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Author Topic: DOHC RODS IN AN F2 ENGINE  (Read 3040 times)

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Offline cavebear

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DOHC RODS IN AN F2 ENGINE
« on: February 11, 2010, 12:44:54 am »
I have a 100 hp F2 race only motor with a wiseco 836cc 12.5/1 ratio with combustion chambers to match. From what I've
been able to estimate using the dohc rods at 1.5 mm shorter would bring the compression ratio down to 10.5/1. Does anybody come up with a different figure?   
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Offline Sam Green Racing

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #1 on: February 11, 2010, 12:48:19 am »
Why do you want to do that ?

Sam. ;)
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Offline lordmoonpie

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #2 on: February 11, 2010, 01:04:30 am »
Dunno about an F2 but the K rods are also 1.5 mm longer than DOHC rods PLUS 1mm longer storke which means if you use a DOHC rod you lose 2mm on the stroke height and a bunch of compression. I had custom rods made 2mm longer than DOHC to fit my DOHC crank into the SOHC engine. Check the F2 stroke as well - if it's not the same as the DOHC your calculations are on the optimistic side...
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Offline cavebear

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #3 on: February 11, 2010, 07:06:32 am »
The reason behind this madness is I intend to convert this engine to street use. The only roadblock is the high compression
ratio. I reread the specs on the engine build and both the head and block  were milled also, so the ratio more like 13:1. Gas with 105 octane is what it's supposed to run now. The only difference Between the F2 and dohc rods is the shorter length of the dohc rods. I,m just having trouble calculating the new compression ratio.

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1975 MotoMartin 836cc
1976 Rickman 1150cc Turbo
1977 Seeley 890cc + 1/4 stroke
3 parts bikes,1970K0 to 1978ss

Get paid to do it,it's work; pay to do it,it's a hobby

Offline Sam Green Racing

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #4 on: February 11, 2010, 07:21:53 am »
Stop pushing me, I was about to do it when I noticed the PM. ;D ;D ;D ;D

Have you not considered swaping the pistons for 836 flat top turbo ones and selling the 12 to 1s to offset the cost. The fact that the head has been skimmed should take the comp to somwhere near std.

Sam. ;)
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Offline cavebear

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #5 on: February 11, 2010, 10:26:24 am »
Sam, thanks for the input. It's now on my option list. 2 other ideas. I have a MTC  forged 823cc kit for a K engine with 12.5:1 comp. With the F2 chambers opened to match the 836cc would that do it Or how about a wiseco 836cc 12.5:1 K piston kit. Would this  option create the 10.5:1 comp I'm looking for? 
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1975 MotoMartin 836cc
1976 Rickman 1150cc Turbo
1977 Seeley 890cc + 1/4 stroke
3 parts bikes,1970K0 to 1978ss

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Offline dusterdude

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #6 on: February 11, 2010, 04:17:44 pm »
ooooh,12.5 to one,that has me salivating
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Offline cb750k77

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #7 on: February 11, 2010, 10:59:40 pm »
why are you sooo concerned with 12:1 just tune the ports and the AF ratio for it and your fine...

http://www.theoldone.com/articles/

read up on the "soft" head design... that guy is amazing!
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Offline Pinhead

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #8 on: February 11, 2010, 11:05:12 pm »
Quote from: cb750k77 on February 11, 2010, 10:59:40 pm
why are you sooo concerned with 12:1 just tune the ports and the AF ratio for it and your fine...

http://www.theoldone.com/articles/

read up on the "soft" head design... that guy is amazing!

THIS!!

Larry Widmer is the man.

Mike Holler's tech additions makes high compression much easier to run. Check out my CB650 rebuild thread. :)
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Offline 754

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #9 on: February 11, 2010, 11:08:58 pm »
Cavebear, just pull the pistons and mill .060 -090 thou, or more off .

 afternoon job, and you are done...
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Offline cavebear

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #10 on: February 12, 2010, 10:02:31 am »
I want to thank all you gentlemen for the excellent input. I will definitely be using one of your ideas instead of mine.
Mike the reason I wasn't concerned about taking the engine apart is when it was set up for full race the previous owner took out the kickstart assembly. >:(.Thanks to OldFett I've got a shotpeened and cryo treated shaft to install. Now I"ll also add the heavy duty primary chain and M3 cam chain tensioner. When I start the build thread on my Moto Martin there will be a full description of engine modifications   
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1975 MotoMartin 836cc
1976 Rickman 1150cc Turbo
1977 Seeley 890cc + 1/4 stroke
3 parts bikes,1970K0 to 1978ss

Get paid to do it,it's work; pay to do it,it's a hobby

Offline dragracer

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #11 on: February 26, 2010, 02:14:14 pm »
Simplest and cheapest thing here is to get Cometic to make you either/ or both a thicker head gasket or a thicker base gasket and simply pull the pistons back in the hole for negative deck height. This will drop the compression to streetable numbers. You could also play with cam timing(adjustable cam sprocket) to pull out compression but then you risk loosing power on the upper or lower end of the rpm band.

My 2 cents worth- and i want change back- lol.
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Offline NitroHunter

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #12 on: February 26, 2010, 03:02:31 pm »
Another .02 here  ;D
I'd love to see the dyno sheet on THAT motor!  :o
I'm not keen on a shorter rod ratio. Thicker base gasket vs. short rods, personally I'd go with the gasket change. I've run .100" base gasket thickness  ::) yeah it put the pistons way down in the hole, and I didn't run out of cam chain with a stock height block.
I'd cc the head myself to verify this high CR, but there are other ways to get REALLY CLOSE before you tear into it.
What is your cranking pressure? a 12:1 motor with a large (late closing intake) cam can have about the same cranking pressure as a 10:1 with small cam. 
There are some really cool calculators online (try RB Racing to start) that can help you figure out all this stuff if you know the numbers and are willing to spend a little time doing the math.
I've used them this way, and by golly they were right on the money...
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Offline turboguzzi

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Re: DOHC RODS IN AN F2 ENGINE
« Reply #13 on: February 26, 2010, 03:46:49 pm »
Quote from: cb750k77 on February 11, 2010, 10:59:40 pm
why are you sooo concerned with 12:1 just tune the ports and the AF ratio for it and your fine...

http://www.theoldone.com/articles/

read up on the "soft" head design... that guy is amazing!

spent some time on that site and could only find 4-valve and car bath tub heads related articles. did you see something there about hemi heads?

maybe i missed it.

Regardless, my much smaller bore 500 racer is on the edge of pinging with 12.5:1 and 100 octane with nice turbulence inducing squish band, so i tend to agree with cave, for a street 750 12,5 is way too much
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