Engine and Transmission Tech
tips, technology, tools and techniques related to vehicle driveline components
|
Compession ratio 300 Heads
Looking for some information regarding compression ratio if I install Buick 300 heads on a stock spec. Rover 3.9 block. I would appreciate it if someone could supply any information. The heads have been ported and have the big valve conversion.
Thanks for the help. |
|
Re: Compession ratio 300 Heads
Just to add to the 300 head compression question - the 3.9 block is a 9.3 compression version (with stock Rover 3.9 heads) and the Buick 300 heads are aluminum. Any idea how the use of the tin or composite head gaskets would effect the outcome?
Thanks again |
|
mgb260 Jim Nichols Sequim,WA (2465 posts) Registered: 02/29/2008 08:29PM Main British Car: 1973 MGB roadster 260 Ford V8 |
Re: Compession ratio 300 Heads
Don't know exactly but, you probably have dished pistons and those chambers are huge. Probably lost one point. years ago I used the 300 heads on a Buick 215 with the 11 to 1 flat tops. The block was decked slightly and heads shaved just to make sure everything was flat. I figured about 9.5 to 1. I think it is common to deck the block up to .040 to zero deck the pistons and mill the heads .030 to maintain your compression. The newer Rover heads I think have 29cc chambers where the Buick 300 has 55cc., the 215's somewhere in between. I think Curtis (our Moderator here) used the later heads to go up about a point.
Edited 1 time(s). Last edit at 08/31/2013 02:41PM by mgb260. |
|
|
Moderator Curtis Jacobson Portland Oregon (4577 posts) Registered: 10/12/2007 02:16AM Main British Car: 71 MGBGT, Buick 215 |
Re: Compession ratio 300 Heads
This online tool makes it easy to see how the variables work together:
[www.rbracing-rsr.com] For example, you can enter different head gasket thicknesses... |
|
Dan Jones Dan Jones St. Louis, Missouri (280 posts) Registered: 07/21/2008 03:32PM Main British Car: 1980 Triumph TR8 3.5L Rover V8 |
Re: Compession ratio 300 Heads
If you start with 9.3:1 3.9L and switch from 36cc Rover heads to
54cc Buick 300 heads, your compression ratio will drop to 7.4:1. Heads can be angle milled and block decked, along wth thinner head gaskets to increase the compression ratio. Dan Jones |
|
mgb260 Jim Nichols Sequim,WA (2465 posts) Registered: 02/29/2008 08:29PM Main British Car: 1973 MGB roadster 260 Ford V8 |
Re: Compession ratio 300 Heads
Wow! Dan, Almost 2 point drop. I must have been around 9 to 1 then. I could use regular gas. A lot of people don't realize how big those chambers are. All they had for head gaskets way back then were the thin steel shim type, didn't Rover go to the thicker ones when they went to the 29cc chambers on the 4.0 and 4.6 heads?
Edited 1 time(s). Last edit at 09/01/2013 01:20PM by mgb260. |
|
Dan Jones Dan Jones St. Louis, Missouri (280 posts) Registered: 07/21/2008 03:32PM Main British Car: 1980 Triumph TR8 3.5L Rover V8 |
Re: Compession ratio 300 Heads
> A lot of people don't realize how big those chambers are.
They work out well on stroker engines with the usual piston choices but with a dished piston on a smaller displacement engine, the compression is quite low. > All they had for head gaskets way back then were the thin steel shim type, > didn't Rover go to the thicker ones when they went to the 29cc chambers > on the 4.0 and 4.6 heads? Yes. The difference in the thickness of the gaskets makes up for the difference in the combustion chamber volume. Dan Jones |
|
|
|
BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6470 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: Compession ratio 300 Heads
You might do well with them in combination with a flat top piston at zero deck and a .040" gasket. That gives a proper squish distance as well though you'll have to look carefully at the CR as it could be higher than you are looking for. On my 5.7L motor the compression came out to about 10.6 with a 13 cc dish which was just about what I was looking for.
Jim |
|
Re: Compession ratio 300 Heads
Thanks for all your replies - got the information I needed and confirmed that the use of the 300 heads would drop the compression to an unsuitable level. I originally planned to use the heads on a stroker engine but decided to go with the original short stroke for higher R.P.M.. I searched the forums trying to find a higher compression piston that would bring the engine back to the 9:1 to 10:1 range with the 300 heads but kinda got lost with all the information available.
Any suggestions?? Looking for the easiest route that doesn't involve a lot of machining. Don't really want to mill the heads too much as I may want to go back to the stroker option at a later date. Thanks again for all your help. |
|
BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6470 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: Compession ratio 300 Heads
For a high rpm engine I highly recommend a forged piston. I broke a piston skirt myself using a 7000 rpm redline with cast pistons. Custom forged pistons made to your requirements generally run about $70 each plus options.
Jim |
|
mgb260 Jim Nichols Sequim,WA (2465 posts) Registered: 02/29/2008 08:29PM Main British Car: 1973 MGB roadster 260 Ford V8 |
Re: Compession ratio 300 Heads
The Keith Black Hyper pistons are a good compromise between stock cast and forged and way cheaper, You just have to keep it under 6000 rpm. If I was building a Rover V8 with 300 heads nowadays I would look at a flat top piston made for a 305 Chevy stroker(.036 overbore) and 6" Chevy style small journal(2")aftermarket rods. 4.6 crank offset ground to same stroke as the 300 Buick. Newer crossbolted 4.0 or 4.6 block. Very tempting but, too many other things going on.
|
|
|
|
Robrover Rob Thornton Adelaide, Australia (20 posts) Registered: 10/01/2009 11:52PM Main British Car: 1978 Rover SD1 4.6 |
Re: Compession ratio 300 Heads
I'm using big valve ported 300 heads on a 4.6 block.with std 9.3:1 Rover pistons in my SD1. There are a couple of ways to get the CR up. One is to swap in 4.0 pistons. The other way is what I have done: mill .080 thou off the 300 head faces, then weld the combustion chamber up (behind the spark plug aperture) to get a 36cc chamber. Use 300 tin gaskets instead of composite Rover ones. End up with about 9.2:1 CR which is ok on the street for the crap fuel we get here nowadays.
|
|
Dan Jones Dan Jones St. Louis, Missouri (280 posts) Registered: 07/21/2008 03:32PM Main British Car: 1980 Triumph TR8 3.5L Rover V8 |
Re: Compession ratio 300 Heads
> then weld the combustion chamber up
I spoke to a local cylinder head shop about this a few years back. I forget the cost estimate but it was quite modest to weld up the chambers using the Tig torch if I did the finishing work. He was actually excited about it as he thought he could do a better combustion chamber shape with a squish pad. Dan Jones |
|
roverman Art Gertz Winchester, CA. (3188 posts) Registered: 04/24/2009 11:02AM Main British Car: 74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L |
Re: Compession ratio 300 Heads, JB weld ?
"then weld the chamber up", Yes this can be done- BUT you have dead soft welded aluminum and reduced hardness, heat affected zones. The aluminum 300 head has a thin deck thickness standard, for high performance use, and only 4 bolts around each bore. Do you feel lucky ? Good Luck, roverman.
|
Sorry, only registered users may post in this forum.
British V8 Home Page
Photo Gallery
Web Forum
Annual Meetings
Contact Info
Site Map
British Race Car
© 2019 British V8™ All rights reserved. Website & graphic design by:
Curtis Jacobson.
BritishV8 Forum is hosted by
SiteWelder LLC,
and is powered by a
Phorum engine.