Author Topic: Big bore and compression ratio question (Read 2963 times)

eurban · « **on:** October 20, 2007, 07:05:15 PM »

I am wondering if the Wiseco 836 kit that I have installed in my 78 750K actually yielded a compression ratio of 10.25:1 (yes it is a 10.25:1 kit). The reason that I am wondering is that the 75/76 F engines and the 77/78K engines had stock higher CR ratios (9.2-1) than the other K (or F2F3s for that matter) engines. It would seem that a change in the stock piston design for these engines was the biggest contributor for the increased compression. However, the heads for these motors were of a new casting and I am wondering if perhaps its design also contributed to a higher CR? If so, then a big bore kit installed in these engines would yield a higher ratio than in an earlier K. Has anyone measured actual CRs of the Wiseco kit in a 77/78K or 75/76F engine? Does anyone know for certain that the head change did not contribute to the bump in CR? Thanks.

paulages · « **Reply #1 on:** October 20, 2007, 07:40:24 PM »

bolt it up and measure it! i'd be curious to see how they measure up to the claimed CR.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #2 on:** October 20, 2007, 10:08:26 PM »

Only way I can tell is to cc the volume of the combustion chambers of both heads. The 75/76F head and the 77/78K head (-392) are hemi design and might have a smaller volume. If so the head contributes partially if not fully to a compression increase. HOWEVER my 75F piston crowns and my 78K piston crowns are shaped differently but both engines have the same CR rating with the same head. PLUS it seems I have seen the same pistons as my 75F in earlier K engines. Perhaps just different piston manufacturers with different crowns but same CR and the hemi head makes the difference. The only bigger mystery with 750 engines is the cams.

My question about the big bore 10.25 CR is: Is the increased CR due only to a larger volume of air (caused by larger piston holes) compressing into the same size combustion chamber in the head and not a higher crown profile??

eurban · « **Reply #3 on:** October 21, 2007, 05:51:36 AM »

Quote from: paulages on October 20, 2007, 07:40:24 PM

bolt it up and measure it! i'd be curious to see how they measure up to the claimed CR.

Can't easily; the motor's been in service for a couple of years now. . . . ..

paulages · « **Reply #4 on:** October 21, 2007, 10:51:36 AM »

yeah, i guess that makes it a bit harder... it would take quite a bit of maneuvering to get those plug holes vertical, and without being able to grease the top ring it would be hard to keep your measuring fluid from draining past the rings. i'm sure you could still do it though--maybe start with an exact measurement of heavy oil to seal the ring, then proceed with your measurement.

eurban · « **Reply #5 on:** October 22, 2007, 06:23:58 AM »

Anyone have pics of the -392 head's combustion chamber vs the earlier heads? How about stock 75F/76F or 77/78K pistons vs earlier?

MRieck · « **Reply #6 on:** October 22, 2007, 07:58:01 AM »

Quote from: eurban on October 22, 2007, 06:23:58 AM

Anyone have pics of the -392 head's combustion chamber vs the earlier heads? How about stock 75F/76F or 77/78K pistons vs earlier?

77/78 chambers are more of a hemi design with less material present in the chamber.....they are definitely different from mid K's/F/F1 and a better chamber IMO. The F/F1 pistons have about .040 higher dome than K pistons. I can take pics but I'd have to send them to you as I don't have a pic reduction service.

mlinder · « **Reply #7 on:** October 22, 2007, 08:48:29 AM »

MRieck:

irfanview will let you resize pictures, and tocuh up contrast/lighting etc.
It's free, http://www.irfanview.com/main_download_engl.htm

Hope this helps.

MRieck · « **Reply #8 on:** October 22, 2007, 09:02:15 AM »

Quote from: mlinder on October 22, 2007, 08:48:29 AM

MRieck:

irfanview will let you resize pictures, and tocuh up contrast/lighting etc.
It's free, http://www.irfanview.com/main_download_engl.htm

Hope this helps.

Thanks. I'll install it.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #9 on:** October 22, 2007, 10:49:57 AM »

Mike,

The 75/75 Fs use the -392 head like the 77/78 Ks. Is there a difference in the chambers?

Bummer, looks like the Rox and the Sox are going to Denver for the game but not us

I've tried to get tix for 2 hours.

KB02 · « **Reply #10 on:** October 22, 2007, 11:09:28 AM »

Doesn't the higher dome lead to a decrease in CR, rather than an increase?

Jerry Rxman Griffin aka MuthaF'er · « **Reply #11 on:** October 22, 2007, 03:36:08 PM »

Here's some pictures (I hope).
ps thanks for asking for the pictures. I'd been too lazy to learn how to use my new blasting cabinet

Both pistons are -392. The one on the left is from my 75 750F. It has 392 and 1 by the pin hole on the skirt while the other is a K8 and it has 392 and 6 on it. The total heights appear to be the same.
http://i233.photobucket.com/albums/ee84/RxmanGriff/DSC01503.jpg
http://i233.photobucket.com/albums/ee84/RxmanGriff/DSC01494.jpg
http://i233.photobucket.com/albums/ee84/RxmanGriff/DSC01496.jpg

MRieck · « **Reply #12 on:** October 22, 2007, 05:10:26 PM »

Quote from: RxmanGriff on October 22, 2007, 10:49:57 AM

Mike,

The 75/75 Fs use the -392 head like the 77/78 Ks. Is there a difference in the chambers?

Bummer, looks like the Rox and the Sox are going to Denver for the game but not us I've tried to get tix for 2 hours.

Jerry....THANKS for trying. My wife OK'd it too. I don't know what it would have been like to get a flight but.......Hell....you only live once (as far as I know). As for the heads.....yes....they are different.

eurban · « **Reply #13 on:** October 22, 2007, 06:57:39 PM »

Thanks RX for the pics. In case you don't know, instead of pasting the link from Photobucket you can actually copy and drag the image code and then paste it into your post. That way the pic will show in your post . . .Don't have an earlier year piston to add to the bunch do you? What year is the head in the first pic? Seems like the pistons with their increased dome height would be a major culprit in the CR bump. Still don't know if there is a head contribution though. KB02, think of it this way, (with everything else being equal) when the a cylinder's valves are closed, the piston's higher dome is now pushing farther up into the combustion chamber compressing the mixture into a smaller place . . . higher compression . . .I would guess this would also lead to slight loss in displacement??

Jerry Rxman Griffin aka MuthaF'er · « **Reply #14 on:** October 22, 2007, 07:33:00 PM »

MIKE !!

The Rockies computer crashed with only 500 tix sold. Back online at noon Tuesday!! I'm outta town working but I'll find a way to get online. What day would be best for you? All 3?! Don't count on Monday! It's going to be 11 playoff wins in a row and 25 out of 26

. I'm trying again. You're taking that head back with you (and your hangover) and I get a discount!

Eurban,

The photobucket is new to me. I was successful with it the first 2 times? I'm wondering if the pix size was too large? I shot it at 4mp. I tried copying and pasting all 3 codes? Digital photog is foreign to me

No older pistons but I do have NOS 900 & 1080 kits just waiting to be used. The head is a 78 K. The total height of both pistons looks the same to me with the most apparent difference being the full dome on the 75 F.

Go Rockies!

paulages · « **Reply #15 on:** October 22, 2007, 07:39:22 PM »

Quote from: RxmanGriff on October 22, 2007, 07:33:00 PM

The photobucket is new to me. I was successful with it the first 2 times? I'm wondering if the pix size was too large? I shot it at 4mp. I tried copying and pasting all 3 codes? Digital photog is foreign to me

use the bottom of the three codes for a board like this. (the one that starts with [IMG]...)

Jerry Rxman Griffin aka MuthaF'er · « **Reply #16 on:** October 22, 2007, 08:03:04 PM »

Paulages/Eurban,

Does it matter what size pix you load to Photobucket? My previous 2 pix of the orange tank and the snow worked fine. I'll have to work on my technique. At least it's MUCH easier than trying to find a program to downsize a pix to use directly on this site! Now that was a nightmare

eurban · « **Reply #17 on:** October 23, 2007, 04:35:03 AM »

I think there might be a max image size that photobucket will accept but I my impression (could be wrong) is that Bucket sizes the pic down if it is too large. If you downsize the pictures small enough to directly fit this sight, nobody will really be able to appreciate what you are showing them! Thanks again for the pics and insights.

KB02 · « **Reply #18 on:** October 23, 2007, 05:52:44 AM »

Quote from: eurban on October 22, 2007, 06:57:39 PM

...KB02, think of it this way, (with everything else being equal) when the a cylinder's valves are closed, the piston's higher dome is now pushing farther up into the combustion chamber compressing the mixture into a smaller place . . . higher compression . . .I would guess this would also lead to slight loss in displacement??

Okay. I heard "Dome" and I was thinking of the top of the combustion chamber rather than the top of the piston. One of those stupid moment, like when I asked a friend one time what the name of the rod that connects the piston to crankshaft was called. He just said, "You mean the connecting rod?"
"Yes."
"Now don't you feel stupid?"

News:

Author Topic: Big bore and compression ratio question (Read 2963 times)