather than duration you won't have a soft engine in the low and midrange. I don't have a down side....my bike makes more power at 6,500 than any stock or mildly modified CB does at redline. You have to have some tuning skills otherwise you can have a bike that is a PITA to ride.
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Offline old750

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Re: Stock heads restrictive?
« Reply #9 on: September 26, 2007, 04:48:36 PM »
Where do you start when porting heads anyways? How the hell does anyone know what they are doing? And how do you do it 4 times without messing up? I've always wondered. It seems to be a real art, more like sculpting than anything else.
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #10 on: September 26, 2007, 05:24:55 PM »
With a book or as an apprentice with a tuning shop. ;D ;D ;D ;D

Sam ;)
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Offline andy750

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Re: Stock heads restrictive?
« Reply #11 on: September 26, 2007, 05:38:27 PM »

I have a stock CB750K4 and the "HeadMaster" Mike Rieck did some mild porting on an otherwise stock head. No change to cc`s and cylinders honed, new valves etc. Stock HM300 exhaust (sounds great!). Bike definitely has more power through the RPMs and Ive yet to find any downside - idles fine and still pulling hard at 6000rpm. Plenty of power left at 90mph.

Although I have to say my stock 49,000 mile K2 has some good top end power but different sprockets (17/48 v 18/48) so this may be a factor?

cheers
Andy
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #12 on: September 26, 2007, 06:37:48 PM »
Quote from: andy750 on September 26, 2007, 05:38:27 PM

I have a stock CB750K4 and the "HeadMaster" Mike Rieck did some mild porting on an otherwise stock head. No change to cc`s and cylinders honed, new valves etc. Stock HM300 exhaust (sounds great!). Bike definitely has more power through the RPMs and Ive yet to find any downside - idles fine and still pulling hard at 6000rpm. Plenty of power left at 90mph.

Although I have to say my stock 49,000 mile K2 has some good top end power but different sprockets (17/48 v 18/48) so this may be a factor?

cheers
Andy

Andy
 You really should go 17/48 with the K4. I didn't know you had an 18 front.
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #13 on: September 26, 2007, 06:41:55 PM »
Quote from: old750 on September 26, 2007, 04:48:36 PM
Where do you start when porting heads anyways? How the hell does anyone know what they are doing? And how do you do it 4 times without messing up? I've always wondered. It seems to be a real art, more like sculpting than anything else.
I've always said doing one isn't that hard.......it's getting the other 3 to match! ;) ;D Actually you do the same work to each port at the same time...i.e. guide boss work, bowl work etc. It makes it easier. As to where do you start? Start at the most restrictive point(s). Don't start hogging the intake spigots out first....they are the least restrictive point.
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Offline andy750

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Re: Stock heads restrictive?
« Reply #14 on: September 26, 2007, 06:42:18 PM »
Ah I have a 17/48 set up now! I changed it before the last trip - now matches the K2  ;)

cheers
Andy

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Offline mattcb350f

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Re: Stock heads restrictive?
« Reply #15 on: September 26, 2007, 06:56:18 PM »
I'm going to hijack this thread for a moment,
MRieck, Have you perfomed any head work on the smaller fours like a 400 or 350? The reason I'm asking as I plan to port my 350F head shortly and was wondering if there is any no-no's I should know about.
 Matt.
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #16 on: September 26, 2007, 07:01:53 PM »
Wow Andy, we have the CR on much higher gearing than that and it smokes the back tyre and then wheel stands ;D ;D ;D ;D ;D

Sam. ;)
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #17 on: September 26, 2007, 07:08:43 PM »
Quote from: mattcb350f on September 26, 2007, 06:56:18 PM
I'm going to hijack this thread for a moment,
MRieck, Have you perfomed any head work on the smaller fours like a 400 or 350? The reason I'm asking as I plan to port my 350F head shortly and was wondering if there is any no-no's I should know about.
 Matt.
Do not cut the floor of the exhaust port. Just clean it up. Stick to removing material from the guide boss and cut into the area just behind the guide (the bowl) on the intake port. Blend the floor of the intake into the seat so it's a smooth transition. There...now you are an expert. ;)
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #18 on: September 26, 2007, 07:09:59 PM »
 ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #19 on: September 26, 2007, 07:10:24 PM »
Quote from: SamCR750 on September 26, 2007, 07:01:53 PM
Wow Andy, we have the CR on much higher gearing than that and it smokes the back tyre and then wheel stands ;D ;D ;D ;D ;D

Sam. ;)
What are you running?? I forget.
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #20 on: September 26, 2007, 07:12:43 PM »
16/39...2.43

Sam. ;)
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #21 on: September 26, 2007, 07:14:59 PM »
Quote from: SamCR750 on September 26, 2007, 07:12:43 PM
16/39...2.43

Sam. ;)
Wow. And you smoke the tire??? That seems real high Sam for 1/4 mile. I'd think around 3.00 unless you have a special gearbox.
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #22 on: September 26, 2007, 07:20:00 PM »
We had to gear it up, he couldn't ride it on the old gearing. It peaks in 4th at the end of quarter now instead of top.

Sam.
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Offline Sam Green Racing

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Re: Stock heads restrictive?
« Reply #23 on: September 26, 2007, 07:36:02 PM »
We were on 15/46 with the old motor..3.06...it peaked in top gear at the end.

It never came up or spun with Chris on it, it spun even in third with me on board.

I hate to think what it would do with me on it now ;D ;D ;D ;D ;D ;D

Sam. ;)
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Offline MRieck

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Re: Stock heads restrictive?
« Reply #24 on: September 26, 2007, 07:36:37 PM »
Another stolen thread. ;)
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