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Thread: Compression/spark plugs

Discuss the Austin Healey Sprite and the MG Midget. Two different but similar cars sometimes referred to collectively as the Spridget.

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    Compression/spark plugs

    Need help on a strange question. I rebuilt a late 1275 engine, the 12V----- one with low compression (dished pistons). As part of the rebuild I had to replace the pistons with Moss replacements (20 over). These pistons were not dished so now I think i have the higher compression and a compression test seems to confirm that (165).(This is good)

    So should I use the plugs for the later low compresion 1275 or the ones for the earlier higher compression 1275 engine or does it not a make a difference at all???


    thanks

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    Re: Compression/spark plugs

    I put a post in the racing section on the very subject. Here is a basic table of applications. Select a colder plug and work your way hotter. Less chance of any permanent unrecoverable damage. Fouled plugs are a lot cheaper than holed/damaged pistons from pre-ignition or detonation.

    8.0 - 9.0/1 = NGK BP5ES or equivalent
    9.0 - 10/1 = BP6ES
    10.0 - 11.0/1 = BP7ES
    11.0 - 12.0/1 = BP8ES
    12.0 - 14.0/1 = BP9ES
    > 14.6/1 = B10ES (Don't know if there is a BP10ES)

    The "BP" are projected nose while the "B" is regular nose. The 1275's tend to run a little better with the projected nose plugs. And, you can get away with them even in race application because you don't need domed pistons.

    All said, I would start with a BP8ES and work my way to a BP7ES if needed. Make sure the carbs are set up properly and you know what needle you have. With flat top +20's you are probably in the 10.5 - 11.5 range. You will likely need some more fuel to go with the extra compression. Do you know the cylinder head volume and the amount the pistons were below deck? If so, then you can do a quick calculation to get a static compression ratio.

    HTH,
    Mike Miller

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    Re: Compression/spark plugs

    Wow thanks for the quick reply and excellent info.
    I have BP8ES in there now and they are too cold so I will try a little hotter. i think I am at about 11 according to the compression test.

    (Don't know dec height but i think the carbs and ignition are spot on..)

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    Re: Compression/spark plugs

    Here's the question I have, you say you replaced the piston with a Moss flat top, there is no such piston offered by Moss, what they offer is the County 8.8 to 1 piston, and that would definately be a dish piston. You compression number are definately good/high for a street engine. Are you sure the pistons are flat top and not dished? Also was the block or head decked and if so how much?

    Mike's advice on the heat ranges vs plug number is dead on.
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    Re: Compression/spark plugs

    Well honestly I am only working from memory on the pistons but they were bought like three years ago from Moss when I rebuilt the engine and the compression test sure doesn't support an 8.8 to 1 ratio.
    I didn't do any head work on it that would affect the compression ratio either.

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    Re: Compression/spark plugs

    A quick peek down the spark plug hole will discern whether flat or dished. But, I agree, with 165 PSIG on your gage, there is some definite compression there. Something around 11 static is a good guess. A shade over or a shade under, but something in the 11 range.

    Most 1275 blocks will give you between 0.030 and 0.060 negative deck in stock condition with stock compression height(~1.495")pistons. The 12V's tend to be on the higher/deeper end.

    My back of the envelope calculation using 0.030" deck, resulted in a 12.05/1 figure with flat tops and a BK450 head gasket. By my figures, a compression point is worth about 3cc's. Likewise if you were closer to the 0.060" deck, then your CR would be around 11.01/1. Every 0.010" negative deck is worth about 1.01cc's.

    Most aftermarket dished pistons show up in the 6-10 cc range. AE stuff(2125X series) is standardized at a nominal 8.4 cc's. They adjust compression height for final cylinder compression.

    With that much CR for the street, I would definitely be using 91-93 octane gas.

    Watch your ignition timing too. I wouldn't run more than 32 total. You might have to take some out of the advance on the distributor cam arm. Weld or braze it up and file/grind back down to get about 10-12 on the arm. That way, you can put more initial timing into it for starting ease. You might have to find a distributor machine in your area to confirm the modifications.

    HTH-Mike

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    Re: Compression/spark plugs

    It seems that the 1275 engine was around 9 CR or 8.8 w/standard pistons which should show a reading of about 165 which is what I had, so I think I was mistaken about there being no piston dish.
    The old ones were more noticeably dished though, as apparently that is how BL adjusted (lowered) the CR on 12V engines (according to my extensive web research).

    What I was trying to figure out was whether I now had the higher compression ratio of the earlier 1275 on my 12V engine as a result of using Moss 1275 replacement pistons, as opposed to the stock CR of the 12V, which was like 8, I think.
    If this was so and I think it is, this would, I think, affect how I tune the car and spark plug choice and stuff.

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    Re: Compression/spark plugs

    Mike can chime in here, but I bleive the most common stock dish piston on the later 1275 was 11cc dish, the rpelacment oare a compression ratio boost with the 6cc dish. AE actualy make a taller compression height piston that will net 9.7 to 1 with stock head and block decks, the AE21253, it also has a 6cc dish the slightly taller compression hiegt is the CR boost, but I never knew of Moss offering this piston. The AE21253 is the pston I used in both Drew and Ray's engine, it's budget HP, you get a nice little jump for spending 100 bucks more on a pston set. The combustion chambers in the 1275 heads are the smallest of the bunch so it takes alot less decking to see the CR jump up.
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    Re: Compression/spark plugs

    Took a look at the BLMC Workshop Manual (AKD4021) last night just to see if it had any information that might help. It listed a cranking pressure for the High Compression HC engine (8.8/1) of 120 PSIG at 350 RPM. Also took a look at the compression gage we have in the shop and it gave me the following comparisons:
    8.0/1 = 118 PSIG
    9.0/1 = 132
    10.0/1 = 146
    11.0/1 = 151
    12.0/1 = 175
    13.0/1 = 190

    The manual also listed HC engines as 8.8/1 and Low compression LC engines as 8.0/1.

    If you were seeing 165 on your gage, then you are somewhere in the 11/1 range. My shop landlord's 1275 LPHP engine, built to be just a tick below the 11.0/1 SCCA limit pumped 165/160/170/165 front to rear. These pressures were obtained with the subject gage and probably a bit faster than 350 rpm. But, they are also from an engine using a cam with around 85 degrees of intake to exhaust valve overlap compared to around 26 for a stock cam.

    Now with all this additional information, it still sounds like you have a small dish piston with about 0.030" worth of negative deck and you are in the upper 10's to low 11's on compression with the +20 overbore and whatever the head is giving you. Try the BP7ES plugs with a properly timed and carb'ed engine and see how the plugs and tail pipe color up.

    Early stock pistons have a shade over 11cc's and the later ones have a shade over 15cc's. Just remember that these engines are over 35 years old and like Forrest Gump's box of chocolates, you never know what you will find in them when you open them up.

    The AE21253 has a nominal 8.4cc dish. It is also taller in compression height by 0.020" from stock. The AE21253 were designed to be the Cooper 'S' replacement piston. Like Hap says, they make a great budget piston for SCCA HP racing and will handle rpms past 8K. If you are building a hot street motor, then these are the ticket. Look at the oil control ring oil drain back design and limit the rpm of the engine according to what you find. If you have big slots and side cuts to limit piston slap noise, then hold the rpms down to less than 6K or you might find you now have 2 piece pistons in a cylinder.

    The most commonly listed number for combustion chamber volume on the 1275 is 21.40 cc's. But like anything BLMC, take it with a grain of salt. Depending upon valve margins, casting shifts and seat cuts, they could be anywhere from 21.1 to 22.0, coming from the factory. 35+ years later, you don't know what you will find.

    Really hope all this new information helps with understanding what you might have. Just put 91-93 Octane fuel in it, drive it and enjoy it over the summer.

    Mike Miller

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    Re: Compression/spark plugs

    I wonder if that holds true to other motors. My 10.3 to 1 Hyundai has 250 psi. Well, at least it's supposed to be 10.3 to 1.

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    RonMacPherson
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    Re: Compression/spark plugs

    Don't forget that cam design(overlap and timing) also has a major input on cranking compression.. Hot Rod Magazine had a very informative article a couple decades ago by their resident engineer(he used to work for Edelbrock) on cranking compression pressures...

    Mebbe see if they have it in their archives?

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    Re: Compression/spark plugs

    It might vary +/- 10% to 15% depending upon cam timing and valve efficiency, but 250 PSI (either Absolute-A or Gage-G) is way too much for anything but a really exotic street vehicle. 250PSIA is about 235 PSIG. That is still way too high for a street type engine.

    I would be thinking "TYPO"! 150 sounds much more likely.

    Per the compression gage dial, 250 is right close to 16/1!!!!!!!!

    Diesels yes, Gas no.

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    Re: Compression/spark plugs

    It's most definalty 250 and a street, factory motor. The gauge isn't off either. No wonder why I spank everyone in my class so bad. Runs on 87 with no problem whatsoever too. It's the fastest thing in it's class. I was suprised when I saw the numbers too.

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    Re: Compression/spark plugs

    Does the Hyundai engine have Variable Cam or Valve Timing similar to a Honda VTEC or Mazda "Miller-Cycle" or others? If so, then that may explain why it could produce such a high cranking pressure and still run on 87. For starting purposes, it may drop the overlap and openings at TDC back to almost nothing and start adjusting once oil pressure comes up. Don't know the engine design, so speculating a bit here. It could be something else entirely as well.

    I was thinking just plain fixed timing on the cam(s). Hyundai and Kia do use other manufacturers engine designs.

    As RonMacpherson above comments, cam design and valve train design can have significant effect on cranking pressure. I hunted for a number of hours one day to try and find a formula/equation that could be used to predict cranking pressures from bore and stroke and un-swept volume, but didn't find anything. I'll have to search for the HOT ROD article

    Thanks for the new information. It would be interesting to know the specifics that generate those kinds of cranking pressures from an advertised 10.3/1 engine.

    Thanks,
    Mike

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