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CR for Mexican FI heads on a 1641cc
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63ratster
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PostPosted: Thu Oct 18, 2018 12:31 pm    Post subject: CR for Mexican FI heads on a 1641cc Reply with quote

I recently purchased a 1641cc DP with Mexican F.I. 043-101-375H (33x30mm valves) heads. This is low mileage rebuild that features a stock crank,stock ( non F.I.)cam,34 PICT-3 carb,Pertronix SVDA, and 1 3/8" dual quiet pack exhaust with heater boxes. My deck height on the 1/2 side is 0.48 & 0.50, while the 3/4 side is 0.47 & 0.47. The heads feature 47cc combustion chamber with an additional 0.50 step(for a total of approx. 53cc). I am planning on running 87 octane that has 10% Ethanol. According to the CB Performance site my CR is currently 8.0. My questions are as follows:

1) Is it worth buying new heads with carb size valves(35.5x32), or will the smaller valves work just fine(there are like new)?

2) If I keep the F.I. heads, should I have the step removed and semi-hemi cut the combustion chamber to 53cc (should yield about 8.3 CR) to give the heads better quench for a more efficient burn?

3) And lastly, would there be a noticeable difference in (seat of your pants) performance with 35.5x32 valve heads over the 33x30 heads with an otherwise stock engine?

Thanks in advance for any advice!!
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74 Thing
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PostPosted: Thu Oct 18, 2018 12:41 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

Save your money and run what you have stock carb, cam, with a small 4 into 1 header. Unless you are going to dual carbs then I would have yours blended under the seats and get a valve job. May want to get a valve job anyways just to be sure.
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63ratster
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PostPosted: Thu Oct 18, 2018 4:23 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

74 Thing- So with the current step in the chamber, that gives a total deck height of 0.97(average). I have read that 0.45-0.60 is ideal for,complete burn. Any additional thoughts on that? Thanks
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ach60 Premium Member
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PostPosted: Thu Oct 18, 2018 5:42 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

OK, I live in the USA, we use SAE, so I'm having a bit of a problem relating to your numbers
But here is what I've assumed, so I could play around with your build
Your measure deck is 0.48 & 0.50, 0.47 & 0.47 and these are mm
You have a 0.50 mm step in the heads.
Your chambers are 47cc
In SAE ideal deck is 0.040" to 0.070" so for metric that's 1.016 mm to 1.778 mm.
Your .97 mm average deck is real tight, and we don't know if the piston will it the step cut in the head.
The stock step is cut for 85.5 mm piston, your 87 mm pistons will hit it!
I dumped numbers into http://cbperformance.com/v/enginecalc.html
and I got your compression ratio is 8.8 to 1, that's too high!
First thing I'd do is cut out the step, and do a valve job, on the heads, and maybe even try to get the chambers to 48-49cc.
Succeeding in that, I'd buy a Engle W90 because they'll work with a compression ratio of 8-8.25:1.
I would use barrel shims to get a deck of 0.065" which is 1.65 mm.
Plugging all that into the calculator http://cbperformance.com/v/enginecalc.html
A deck of 1.65 with 47 cc chambers, 69 mm stroke, 87 mm bore.
I get your compression ratio is 8.2 to 1, which will work fine with a Engle W90 cam.
I suspect that 8.2 to 1 compression with a stock cam will not work well,
because a stock cam is used with 7 to 1 or 7.5 to 1 compression.
That's why I'm suggesting the Engle W90. Which means you need to get the Engle lifters too.
You can read about this stuff here:
http://vwparts.aircooled.net/Engle-W90-Type-1-Camshaft-1-1-or-1-25-Rockers-p/e6090.htm
http://vwparts.aircooled.net/Engle-Lifters-Cam-Fol...sphate.htm
http://vwparts.aircooled.net/SCAT-HD-Single-Valve-Springs-Set-of-8-20011-p/20011.htm
http://vwparts.aircooled.net/SCAT-Bolt-On-Rocker-Shaft-Kit-20129-p/20129.htm
http://vwparts.aircooled.net/Chromoly-Valve-Spring...-set-8.htm
http://vwparts.aircooled.net/Hardened-Valve-Keepers-Valve-Locks-8-Pair-p/valve-keepers-8mm.htm
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PostPosted: Thu Oct 18, 2018 6:45 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

I read his deck as in "thousandths". So he needs to clarify that. Normally the step is .050", not .05mm.
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63ratster
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PostPosted: Thu Oct 18, 2018 7:09 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

My apologies to everyone...I wrote my original post in a hurry, and accidentally forgot a "0". Yes, my deck heights should be 0.047"-0.050", with my step being 0.050". And to clarify, my heads are 53cc's including this step, and approx. 47cc's just filling the bowl around the valves themselves. I am sorry for any of the confusion from my original post. Embarassed
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63ratster
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PostPosted: Fri Oct 19, 2018 10:56 am    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

John.....with my numbers punched into the CB Performance calculator, my current CR is 7.0-7.1, which is good for a stock cam. My thoughts were to minimize the burn in the top of the cylinder by having the step removed, then opening up the chamber(with a semi hemi cut) to get the chamber volume to 56cc's, which should yield a 7.5-1 CR. Do you guys think that this is a cost effective plan for small valve heads, or would I be better off just buying a set of 35.5x32 valve heads without the step in them?

Ach60- thanks again for your detailed plan above. Sorry for the numbers error
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74 Thing
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PostPosted: Fri Oct 19, 2018 12:50 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

I would not go semi hemi. If you use your current heads have the step cut out and get a three angle valve job-Should cost you around $150 or so from the machine shop.

Wait your calculations are wrong-I just ran the number on the CB Engine Calculator-with a 87 bore and 69 crank throw with .047" deck height and 47cc combustion chamber your compression ratio without the step will be 8.6:1 (too high for a stock cam and stock carb IMO)

By leaving the step in you have a total of 53cc with a .047" deck height so your compression is 7.8:1 (which is fine for your set up).

I would just get a valve job and leave the step and run it as it at 7.8:1.

If you are buying new heads you are looking at spending a lot more money.

With a stock cam and stock carb I would shooting for around 7.7-8:1 compression range and your current heads fall right into that. With low compression and semi hemi you are going to really have to advance the timing and tuning becomes difficult as well.

I have seen guys run high compression with semi hemi and run big numbers (Joel Mohr), but that is a different topic all together.


Last edited by 74 Thing on Fri Oct 19, 2018 12:58 pm; edited 1 time in total
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PostPosted: Fri Oct 19, 2018 12:54 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

My suggestion is to shoot for 7.5-8.0:1 static cr, remove the step and have your chambers opened accordingly WITHOUT semi-hemi cutting them. If you bring the piston closer to the plug, but destroy your quench pad by dishing your chamber, you haven't accomplished much. Your mixture approaching TDC will still be pretty stale.

We have an excellent head guy in our midst here on thesamba. Brain_e can take your heads and make them flow better than 35.5mm stock heads, and open your chambers up for a very reasonable price.
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PostPosted: Fri Oct 19, 2018 1:14 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

63ratster wrote:
I recently purchased a 1641cc DP with Mexican F.I. 043-101-375H (33x30mm valves) heads. This is low mileage rebuild that features a stock crank,stock ( non F.I.)cam,34 PICT-3 carb,Pertronix SVDA, and 1 3/8" dual quiet pack exhaust with heater boxes. My deck height on the 1/2 side is 0.48 & 0.50, while the 3/4 side is 0.47 & 0.47. The heads feature 47cc combustion chamber with an additional 0.50 step(for a total of approx. 53cc). I am planning on running 87 octane that has 10% Ethanol. According to the CB Performance site my CR is currently 8.0. My questions are as follows:

1) Is it worth buying new heads with carb size valves(35.5x32), or will the smaller valves work just fine(there are like new)? YES

2) If I keep the F.I. heads, should I have the step removed and semi-hemi cut the combustion chamber to 53cc (should yield about 8.3 CR) to give the heads better quench for a more efficient burn? NO

3) And lastly, would there be a noticeable difference in (seat of your pants) performance with 35.5x32 valve heads over the 33x30 heads with an otherwise stock engine?

Thanks in advance for any advice!!


A pair of my 040's would instantly solve all your problems. My heads flow better than any other stock valved head on the market without any additional work. Set the CR at 8.0 to 1 and I guarantee 100% without question that you will feel a seat of the pants increase. $460 for the pair and I will cut the for the correct chamber size for no additional cost.

You can go the other direction and "fix" your heads and it will work fine and be better than what you have now BUT why spend $200-$300 and have fixed used heads when you can buy new for a little bit more?
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PostPosted: Fri Oct 19, 2018 5:55 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

You can run 8:1 with the stock cam.

That's 51cc at .050" DH. Cut the step out, minor work and you are GTG.
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PostPosted: Fri Oct 19, 2018 8:55 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

If he does not cut out the step, it's possible the pistons will hit the step.
If he cuts the step out, and does nothing he has 47cc chambers, at least that's the way I read it.
I agree the step need to be removed, but I don't see where the 51cc came from?
But If he works the chambers to get 51cc, then sets the DH at 0.050" I see what John's talking about.
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PostPosted: Fri Oct 19, 2018 9:02 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

I simply calculated the CC needed to get 8:1 on a 1641.

IMO the step has to go.
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PostPosted: Fri Oct 19, 2018 9:13 pm    Post subject: Re: CR for Mexican FI heads on a 1641cc Reply with quote

The FI heads would be ok but if you want to worry about getting the step out I would just go by new L3's or Mofoco 40's super or 41's 53 or 54cc chambers. Set deck at 40 or 50 thou.
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