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compression ratios for pump fuel?
lohoon - January 25th, 2006 at 08:08 PM

Whats a good compression ratio for pump fuel?Where can i get barrell spacing shims?

hellbugged - January 25th, 2006 at 09:52 PM

worked out to 8.5:1 for 98 octane for me.

i'll email you about how i came to this conclusion

dangerous - January 26th, 2006 at 06:44 AM

I would like to hear what you have found about this.
For years people have been using a "certain guy's" recomendations, and there are many ifs and buts.
We have a few customers that run 10:1 on Optimax (98?), and engine life is very acceptable.
I think that if the engine CANT be built with less than 1.5mm piston to head clearance, or has a semi-hemi
chamber, then "that guy's" recommended 7:1 is a safe comprimise.
If you can build the engine with around 1mm deck, then I am thinking at this stage that another point higher would be safe.
Apparently dual carb set-ups can handle more.
Of course, if the engine temps are high something needs to be dealt with.
There is alot of info about this topic that people can share their opinions and experiences.
There are detonation issues, and engine temp issues of various parts of the engine.
What ever ratio you choose, always run less than 1.5mm deck and use chamber CCs, or a piston dish to acheive what comp you want.
I would personally run 1mm plus or minus .05 deck and use a piston from a type 4 or Waterboxer
to get the CC,s I want. You can use a Waterboxer rod with better bolts, to make this fit, or buy custom pistons.

[ Edited on 25-1-2006 by dangerous ]

[ Edited on 25-1-2006 by dangerous ]

hellbugged - January 26th, 2006 at 08:03 AM

what about 92mm, Dave?

any reccomendations specifically for them?

dangerous - January 26th, 2006 at 10:09 AM

Oooh, this could be started in a whole new post, and realy stir up some controversy!
Can you still get them?
The sets that I have measured stayed just as round as any other size,
unless they had been submitted to over heating and detonation.
Most bad reputations came about due to poor assembly and poor tuning.
Except for their reduced availability, they will make more power than 90.5s,
and have the same material removed from the case.
They also have more BDC cylinder support than 94s.
Don't be afraid of 92s...its just a number.

VWCOOL - January 26th, 2006 at 11:52 AM

a lot to do with cam, too - cylinder pressure at max torque has as much to do with pinging etc than static compression ratio

And no, I don't know the answer!


92s... lol... what about less support at bottom of barrel than some other sizes

[ Edited on 26-1-2006 by VWCOOL ]

lugnuts - January 26th, 2006 at 01:08 PM

Cool info Dave,'I would personally run 1mm plus or minus [.05] deck,is this a typo,can u spell it out for the dummies,sorry dude i needs to know :thumb

dangerous - January 26th, 2006 at 02:27 PM

38 to 42 thou piston to head clearance.
= .95mm to 1.05mm.
The 94 barrel only has 16mm in the case (and thinner), the 92s and 90.5 has an 18mm spigot.
Yeah only minor details.

lohoon - January 26th, 2006 at 05:06 PM

The situ is: I have 1.7 deckheight,92x69,52cc heads=8.2:1 compression.Is this going to detonate an ping its heads off?
Will i be able to put normal firing time into it?

VWmotorvation - January 26th, 2006 at 09:11 PM

Hi, I agree with Dave with the deck height thing but i wouldn't go under 1mm. I definitly don't want to start any big arguments about 90.5s vs 92s as both work fine. One advantage of 90.5s for me is the ability to turn down the spigot to 88mm thick-wall size to give a wider seating surface on both the cyl. and case. Also, 90.5s won't dig into the head quite so much and can withstand a fraction more comp. (shorter distance from pl:blush:ug to edge of chamber). But dont worry, your 92s will be fine. To the point, comp. ratio should be guilded mainly by cam, although as Dave said, dual carbs will allow a little more. I've refered to engle cams but you can look up various charts to convert other cams.
Assuming dual two-barrel carburetion and 98 octane unleaded:
Stock cam: up to 8.5:1
Engle w100: 8.0 to 9.0:1
" w110: 8.5 to 9.5:1
" w120: 9.0 to 10:1
" w125: 9.5 to 10.2:1
" w130: 10.0 to 10.5:1
" w140: around 11:1
Just my experience. If the comp. is not raised according to cam duration, the engine will not feel crisp and will have a soggy bottom end with power that never quite comes "on song". As for detonation, as cam duration increases, so does the RPM point where maximum cyl. pressure occurs and there is less actual time between when the plug fires and when the piston is actually on it's way back down. This means detonation is less of an issue. Sorry this post is far too long!!
Just my thoughts, hope it helps. See ya

hellbugged - January 26th, 2006 at 09:19 PM

what are peoples thoughts on copper head gaskets/shims?

hellbugged - January 26th, 2006 at 10:07 PM

Quote:
Originally posted by dangerous
I would like to hear what you have found about this.
For years people have been using a "certain guy's" recomendations, and there are many ifs and buts.
We have a few customers that run 10:1 on Optimax (98?), and engine life is very acceptable.
I think that if the engine CANT be built with less than 1.5mm piston to head clearance, or has a semi-hemi
chamber, then "that guy's" recommended 7:1 is a safe comprimise.
If you can build the engine with around 1mm deck, then I am thinking at this stage that another point higher would be safe.
Apparently dual carb set-ups can handle more.
Of course, if the engine temps are high something needs to be dealt with.
There is alot of info about this topic that people can share their opinions and experiences.
There are detonation issues, and engine temp issues of various parts of the engine.
What ever ratio you choose, always run less than 1.5mm deck and use chamber CCs, or a piston dish to acheive what comp you want.
I would personally run 1mm plus or minus .05 deck and use a piston from a type 4 or Waterboxer
to get the CC,s I want. You can use a Waterboxer rod with better bolts, to make this fit, or buy custom pistons.

[ Edited on 25-1-2006 by dangerous ]

[ Edited on 25-1-2006 by dangerous ]


lets not forget that the semi-hemi was introduced originally when the octane of pump fuel dropped from over 100 to less than 90.

this "scooping" of heads was originally to give existing engines the head cc to lower the compression BUT maintain the desired deck height as Dave is talking of.

i too would not go for a semi-hemi

when the octane was good, like everyone, GENE BERG, there i said it, was happy to run high compression, I'm sure if he had the chance still, with todays improved fuels, he also would be again recomending the good old figures.


not bitching or winging, crying or upset, nor contradicting anything of the above posts.

wonderful information from Dave as usual, he is certainly an asset to the forum, it also looks like Mr VWmotorvation knows his stuff! thanx

lohoon - January 27th, 2006 at 06:54 AM

Yeah,Thanx all for your knowledge.I'll be running twin Weber 44s,a mild cam and 1.25 ratio rockers.So the compression seems fine!cool i can bolt it all together!

dangerous - January 27th, 2006 at 09:30 PM

Just some personal preference here, but I would reduce that deck height a little,
and unshroud the chamber a little on the inlet side to get your CCs back. 8.0 to 8.5 sounds fair if your cooling system is in good order.
Either way, let us all know how it performs.
Oh yeah, I like to set the piston flush with the top of the barrel and get the deck height in the head,
with a machined step, or a copper gasket if there is some head material for support around it.