1968 compression height for 383, anyone know

[Fast One]

I was just needing to know the piston compression height for 1968 , 383 cast pistons / factory stock, regular pump gas engine, thanks heaps.

[ehostler]

All that I know is that the factory piston in the '68 383 had a positive deck height. You can't put a closed chameber head on this engine without replacing the pistons or using a very thick head gasket or a spacer.

[gotcha]

The 68-69 383 4bbl. is the only 383 to have been built with positive deck height from the factory. The blueprinting manuals list it as +.020 to +.021. Hope this helps. Remember that alot of machine work can happen to a block in 35 years, unless it is totally original.

[Fast One]

Ok thanks

[Doug Wilson]

God!!!! This is a GREAT website!

[ehostler]

Gotcha - Thanks for posting the actual specs. I've been trying to find that information for a few months now.

[Fast One]

So just to test my luck further, any ideas on the compression height from the center of the pin to the top of the piston crown?

I'd say it would be over 1.80's surely??

[ehostler]

Gotcha - I have another question for ya. Would you know how much of an over bore a 383 would need, to be able to run 2.14/1.81 valves?

[Tarrbabe]

My book says 1.93"

[Tarrbabe]

My engine book says, "These two oversize valves will fit in the B-RB engines with cylinder bores of 4.32" (stock 440) or larger without a bore notch."
That comes to .070"

[gotcha]

Ed, from the information I have, the 4.320 number is dead-on. A slight misconception is that the Max Wedge heads on a 426 (4.250) bore were 2.14-1.88. They were in fact 2.08-1.88. As you know, Mopar discovered later that the 1.81 was a better exhaust valve combo.

[gotcha]

The compression height on the 68-69 383 is 1.934 as stated by Tarrbabe.

[ehostler]

Maybe it's time for me to buy a new MOPAR engines book. The one that I have (late 80s) just says that the '68 383 had a positive deck height and that if you want to use 2.14/181 valve that you need notches in the bore, or have it over bored. It doesn't have the specific numbers.

[Fast One]

Thanks again guys, 1.934 comp height, sheesh! they must be a heavy piston.

I'll have to get my 383 block sonic tested & see how much overbore I can get, bigger valves would be nice, being a 60's vintage block I'd say the bores will have a reasonable amount of metal thickness.

[gotcha]

Fast One...the overbore should not be a problem on the 383 and some have gone as far .090, but Sonic testing is the answer. The problem becomes having to find a piston-ring combo for odd sizes.
Custom made pistons are relatively expensive for one-off deals. You can, as Ed mentioned.....notch the block for valve clearance. Another choice would be a 400 block with either stock crank, 383 crank, or cut down 440 crank.

[Fast One]

I have a steel 440 crank to put in but it's a pretty good one, standard journal size & very clean so I'm a bit reluctant to grind it down for the swap.

The other idea I considered was to use the 383 steel crank that's already in it & offset grind it using the Manley PN14279 con rods 6.765 length with the B/B Chevy (2.20in) journal size & pistons to suit, with 0.060 overbore I think comes to about 405 ci

400 blocks are hard to come by down this end of the planet.

[gotcha]

I have done several 440 cranks on the 2.20 rod journal and love this combo in the 400 block. (474) 3.90 stroke. There will probably be some who disagree, but another advantage to the 2.20 rod journal is less swept area for oil, meaning better coverage, but less parasitic drag. I have never done it on a 383 crank but it could be interesting. You would pickup about .150 in stroke. About a 3.53 stroke and a bore of 4.310. That works out to about 412 cubic inches depending on actual final stroke. Quite a bit of work and expense for 412 CID. A 360 with a $300.00 4 inch stroke crank will give you 408 with a stock rod. Build it the way YOU want it, 'cause it is your car.

[Fast One]

Yes the 408 is a very tempting idea but I'd like H-Beam rods with the rpm I'd be gunning for, plus the pistons to suit etc, so it doesn't work out that cheap in the end, then there's the heads.

From my past experiences the costs always seem to go over budget because I get greedy for power (?)

This is what I thought about the 2 ideas, the 383 block is a fairly sturdy piece of Mopar engineering & will take gobs more stress than the 360 block in factory form, depending on the HP level I end up with.

Even though there is a much debate on rod / stroke ratio's the 360 S/B stroker has a 1.53 ratio & the 383 stroker with the 6.765 length rods will have a healthy 1.91 ratio, so at this moment I'm leaning more towards the 383 stroker

[gotcha]

Fast One....the rod ratio is a factor, but the opinions are another story. The 383-400 blocks are without a doubt the strongest. Deep skirted, low deck height, and lots of headbolts.

I am one of the most dedicated smallblock guys, but have moderated over the last 10 years. For all my high horsepower applications, I have switched to the low deck big blocks.

Remember with that stroke and that rod length, you will may have some piston design flaws. The pin will be very near the bottom ring. Buttons may be required, and the piston will be very short. However, rotating weight will be much lighter than stock. BTW, gather up any 400's you find along the way.

Just for info purposes, Reher-Morrison did extensive dyno testing recently on their pro-stock GM engines with exactly 500 cubic inches in 5 different rod ratios. There was less than 2 hp difference in any of them.

Do your engine the way you want it.

[Doug Wilson]

Curious to know if there were any significant torque or torque curve differences??

tHANK YOU.

[gotcha]

Doug, I will try to get that exact info for you. I believe I can get you a reprint of the test. As I recall, they stated there was no measureable benefit to the long rod theory. The test was done to dispell rumors to the contrary. I will try to find the info for you. I was kinda shocked to see the results.

[Fast One]

Trying to have your cake & eat it so to speak with an engine combination isn't easy, yes I'm very much aware of the debate around rod / stroke ratio's & with the ring being close to the pin.

The Chevys have some aweful rod / stroke ratio's & unfortunately do go well making good power, but in a way I did consider the Mopar concept on engineering the better, Chrysler engineering believed anything more than 17 degree rod angle was bad for a high performance engine, well back in the earlier days anyway, the rod angle for a 340 example is 16.689 degrees & we all know how well & reliable the 340 has been over the years.

I wasn't really thinking of HP benefits with the rod / stroke ratio but do believe it has some effect on the torque curve, I too have some info somewhere on a Chevy using different rods, just another way of moving torque around with certain combinations where you want it, the way I see it anyway, at the end of the day I can't see using longer rods hurting anything, if it means less wear on the bores or strain on certain parts it could only be a good thing.

We all have different reasons for & against when building a certain combination but if it works for you & it means a few wins, then you feel reasonably happy & if it's a Mopar that's even better!

[Tarrbabe]

I am sure that with some engine combinations that rod/stroke ratio may not make big power differences. But on some engines, and Mother thinks it does on their engines, it must make a difference. To my way of thinking, for a pure race engine, it may not make that much difference. BUT if you are talking an engine that is expected to hold up over a period of time, (more that just a few trips down the track) then the longer rods have some advantages.
1. - less angle on the cyl. walls and less wear.
2. - more stability on the piston, thus less rocking and greater and longer ring seal.
These may seem very small to some of you and to you I just say, I am glad your wallets are bigger than mine.
To the others, I think you understand why I like to build something that last longer than a few races.
I'm not sure about the numbers you posted, but the small blocks had a rod ratio of 1.84 and that is very good for a stock engine. The 360 has a rod ratio of 1.71 and that is good too.

Just thinking out loud again.

[Fast One]

Nothing wrong with thinking aloud, you're entitled to like anyone else, yes sometimes it can be a bit odd how things work for one engine & not another, take 1.5 & 1.6 rocker ratio's for example, one person will say the swap from 1.5 to 1.6 was great for his or her engine, someone else will do the same & say it had no noticable effect, which could be very true.

Just one passing thought on the longer rods, if youre playing with longer strokes etc & your pistons are starting to look like Hockey Pucks I'd think the longer rods are a good idea also.

[Doug Wilson]

There was a rather lengthy discussion on this forum about rod lengths, several months ago, and my recollection is that the result was inconclusive.

I'm also curious about the real-world effects of rod length, compared to theoretical math.