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MG Midget and Sprite Technical - Compression Ratio Wont work ...I need help FAST!

I got a problem, I just realized My new pistons (21253 mini coopers at .60 over) are only rated from 9.75:1 to 10:1 Compression Ratio... So whats the Problem??? I planned On a Compression Ratio of 10.5:1....DUH!!!

Now what Do I do?

I had talked to Mark B. about my engine set up some time back and I decided to take his advice and shoot for a 10.5:1 compression ratio.

Is that last .5 CR really that big of a deal, Should I make everything easy and RE-Adjust to a 10:1 CR or should I go ahead and shoot for a 10.5 and hope the pistons will hold up...what would happen if i shoot for a 10.5:1 CR instead of staying in the piston CR limits ....worst case secnerio, what would most likely happen


Im in sharky water, and have no idea what to do... any advice, BTW. Im NOT going to do a differant piston, I will be using these. I just dont know HOW to use them.

Here are my spec.

local fuel is max. 91 octan...93 at 3 pumps on the outskirts of town
head... bigger rimflow valves (knock offs), ported, 1.5 roller tip rockers, duel HS4 carbs w/modified MGB intake all vizardised

bored to 1330 aka .60 over, swiftune SW5-07 camshaft, with matched valve springs, crank will be wedged, rods, crank pully, clutch, flywheel, all lightened and balanced. 3 into 1 pace setter knock off header.

Im trying to get all my notes togather fot the machine shop...(been doing this 3 weeks and counting) and wanting to wrap this up and get to the machine shop tomarrow or at least by friday...so any fast advice would be really appericated

Prop
Prop

What do you mean "rated" ?

Are you sure its not just the specs saying thats the cr you'd get in an other wise std motor with those pistons ?
Dean Smith ('73 RWA)

rated = the CR standards for which these pistons have been judged to operate in.... 9.75 to 10.0:1 CR
Prop

Sorry dean,

I didnt see the last part of your posting...

I dont know the answer to that question, Ill see If I can find a link to the spec.

BTW...standard CR on stock was 8.8 CR and those are deemed as 21251 pistons

Prop
Prop

Prop. I agree with Dean, "What do you mean by 'rated'"?

To the best of my knowledge, pistons are designed/rated to give a certain compression ratio when used with a standard cylinder head. There is no mention, on any of the pistons I have examined, that they would not hold up if the cylinder head had been machined, reducing the size of the combustion chambers and raising the actual compression ratio for the modified system. Thus, the "rating", the one that is most frequently used, provides you with a rough starting place for your modifications.

But, these are the types of questions you need to be discussing with a manufacturer's representative. They are the ones who, officially, and preferably in writing, can tell you what their "rating" means and if the pistons can be used at a higher compression ratio with no problems.

Les
Les Bengtson

http://www.minimania.com/web/Item/21253%2F60%2FEA/InvDetail.cfm

http://www.bpnorthwest.com/.sc/ms/dd/ee/6974/PistonSetAE1275%209.75-1

I may have used the wrong terminalogy....when I said "rated"

these pistons are for a higher compression ratio engine unlike the standard pistons that where good for only an 8.8 CR also known as 21251 pistons

I hope the 2 top links can answer any questions, but let me know

Thanks guys
Prop


Prop

well almost there ...

on the mini mania site, you will have to type manually in the search box "21253" even tho it says nothing is there. its a glitch in there new website that they havent fixed yet.

On the BPnorthwest site...its a typo in the heading it says "9.75 to 1" but it supposed to read "9.75 to 10)


prop
Prop

might try this site for the same piston

http://www.minispares.com/Product.aspx?ty=pb&pid=34819

Prop
Prop

Okay having stepped away for a few minutes and letting my brain cool down from being over worked....LOl


Would this piston Im decibing, That I own, If installed without any modifications, no decking the block, No skiming the head, a straight stock set-up ...would produce around 9.75-10.0 compression Ratio, meaning I CAN use these pistons and still achive a CR of 10.5 by skiming the head

Important point here.....

Meaning that the 9.75 - 10 CR is NOT a "safty/tolerance" concern aka I can set the CR to 11.5 or higher by skiming the head and it will NOT Blow up, Blow a hole in the top of the piston, sieze up in the cly, rape my dog, ect. ect. ****** But rather what the compression ratio would be (9.75 - 10) if installed on a pure stock un-touched engine, Non-machined engine???


Is that my understanding??...If so then I can have the head shaved and the block decked to achive a 10.5 CR withOUT worry that the enhine will go KA-BOOM, correct???

Prop
Prop

Prop. What part of "Ask the manufacturer" do you not understand?

Les
Les Bengtson

Prop, I wouldn't worry about blowing up because of the compression ratio unless you really get it up there way above what you can run with pump gas. What you do need to check with a skimmed head and low dish or flat topped pistons and a performance cam with high ratio rockers is valve to piston clearance to make sure the valves don't ever come into contact with the piston crowns. As far as compression ratio goes, you really need to cc the combustion chambers after the head is skimmed and then calculate the total chamber volume between the head, the head gasket thickness, and any piston dish or valve reliefs. That's the only way you'll really know what the static compression ratio is. Notice I said "static" ratio, the actual ratio will be somewhat different because of the valve timing and pumping efficiency. The more radical the cam grind on the intake timeing the lower the effective compression ratio you wind up with. This is what really allows you to run 91 octane pump gas with static ratios up around 10 to 10.5 without detonation.
As Les said, consult the manufacturer, they will have the information on the piston dish volume as well as the amount of cylinder pressure it can withstand. Generally a cast piston can take most anything a normally aspirated street engine can dish out, for blowers then you would want a forged piston. If you can or already have cc'd your heads then the piston manufacturer would need to know that figure as well I'd think to make an informed answer.
Bill Young

I think this is what you are worried about Prop

Manytimes overreved and turboaspirated before this happened to the Saab 9000 Turbo *

This would have been a forged piston, but it wasnt a fake!

Possibly counts as metal abuse, t'was a damned fast Sayab

I wouldnt allow it to worry me, upping the comp ratio is my late summer job for Lara.

* © Mark in Ireland


Bill sdgpm

Prop, I'd like to stress something Bill said.

Valve to Piston clearance!

You have have mentioned high lift ratio rockers and various cams and skimming the head and large diameter valves and HC pistons. I'm thinking you must be getting close to smacking a valve into the piston.

Trevor Jessie

Thanks everyone,

I had an oppertunity to talk to several people here on the board and those in the know, And luckly my fears are NOT a concern, I had mistaken what the advertised CR of 10.0 was implying and mis-understood its meaning and have learned Im more then okay with bumping it up to 10.5, Whew, thank god!!

however several people have made a good observation that I need to apply some extra thought to and that is the "Octane rating" of the local fuel supply its now droped to 91 for primium and that as down from a year ago when it was at 93 and that is down from 4-5 years ago at 95 octan.

So Im thinking seriously of taking Mark Boldrys advice and raining the CR back to 10.25 instead of 10.5, Really hard to argue with Mark after all he is sort of like E.F. Hutton (in a good way), So I guess I will have to have another talk to my machine shop and see what they think.

Oh I will certianly watch those valves wanting to smack the tops of the pistons...perhaps I should add in a bit of clearance between the pistons and the deck. esp. if for no other reason then to keep the pistons from smacking the cly. head under high rpms and heat build up.

thanks everyone
Prop

Prop

Prop, you can check the valve to piston clearance pretty easily by doing a trial fit with one piston, the cam in place and the valves and valve gear adjusted to the proper lash. Just put a dab of modeling clay on the top of the piston and turn the engine through a complete cycle. The valves will press the clay to the depth necessary and you can carfully slice a cross section of this area and measure with a caliper to get the clearance. If it stops turning at any point don't force it, just remove the head and find out what the interference is. If it's from the valves it will be easy to see. You can also check the clearance to the spark plug in the same manner if that might be an issue. For some high compression engines the plugs need to be indexed so that the ground electrodes are pointed away from the piston to keep it from touching and closing the plug gap. I doubt on your engine you will have any problem areas but it's easy to check and oh so expensive if you don't.
Bill Young

Hey Bill,

We are wearing the same ruby red slippers, Thats exactly what I am going to do when I build the engine, already got the green play doh, last week while in walmart.

seriously I dont see a problem with valve to piston smack down. With the cam lift and the 1.5 ratio of the rockers it comes out to a .430 total lift for both intake and exhaust. IIRC thats set up at 112 degrees full lift which is around 1/3 of the cycle,from TDC, so My thinking is the piston will still be along way from the top as the valves are closing...but then agian, thats why grown men buy play doh. LOL

Prop....Ill trade you the left side slipper for the right side, its stating to give me corns...Prop
Prop

Hi Prop

I can't see you having enough cam lift at TDC to hit a piston, most cars rarely go over 3 mm lift at TDC even with a Race cam...mind you some Harleys have amazing lift at TDC! Check with the cam grinder for TDC lift in and ex then add the requisite amount of clearance if you use higher ratio rockers. It is very difficult to measure that accurately all the ccs to get 10.25 or 10.5, I cannot help but think you are worrying too much, plugs can have 0.3 cc difference depending on the grade selected, you have a choice of 2.8 or 3.8 cc head gaskets, you would need a posh calibrated burette to achieve accurate ccs. You also need to work out the area between the top of the piston and top ring, up to 0.5cc!

Peter
peter burgess

Well... there you go. Peter Burgess has spoke (posted) now you can get on with the build and let us know how it turns out.
I was concerned about interference since you seemed to be maximizing everything to the nth degree. But I'm guessing it is not going to be an issue here based on what Peter said.
Trevor Jessie

There you go, what Peter says....LOL, I think Im on the same page as pPter, he just got there alot easier and faster then I did.

Thank you peter, its an Honor to have your thoughts.

Prop

Prop

Hi prop

We seem to have been quite busy with midget heads and engines for a couple of years, they seem to be more popular with owners wanting to modify them than used to be the case, so we find ourselves playing with CR figures frequently.

Peter
peter burgess

Prop, I have the pistons you are using and they are decked flush with the block. I'm using a Kent 286 with 1.5 rockers and had no issues with clearance. Its running 10.8:1 cr and although set up when on 99 octane (on Peter B's RR!) it still runs fine on the standard supermarket stuff which is about 95 I think.

John
John Payne

Of course it does John, it falls inside all the criteria :-))
11 to 1 on 95 is about where you would need to be. By the way Prop don't get to upset our 95 octane is different to yours, Not sure by how much but your 89 could be the same as our 95. :-)
Anyway long time no see how are you doing?

Will you be going down to Woodall again this year for our Summer Concours?
Robert (Bob) Midget Turbo

Hello Bob, yes fine thanks. Might make it to Woodhall - might polish up the Midget and enter! Will my old Mustang be there or did he sell it?
John Payne

Sorry to say but it got flogged. Dave put the dosh into a Cobra kit car that he has just finished and got the MOT, so perhaps he could be in that beast. :-))
Carol and I were thinking of going in fancy dress this year. Having just got back from "TEgypt" we have a couple of outfits that could be just the ticket. :-)
Robert (Bob) Midget Turbo

Robert (Bob) Midget Turbo, England,

WOW,

Thanks for that heads up on the octan rating differances,(Uk 89 = USA 95) I will bet when I talked this over with Mark T. he was refering to UK octan and not USA, I was not aware that there was a differance in the defination between the UK and USA on what is octan...sounds like 10.5 CR is back on the menu...this is getting easier by the minute.

Thank you SO much for that bit of great news, Robert

Prop
Prop

>>I can't see you having enough cam lift at TDC to hit a piston, most cars rarely go over 3 mm lift at TDC even with a Race cam<<

Just for comedy value (as it doesn't help the debate) with the BMW head I'm timing at 2.5mm lift on overlap and from memory the total lift is something like 8.8mm

Thats when you start to have piston valve clearance issues....
Toby Anscombe

Prop,

Part of the reason for picking the Hepolite 21253 pistons is that the top compression ring is mounted lower than many other pistons. Thus, the crown above the ring is less likely to be crushed by high compression.

Mine is running 0.040 over 21253 with the block decked flush with the top of the pistons. My pistons have an 8 cc dish, the combustion chambers are 21 cc, the head gasket is 0.035 inch which I calculated at 3.8 cc, and I calculated 1.01 cc above the top ring.

With those values I came up with a static CR of 10.69.

It is running a VP7 cam from APT along with offset rocker bushings, and is timed at 102. The play dough indicated no problems with interference.

Charley
C R Huff

As Peter stated interference between valve and pistons on A Series engines is just a non event.

Toby I think you missed the whole point of what Peter stated, He said basically that when the piston is at TDC and the valves have overlap (at this point a collision could theoretically occur) the valves are only open a maximum of 3mm even with a high lift full race cam.

Yours on the other hand is a mere 2.5mm lift at the same point. Not sure how that makes yours more likely to have clearence issues?
Robert (Bob) Midget Turbo

Just what I was going to say when I saw your post Bob....Peter
peter burgess

Prop
"(Uk 89 = USA 95)"
NO! What Bob said was "...your (USA) 89 could be the same as our (UK) 95." Note the 'could'
Regards
Roger

Roger T

This thread was discussed between 30/04/2009 and 02/05/2009

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