Limited BB (427) piston choices: Opinions?

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The Artist formerly known as Turbo84
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Looking at the TRW/SpeedPro charts for standard bore pistons for my next engine, it looks like my choices end up resulting in either a 9.07 or a 10.83 compression ratio. I would have preferred something in the middle. I could run any cheap crap fuel with the 9.07 pistons, but I'm wondering with the aluminum engine (block and heads), the 10.83 CR might be somewhat knock insensitive. Opinions? What are some of you BB guys running for compression these days? The combo I'm looking at is: 110cc chambers, 10.5cc head gasket volume, 1.765 piston compression height (resulting in .020 inch below deck level?), 36.3cc dome in one set of pistons, 16.8cc in the other, 6.135" rods, 3.76" crank.
I welcome any checks on my math here, and thoughts on the piston choice.

Mike
 
There sure isn't much choice there for standard bore. Looks like if you go to 4.280 you will have more options.
 
There sure isn't much choice there for standard bore. Looks like if you go to 4.280 you will have more options.

I hadn't looked at any 30 over pistons so far. It's a new block, so I hate to take out some of the wall thickness if it's not necessary. But then again, I suppose the up side would be a net weight reduction of about ten cubic inches of cast iron. LOL
I'll take a peek and see what I find out.

Thanks,
Mike
 
I've got 118cc heads and the dome on the pistons is big enough I get 12.25 for compression. No idea what the dome volume is but they look like this:
( BTW, the first picture is #1 cylinder AT TDC!!
23484f2b7bd1c23.jpg

23484f2b7c4199b.jpg
 
As usual, I got to thinking. A 427 is 4.25 bore and 3.76 stroke. A 454 is a 4.25 bore and 4.00 stroke. The crank is the difference. It may be easier to find pistons by looking for 454's? :smash:
 
As usual, I got to thinking. A 427 is 4.25 bore and 3.76 stroke. A 454 is a 4.25 bore and 4.00 stroke. The crank is the difference. It may be easier to find pistons by looking for 454's? :smash:

I've already got the (427) crank and neutral balanced flywheel/balancer, so I hate to spend the money replacing good parts. Plus, being a '69, I just like to say it's a 427. And, to be honest, on track days I rarely got outpulled down the straights with the present motor setup. My only complaint (with my C3) has been corner exit speed, requiring me to have more hp (down the straights) just to compensate, versus the C4, C5 guys who can get through and out of the corner quicker than me. Assuming things go well, and I'm able to get the car back together this year, I'll have taken an additional 270 pounds off the car (160 off the block, 110 off the suspension).
Tim, just for clarification, do I understand correctly that you presently have a 12.25 CR? Are you having any fuel/knock issues there?

Thanks,
Mike
 
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You are correct-- I'm running 12.25 compression. No problems with it pinging, but that's due to using VP 110 fuel. Not cheap, but it smells nice. I have mixed it 2:1 with 91 pump gas and had no problems..

If I had an original block I'd go with the 427 too. I was thinking that if you're looking at a catalog there may be a larger listing for 454 pistons than 427's, and the bore diameter is the same. I don't know if the pin height or rod length is the same..
 
Have you checked with venolia? they have anything ranging from flat tops to dishes and domes, std - 4.440 bore.
 
Have you checked with venolia? they have anything ranging from flat tops to dishes and domes, std - 4.440 bore.

No, I haven't. I'll check their site. My only personal experience in piston choice (paying the money and sticking them in the holes) has been with TRW/SpeedPro pistons, so I usually gravitate towards them as I've had good luck with them. I'm familiar with Venolia products in racing engines, but didn't know what their reputation was with (longer mileage) street engines. I'll see what I find out.

Thanks.
 
Hi

If the theoretical compression is 11.5, you probably never will see this in real life because all calculated compressions are on the very high margin.
You can easily deduct .5 from this calculated compression and say that's the real one.
Therefore , with the 427 standart 11,5 pistons, this will hardly be 11.0 and with alu heads and the ignitions not too much advanced, will not knock.

Günther
 
Hi

If the theoretical compression is 11.5, you probably never will see this in real life because all calculated compressions are on the very high margin. You lost me here. I've never seen any measurements that indicate what the manufacturing tolerances are. Without actually measuring each part, why would I not think that the tolerance stackup might go the other direction, and end up a half point the other direction (higher)?You can easily deduct .5 from this calculated compression and say that's the real one.
Therefore , with the 427 standart 11,5 pistons, this will hardly be 11.0 and with alu heads and the ignitions not too much advanced, will not knock.

Günther

I'm pretty much in agreement with your opinion on the knock issue, given my calculations of a 10.8 CR and running aluminum heads and block. I just wanted to make sure there weren't any real world examples of bigblocks out there with problems running over 10:1 ratios these days.

ps: I've been to Luxembourg a few times on business at my former employer's technical center there. Always enjoyed my stay there.
A business colleague gave me a tour of the area, including the American Military Cemetery there. I'm very grateful for how well your countrymen take care of that place. It was clean, quiet, and dignified. Thank you.
 
Hi

If the theoretical compression is 11.5, you probably never will see this in real life because all calculated compressions are on the very high margin. You lost me here. I've never seen any measurements that indicate what the manufacturing tolerances are. Without actually measuring each part, why would I not think that the tolerance stackup might go the other direction, and end up a half point the other direction (higher)?You can easily deduct .5 from this calculated compression and say that's the real one.
Therefore , with the 427 standart 11,5 pistons, this will hardly be 11.0 and with alu heads and the ignitions not too much advanced, will not knock.

Günther

I'm pretty much in agreement with your opinion on the knock issue, given my calculations of a 10.8 CR and running aluminum heads and block. I just wanted to make sure there weren't any real world examples of bigblocks out there with problems running over 10:1 ratios these days.

ps: I've been to Luxembourg a few times on business at my former employer's technical center there. Always enjoyed my stay there.
A business colleague gave me a tour of the area, including the American Military Cemetery there. I'm very grateful for how well your countrymen take care of that place. It was clean, quiet, and dignified. Thank you.


I have 10:7 on my rebuild with the stock cast iron heads. Timing 34 all in at 3000 with a 12 vacuum can in 3rd/4th gear. Running 93, no problems.
 
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You could make up the diff in pin height with a longer rod.:mime:

A longer rod might seem one solution, but unfortunately the next common BB rod length is 6.385" rather than the 6.255" you'd need. And, even if you got them you'd remain at the mercy of off the shelf selection, albeit a different one. Not that your idea isn't theoretically sound, but a custom set of pistons (4032 forged alloy) is likely a more practical solution in lieu of finding a "stock" slug that works.
:beer:


BTW, isn't the octane in Europe considerably higher than we can get at the pump?
 
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I have 10:7 on my rebuild with the stock cast iron heads. Timing 34 all in at 3000 with a 12 vacuum can in 3rd/4th gear. Running 93, no problems.[/QUOTE]

Thanks. I appreciate the info. Looks like I'm probably going to pull the trigger on the 10.8 parts. It appears that a few guys with big blocks are doing okay with todays fuels with these compression ratio levels. Ideally I'd like to cruise around town on 89 octane, and only have to run the pricier 93 or 94 octane stuff on track days.

Thanks again,
Mike
 
FWIW--

I put the 12.25 pistons back in the engine last winter. The ones I took out were around 10.5. No problems on pump gas (91 octane) with aluminim heads and 36* @3000..

Still the same timing and everything, just the 12.25 pistons and VP110..

:bounce:
 
Hi

BTW, isn't the octane in Europe considerably higher than we can get at the pump?

Yes, the numbers are higher, but that is because we use different Oktan measurements:

For example your MOZ 93 is sold in Europe as ROZ 98
We also have ROZ 95 , in USA this would be MOZ 90
New fuel in Europe now is sold as ROZ 100 , in USA this would be around MOZ 95.

I drive my L71 with a theorethical compression of 11.5:1 with ROZ 98 and use blumb additive once in a while.

I'm very grateful for how well your countrymen take care of that place. It was clean, quiet, and dignified. Thank you.

Thank you for taking all this pane to free us from the Nazis over 60 years ago.

Rgds. Günther
 
NO!

The octane here is displayed as RON, which is RESEARCH OCTANE NUMBER, the US Uses PON which is PUMP OCTANE NUMBER. It's (RON + MON)/2 MON is MOTOR OCTANE NUMBER. Generally the MON will be a good amount of points lower usually around 7 for the normal pump ratings we all use.

MOZ, ROZ = German, Z= Zahl (number)

Here's a chart, the numbers can be off by as much as 2 points though depending on circumstances

[tr][td]RON[/td][td]MON[/td][td]PON[/td][/tr][tr][td]90[/td][td]83[/td][td]86.6[/td][/tr][tr][td]92[/td][td]85[/td][td]88.5[/td][/tr][tr][td]95[/td][td]87[/td][td]91[/td][/tr][tr][td]96[/td][td]88[/td][td]92[/td][/tr][tr][td]98[/td][td]90[/td][td]94[/td][/tr][tr][td]100[/td][td]91.5[/td][td]95.8[/td][/tr][tr][td]105[/td][td]95[/td][td]100[/td] [/tr][tr][td]110[/td][td]99[/td][td]104.5[/td] [/tr]


Here's the difference between testing:

[tr][td]-[/td][td]Motor Octane Test (MON)[/td][td]Research Octane Test (RON)[/td][/tr][tr][td]Inlet air temperature[/td][td]148.9C[/td][td]65.6 C[/td][/tr][tr][td]Engine jacket temp[/td][td]100 C[/td][td]100 C[/td][/tr][tr][td]Engine RPM[/td][td]900[/td][td]600[/td][/tr]
 
Not all gas with the same PON is created equally. Mr. Vizard published a chart in one of his Horsepower books which told what brands held up better in the real world under the hood, and for which MON is the better indicator. Not sure I'm comfortable with posting the info and having the offending brands come after me...
 
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