Welcome to our resource for MG Car Information.
Triumph TR6 - Compression ratio??
|Need help from the tech guys. Received the head from the shop, shaved by 60 thou. The shop cc'd the head and determined volume is 54 cc's. We know the bore on a standard TR6 is 2.94 " or 74.7mm, the stroke is 3.74" or 95mm. We don't know the head gasket thickness but it's part of your standard stock TRF set. Can anybody out there add the standard head gasket thickness to my data and compute the compression ratio or tell me what else I need. I have searched the archives but it appears I do not have all the necessary data. Any help would be greatly appreciated. There is a site that has a compression ratio calculator if anyone is interested and knows how to do it. It's http://www.csgnetwork.com/compcalc.html|
Sorry, looks like we're doing this on 2 threads! Check this : http://www.goodparts.com/tech_docs/TR6_Compression_Ratio.html
Might give you everything you want to know?
2 threads? Didn't see the other. Quick calc based on Richard's chart til I can get a head measurement indicates that I've only increased the compression on my 1975 head to between 8.0:1 and 8.25:1. Where's the other thread?
I was posting on the "Scott Helms Thrust Washer" thread while you started this thread.
I am planning to shave mine .010 inch, (ten thousandths) for about 9.5 to 1. Am I mistaken here? I'm glad it's Friday...........
Here're some data (from where I'm not not sure, but one of my references) that may be of value in your calculations. Steve Pike could tell you in a heart beat what you need to do to the head for a specific CR, but it'd seem to me that you just take the stroke and do a volumetric of the cylinders and from there it's arithmetic. Steve'd do math, but that's over my head:) BTW, these data are EITHER an early head or later, not both, but I don't recall which is which???
Displacement/Cylinder Standard Bore: 416.33cc x 6 = 2497.98cc (2498cc)
Standard Stock Compression Ratio: 7.62
Head Thickness: Initially 3.550
Compression Ratios Amount removed and remaining head thickness
8.5:1 -0.071 = 3.479
9.5:1 -0.13 = 3.416
10.5:1 -0.18 = 3.366
Bob, I'm thinking 0.60 will put you somewhere beyond 12:1 which is probably not streetable. From my many discussions and reading about CR, I'm convinced that anything beyond 9.5:1 is inviting nagging and constant maintenance just to keep it running and you'll probably need petrol at 120 octane which you ain't gonna get!! You may be looking at burned pistons (unless yours are billet and hardened), and maybe worse.
Rod, 0.10" would give you pretty close to 9.0:1, if the above data are correct. Steve???
Do not wish to be picky but since we are talkin' numbers here I think you might have put the decimal place in the wrong place. .10" is a tad bigger than .010"
If you shaverd .60" off the head you would have the valves helping to push the piston back down.......now there is an idea to help raise HP!
I have only heard about .010" being shaved off. I agree, 60 thou seems a bit too much.
|You also have to take the deck height into account as well as the actual bore in the event you had to/wanted to bore the cylinders and are runnning oversize pistons.|
The nominal gasket thickness for your basic Payen brand head gasket is .045" and on installation and with "squish" it should be somewhere in the .033" range.
The head thickness varied between the early NA spec cars and the later NA spec cars with the early ones being 3.46" thick* and the later ones being 3.55" thick*. You are on your own for ROW cars, I have no idea on head thicknesses there. Given the age of the cars, get a good measurement on the head thickness since they may have been hit by the surfacer already. Since it is a wedge shaped combustion chamber, any thickness you take out can be calculated using a circular diamater. If get down to where you accounting for volume changes in the wedge, you've gone too far.
So in summary, you need the swept volume of the cylinder, any unswept cylinder volume (or potentially overswept volume), the volume taken up by the head gasket and the volume of the combustion chamber itself to calculate the compression ratio.
Be careful about the compression ration that you go for. Stretch it too high and you will not be able to run pump gas. A friend of mine went up to ~11:1 on his TR250 and has to either use racing fuel or lots of octane booster to keep from destroying his engine. It makes for a mighty expensive fill up. On the other hand, it is also one scary fast TR250.
*source: Kas Kastner
|OK Rick. Leave it to you to clean up the battlefield. I caveated my post that I could handle arithmetic OK, but math was beyond my ken. Move everything one decimal point to the right!! I don't know wha'tha'hell Steve is talking about sweeping.|
|Swept volume is the volume taken up by the movement of the piston in the bore. Lots of volume at BDC, not nearly so much volume present at TDC which allows us to calculate a compression ratio. |
I did go to the website mentioned in the first post, nice little calculator set up there. Based on my engine build notes I used it to come up with a compression ratio of 9.6:1, which matched up rather well with what was originally calculated the long way and close to my build target of 9.5:1.
Regarding the information on Richard Good's site, it is nice information to have if you do not have access to some basic chem lab equipment. It provides a decent mechanism for getting to a nominal compression ratio without having to do all the combustion chamber volume measurements and the numbers look to match up with our experience in taking combustion chamber measurements. He still requires that you take two measurements so you are not completely off the hook measurement wise. One is the cylinder head thickness, the other is the deck height. What it doesn't do is account for any slight chamber volume differences that might have resulted during the manufacturing process and for that......
You always have the option of going the buret route to determine each combustion chamber volume, matching them up, verifying and then crunching through the numbers the old fashioned way like us retro-grouches.
You did not comment on the shaving 60 thou....or did you with your comment on your buddys 250?
It's not so much how much is shaved off, it boils down to what is the combustion chamber volume. Given that there is difference of .090" nominal between a factory spec early carb car head and a late carb head, taking .060" off each head would yield dramtically different results. In the case of a late head, it would raise the compression ratio, but it would still be lower than that using an untouched early head. To just blindly go take meat off the head without measurements first opens one up to the law of unintended consequences.
The key is know what you have and them modify accordingly. As an example, measurements made to the head on my current engine, and backed up by combustion chamber volume, indicated that it had already been shaved so to get to my desired compression ration only .015" was removed instead of the .044" (based upon nominal new head thickness) that one could pull off of Richard Good's table. If I had just blindly gone and taken .044" off the head I would have wound up with a compression ratio in excess of 10:1. You will note in that table that Richard has provided nominal combustion chamber volumes that can be used for plug and chug calculations if you want to go that route. All in all, I like his table. Unless you just want to completely peg the weenie meter over your engine build, that table saves you mucho work.
I don't have the build notes on the TR250 engine, but seem to recall him saying that it was about .100" off the nominal stock height and with the .040" overbore, he had calculated it out at about 11:1. Knowing him, it was not based off a table, he's another retro-grouche and he would have drug out the buret and got a good measure of the combustion chamber volumes to use in his calculations for the desired compression ratio and by extension, determine the amount of meat to remove from the head. By the same token, it is only fair to point that many bits of information were not out there when he did his engine build so odds are he could not have consulted such a handy table even if he wanted.
How about e-mailing me so I can get some more advice on this this compression ratio business? Would very much appreciate it.
|Bob, you should have mail. Post back if you do not.|
|FWIW, I took 0.120 off my stock 7.78:1 CR head (4.550 stock thickness) to give me an calculated CR of 9.62:1 w/0.030 oversize pistons. Driving experience was immediately improved. No negatives other than having to buy premium fuel.|
It looks like I am looking at having some work done on my head in the coming months. Could anyone offer some advice or experience about what this will cost? I need valve seats installed, and I am thinking of shaving the head to increase compression. I also have a set of the bronze valve guides and seals which I guess I should have installed at a machine shop? Do I need a 3 angle valve job? If so, how much should it set me back?
My machine shop charged the following (Canadian $$, which are rapidly approaching parity with US$$):
Recondition cylinder head - $96.00 (dip & strip)
Install 12 bronze valve guides - $42.00 (Goodparts with teflon stem seals)
Cut & install 6 valve seats - $48.00
Plane head - $80.00
Install Goodparts hi-perf valve springs - no charge???
They also charged me $122.00 to clean up pistons and con rods, install small end bushings and hone pistons for wrist pins. Note that I only installed hardened seats for the exhaust valves as we did not see the need for the intakes. The jury is still out on whether I screwed the pooch on the 60 thou shave off the head but we will see. I still have the head from my daily driver engine so I do have a backup. Only problem there is doing it all over again. It's only money!! Hope this helps and if you are shaving the head, follow this forum and do all your measurements up front. My only consolation here is that I know this head was not previously shaved so I am still optimistic.
|I looked up a CR calculator online and measured my combustion chamber volume and did all the calculations to give me 9.5 to 1. It ended up being ~ .055" shave. I calculated the new CR with the new calculated combustion chamber volume and it came out the same. Doing the calculation forward from swept volume etc and then backward from new combustion chamber volume gave me the confidence to take the head to a machine shop to have 0.05 shaved off. I currently have the ignition set at 12 degrees advance at idle and 9.5:1 CR, this gives a nice sharp note to the exhaust and pulls real hard. At low RPMs with the throttle down it does clatter with premium gas but I just keep the RPMs up and no problem (and I love it)|
|Hi Guys, I haven't been around for a while, but couldn't resist adding my 2 centsworth to the discussion.|
Agree with Steve, very wise if not essential to measure (cc) the combustion chambers first so you know what you have to start with. The head may have warped from overheating and been shaved 25 years ago without your knowledge. Any machine shop can measure this for you.
Second, how much material are you going to remove from the chamber in order to unshroud the intake valve?
Third, if you measure with valve seat recession then put in nice new valves and valve seats they will reduce the volume of the combustion chamber.
I have a used Payed gasket in my basement, when I did my head I called their tech line (Federal Mogul, who distribute Payen) and was told to measure the thickness at the fire ring and subtract 5 thou for compression when the gasket is clamped down by the head. Mine measured 51 thou, but this was with a caliper not a micrometer, which would have been more accurate. I have read about 40 thou is the thickness of a compressed gasket.
The pistons at tdc may not be in line with the deck of the block, but may be either proud of the block, or "in the hole"- don't assume, measure!
The sides of the combustion chamber will taper inwards a bit, but this will give a small safety factor when calculating, so I would ignore that.
Finally, remember to add the volume of the combustion chamber and gasket thickness to the swept volume before dividing that total by the chamber and gasket volume to get the compression ratio. In other words if each cylinder is 416.33 ccs and the volume above the piston is 52.04cc for a total of 468.4, then your ratio is 9.0:1, not 8:1.
Buddy Steve has had family visiting so have not been at the engine for a week or more but really appreciate all the info and mail from Steve P. Hopefully back at it this weekend and will be doing calcs and adding cam & related parts to block. Will post results.
|Alistair - at the risk of hijacking this thread (sorry!) it may be wise to seek opinions on the bronze valve guides. I did them and 20k miles of hard driving has worn them to a point that I am getting p**d off with the oil smoke puff on start-up. They are soft and wet which may be better suited to short term performance rather than longevity. I will be replacing mine with steel ones but not changing my driving style!!!|
Thanks for your your post - sorry about the delay - I've been away for a while.
I am interested to know whether anyone else has seen this problem with the bronze guides? The chap at TRF sold me them with great promises of improved sealing and lubrication, and they turned out to be the ones from Richard Good. I have never heard a bad word about anything from Goodparts, so I figured I had made the right choice. Anyone else suffering premature wear with these guides?
|Haven't heard anything myself other than the above post on the bronze guides being a problem. How valid not hearing anything though is anyones guess. For most of us the cars see somewhat limited use which might mask a problem. Another thing to consider is which bronze. |
There are a number of copper alloys and even different heat treats within the same alloy out there and some are rather soft while others are reasonably hard. Then you get into the "B-word" thing. In the last few years things have gotten a bit out of control with respect to copper-beryllium alloys. People hear the "B-word" and freak out. There is no real need to do that as long as proper care is taken when working the material. The shame is that a number of applications have shifted away from that material as a result. This has led to some pretty screwy material selections as far as I am concerned. People have been specing out steel bushings to get the strength with all of the resultant hassles you would expect from a steel bushing or they have gone with a different copper alloy and then have issues when it wears prematurely. What's a poor boy to do.... on the other hand it does provide me with some level of job security knowing that there are going to be "issues."
This thread was discussed between 01/06/2007 and 20/06/2007
Triumph TR6 index