I am trying to establish a static comp ratio for my 5k. I am getting 11:1 as an outcome and I'd just like to confirm this

 

Cylinder volume

Bore 81.7 & stroke 73 = 382.7cc

Add comp chamber 30cc

Head gasket 3.9cc

Piston dish 4cc

Deck height is 0

Total is 420.6cc

 

Divided into comp chamber, head gasket & piston dish.

Total is 37.9

 

420.6/37.9 = 11.09

Edited August 1, 201511 yr by GJM85

This one is good...

 

http://www.summitracing.com/expertadviceandnews/calcsandtools/compression-calculator

Well that's telling me I'm correct.

The current head has comp chamber of 35cc so I'm currently at 9.9:1

 

When I put the new head on I'm going to elevenseez.

 

Oh dear

 

 

You might be 2cc light on that piston dish, I thought they were 6cc.

 

..and the head gasket is also around 7cc depending on how thin you reckon it gets. I measured the last one I took off as 1.5mm thick.

 

That would make the TDC volume 45 going into 428, a lot lower at 9.5 .... I thought 30-31cc was about normal for a combustion chamber, and I know when you put the 5K pistons in a 4K you don't get any increase in compression even tho' they're bigger. The increase in compression from that is taken out by the dish.

I measured 2 4k heads here and both had 35cc comp chambers. The 5k head I just got back from the shop is 30cc.

I have a used set of factory aisin pistons from a 5k and came in at 4cc. The Nason Automotive pistons currently in the engine could be larger.

The gasket I measured is an unused Fuelmiser item and is 1mm at the ring. I deducted .25mm for compression.

When you put oversize pistons in an engine compression naturally increases.

 

But you are right. It will likely be lower. I haven't yet had the chance to measure my pistons or a used gasket.

I'm hoping to come in around 10.

I'm hoping to come in around 10.

It should live happily on 98 fuel at 11:1. Small(ish) cam and stock ports would make for an F'tonne of torque.

The Nason Automotive pistons currently in the engine could be larger.

 

I don't know which brand Richo bought for the 345K, but I'm sure we got 6cc. Measure it up when you've got it all, it will be interesting to see how it works out. Someone used 80mm flat-top VW pistons apparently, but I've never managed to find them.

 

Anyway, if you're skimming from 35 to 30 it will make a tremendous difference. He went from 4K to 5K bore, dropped some compression from the dish, then skimmed 20thou off the head. It was chalk and cheese from before...

Edited July 31, 201511 yr by altezzaclub

Today I set about swapping over to the new head.

I cleaned the piston tops in the engine and did my best to cc them. With 5mm in a ceringe and a small fish tank lid I managed to fill the piston top with fair amount of leakage so I'm calling it a 4.5mm dish.

 

With that I measured the combustion chambers in the head coming off at 35cc, measured the old gasket at 0.9mm and confirmed the zero deck clearance and bore size

 

The total displacement is 1533cc. Previous compression was 9.54:1 & the new comp is 10.63:1

Edited August 1, 201511 yr by GJM85

Fair enough- You'll feel the difference!