Raising the compression.

[regis101]

All things considered, if a stock 1600 bus engine has 7.5 CR, what would it be if the P&C's were changed to 90.5's. Nothing else is to change. No spacers and the deck height stays the same as the 1600. Thanks

[JC-ATL]

Should come out to about 8.2 if nothing else changes, but that's highly unlikely. Going to 90.5, you're using different pistons and cylinders. Variations in pin height and cylinder length alone will vary this. Measure.

[Guest]

You will need to have the case machined to fit the 90.5's - they will probably deck the case at the same time which will effect the deck height as well.

[Marc]

You will need to have the case machined to fit the 90.5's - they will probably deck the case at the same time which will effect the deck height as well.

Good point. Most cases won't have much of a barrel seating surface left when cut for late 90.5 (or larger) cylinders and they'll probably need to deck it a considerable amount so it'll need shims just to get back to the minimum safe deck height (which is .050" for a street engine IMO, you definitely want at least .040"). Also, flycutting the heads for the larger cylinders will result in some loss of volume, from negligible to maybe 2cc.
A 7.5:1 1600 with .060" deck would have 52.2cc heads; figuring .050" deck and 51cc heads a 1775 would end up with 8.5:1. Compared to that scenario you'd need to come up with an additional 9.1cc to hold the CR at 7.5:1.

[Guest]

So, just so I understand. When they bore the ( new ) case for the 90.5's, they will also need to machine a tad bit more on the case so to create a nice flat sealing surface for the barrels to rest upon? And this in turn will affect the deck height because the area they cleaned up for the new sealing surface may or probably will be below the original case surface. And this finally means that the sinking of the barrel into the case, a tad bit, will tighten the deck height. Meaning that spacers are needed to bring the deck height up to a respectable .050". Now, once the deck height is restored, I will have to play with head cc's to get back to at least 7.5 CR or better. Using the numbers from above will be of great value. Thanks

[MASSIVE TYPE IV]

Try to run tight deck height, and keep the CR where the cam dictates by modifying the combustion chambers...Thats a big key to power and efficiency.

[regis101]

Good point. Thanks for the tip

[Marc]

So, just so I understand. When they bore the ( new ) case for the 90.5's, they will also need to machine a tad bit more on the case so to create a nice flat sealing surface for the barrels to rest upon? And this in turn will affect the deck height because the area they cleaned up for the new sealing surface may or probably will be below the original case surface. And this finally means that the sinking of the barrel into the case, a tad bit, will tighten the deck height. Meaning that spacers are needed...

You've got it. It isn't always necessary to remove all of the raised lip around the hole in the case in order to establish an adequate seating surface for the 92/late90.5 bore (it's mandatory for 94s)...depends upon the casting and how wide that lip is.

[RoWaMe]

When they bore the ( new ) case for the 90.5's, they will also need to machine a tad bit more on the case so to create a nice flat sealing surface for the barrels to rest upon?

Guest- Make sure you inform the machine shop that'll do the work in writing that they must machine the seating surface on the 90.5's (or any large bore). Then call them when they have received your case to remind them, and get the guy's name who you talk to. I've had a few cases machined for 90.5's by anotherwise reputable company whereby they did not machine the surface despite my written instructions... For me in NY it's a hassle to ship a case across to CA to have that "forgotten" operation done...

[regis101]

It is a crying shame that we have to do their friggin' jobs also. But I will mention this to them. Thanks