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  1. #1
    Pistonwash's Avatar
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    About running higher compression.

    Just some observations from testing.

    When I went from 135 to 155 I picked up 30 rpm's. Went back to 140 and lost 30. The only concern is, I will have to run a half & half race/pump from now on. At wot w/ the 155 I could hear the deto w/ 93 octane.

    I think there are gains to had with squish design and cranking compression. Make sure your machinist gets it right.

    I remember back in my racing days, we'd run insane compression for quickness, no real gains (if any) in rpm's with the higher comp heads. Probably because of the deto happening with very tight squish bands and high msv numbers.

    On the GPR's, they seem to have a cut-off point of diminishing returns. I now understand why race tuners used the Kawi flat tops on the 1200's, trying to get the initial hit from the powerband. But they trade off top-end help from squish. If that makes any sense.

    I think with the right timing curve and right squish/compression numbers, even the single-pipers can achieve gains. If we're trying to go faster, we want to find the sweet spot and not have to trade off our holeshot for top-end. But in the process we must move the powerband up the scale in order to make more power up top...hence, more top-end speeds.

    Anyway, the point of this is, explore the rise in cranking compression carefully because there are gains to be made with the right combo. Call Lowell Horning to get a better handling on what you want in your set-up.

    In conclusion, the Nando reeds/Jetworks mod/handling set-ups will net you phenom gains. When you are trying to tweek a few more inches, send those heads and get em' done to your application.


  2. #2
    Hydrotoys's Avatar
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    Mike,
    Have you tried straight 93 at 140psi?

  3. #3
    Pistonwash's Avatar
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    Quote Originally Posted by hydrotoys
    Mike,
    Have you tried straight 93 at 140psi?
    Yes..I was'nt comfortable with the water temps coming from the head, then again I was running leaner than I needed. I think it is safe for singles to run it, but when I am hitting 7900 w? triples I think it becomes an issue. I am chicken, I know. But I have run for 2 or 3 tanks with 93 and not suffered any consequence. I am going to run the higher comp just because of the increase in rpm's. Believe it or not, those 30 rpm's are huge when everything is clicking.

  4. #4
    Hydrotoys's Avatar
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    Consider calling Randy at watcon.com and getting a pack of temp stickers. Use them to track the head temps. They will stick on the maximum color showing you the hottest cylinder that day or if you so care, that run.

    Whatcha think? 160-180 deg sounds about right for efficiency.

    If you know EXACTLY what temps your cylinders are running, you might have a better understanding of it, and then know what to do with the compression versus the gas quality.

  5. #5
    Moderator RX951's Avatar
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    ttt

  6. #6
    TJBrad04's Avatar
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    Quote Originally Posted by hydrotoys
    Consider calling Randy at watcon.com and getting a pack of temp stickers. Use them to track the head temps. They will stick on the maximum color showing you the hottest cylinder that day or if you so care, that run.

    Whatcha think? 160-180 deg sounds about right for efficiency.

    If you know EXACTLY what temps your cylinders are running, you might have a better understanding of it, and then know what to do with the compression versus the gas quality.
    150-180 is a generic temp they told us when i was going to MMI

  7. #7
    GPRPITMAN's Avatar
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    Like the idea of the stickers mark. And I was running 93 oct at 140psi on my triple application with the satndard curve for triple pipe pump gas. Now change that and your mixing fuel period. If not it will only be a matter of time before you stick a ring

  8. #8
    Hydrotoys's Avatar
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    I'll post some up used here in a few weeks... I've had them a while, but havent installed them. I'm dying to know about a couple of areas on the exhaust ports.

  9. #9
    checking the temp on the head wont help you guys to the real deal that is the temp of the dome the only way is a prove on the ex manifold water temperature on the top of the head is not accurate even the temp will change bet the first and last cylinder
    I build gprs engines whit over 200 psi on flat top kawi pistons and very good top speed but like pistonwash mentioned if you go to hi you will lose rpms but same times you need to loose same rpms to reach the tuning point on the pipes and prop the ski according to these rpms
    so the formula more rpm more speed is not all ways true I experience same engines rev over 8200 rpms and I have to use over props to brig the engine to 7900 since these was the rpm I like for these engine and to get the best temperature on the domes
    I remember on 8200 rpms the ski will reach the top speed for 2 o 3 sec and then fall to 7600
    on the 7900 range the ski will hold these rpms for 7 sec before drop
    the over role speed was more
    the squish is very important to and every setup is different in my super jet I run 235 psi the arrow on the piston make a impression on the dome these ski run good on these setup but you don't want to try these on a 3 cylinder
    porting time affect the compression so these is one of the more important factors you can run a gpr eng 155 psi on 92 oct like I do but I can race the port 1mm now my compression maybe will be 135 these don't mean I can run 87 and now you change the domes and com again reach 155 psi if you use 92 you will need a new pistons in 5 minutes you may have to use 97 so what I try to say don't use a compression us a tool to see which gas to use and different gas companies will give you all different results

  10. #10
    ABBOTT's Avatar
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    good info here!!

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