11:1 compression too high for pump gas?

SadbutTrue · Apr 2, 2008

With aluminum heads and flat top pistons, would an 11:1 compression ratio be too much for 91 pump gas (california )?

68keyblr · Apr 2, 2008

It's always a crap shoot, but I think you'll be OK. A friend is running 10.25:1 compression with aluminum heads, flat top pistons, and can run 89 with no problems. Depends on your tune too (timing and fuel curves).

57fairlane · Apr 2, 2008

get a custom cam . . .

65fastbackresto · Apr 2, 2008

Mine is 10 to 1 and I cant run 87

but 89 or 91 works out fairly well.

Shell stations here in AR carry 93 octane, ya might look around out there and see if someone around sells it.

mustbereel · Apr 2, 2008

65fastbackresto said:
but 89 or 91 works out fairly well.

Shell stations here in AR carry 93 octane, ya might look around out there and see if someone around sells it.

There's a place in Rancho Santa Fe that sells 100 octane for $7.99/gal. It's for all the high powered high dollar exotics all those millionares drive. That's $7.99 full serve of course.

Vinyl66 · Apr 2, 2008

The cam means everything. You could pick a cam with that compression that would ping like crazy on 93 octane or run perfectly fine on 89.

I'm with 57fairlane: Get a custom and tell them everything vital about your combo and that you have to run on 91 octane.

2bav8 · Apr 2, 2008

With the right setup it would be ok.

My 347 has 10.8:1 and its fine on 91 octane.
I've even used 89 octane on road trips before.

rbohm · Apr 2, 2008

with the right fuel and ignition curves, and the right cam timing, as well as the best exhaust efficiency, 11:1 on the street is doable. by the way i would tune to run 89 octane, that way if 91 goes away, you can still find gas to run.

Decurion · Apr 2, 2008

If you ran E-85 you wouldnt have a problem at all! Even with timing and fuel set for maximum power! Ill assume you already have the engine built, so thats sort of out of the question, but its a thought. Its something Ive been thinking about anyway.

But you should be ok on premium, just be careful at first to listen for knock. Also make sure that all edges in the combustion chamber are WELL rounded.

SadbutTrue · Apr 2, 2008

The engine isn't together yet, I'm just having another mental battle debating the pros and cons of building my 351w or getting a crate motor/long block from someone else. My current 351 has really low compression pistons (well, I think it does anyway, if it was rebuilt to the stock specifications), and I either need to mill the heads or get new pistons to get the compression up.

I'm trying to do the job right but still do it in a cost-effective manner... as you may expect the two goals conflict a bit.

SadbutTrue · Apr 2, 2008

If I wanted a custom cam, who would I talk to? Ed Curtis? Are there specific camshaft specifications that imply high-compression friendliness? (ie what #s would make a cam agree with high compression/pump gas)

dodgestang · Apr 2, 2008

SadbutTrue said:
If I wanted a custom cam, who would I talk to? Ed Curtis? Are there specific camshaft specifications that imply high-compression friendliness? (ie what #s would make a cam agree with high compression/pump gas)

Valve overlap in the timing of events bleeds off compression. Ed is a good guy and will certainly help you. He has speced both custom cams in my stroker (one that went when the motor bottom end did and one that is sitting on the shelf waiting for to install it one of these days)

rbohm · Apr 2, 2008

start with denny wykoff at motor machine and supply here in tucson. he has developed a cam program that will give you a custom profile that will give you best power for your combination. all you need do is tell him what you have, and what you are looking for.

blown65 · Apr 3, 2008

I'd go the Ed Curtis route and just have it set so you can run 100 if your at the strip with extra timing, and back off to run 91 on the street. Our Pantera is that way, its at 12-1 and runs on 93 octane, but its real touchy to timing.(408w/AFR heads) Add some race gas when ya wanna run it hard, but be very observant of detonation on the pump gas.

65ShelbyClone · Apr 4, 2008

There is also Jay Allen at Camshaft Innovations. He seems to be popular on the ******* forums, but I have no personal experience.

EDIT: I see how it is. We can't even mention other forums like SBF Tech, huh? I remember many of the Honda forums being immature like that...

jerry S · Apr 4, 2008

I ran 11:1 on my 351W with absolutely no problems.

Edelbrock RPM Performer alum heads
Crower Flat tappet Cam

ADVERTISED CAMSHAFT SPECIFICATIONS:
INTAKE: Duration: 297º Lift: 0.538 Clearence Hot: 0
EXHAUST: Duration: 308º Lift: 0.534 Clearence Hot: 0

Duration at .050: Intake 236 / Exhaust 242

I now have a roller cam with more aggressive lobes (and by that I mean the side profile) and higher lift that is going in. Troy Bowen tells me I should have no issues with detonation with this cam.

11:1 compression too high for pump gas?

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