10.3 to 1 compression on pump gas at 4000ft

[Hauls Assington]

I got a stroker motor 414w, and it has 10.3 to 1 compression. The guy said it did fine on 91 octane, we are at 4000 ft.

I am wondering how fine that is really going to be. It has Iron heads, windsor sr. Mild cam, probe bottom end with forged pistons.

Can I take out some timing if it is getting hot for dd use?

I know many people say that 10:1 is too much on pump gas, but most of those people are at sea level, I am wondering if 4000 ft above sea level is going to be enough to make this set up ok.

I was wondering if maybe they made sparkplugs that were a tad shorter, and maybe it would reduce compression ratio a little.

[philofab]

Cam specs make a big difference. Saying mild does not tell us enough.

[Rmc]

You can change to a thicker head gasket to lower the compression. Keep same plugs. Google setting the deck height on a 351w, you should find an answer.

[Rmc]

Keep in mind I'm not an engine builder or anything. So go easy on me if I'm wrong.

[Hauls Assington]

It has a comp cams:
gross lift int .477 ex .493
duration @.006 254 262

with cam installed

duration @ .050 int 210 ex 218
lobe lift .298 .308

lobe separation 111

If that helps at all.

There is also a valve timing section on this comp cams sheet if that will help.

[Hauls Assington]

You can change to a thicker head gasket to lower the compression. Keep same plugs. Google setting the deck height on a 351w, you should find an answer.

Nor am I an engine builder! That is why I really don't want to tear into this thing if I don't have to lol.

This thing runs like a raped ape, and I don't want to monkey around with it anymore than I have to.

But it seems like thicker head gaskets would be a lot better than new pistons!

[MOSS2]

Should be able to tune it do fine on 91 at that altitude. If you want to try and run 87 though it could need some work. Might try doing a compression test to see what your cranking compression is to see what ballpark your in. Sometimes too mild or stock cams can give you really high cranking compression. May get away with more with a more radical cam and less separation that is not so efficient at the low rpms. Find somebody good at setting up ignition curves which can make a big difference. Is it carburetor?

[hobbyturnedobsession]

I'd run it. The can isn't crazy and as dumb as it sounds my jet boat has quite a bit of motor work done (BBC 454 healthy can and internal work) and I run 91 with no issues.

[philofab]

High compression needs a cam not optimized for low RPM to get away with low octane. If you can get the ignition curve to keep the timing retarded until 3500 rpm or so you should be ok. I wouldn't try and get away with 87 or regular Pemex with that motor though.

[hobbyturnedobsession]

I agree with Philo. That's a premium unleaded motor only

[Hauls Assington]

Should be able to tune it do fine on 91 at that altitude. If you want to try and run 87 though it could need some work. Might try doing a compression test to see what your cranking compression is to see what ballpark your in. Sometimes too mild or stock cams can give you really high cranking compression. May get away with more with a more radical cam and less separation that is not so efficient at the low rpms. Find somebody good at setting up ignition curves which can make a big difference. Is it carburetor?

Yeah, it has a holley vac secondary 750cfm.

Definitely going to run premium.

[Hauls Assington]

High compression needs a cam not optimized for low RPM to get away with low octane. If you can get the ignition curve to keep the timing retarded until 3500 rpm or so you should be ok. I wouldn't try and get away with 87 or regular Pemex with that motor though.

Based on those cam numbers, would you say this is a low rpm cam? definitely going with 91 octane, if not a splash of oct boost.

[philofab]

Pretty mild for how many cubes that motor is. The bigger the motor and heads the smaller the cam is. That cam would be way bigger on a 302 if you get what I am trying to say.

[Hauls Assington]

Yeah, I think it is a low end monster of a motor, it is supposed to be well above 500 ft lbs.

[Seabass]

Elevation won't change what octane you have to run just jetting/tuning. 10.3 insn't really crazy compression, high for iron heads but not crazy. If you have to change compresion or want more out of it your money would be better spent going to aluminum heads over changing pistons.

[baja-chris]

Pretty mild for how many cubes that motor is. The bigger the motor and heads the smaller the cam is. That cam would be way bigger on a 302 if you get what I am trying to say.

The same holds true for intakes. The rated rpm range on windsor intakes are often based on a 302 inch engine but what's needed for a 408 is much larger.
Our 427 crate engine from Ford has a 5600 rpm redline and Ford says to use the Super Victor intake which is rated at 8500 rpm.
A 427 at 5500 rpm needs roughly as much air as a 302 does at 8000 rpm.

[Hauls Assington]

Pretty mild for how many cubes that motor is. The bigger the motor and heads the smaller the cam is. That cam would be way bigger on a 302 if you get what I am trying to say.

The same holds true for intakes. The rated rpm range on windsor intakes are often based on a 302 inch engine but what's needed for a 408 is much larger.
Our 427 crate engine from Ford has a 5600 rpm redline and Ford says to use the Super Victor intake which is rated at 8500 rpm.
A 427 at 5500 rpm needs roughly as much air as a 302 does at 8000 rpm.

Ahh, this is good info! I didn't know that. Do you think I would get more hp with a decent single plane? It has a weiland dual plane on it.

The only thing is, I really want to be able to use the super low stall converter I have, and be able to tip toe around in nasty rocks if i need to.

[Hauls Assington]

Well, it looks like I'm going to run this puppy as is. I am going to have someone set the timing and do a tuning job on it, and hope for the best lol.