Author Topic: Best way to REDUCE compression ratio? (Read 12687 times)

Sam Green Racing · « **Reply #25 on:** March 19, 2007, 05:35:51 PM »

Is it not still going to throw the cam timing out though Mike

Sam.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #26 on:** March 19, 2007, 05:39:48 PM »

How much do you think the double gasket will reduce the CR and will this be enough?

MRieck · « **Reply #27 on:** March 19, 2007, 05:43:23 PM »

Quote from: samcr750 on March 19, 2007, 05:35:51 PM

Is it not still going to throw the cam timing out though Mike

Sam.

Right you are Sam...anytime you increase or reduce the deck height or head thickness you'll change cam timing. Get a Falicon sprocket or slot an OEM unit. The cam chain does get tough to install with the extra height.

Ricky_Racer · « **Reply #28 on:** March 19, 2007, 05:48:00 PM »

Quote from: MRieck on March 19, 2007, 05:29:15 PM

A .060 copper base gasket combined with the .043 head gasket would work OK.

Yesssss! I knew someone out there had the right numbers... Thanks for chiming in, Mike! You too, Sam. So, with the .060 and .043, what do you think the CR would work out to be? Would it drop a full point? I've got a couple ported heads I could burette. Do you happen to know the volume of the stock 750K-model combustion chamber?

Also, Curt Jordan, second mechanic at the old Action Fours lives nearby. I can have him take a look at the piston crowns and advise whether there's sufficient meat to take off a slice. Man, I love a good challenge and this topic is bringing out the best of the best! Thanks to everyone who's participating! RR

Roach Carver · « **Reply #29 on:** March 19, 2007, 05:57:28 PM »

I have one other idea not covered here. The f heads had a larger chamber didnt they? I say this b/c I have one on a stock K motor and I was under the assumption that this would decrease my c/r due to the lack of dome pistons.

Ricky_Racer · « **Reply #30 on:** March 19, 2007, 06:16:08 PM »

yes, 71cb750. I was under the same assumption; I for some reason thought the use of the '77-78 "F" cylinder and head dropped the "K" CR by about a point, but no one else mentioned it so I assumed my info was incorrect. I understand there are some design and parts availability issues with the Late "F" head, but it might be a necessary option if I'm going to run these pistons...

Thanks for bringing it up. Any one out there tried this combo to reduce CR? Any success? RR

MRieck · « **Reply #31 on:** March 19, 2007, 06:50:35 PM »

The late F head would probably drop just a little over 1 point. It's doable and it wouldn't kill squish as much as raising the cylinder though I'm no fan of the F2/3 head.

HondaMan · « **Reply #32 on:** March 19, 2007, 06:59:18 PM »

Quote from: Ricky_Racer on March 19, 2007, 10:57:41 AM

I have a set of NOS Action Four 836 pistons I'd like to use in a '72-76 K motor... Problem is that they are 12:1 CR and I don't fancy spending for, or hassling with, fuel enhancers unless I absolutely have to. So, what's the preferred method of reducing the CR? '77-78 F cylinder and head? Cylinder base plate? Thick head gasket?

Anybody got an answer to this dilemma? I'd really like to use these slugs! Thanks in advance. RR

Ricky: scout out the thread where I hemi-d the K head on my bike. It dropped the CR almost a full number with a very conservative smoothing. I had semi-hemi-ed the head in the mid-1970s, just enough to remove the valve shrouding, and that dropped it from 9:1 to about 8.5:1 at the time, necessitating a head mill to get 9:1 back. Last summer, I hemi-ed the same heads and had to remove about .010" again to get it back into the low 9:1 (it calcs to 9.1:1, I think, measures like 9.6:1). If you started at 12:1 on 836cc bores, then hemi-ed the head adding a CC or a little more, it will drop toward 10:1 real quick.

And, it will breathe better.

And, mine runs on 85 octane now with no complaints, runs dark plugs with premium gas. It's happy!

Ricky_Racer · « **Reply #33 on:** March 19, 2007, 09:14:55 PM »

Thanks, Mark! I'll review the Hemi thread again; it was a great discussion. Looks like I may be able to pull this piston installation off after all. I'll plan on digging out the burette and calculator.

How many CC's do you feel I will need to add to the standard combustion chamber volume to drop the CR acceptably? RR

HondaMan · « **Reply #34 on:** March 20, 2007, 12:04:33 PM »

Let the calculator be your guide: use the number .028" thick for the head gasket volume.
10:1 CR should work out fine with pump premium gas.

Ricky_Racer · « **Reply #35 on:** March 20, 2007, 03:15:55 PM »

Will do, Mark. Thanks again. I appreciate your advice! RR

merc2dogs · « **Reply #36 on:** March 20, 2007, 04:44:55 PM »

Double up the base gasket, depending on how low you want to go, maybe use a sheet of aluminum between the gaskets then either slot the cam gear holes to adjust the valve timeing to where you want it.

if you slot the holes, after proper timing, cut thin 'fillers' to take up the space in the slot to keep the gear from walking, otherwise the only thing holding it will be the clamping force of the bolts.

I think the valves operate at too tight of clearance to just space it and call it good, which is why you'd want the slots in the cam gear.

Spacing at the baseplate leaves the actual combustion chamber in stock conmfiguration, gaskets will be at least as strong as stock without the potential weakness of stacked gaskets in the highest pressure areas.

Ken.

Jim F · « **Reply #37 on:** March 20, 2007, 08:39:59 PM »

hell just grind the rods in two take about a half inch of the length
and weld them back using coat hanger wire
good as new and less compresion.................

always a smart ass in the bunch.......ME

just double up on the head gaskets or run 110 race fuel like I do

love the smell of race gas..................anytime

Jim

cafebob · « **Reply #38 on:** August 13, 2007, 12:39:33 PM »

just wanted to add here that it sounds like your pistons might be a set that action four made for the F motors.... if the heads are domed this is very likely. I am currently trying to track down one. PM me if you want more info

C.B.

aptech77 · « **Reply #39 on:** August 13, 2007, 08:46:45 PM »

I'm running 11:1 in my 400F and 93oct is fine. I might/should be fine IMO.
My triumph is running 12:1 and they recommend 89oct. It IS however an oil/liquid cooled beast.... $:-\$

MRieck · « **Reply #40 on:** August 13, 2007, 09:04:16 PM »

Quote from: aptech77 on August 13, 2007, 08:46:45 PM

I'm running 11:1 in my 400F and 93oct is fine. I might/should be fine IMO.
My triumph is running 12:1 and they recommend 89oct. It IS however an oil/liquid cooled beast.... $:-\$

Right....big difference. Oil cooled Gixxers we ran at 11:1 no problem. Better chamber, valve angle, piston etc, etc. My FJ is about 11:1 and it gets HOT. The 400F is a very small bore....you can get away with more.

Pinhead · « **Reply #41 on:** August 13, 2007, 10:25:28 PM »

Personally, I'd do Hondaman's Hemi-ing to the head.

_{Actually, PERSONALLY, instead of lowering compression, I'd try to lower detonation... But we all know how that discussion went last time I brought it up on here...}

Sam Green Racing · « **Reply #42 on:** August 13, 2007, 10:46:32 PM »

Drill some very small holes in the piston crown maybe ?

Sam.

nippon · « **Reply #43 on:** August 13, 2007, 11:51:29 PM »

Hi,
in order to reduce compression ratio, take two base cyclinder gaskets,...the cheapest and best way.
I'm shure, two head gaskets don't work.

Use high octan gasoline in order to avoid jingle. If your engine jingles, set your ignition to an earlier point.

In order to get the volume of the head chamber, take a plexi glass. Drill a small hole in it.
Put the head upside down, grease the edge of the chambers and put the plexi glass on it.
Of course, your valves should be installed and the valve should close complete.
Then fill the chamber with water out of your injection through the hole of the plexi glass, so that there are no air bubbles in the chamber anymore. Soak the water with the injection and read the scale of the injection, that's all.
I used this method in order to get the same volume on every chamber after some head work.

nippon

mark · « **Reply #44 on:** August 14, 2007, 01:19:21 AM »

HondaMan's hemi job sounds like the way. Add a thicker base gasket if that isn't enough.
To realize the full potential of your cam, an adjustable sprocket is a good idea anyway.

or..

look up the recent thread about trailers. load one up with your jugs of race gas. nothing like the smell of race gas

Happy trails.

Double head gaskets?

doobiebro · « **Reply #45 on:** August 14, 2007, 09:35:20 AM »

Please understand that I am an accountant by trade so my mechanical skills are far less than the many masters in this forum, but I thought I would offer a suggestion to ponder.

I know Big-Jim was just joking about cutting the rods, yet my first thought was regarding the rods. Perhaps there are some slightly shorter rods available that would fit your engine. The shorter rods would shorten the stroke and provide more cylinder volume, thereby reducing the compression ratio? Also, the shorter rods would have less weight which is generally a good thing.

oldfordguy · « **Reply #46 on:** August 14, 2007, 10:36:12 AM »

Shorter rods will not change the stroke, the crankshaft determines the stroke. However, shorter rods would lower the compression ratio, just as raising the piston pin journal, raising the head, etc. I think Hondaman's suggestion would be the most cost effective way to accomplish this, and not have to worry about head/base gasket sealing, etc.

Sam Green Racing · « **Reply #47 on:** August 14, 2007, 02:08:35 PM »

The most cost effective way is not to use the 12 to 1 pistons in the first place.

Sam.

peter c · « **Reply #48 on:** August 14, 2007, 02:16:16 PM »

You know Sam that shows a hell of a lot of sense to me. Saves a lot of work too.

peter c

c91x · « **Reply #49 on:** August 14, 2007, 03:52:41 PM »

do you really need to run hi octane or hi oxy fuels with 12:1

? i run a 13.5:1 and run the 91octane pump gas???

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Author Topic: Best way to REDUCE compression ratio? (Read 12687 times)

peter c