How does a dome increase compression?

[Circlotron]

Me just saying the same thing as everyone else:
For the sake of round figures, say you have a cylinder volume of 1000cc and a combustion chamber volume of 100cc.
Total volume at BDC = 1100cc, total volume at TDC = 100cc.
1100 squeezed down to 100 = 11:1 CR.
Now put a 50cc pop top on the piston.
Total volume is 50cc less so we squeeze 1050cc down to 50cc. That’s 21:1 CR.

[Racer18]

The reason for my post- few years back we had a customer that was point racing. Dropped a valve damaging a couple pistons. He had custom made domes, we couldn’t get replacements in time, so we got a set of flat tops and bandaged it back together so he could keep his championship hopes alive. To my surprise, it made significantly more power and torque on the dyno with the flat tops. Customer also said it raced much better. I could never wrap my head around it. The difference was 13.8:1 down to 11.2:1.

[Ericnova]

The reason for my post- few years back we had a customer that was point racing. Dropped a valve damaging a couple pistons. He had custom made domes, we couldn’t get replacements in time, so we got a set of flat tops and bandaged it back together so he could keep his championship hopes alive. To my surprise, it made significantly more power and torque on the dyno with the flat tops. Customer also said it raced much better. I could never wrap my head around it. The difference was 13.8:1 down to 11.2:1.

You need to be thinking about things like flame front inhibited by too tall a dome blocking flame travel, or camshaft lobe events not correct for the 13.8:1 combination but they matched up better with the 11.2:1 combination....and I'm sure others can fill in something I'm not thinking of at the moment. Fuel type maybe a factor too?

It was nothing to do with compression ration per se....the combination just wasn't right...."engine builder failure"

[Steve.k]

This was the exact reason I posted the thread about domes pistons and extra compression. Sometimes its not as simple as squeezing more compression to obtain more tq. I've done a few engines now with flat tops and they were mild in nature and the outcome surprisingly did better than we expected.

[ClassAct]

The reason for my post- few years back we had a customer that was point racing. Dropped a valve damaging a couple pistons. He had custom made domes, we couldn’t get replacements in time, so we got a set of flat tops and bandaged it back together so he could keep his championship hopes alive. To my surprise, it made significantly more power and torque on the dyno with the flat tops. Customer also said it raced much better. I could never wrap my head around it. The difference was 13.8:1 down to 11.2:1.

Did you dyno the engine after the Pistons were changed to VERIFY you made more power with less compression? I'd bet you didn't. I've personally seen so many cars go faster with less power it's just silly. And the guys who run their mouths the loudest about how there isn't an engine builder who can't make more power than they can get to hook are the exact ones who can't tune a chassis.

My suspicion is your engine lost power (a good bit) and the chassis and everything else was happier with less power, hence it went faster.

[Racer18]

Class Act- YES we did dyno it, otherwise how would I have known the numbers? Not guessing, it was factual reality that I saw with my own eyes! We at first thought there was something wrong with the dyno/data, but after freshen with domes, it was back to original numbers (or very close). And yes, most dirt cars are way overpowered for the average saturday night guy, but this one had his ducks in a row, and had won several championships and races over the years.

[Steve.k]

He says it made more tq and hp on dyno.

[GARY C]

My thoughts were, a domed piston has less cylinder volume at bottom dead center ( by the amount of dome) than say, a flat top piston. Again, just curious as to how subtracting x amount of volume at bdc makes more compression than subtracting same amount at tdc.

This is one of those things that visually it appears that way but mathematically and in practice shows otherwise.

If you did a cylinder pressure test on a flat top engine and then only reduced the chamber size you would see an increase in cylinder pressure, so in simple terms it's the size of the area you have left to compress everything into that makes the difference.

[Steve.k]

The reason for my post- few years back we had a customer that was point racing. Dropped a valve damaging a couple pistons. He had custom made domes, we couldn’t get replacements in time, so we got a set of flat tops and bandaged it back together so he could keep his championship hopes alive. To my surprise, it made significantly more power and torque on the dyno with the flat tops. Customer also said it raced much better. I could never wrap my head around it. The difference was 13.8:1 down to 11.2:1.

It could've been cam was spec'd a bit off for compression/head flow and liked less compression. But who knows? Could be flame travel also.We recently did a mild cleveland 358 open chamber headed 4v motor with 9.3:1. Goal was to crack 400hp but to everyone's surprise the motor made 480@6000 and was still climbing but owner didn't want to rev past 6000 so thats where we stayed. Ive seen higher comp motors struggle to crack 500 so it got me scratching my head.

[GARY C]

The reason for my post- few years back we had a customer that was point racing. Dropped a valve damaging a couple pistons. He had custom made domes, we couldn’t get replacements in time, so we got a set of flat tops and bandaged it back together so he could keep his championship hopes alive. To my surprise, it made significantly more power and torque on the dyno with the flat tops. Customer also said it raced much better. I could never wrap my head around it. The difference was 13.8:1 down to 11.2:1.

A buddy of mine changed to a lower ring pack in his nitrous BBC and to maintain the same compression they increased the size of the dome, it ended up hurting the power so he re-machined the dome to match the other and lost some compression and regained the power.

This was caused by poor flame travel of the bigger dome.

Cam timing between the two compressions can have an effect also. In general you need less overlap to scavenge a small chamber volume than you do for a large one.

[swampbuggy]

I have been told by people more engine smart than myself that increased compression ratio is one of the changes that will increase power over the entire RPM band. This is IF ALL engine parts are maximized for the combination (work in harmony together) . Mark H.