Author Topic: compression ratio (Read 5661 times)

Mitchell · « **on:** May 05, 2015, 02:09:22 PM »

How much would a set of dohc rods lower the c/r in a stock k3 with stock pistons. If im not to far off the dohc rods are 1.5mm shorter?

Old Scrambler · « **Reply #1 on:** May 05, 2015, 02:58:35 PM »

My guesstimate is 7.5:1..............................you could run 65-octane fuel and have less power than an A-model...........or use a blower/super-charger safely for a modest power boost with relatively weak rods. But why? It may be easier to leave the bottom-end as-is and change the head to an F2 type that will get you about 8.0:1 and keep the motor balanced.

bear · « **Reply #2 on:** May 05, 2015, 04:04:58 PM »

DOHC rods are 1.5mm longer than the SOHC rods.

Cheers,
Brian

sohc boy · « **Reply #3 on:** May 06, 2015, 02:41:33 AM »

..as Brian said . they are longer not shorter , maybe 750 DOHC rods are shoter . but doubt it

mick7504 · « **Reply #4 on:** May 06, 2015, 05:42:07 AM »

750 .060 shorter 15mm pin
900 .060 longer 15mm pin
1100 .060 longer 17mm pin

PeWe · « **Reply #5 on:** May 06, 2015, 10:14:42 AM »

Some photos of CB750 sohc vs CB900 dohc rods
I bought them for a future CB750 engine project, cheap rod upgrade

Badluckhonda · « **Reply #6 on:** May 06, 2015, 10:43:04 AM »

Have a look here http://www.webring.org/l/rd?ring=thehondasohc4web;id=1;url=http%3A%2F%2Fwww%2Esatanicmechanic%2Ede%2F

PeWe what do expect you compression ratio will be with those rods ? Theure about 1.5m longer right ?

PeWe · « **Reply #7 on:** May 06, 2015, 12:49:53 PM »

I have no ideas about CR. I guess stock piston will not work when the pockets for valves are almost nothing. It depends on the cam, lift and duration so the pistons will not hit the valves. Why more compression with stock pistons?

I have thoughts to use them in future engine, 900 + pistons with valve pockets for hot cams. Probably thicker base gasket like 1mm extra plus o-rings that can support the thicker base gasket. 1.5mm rasie of pistons will probably be too much.
Eventually carve cam sprocket inside area for cam mount so it can go deeper on cam when mount the cam chain when everything moves upwards with thicker base gasket.

1.5mm longer rods must be like deck the head 1.5mm.

bear · « **Reply #8 on:** May 06, 2015, 01:33:52 PM »

The DOHC 750/900 rods are not much of an upgrade on the SOHC units, considering the stuffing around involved.
It's the MA3 rods used in the 1100's and early RC's that provide an upgrade.

We have used them on our stroker motors to give us an acceptable rod to crank ratio.

Cheers,
Brian

mick7504 · « **Reply #9 on:** May 06, 2015, 02:15:47 PM »

It certainly is a lot of stuffing around.
Don't know how many hours I spent attempting to "reinvent the wheel"
Then putting a fly cutter straight through the top of a new piston while opening up the valve pockets for clearance was enough to get the message through.

Badluckhonda · « **Reply #10 on:** May 06, 2015, 03:29:48 PM »

I figured they wouldn't work with stock Pistons. I guess you could get custom copper gaskets made which would be the easiest.

Mitchell · « **Reply #11 on:** May 06, 2015, 05:19:12 PM »

Sorry guys i should have gave a little more info. Dohc 750 rods, i have a set and they are shorter. I plan to see how much boost i can throw at a stock sohc 750 motor Before she Goes Bang. I Have Enough Stock Motor parts To Build A Few Motors. So My Plan Was To Pretty Much Build a Stock motor With A 750a cam. Maybe Ceramic Coat The Tops Of The Pistons And Valve With Some Mild Head Cleanup And Water Injection To Help Keep Knock At Bay

Old Scrambler · « **Reply #12 on:** May 06, 2015, 07:08:24 PM »

That's what I surmised...............but those rods will be on the weak side

Mitchell · « **Reply #13 on:** May 06, 2015, 07:51:26 PM »

yea but im really not concerned if it flies apart or not ive had most of these parts in the same box on the same shelf for 6+ years just as well do something with em. i wanna see how much the stock honda motors can handle. so how would i go about calculating compression with dohc750 rods? i want it low but not to low,

754 · « **Reply #14 on:** May 06, 2015, 09:23:08 PM »

10.05 in the 1/4 mile is what Nitrohunter on here got with it believe Stock pistons....before the big bang..

PeWe · « **Reply #15 on:** May 07, 2015, 03:11:45 AM »

Quote from: bear on May 06, 2015, 01:33:52 PM

The DOHC 750/900 rods are not much of an upgrade on the SOHC units, considering the stuffing around involved.
It's the MA3 rods used in the 1100's and early RC's that provide an upgrade.

We have used them on our stroker motors to give us an acceptable rod to crank ratio.

Cheers,
Brian

I have got a voice in my head whispering to me, get the APE ultracrank with Carillos + undercutted gears, at least 2:nd and 4:th.
It began after some road tests and dyno with a fun cam.
The boring thing is that the engine is just restored, head torqued 3 times and no oil leaks at all. Boring to pull engine out of the frame again.

Mitchell · « **Reply #16 on:** May 07, 2015, 06:47:35 AM »

10.05!!! holy cow thats scremin. i was also thinking last night instead of running water injection to run 100% methanol to keep things in check

SOHC4 Cafe Racer Fan · « **Reply #17 on:** May 07, 2015, 12:48:18 PM »

Quote from: bear on May 06, 2015, 01:33:52 PM

The DOHC 750/900 rods are not much of an upgrade on the SOHC units, considering the stuffing around involved.
It's the MA3 rods used in the 1100's and early RC's that provide an upgrade.

We have used them on our stroker motors to give us an acceptable rod to crank ratio.

Cheers,
Brian

Brian,

Can you provide years and models for the 1100s and RC's that provide an upgrade. Is this a plug and play solution or one that requires modifying the con-rod?

I am already using Cycle X superrods on my next build, but I am sure there are members who wouldn't mind a more affordable and easy upgrade for their engine rebuilds.

Thanks,
Don

Badluckhonda · « **Reply #18 on:** May 07, 2015, 02:25:13 PM »

I think he said he used them in a stroker, so they shorter then stock

Bill/BentON Racing · « **Reply #19 on:** May 07, 2015, 02:31:34 PM »

CB1100F rods have MF3 stamped on them. Only made in the states in 1983. They are 1.5mm/ .060 longer!

SOHC4 Cafe Racer Fan · « **Reply #20 on:** May 07, 2015, 02:42:42 PM »

Quote from: Badluckhonda on May 07, 2015, 02:25:13 PM

I think he said he used them in a stroker, so they shorter then stock

Didn't catch that last sentence on the first read. Thanks.

kmb69 · « **Reply #21 on:** May 07, 2015, 03:29:00 PM »

Quote from: Badluckhonda on May 07, 2015, 02:25:13 PM

I think he said he used them in a stroker, so they shorter then stock

They are part number 13203-MA3-000 or 13203-MA3-405. Made for US 1 year only, 1983, for DOHC CB1000C and CB1100F.
They are 1.5mm LONGER than stock SOHC 750 rods and use 17mm piston pins. Not a straight swap - several issues to deal with.
You must raise the cylinders and/or custom pistons to use them in standard OR stroker SOHC 750 motors.

They are part number 13203-438-000, 13203-438-405, or 13203-438-405. Made for DOHC CB900C and CB900F.
They are 1.5mm LONGER than stock SOHC 750 rods and use 15mm piston pins.
You must raise the cylinders and/or custom pistons to use them in standard OR stroker SOHC 750 motors.

They are part number 13203-425-000 or 13203-425-405. Made for DOHC CB750 C, F, K, L, and SC.
They are 1.5mm SHORTER than stock SOHC 750 rods and use 15mm piston pins.
Will work with cylinder/piston/crank mods but rod length to stroke ratio is on low side for standard 63mm stroke.

bear · « **Reply #22 on:** May 07, 2015, 03:50:59 PM »

Sorry fella's I should have kept my nose out of this thread I seem to have muddied the water a bit.

Keith is right they not really an option for a cheap rod upgrade for a std stroked motor.

But I would like to highlight his point regarding rod/stroke ratio.
In my opinion you will create more problems than you will solve by straying to far either side of a 1.7:1(fixed

) ratio on these motors.

Quote from: kmb69 on May 07, 2015, 03:29:00 PM

Quote from: Badluckhonda on May 07, 2015, 02:25:13 PM
I think he said he used them in a stroker, so they shorter then stock
They are part number 13203-MA3-000 or 13203-MA3-405. Made for US 1 year only, 1983, for DOHC CB1000C and CB1100F.
They are 1.5mm LONGER than stock SOHC 750 rods and use 17mm piston pins. Not a straight swap - several issues to deal with.
You must raise the cylinders and/or custom pistons to use them in standard OR stroker SOHC 750 motors.

They are part number 13203-438-000, 13203-438-405, or 13203-438-405. Made for DOHC CB900C and CB900F.
They are 1.5mm LONGER than stock SOHC 750 rods and use 15mm piston pins.
You must raise the cylinders and/or custom pistons to use them in standard OR stroker SOHC 750 motors.

They are part number 13203-425-000 or 13203-425-405. Made for DOHC CB750 C, F, K, L, and SC.
They are 1.5mm SHORTER than stock SOHC 750 rods and use 15mm piston pins.
Will work with cylinder/piston mods but rod length to stroke ratio is on low side for standard 63mm stroke.

kmb69 · « **Reply #23 on:** May 07, 2015, 04:48:54 PM »

I want some of what Brian is drinking!

I think he meant 1.7:1 ratio which is the ratio with 117.5mm rods and 69mm stroke.
The stock SOHC is 1.84:1 ratio which is considered by most to be a "long" rod motor.
And I actually got that wrong. The DOHC 750 rods are 1.81:1 on the 63mm stroke which should be just fine.
They are getting to be on the short side for a 69mm stroker motor.

Badluckhonda · « **Reply #24 on:** May 07, 2015, 05:01:57 PM »

always thought strikers used shorter rods. To counter act the piston going up higher into the cylinder.

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Author Topic: compression ratio (Read 5661 times)