Compression

Getting your blazingly fast Suzuki powerplant to perform even better!

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water cooled
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Compression

Post by water cooled »

I am in the process of adding a lift plate under my barrel and at the same time want to increase compression slightly so I have calculated that the deck needs to be cut the difference between a stock base gasket and the lift plate plus its two gaskets....plus an additional 10 thous to increase approx 5 psi.

Just want to check and see if actual experience bears out a corrolation of .010" off the deck equating to about 5 psi increase.

I have a cylinder bore of 2.7759 inch and actual (measured) volume of 23.7 cc (1.4463 cubic inch ) at TDC. My current cold compression is 135 psi on each cylinder.
Last edited by water cooled on Fri Apr 03, 2009 9:32 am, edited 2 times in total.
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tz375
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Post by tz375 »

Kevin, PSI cold cranking is not the best way to work out your compression. it's better to know the uncorrected (geometric) ratio and corrected. The latter is more important on the street.

Just as a point of reference, my last street motor ran 150PSI.

For a drag motor I'd start at 13:1 and work UP from there. The bore size is very large so you will have problems getting it run cleanly at 15:1 that I was thinking earlier, But it might work there with the right fuel and head shape. Try VPMR11, it burns fast and has high enough octane to resist detonation.

If your block and head are stock, you will have a head problem. At the very least, the heads and barrel need to be doweled and matched, but I'd recommend squish heads.
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Post by water cooled »

Hi Richard,

The cylinder is a '74 modified and the cylinder head is stock except being skimmed. Exhaust and Intake ports were opened and the transfers were left alone.

The cylinder has already been decked once. Presently, exhaust ports have a roof at 36mm and intake floor are 105mm with the bridge removed.

The uncorrected compression is 11.54:1 and corrected is 6.93:1 with these exhaust ports. The volume was measured fairly accurately on all three cylinders with very little variance.

I've decided not to port the cylinder further for this racing season but did notice that the cylinder should be raised at least 52 thou to fully open the floor of the transfers and exhaust. The lower piston ring is still contained inside the cylinder but with a lift plate, the ring is right on the roof of the intake port.

So the lift plate is needed and Ihave to send the cylinder out to get decked next week and would like to increase compression a bit at the same time.
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Post by water cooled »

Richard,

I'm interested in learning more about doweling and matching the cylinder to the head. I'm begining to see that for several reasons, indexing the cylinder and head to each other is important if nothing else than to locate the copper gasket more accurately.

Do you have any information on locating and pinning the cylinder head??
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Post by water cooled »

Richard,

To get to 13:1 would require removing 2.7 cc's from the combustion chamber which could be had by removing .027 thou from the deck. I assume a squish design is the otherway to get there but only if the barrel hasn't already been decked too much.
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Post by water cooled »

My Google Search for GT cylinder head alignment turned this up...

http://www.pinkpossum.com/GT750/GT750head.htm

Richard, I hope its ok to post this.
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tz375
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Post by tz375 »

Kevin,

Yes, that the page I threw a quick page together a couple of weeks ago at: http://pinkpossum.com/GT750/GT750head.htm

Not all heads and barrels are as bad as this pair, but both I have here are this bad. It causes blown head gaskets, detonation and melt down like a Russian nuclear reactor.

I used a pair of Honda dowels I happened to have in the shop, but there are many alternatives. You could drill new holes in the head, then use transfer pins (pins with a ground point) to transfer the location to drill the barrel.

11.54:1 should be closer to 150 PSi kicking it over. I wonder if your rings aren't sealing or if the gauge is reading low. My Motion Pro gauge give a higher reading that the one I got from Summit. It's a function of where the pressure release valve is located I believe.

I typically will machine the block to get a deck height of 0.00mm, so I start with the block and then move to the head to get what I need in the way of compression. It's 3.847cc per mm, so .027" (.69mm) = 2.7cc - your calculator is right.

The transfer and exhaust port floors are usually lower than the piston crown. I seem to recall reading somewhere that flow is better that way, but it's not clear why that should be. I think it's because there is so little flow at the bottom corner that dropping it has no bad side effects.

For you motor, you can go beyond TR750 type porting because you use the throttle like an on-off switch. You aren't looking for a smooth idle or great midrange - just top end power. With huge exhausts you really need to pen it up at the header flange as far as it can go otherwise it's a constriction.

The spacer will give you more exhaust and transfer duration and area and will keep the long blowdown you already have which is OK. I suspect you'll run out of intake area with those settings. you could be aiming for 120-140 at the crank with the right pipes, but is going to try to rev to very large numbers - ask Steve H for details!

But let's get the head right first. Do you have access to a squish head at a reasonable price? You must use a good head gasket - get one from Suzukidave and match it to your head/barrel combination.


BTW, what are the port widths?
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Post by water cooled »

Thanks Richard,

Its funny that you mention access to a squish head. I was just contacted today about one...price seemed reasonable. I also have two of the copper head gaskets from Dave but one is spoken for.

There is a good chance that my compression readings are off. I bought the cheapest compression gauge I could find at NAPA and it was freezing cold in the shop when I took the readings..At the time, I was simply trying to compare cylinder readings. They were very consistent between all three.

I didnt calculate the aperture width but the circumferential segment width is:

Exhaust: 47.2mm
Transfer: 36.0 mm
Intake: 53.0mm edge to edge with no bridge

The rings are new and gap tolerances are all well within Suzuki spec. though I was going to ask at what point stock piston rings become an issue on a race engine where piston acceleration might cause ring-flutter.

Is there a modern (lower inertia) ring that can be used on stock or modified stock pistons for a race version GT750?
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Post by tz375 »

Kevin,

Port width
When you say circumferential segment, do you mean measured around the surface of the barrel as in a port plot flattened out? Or do you mean straight across as measured with calipers across the aperture?

Rings
Good point. Those rings are thick and heavy and will flutter if the revs go high enough. Interpolating an old table of acceptable acceleration rates, 1.5mm rings can stand piston acceleration of around 94,000ft/sec2 which equates to 7,945 RPM - repeatedly.

I'd be happy to run that configuration up to 9 or even 10k for short distances as long as I pulled the pistons to inspect for wear (the ring groove would get hammered and open up) and I'd replace rings regularly (whatever that means in your context :lol:

On 4 smokes they add gas ports to add more ring pressure, but I have no experience with them on a hi-po 2 stroke to know if they work. What are teh H2 guys doing about rings and gas ports? and do the top guys run single rings? What do B Baxter and Chris Ritchie, or other Kawa pilots, recommend.

This all comes down to budget and as you start to go faster you are on that slippery slope of decreasing returns. It's the old story about speed costs, how fast can you afford to go. :lol:
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Post by water cooled »

Port width: I measure around the port on an arc and layed flat. I have not yet converted the dims from the port rub over to a chord length. The dims were measured from the flat paper.
rngdng
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Post by rngdng »

You guys are really getting into this! On mine, it was all guesswork; a little of this, a little of that, not so much of that...... :D :D :D :D



Lane
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Post by tz375 »

Lane,

it's really more a case of me trying to be 100% sure that my understanding of what Kevin is saying is the same as what he thinks he is saying.

The empirical way has a lot of merit and it the final analysis it's the only way to get real results. I believe that Brett in Sydney works that way. No computers, just try it and see, and try some more. Some guys just have the experience to make that work.

I have neither the time nor the skill of a guy like Brett, so I use tools that get me a little closer.

Of course, in the real world, engines never behave the same way as the computer predicts, but it's a useful tool to see what works and what hurts.

I ran a set of simulations for someone recently and as I changed one variable after another, what I found was that I could move the whole curve around as you might expect. What I also got were "holes" where everything was slightly out of synch and the holes moved all over the powerband. Sometimes a seemingly small change resulted in a huge hole at say 3500, or a dip at 5000.

What I had to do was to work out what was out of synch and tweak it until we had a reasonably smooth powerband with fatter shape below peak revs.

If the truth were known, most of us could port a set of barrels and have a motor that feels good and never know or care if it could theoretically be better. Sometimes a hole before the power comes in makes it FEEL more powerful and in the final analysis that's what we want.

Let's face it, if a TR750 managed to go 180MPH with road race fairing, do we really NEED that much HP (or more) on a street bike with heavy flimsy suspension and a bendy frame?

Probably not, but it's fun to see what we can do. :)
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Post by jkevinlilly »

Somehow I feel I am back in college trying to understand calculas and organic chemistry. :shock:

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Post by jaybob »

Yeah same here. Only it's not the first time on this board that the technical discussion has gone waaay over my head.
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Post by tz375 »

Kevin,
is that the right spelling?

I thought it was Orgasmic chemistry that we studied at uni. Maybe that's why I was invited to come back and do it all again. :oops:
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