Compression ratio calculation

[Jakes66snake]

Oh yeah, Option 4, from a while back. I suddenly remembered this one, which is almost the same as the Rover 4600 piston balldrick uses in his supercharged XKNO1 1961 Falcon. Since the Buick V6 and Rover engines from the same GM family. Its the N code 89-93 GM 3300 piston, the little brother to the GM 3800 found in all the front drive Buicks and Oldsmobiles

See viewtopic.php?f=3&t=67792, Crossflow tips!!

Yeah I am guessing its a mistake on their part too, but new plan that wouldnt involve expensive sleeving. These pistons http://www.summitracing.com/parts/UEM-1743H-5MM/ have a compression height of 1.310 in and a bore of 3.720 only requiring a .040 overbore which is very conservative. If you used these pistons with 300 rods which have the same big end dia as the small six engines the piston would only poke out -.0057 inches before machining. Which is pretty darn close to zero decked. the piston pin hole would have to be opened up by .007 inches so very minor, and with a 62cc head 9.5:1 compression ratios are a possibility because these pistons have a 12.67 cc dish!

This piston allows the stock shallow deck GM 3800 piston with 12.67 cc fit a 6.21" Ford 300 rod if its wrist pin is bronze bushed to suit

This looks like an affordable way of correcting piston height but are there any negitive effects of using a longer rod on an already long rod engine? Does it change the peak hp at a given rpm level? Oh I assume we are still talking about a US 250 right?

Also this combo doesn't help if you are looking to run a CI 50cc head tho if the stated numbers are correct at 9.5-1 at 62cc....right?

 

[xctasy]

It's specs relate to the little 3300 GM 3300 only, so on a 250,it would be very good. Longer rods create a power increase at maximum pison speed of half the elgeraic difference. So if your rod ratio before was 1.5:1 with a 3.91" stroke and 5.88" rod and you go to a 3.91" stroke with a 6.21" rod for 1.588:1, then you can expect 1.588/1.50= 5.9% improvement, divided by two, or 2.9% at best.

There is are other Options, that get shallower, like the http://www.summitracing.com/parts/UEM-3212C-75MM/, the 3.3 ohv Voyager piston replacement. It has a 90 thou trough, about 8 cc's, or more of a trough than the stock piston, but it needs a 6.21" 300 or 6.24" Nissan 240sx/Z24/KE 24 rod

The modular 4.6 5.993" and Nissan NAPZ/z24i/240 SX 6.24" rods are 866 thou at the wrist pin and can take a 900 thou piston pin.

All a bit of a head bender, isn't it?

 

[Jakes66snake]

All a bit of a head bender, isn't it?

To much! Like the OG poster I'm looking to use a CI alum head on a 250. I'm looking to get "mild 302" hp levels. Only real stipulation is I'm limited to 92 gas. It would be nice to use the most of the parts I already have but willing to get what I need to optimize power output.

 

[xctasy]

I think I've given you the do everything options, but in practice, any persepctive Classic Inlines 250 will be an amicable compromise. As long as the head doesn't have a stock 252 or 256 cam which could cause severe detonation, and as long as you use a 264 or more Clay Smith cam, you shouldn't have any problems.


The problem is Ford US wanted to use stock 200 pistons, cylinder head and gasket

In order to do that, Ford's Cleveland engine plant lads added just 89 thou to the deck height of the 250 engine, and then used a shorter than 5.88" conrod.

The L code in Austrailia and even the last US 250 based 1986-1990 HSC 2500 4 cyl used 9.38 and taller con rods, while the US 250 itself used 9.469" and shorter conrods.

So to fix it, you have to use either
a longer conrod
and/or deck the block
and/or use a 12.5 cc piston to get it back to a good combination.

If you do a dollars verses result calculation, then you'll just end up massaging what you can afford untill you've got an improvment on the stock combination.

I know that the CI head has world class mixture motion, intake manifolding, thermodynamics and flow rates that will look after detonation if you use a nice big cam on it. I doubt you'd ever over cam a big port Classic Inlines head even on a 250 six with a stock 1650 stall converter and a 278 cam. Even that combo leaves you free to get a more than a 60 hp flywheel boost on something that really only had 100 to 110 net horsepower at the flywheel. 160 to 170 rear net flywheel horsepower is in easy reach even with stock exhast system.

If the replacement pistons are slightly better, the deck is slightly better and you've got more compression and a higher duration cam, the worst you can do is get a 60 odd hp boost with the right carb and exhast, and still be able to run it on whatever pump fuel you come accross.

I'd say Option A, do nothing and just live with it seams a pretty darned good option. Option B, do what Davis S says in the Falcon Six performance book. Option C, look at my suggestions.

Option a and b won't send you broke, and there's little chance you'll mess up on any 250 with a Classic Inlines head....